BEST PRACTICE DESIGN GUIDEFOR PUMPING SYSTEMSVersion 1.0October 2011

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BEST PRACTICE DESIGN GUIDE FOR PUMPING SYSTEMS VERSION 1.0 Date: October 2011

Quality Assurance Statement

Office Address DEN-1

Prepared by Timur Ayvaz (see list of contributors)

Reviewed by Tino Senon, George Tey

Approved for issue by Tino Senon

Revision Schedule

Rev No. Date Description

Prepared By

Reviewed By

Approved By

Disclaimer

This document contains information from MWH which may be confidential or proprietary. Any unauthorized use of the information contained herein is strictly prohibited and MWH shall not be liable for any use outside the intended and approved purpose.

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1. INTRODUCTION The design of pumping systems is an important component of MWHs core business in the municipal water and wastewater industry. Pumping systems convey a variety of liquids into, and out of, treatment plants as well as conveyance systems. It is the responsibility of the pump system design engineer to fully understand the system hydraulics and operating scenarios required by the project.

Although many textbooks, reference guides, standards and other publications describing how to design pumping systems exist, most of these publications focus on either general or very specific pumping applications. In addition, some of these publications reflect the authors experiences and opinions, but may not necessarily apply to project design applications. This Best Practices Design Guide ("the Guide") provides the design engineer with the relevant information and MWH Best Practices required selecting and sizing the right pump for the required application.

The MWH Best Practice Guide was originally developed to document best practices in early 1990 by Tino Senon, Julian Strassle, and Brian Stone under the direction of Dewey Dickson, JMMs Engineering Director. The first edition was completed and distributed in the Americas but later withdrawn because of some issues in Chapter 1, Hydraulic Considerations. Soon after that, there were several attempts to finish the Guide but were not successful.

The 2011 Edition of this Best Practice Guide should be considered the first release and it is a compilation of experience in the US and overseas in designing small, medium and large size pump stations with an aggregate total connected load in excess of 2,000,000 horsepower.

In 2008, MWH became a Hydraulic Institute Standards (HIS) Partner and has provided valuable time to review and provide input to the Standards. The Engineer should use the HIS as a reference.

This Guide would never been completed without contribution of the following MWH Staff:

Atul Yadav

Chris Michalos

David Baar

David Sudibyo

Ed Pascua

George Tey

Jagannath Hosmani

Jakub Adidjaja

Lou Yaussi

Michael Arroyo

Rich Atoulikian

Shane Ramcharansingh

Steve Hyland

Steven Hinman

Sushrut Joshi

Tim Ayvaz

Constantino Senon

1.1. Scope

The purpose of the Guide is to provide guidance for the design of pumping systems for water and wastewater treatment plants and water conveyance systems. It focuses on eight areas: development of the design criteria, sizing and section of pumps, system and efficiency, construction optimization, ancillary systems, interdisciplinary coordination, specification development, and safety.

This Guide also includes the following:

MWH Standard P&IDs for pumping systems Pump data sheets Best practices for pumping system layouts Reference Attachments

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Attachment No. A Pump Station Design Procedure Flow Chart Attachment No. B Typical Treatment Plant Process Flow Diagrams

(C.1, C.2, C.3 and C.4) Attachment No. C Design of Trench Type Wet Wells by Sanks Attachment No. D CFD Modelling of Proposed Pump Station by Constantino Senon Attachment No. E - Design Worksheets Attachment No. F - Example Drawings

o Not included in this version of the Guide are valve selection, electrical schematics and conduit drawings, Input/Output (I/O) lists, and valve and auxiliary equipment lists. This information is currently being developed and will be included in future editions.

The Guide is applies only to fluids (liquids) typically encountered in water and wastewater treatment facilities, including potable and non-potable water and dilute water/chemical mixtures (for the pumping of chemical solutions, refer to MWH Best Practice Design Guides for Chemical Feed System).

1.2. Usage

1.2.1. How to Use this Guide It is impossible for one guide to cover all aspects of pump station design. The Guide is a starting point for the design of pumping system projects designed by MWH Americas, and combines best industry practices with MWH experience. Refer to Attachment A Pump Station Design Procedure Flow Chart.

In addition to this Guide, the design engineer should consider project-specific requirements, MWH quality control, client input and manufacturers recommendations. Furthermore, the design engineer must be cognizant of and carefully review requirements mandated by local codes, the governing regulatory agency having jurisdiction and client preferences. Where compliance with such requirements appears to be in conflict with the Guide, the MWH Chief Mechanical Engineer and the MWH Water or Wastewater National Practice Leader must be consulted to reconcile those differences.

Each project has its own unique and specific requirements, which may require customization of the Guide's recommended practices. The design engineer should always verify the clients operational, performance criteria and review them with the design team throughout the design process. Customization of design may be required in order for the pumping system to meet the required operational performance as well as the clients expectations. However, the design engineer should not deviate from the Guide without the review and acceptance of the MWH Chief Mechanical Engineer or an approved alternative reviewer. The design engineer should provide the Chief Mechanical Engineer with a list of any deviations on the modification form located in the Appendix G.

Although an attempt is made to discuss hydraulic theory, the Guide assumes the design engineer has a fundamental understanding of hydraulics and design. Basic hydraulics, pump characteristics and pump theories are included in the Hydraulic Institute Standards as reference.

1.2.2. Design Philosophy Regardless of the level at which an engineer becomes involved in the design of a pumping station, whether it be at the conceptual, pre-design or final design phase, it is important to have as complete a picture as possible of the entire system. The Guide attempts to discuss the major topics relevant at all phases of design. Topics include recommendations relative to consultation with the client to determine preferences that may affect the design of equipment. This process involves review of pertinent data, referral to standard designs, manuals and previous designs, discussions with accompany experts and site reconnaissance. Once all data is collected, the engineer should then determine the magnitude and principal features of the pumping station; such as location, capacity,

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suction and discharge conditions (including transmission pipeline diameters and lengths), power requirements, and type of pumps (with reasonable alternatives). In addition, all available data regarding the system should be obtained. These data allows the design engineer to determine the magnitude of the total capacity, power requirements, type and number of pumps, type of driving units, and other major features sufficient for preparation of preliminary design.

Data collection and calculations involved at all stages of the investigation and design should be summarized and recorded. Final calculations (civil, mechanical, structural, electrical, controls, surge, and cost estimates) should be checked and well documented. A project file should be maintained and records of all computations, memos and letters should be kept in accordance with the project and MWH standards.

It is MWHs policy that pumping systems be as simple and maintenance free as possible, equipment and materials shall be selected to be long-lasting and, in general, to employ designs that have a long record of success. Innovative designs and equipment selection is not discouraged when there are compelling reasons, such as a , project specific requirement, clients requirements or significant cost savings, but with the knowledge that there may be limited risks involved. In which case, the Engineer must seek the assistance of the Chief Mechanical Engineer or his designated pump station design specialist and exercise due diligence in working with the pump manufacturer to insure that all aspects of design have been addressed to make sure that the pumping system work.

Table 1-1 indicates the typical scope of design services relative to the size of the pumps at the station.

Table 1-1 Typical Scope of Design Services Relative to Pump Station Size

Description Typical Scope of Design Services

Fractional horsepower pumps (less than 1.0 HP (0.75 kW) such as sump pumps, sample pumps, small lift stations, utility water pumps, etc.

Utilize package system as much as possible. Determine flow, head, horsepower and controls requirement. Select pumps and manufacturer. Establish foot print and show single line piping layout. Normally no detail layout is required other than reference to standard details. Indicate in schematics, if appropriate. Use standard specifications

Water and wastewater pumps 2 to 40 hp (1.5 to 30 kW)1

Utilize standard designs if available. Show simple outline layout of equipment and single line piping details. Utilize standard specifications.

Water and wastewater pumping facilities 50 to 1000 HP (40 to 750 kW)

Proceed on the basis of pre-design then detail design. Utilize Criteria Committee Meeting review (CCM) as discussed in the Delivery Framework. The designs may require additional services such as geo tech, surge analysis, vibration analysis, architectural input and constructability review.

Water and wastewater pumping facilities with installed total capacity in excess of 1000 HP (750 kW)

Design team should include the Mechanical Chief Engineer or his designated pump station design specialist. All design discipline shall be involved especially electrical and I & C as early as the predesign phase. Alternative layout studies consultation and extensive coordination with equipment manufacturers and substantial (electrical and mechanical) input.

1 Low lift pumps (head less than 60 ft (20 m)) require careful review of the system head loss calculation hydraulics. Include velocity head through the pump column in the pump TDH. All pump station hydraulics shall be supported by a system head curve with pump curves superimposed over the system curve.

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1.2.3. Abbreviations and Definitions Pump nomenclature, abbreviations and definitions as used by the Hydraulic Institute Standards are provided in Appendix F.

1.2.4. Symbols and Specification References The MWH General Drawing Sheets included pump symbols and can be accessed from the CAD Drafting Standards in the Delivery Framework. The MWH Guide Specifications for pumping equipment is provided in the Delivery Framework arranged by pump type.

1.2.5. Glossary Appendix E includes commonly used terms through this guide and within the Water and Waste-water Industries.

1.3. Codes and Standards

The codes and standards listed in our Guide Specifications are available through MWH's IHS subscription service as indicated in the Delivery Framework.

In addition, the design engineer should become knowledgeable of project-specific and local codes and ordinances within the jurisdiction of the project.

1.4. References

The Guide is based on the references listed below. The design guide attempts to summarize the main discussions in the references; however the design engineer is encouraged to become familiar with these references. In order to determine if there is a copy available in your local office, please contact your supervisor.

For further information regarding the delivery process, please see the delivery framework at the following link: http://design-framework/

1.4.1. Industry Hydraulic Institute Standards, Parsippany, New Jersey Internal Flow Systems 2nd Edition, D.S. Miller (2009) Pumping Station Design, Robert L. Sanks, et al. (1989), Butterworths, Stancham, Massachusetts Cameron Hydraulic Data, 18th edition, Liberty Corner, New Jersey Pump Handbook, 4th edition, Igor J. Karassik, et a.l, McGraw-Hill Book Company, New York,

New York

Design of Trench-Type Wet Wells for Pumping Stations, Robert Sanks, reviewed by Senon (May 2008 Pumps & Systems magazine)

Computation Fluid Dynamic Modeling of a Proposed Influent Pump Station, Wicklein, Sweeney, Senon, et al., WEFTEC 2006

Machinery Malfunction Diagnosis and Correction, Robert Eisenman, Hewlett-Packard Professional Books

Friction Factors for Non-Newtonian Fluids (Sludge), Design of Municipal Wastewater Treatment Plants, 4th Ed., WEF Manual of Practice 8, ASCE and Report on Engineering Practice No.76, Volume 3, Chapter 8: Solids Storage and Transport

Flow of Fluids Through Valves, Fittings and Pipes Crane Technical Paper No. 410.

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1.4.2. MWH Contacts

NAME TELEPHONE E-MAIL

Constantino (Tino) Senon Cell 425 421-6842 Direct 360 387-7851

Constantino.m.senon@us.mwhglobal.com

Timur Ayvaz Cell 713 501-6784 Direct 303 291-2124

Timur.ayvaz@us.mwhglobal.com

George Tey Direct 626 568-6259 George.c.tey@us.mwhglobal.com

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2. DESIGN CRITERIA DEVELOPMENT

2.1. General

The following information is presented as a standard methodology in order to ensure consistent and accurate MWH designs. Information is presented in a sequence typically encountered during a design project.

2.1.1. Client Preferences With every new project, MWH employees have the opportunity to work with multiple clients throughout the world. Each client has a unique perspective of their environment. As consultants we must listen to our Clients in order to ensure our Clients success. Many times, Clients have very specific preferences regarding the type of design, equipment used or how the system is controlled. These preferences could be based on a new direction the client is interested in pursuing, past experiences or even the level of local vendor support. In developing the design criteria, it is essential to first determine the Clients preferences with regards to the project.

2.1.2. Site Constraints A design engineer must also understand the site details surrounding the new or existing pump station. In some instances, there may be limitations influencing the design of the station. These constraints could range from geotechnical information, such as settlement, seismic requirements, flood elevation or high ground water to neighbors on adjacent properties. The design engineer should always be mindful of the environment the pump station is being constructed.

2.2. Fluid Properties

Determining the process fluids properties to define the fluid service and pump materials is the first step in developing the pump station design criteria. The properties of fluids which are of fundamental importance to the subject of the Guide are discussed in the following sections. These properties are specific gravity (based on density or specific weight), viscosity, temperature and corrosion and erosion potential.

2.2.1. Specific Gravity The density of a substance is a measure of the concentration of matter, and is expressed in terms of mass per volume. The specific gravity is a term used to compare the density of a substance with the density of water. Because the density of liquids depends on temperature, the temperature of the liquid in question as well as the reference temperature of water should be stated in giving precise values of specific gravity.

When dealing with fluids other than water, identifying the specific gravity is essential as it directly affects the pump station power demand. The power input (horsepower) of the pump is directly proportional to the specific gravity (S.G.) of the fluid. Therefore, if the horsepower for a pump conveying water (S.G. =1.0) is 100 HP, then a liquid with a specific gravity of 1.2 requires 120 HP.

The following are two reference tables for the design engineer. Table 2-1 indicates the density of water with respect to temperatures. Table 2-2 identifies the specific gravity of typical fluids the design engineer may come across. More detailed information is available in the Cameron Hydraulic Data reference book.

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Table 2-1: Density of Water at Various Temperatures

Temperature F Density (slugs/ft3) Specific Weight (lb/ft3) 32 1.94 62.41 40 1.94 62.43 50 1.94 62.41 60 1.94 62.37 70 1.94 62.31 80 1.93 62.22 90 1.93 62.12

100 1.93 62.00 120 1.92 61.71 140 1.91 61.38 160 1.90 60.99 180 1.88 60.57 200 1.87 60.11 212 1.86 59.81

Table 2-2: Specific Gravity of Some Liquids at 60F

Liquid Specific Gravity Gasoline 0.66-0.74 Kerosene 0.78-0.82 Sea Water 1.03 SAE Oils 0.88-0.94

100% Glycerin 1.26 Ethyl Alcohol 0.79

40% Caustic Soda 1.43 Mercury 13.57 Water 1.00

2.2.2 Viscosity Another major factor affecting pump behavior and system response is the fluid's viscosity, which is the fluid's resistance to shearing. Fluids differ from solids by continuing to deform in the presence of a shearing stress; when a shearing stress causes a liquid to flow, it continues to flow as long as the shear stress acts on it. Therefore, viscosity can also be defined as a fluid's resistance to flow. A general formula developed by Isaac Newton is:

Where

= the shear stress exerted by the fluid (its "drag") = the fluid's viscosity (a constant of proportionality)

= the velocity gradient perpendicular to the direction of shear

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Fluids which behave in the above manner are called Newtonian fluids, and continue to behave this way no matter how fast it is stirred or mixed. With a non-Newtonian fluid, on the other hand, stirring can leave a "hole" that gradually fills in over time, or cause the fluid to become thinner.

The viscosity of a Newtonian fluid depends only on temperature, pressure and the chemical composition of the fluid. Therefore, for a given substance and pressure, these fluids have a straight-line slope when plotting viscosity against temperature on the viscosity charts included in standard references.

Non-Newtonian fluids are classified as thixotropic, dilatent, or rheopectic, depending on how the viscosity changes with respect to the rate of shear (see Figure 2-1). Of particular importance in wastewater engineering are thixotropic fluids, which include thick sludges and some chemical precipitants. A thixotropic fluid is one in which the viscosity decreases as the rate of shear increases (a characteristic of ketchup, difficult to start, but once started it is difficult to stop shaking the bottle first "pre-shears" the ketchup, making it easier to pour).

Figure 2-1: Newtonian versus Thixotropic Materials

The "pumpability" of a thickened sludge, dewatered sludge or a chemical precipitant depends on many factors and should be assessed by specialists in this type of pumping. In some cases, a representative sample of the fluid is sent to the laboratory for testing. Based on the test, the pumping design criteria can be established especially for mine slurry and sludge. Another good reference is Moyno Pumps by Robbins Meyers Company. They have compiled actual test data of different Newtonian fluids especially for sludge. In fact Moyno will test any sludge for a minimal fee. For this type of analysis, the design engineer should seek assistance from the Chief Mechanical Engineer.

Viscosity is expressed as either dynamic or kinematic viscosity. The kinematic viscosity is the ratio of dynamic viscosity to density:

Where = kinematic viscosity = dynamic viscosity = density As with specific gravity the viscosities effect on the pump performance. There are many guidelines and tables published to predict pump performance when pumping highly viscous solutions. The design engineer should seek assistance from the Chief Mechanical Engineer when dealing with viscous solutions. The following Figure predicts the affect of a viscous solution on the performance of a pump. The effect on pump performance is presented in the form of percentage changed as compared to pumping water.

VISCO

SITY

SHEARRATE

Newtonian

thixotropic

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Figure 2-2: Viscosity Corrections for Large Pumps

Obtained from Cameron Hydraulic Data, 19th Edition

2.2.3. Temperature The previous sections discuss the impact of specific gravity and viscosity on the pumping system. These properties are not always constant; temperature affects both the density (specific gravity) and viscosity of the pumped fluid. A design engineer must incorporate the affects of temperature in the hydraulic calculations especially if the process covers a wide range of temperatures greater than 68 F.

Furthermore, temperatures may directly impact the Net Positive Suction Head (NPSH) for the pumps. (NPSH is discussed in greater detail in section 2.6.4). Hydraulically, for optimum pump performance, the pump requires a minimum amount of pressure at the eye of the impeller. This minimum pressure at the eye of the impeller is affected by the temperature of the solution. At higher temperatures, the vapor pressure of the fluid increases thereby decreasing the overall inlet pressure. The design engineer should always consider the fluid temperatures and vapor pressure when determining the NPSH available in a system.

Temperature can also affect pump efficiency. Usually, pumps are tested at the factory using water at ambient temperature. If the test water temperature is higher or lower than the ambient temperature of approximately 50 F, the pump efficiency shall be corrected to ambient temperature. Conversely, if the test pressure is lower than the water temperature at field condition such as for hot water circulating service. The test efficiency should also be corrected to field condition. Refer to the Hydraulic Institute Standards, Rotodynamics Acceptance Test Criteria.

When designing a system, certain materials or components are temperature dependent. The MWH Guide Specifications provides guidance in the form of Notes to Specifier when dealing with special considerations for high temperature applications. For example, Section 43 10 50, Piping General provides guidance on the selection of gaskets, couplings, connectors and other piping components for various temperature conditions.

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2.2.4. Corrosion and Erosion Considerations The discussion presented in the previous sections considers the fluid propertys effect on the hydraulics of the pumping system. Corrosion and erosion are a fluid characteristic with no effect affect on the hydraulics, but if not considered may be detrimental to the life of pumps, valves and piping. The effects of corrosion and erosion should always be considered when dealing with fluids other than potable water. Corrosion is an undesirable degradation of material resulting from a chemical or physical reaction with the environment. Erosion is the deterioration of metals buffeted by the entrained solids in a corrosive medium. The corrosive or erosive potential of a service would dictate the materials of construction, hardness and ductility of material and special liners such as rubber are required.

Figures below show an example of corrosion and erosion on the pump impeller.

Figure 2-3: Corrosion on Pump Impeller

Figure 2-4: Erosion on Pump Impeller

The following is a brief list of potential corrosive services the design engineer may encounter. Note this list is a small excerpt of various corrosive services defined in the Pump Handbook by Igor Karrassik. When designing a pump station with a fluid containing corrosive constituents such as the Colorado River Water, water known to be corrosive, or fluids other than water, a sample must be taken and tested. Results should be reviewed by the corrosion engineer and the pump manufacturer

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for proper material selection of pump components. An example of water constituent analysis is included in the MWH Guide Specifications for Pumps.

Common Corrosive Applications o Sea Water o Water with high sulfides (hydrogen sulfide in wastewater) o Chemical Acids and bases with lower and higher pH

The following is commonly used terms the design engineer should be familiar with.

Erosion Corrosion The deterioration of metals buffeted by the entrained solids in a corrosive medium. This corrosion also depends on flow angle of attack of liquid relative to the component

Abrasive wear Erosion of any material as a result of the following suspended solids characteristics o Solid concentration o Solid size and mass o Solids shape o Solid hardness o Relative velocity between solids and surface

Abrasive material Suspended solids which contribute to abrasive wear such as grit and sand. Cavitation erosion Pumps experiencing inadequate NPSH margin, air entrainment, free-

surface and sub-surface vortices are susceptible to cavitation erosion. Cavitation erosion is the degradation of the material surface due to cavitation. Pump materials resistance to cavitation erosion in increasing order are as follows: o Cast iron (least resistant) o Bronze o Cast steel o Manganese bronze o Monel o 400 Series stainless steel o 300 Series stainless steel o Nickel-aluminum bronze o Ni-resist ductile iron ( Ni-Hard) (most resistant)

Corrosion Fatigue Related to the endurance stress of material based on cyclic reversal of load applications.

Galvanic Corrosion Galvanic corrosion occurs when two dissimilar materials are in contact or electrically connected in a corrosive medium. Corrosion of less noble material is accelerated and corrosion of more noble material is decreased.

Graphitization In the presence of an electrolyte, a galvanic cell exists between the cast or ductile iron and the graphite particles. In the galvanic cell of iron and graphite, iron becomes the anode and the graphite becomes the cathode. A galvanic current flows from the iron to graphite; therefore, the iron goes into solution resulting in gradual depletion of iron until only graphite remains. While the casting appears sound on the outside, pieces may be broken off with the fingers.

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Concentration cell, or crevice, corrosion When an electric current flows between two areas causing a localized attack. This usually occurs where water is stagnant, such as threads, gasket surfaces, holes, crevices, surface deposits and in the underside of bolts and rivet heads. When concentration of corrosion occurs, the concentration of metal ions or oxygen in the stagnant area is different from the concentration in the main body of the liquid.

Selective Leaching Removal of one element of material from solid alloy in a corrosive medium such as the process of dezincification, dealuminumification, and graphitization. For example where a certain water source is known to have dezincification characteristics, low zinc bronze is normally recommended

Intergranular Materials can look sound on the surface but intergranular corrosion can progress to a point that the material literally disintegrates. Intergranular corrosion of austenitic stainless steel occurs as a result of carbides precipitating out the grain boundaries during slow cooling of the casting.

Buried piping Corrosive soils are a factor when designing buried piping systems. The geotechnical reports shall consider the overall corrosive properties of the soils, and recommend a method to mitigate its affect on the buried piping.

Corrosive Constituents in Municipal type Wastewater Systems

When designing a wastewater pump station, the design engineer shall pay special attention to any air/wastewater surfaces. Anaerobic sulfate-reducing bacteria (such as Desulfovibrio) thrive in wastewater. This bacteria utilizes the oxygen in sulfate (commonly found in wastewater) to create hydrogen sulfide, which escapes from the wastewater to the atmosphere above. At that point, an aerobic bacteria (Thiobacillus) converts the hydrogen sulfide to sulfuric acid. Sulfuric acid is extremely corrosive whether it is concrete, steel or ductile iron. Process design should minimize hydraulic jumps, turbulence which could cause off-gassing of H2S. If off-gassing cannot be prevented by alternate design, materials resistant to H2S shall be specified such as 316 stainless steel.

Iron Reducing Bacteria found in Deep Wells

Iron reducing bacteria have been found in deep wells which corrode pumps components made of cast iron, ductile iron, carbon steel or even 304 stainless steel. Recommend using 316 stainless steel. During development of deep wells, a water sample shall be tested for its corrosivity and look for the presence of iron reducing bacteria as well.

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>>>

>>>>

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Table 2-3 Galvanic Series of Metals and Alloys

Sacrificial Anodes Magnesium Magnesium alloys Zinc Aluminum 2S Cadmium Aluminum 17ST Steel or iron Chromium stainless steel, 400 series (active) Austenitic nickel or nickel-copper cast iron alloy 18-8 Chromium-nickel stainless steel, Type 304 (active) 18-8-3 Chromium-nickel-molybdenum stainless steel, Type 316 (active) Lead-tin solders Lead Tin Nickel (active) Nickel-base alloy (active) Nickel-molybdenum-chromium-iron alloy (active) Brasses Copper Bronzes Copper-nickel alloy Nickel-copper alloy Silver solder Nickel (passive) Nickel-base alloy (passive) 18-8 Chromium-nickel stainless steel, Type 304 (passive) Chromium stainless steel, 400 series (passive) 18-8 Chromium-nickel-molybdenum stainless steel, Type 316 (passive) 18-8-3 Chromium-nickel-molybdenum stainless steel, Type 316 (passive) Nickel-moly Silver Graphite Gold Platinum Protected End (cathodic, or most noble)

Reprinted by courtesy of The International Nickel Company Inc.,67 Wall St., New York, NY

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2.3. Pump Station Configuration

Unfortunately, no single pump station configuration fits every application. The design engineer is urged to review the pump station examples provided in this Guide as a starting point. If the required features do not closely resemble the examples, contact the Chief Mechanical Engineer or any of the persons listed in the contact list for any other examples. The type of pump station configuration varies greatly depending on different factors such as hydraulic considerations, Clients preference, and the type of pumps, the control and maintenance expectations and the size of the station. The following sections provide a brief overview of the various types of pump station configurations available.

2.3.1. Suction Configuration Depending on the hydraulics of the specific application, a variety of pump suction configurations may be used.

Wet wells are typically used to provide storage volume or a hydraulic break. Typical pump stations which use wet wells are sewage lift station, treated water high service pump station, raw water booster pump station or submersible sewage pump station.

A can (also called a barrel) is typically used for a vertical turbine pump connected to an adjacent forebay or reservoir.

Piping manifold suction headers are typically used for horizontal centrifugal pumps stations connected to an adjacent forebay or reservoir. This configuration may also be used for inline booster pump stations. (Inline booster pump station is usually discouraged because it complicates the surge protection and controls)

2.3.1.1. Wet Well A wet well is a below-grade structure (above grade is possible, but not typical) of a pumping station. It is the structure into which the liquid flows from, and where the pumps draw water. The wet well serves three purposes. First, it creates a hydraulic break minimizing the effects of upstream system on the current system. The free water surface is allowed to rise and fall buffering the system from any fluctuations in flow and pressure. Second, it provides storage volume to allow constant speed pumps to start and stop without exceeding the number of starts required for a certain size motor. Third, it provides adequate submergence above the suction bell of pump to prevent formation of vortices and provide adequate Net Positive Suction Head (NPSH). Fourth, it provides free-board to allow the level to rise during upset or emergency operation without overflowing. Additionally, the wet well shall be configured to preclude formations of free-surface or subsurface vortices or prerotations that can be carried through the pump suction volute. At a minimum the wet well design shall meet the flow distribution based on the accepted criteria recommended by the Hydraulic Institutes Intake Design Standard. These recommendations mitigate adverse hydraulic phenomenon that may occur in the pump station wet well. In summary, the geometry of the wet well, operation of the pumps, and the depth of water in the sump influence the approach flow hydrodynamics and can result in the following adverse hydraulic phenomena (Sweeney and Rockwell 1982):

Pre-swirl of flow approaching the pump impeller Free surface vortex formation Spatial asymmetry of the flow approaching the pump impeller Temporal fluctuation (turbulence) in the flow approaching the pump impeller Air Entrainment

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The following section describes typical wet well configurations used by MWH. The appendix includes example drawings of these configurations.

Rectangular with flat bottom used for clean water application such as treated water high service pump stations, and potable water storage reservoirs.

Rectangular with hopper bottom used for solids bearing fluids such as raw sewage, grit chamber, sludge, raw water, etc.

Rectangular with sloped bottom and flow distribution inlet channel for sewage and sludge pump stations. Refer to Wet Well Design Guide for Large Submersible Pumps by ITT Flygt Pump.

Open-trench Type wet well with an Ogee weir for liquids bearing solids especially raw unscreened municipal sewage.

For design guides, refer to the following references: ANSI 9.6-1998 HI Pump Intake, Design of Trench-Type Wet Wells for Pumping Stations, Robert Sanks, reviewed by Constantino

Senon (May 2008 Pumps & Systems magazine)

Computational Fluid Dynamic Modelling of a Proposed Influent Pump Station, Wicklein, Sweeney, Senon, et al., WEFTEC 2006

MWH exceptions and improvements to HI recommendations based on lessons learned from Durham and Tacoma IPS. For list of exceptions, refer to Computational Fluid Dynamic Modeling of a Proposed Influent Pump Station, Wicklein, Sweeney, Senon, et al., WEFTEC 2006. For detailed drawings, refer to example drawings from Durham and Tacoma IPS. Use minimum submergence S=4x suction bell diameter D or greater. Use long radius suction elbow with flare bottom. Provide flow straightening device downstream of the suction elbow Use long radius reducing elbow to connect the horizontal suction pipe to the pump suction. When is Physical Hydraulic Model Study Required?

A physical hydraulic model study can be expensive, but in many cases will provide insight into specific hydraulic issues that may adversely affect the pump station. Based on ANSI/HI 9.8-1998, the Hydraulic Institute Standards recommend physical model testing if one or more of the following features exist in the project:

Sump or piping geometry deviates from the intake design standards. Non-uniform or non-symmetric approach flow to the pump sump exists. Pumps have flows greater than 2520 l/s (40,000 gpm) per pump or the total suction flow with all

pumps running would be greater than 6310 l/s (100,000 gpm).

Pumps with an open bottom barrel or riser arrangement with flow greater than 315 l/s (5,000 gpm) per pump.

Proper pump operation is critical and pump repair, remediation of a poor design, and the impacts of inadequate performance or pump failure all together would cost more than ten times the cost of model study.

When is the Computational Fluid Dynamics (CFD) Study Required?

The computational fluid dynamics study could be performed as a pre-requisite to the physical model study for the following reasons:

The CFD model is less expensive as the physical model study.

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The CFD model can be used to optimize the wet well design for least cost. The model can be revised easily in the computer and operational flow scenarios can be simulated without the need to modify fabricated physical model.

Although the result of the CFD model have not been endorsed by HI as a substitute to the physical model because of the accuracy of the software available in the market, MWH experience indicate that if the result of the CFD model is gauged against the HI acceptance criteria used for physical model, the result of CFD model could be conclusive in lieu of conducting physical model study if approved by the Senior CFD modeller and Chief Mechanical Engineer. The approval criteria shall be based from previous experience of similar wet well configuration that had been proven to work.

An example of the CFD Study of the wet well configuration that demonstrate non-conformance to the HI acceptance criteria is shown in Figure 2-5 below. This wet well was an existing wet well designed by another consulting engineering firm and it is in the process of being modified by MWH using physical model test.

Figure 2-5

Example CFD Study of a Wet Well Configuration with Flow Distribution Deficiency

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MWH has taken exception to the HI guidelines regarding when to perform a CFD or physical model study under the following circumstances. If the wet well design is identical in flow, wet well configuration with a pump station that has been previously modeled, constructed and have a record of successful operating experience such as the Durham IPS and City of Tacoma IPS, etc., a physical hydraulic model test may not be required. Please contact the Chief Mechanical Engineer for the additional list of projects. Example of a physical model is shown in Figure 2-6.

Figure 2-6

Example Physical Model

What is Model Performance Acceptance Criteria

The Hydraulic Institute Standards established criteria for evaluating performance of pump station designs through the use of physical model studies. ANSI/HI 9.8-1988 details the physical modeling procedures and the interpretation of the results. The following is a list of minimum performance criteria for a physical model:

Free surface and subsurface vortices entering the pump must be less severe than vortices with coherent (dye) cores (free surface vortices of Type 3 and sub-surface vortices of Type 2 from HI 9.856). Dye core vortices may be acceptable only if they occur for less than 10% of the time or only for infrequent pump operating conditions.

Swirl angles, both short-term and long term as defined in the Standard. Time-averaged velocities at points in the throat of the bell or at the pump suction in a piping

system shall be within 10% of the cross-sectional area average velocity as defined in the Standard.

For special case pumps with double suction impellers, distribution of flow at the pump suction flange shall provide equal flows to each side of the pump within 3% of the total flow.

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2.3.1.2. Design Considerations The wet well or forebay volume should be designed with adequate storage to prevent frequent starting and stopping of the pump. This starting and stopping is called cycling of the pumps. The maximum number of allowable starts is typically dependent on the characteristics of the electric motors and typically ranges between 6 for large motors and 15 for small motors. The design engineer is responsible for contacting the pump/motor manufacturer to obtain the minimum cycle time. Furthermore, the wet-well should be sized to allow for the pump starting sequence. The starting sequence usually takes between one to three minutes, depending on the required opening and closing time of the pump control valves. The opening and closing delays may be field adjusted to prevent extended operation of the pumps between shut off and operating duty point. The starting and stopping times for pumping units equipped with check valves, is usually less than a minute. The wet well should be sized to provide adequate storage during this time period.

For multiple-speed pumps, the available storage volume in the wet well does not need to be as conservative. As flow rate is controlled by the speed of the pump, the pump does not need to start against a closed valve. The pumps can start, and increase speed to immediately contribute flow into the system.

One design criteria often overlooked is the storage volume required in the event of a power out-age. With a constant flow rate entering the pump station wet well, a disruption in local power will immediately be reflected with a rise in the water surface elevation as in the case of booster pumps in series. In this example, it is impossible to provide storage for an extended power outage. Therefore, the SCADA system shall be configured such that in the event of power failure in a downstream pump station, the upstream pump station shall be signaled to stop. In collection system applications, the flow can be allowed to back-up into the system, otherwise the wet well should be designed with adequate storage volume or overflow potential during a power outage. The design engineer shall review the local codes and clients preferences regarding the design power outage duration.

Most state regulatory agencies in the US include maximum retention time in the wet well design criteria, when pumping wastewater. The intent is to minimize the potential for the development of septic conditions and resultant odors. A maximum retention time of 10 minutes, at average design flow rates is often quoted. Unfortunately, this requirement may conflict with the need for adequate volume to prevent short-cycling of the pumps. In such cases, multiple pumps or variable-speed pumps should be considered to reduce the required volume. Furthermore, in addition to minimizing retention times, odors can be minimized if the lowest liquid level in the well is set above the sloping portion of the wet well. This can be accomplished by making this level the stop point for the lead pump in the sequence.

For Sizing of Wet well or sump Volume, refer to HI Pump Intake Design, Appendix B

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Figure 2-7: Hydraulic Institute Wet Well Volume Calculation Procedure

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When designing the wet well, the design engineer shall consider the following: Provide an opening in the deck with adequate clearance to allow removal of any pump

components or piping from the wet well.

The wet well shall be provided with an air vent sized to release or admit outside air due to the rise and fall in water levels. Area of vent is typically equal to at least half of the inlet pipes area this dimension is a minimum, the required dimension may be larger. For se-wage or sludge pump station, vent pipe shall be connected to the foul air scrubber

Provide a hatchway for access to the wet well. Hatchway size to be at least 4 ft by 4 ft with appropriately sized safety net or equivalent safety system.

Permanent ladders shall NOT be included in the wet well due to corrosion and the potential safety concerns.

Address Confined Space requirement and fire and safety requirement per NFPA 820. Consult the MWH HVAC Lead Engineer.

2.3.2. Dry Well or Dry Pit As mentioned previously, pumps draw water from the wet well. Locating the pump adjacent to the wet well minimizes the suction losses. The pump would need to be installed below the water surface elevation. To accomplish this, a second structure is installed adjacent to the wet well. This below grade structure, called a dry well, contains the pumps, drive shafts, valves and piping. For this configuration, there is no liquid surrounding the pump and valves, therefore the equipment are accessible for maintenance. This maintenance accessibility is the main advantage of the dry well configuration.

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Figure 2-8: Typical Dry Well Configurations

2.3.3. Submersible Pump Station Another type of pump station configuration typically used for sewage lift stations is a submersible pump station. A submersible pump station does not have a dry well. The pump and piping is located within the wet well. The pump and motors is specially designed for submergence in the water or wastewater.

Figure 2-9: Typical Submersible Pump Station

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The advantage of this type of station is cost. The overall building footprint is much smaller than a dry pit station. Furthermore, the entire pump station, excluding electrical panels and SCADA system, can be located below grade, which might have its advantages if the pump station is located in a visually sensitive area.

Unless otherwise preferred by the Client, MWHs preference regarding when to utilize a submersible configuration versus a dry well configuration is related to the pump horsepower. For pumping stations equipped with pumps sized up to 700 HP the installation costs of a submersible pumping system is normally less than the dry well pumping systems because the pumps can be installed without the need of building a dry pit. However for larger installations with pumps larger than1, 000 hp, a dry-pit pumping system is preferred by most Clients because it is easier to inspect and maintain. Dry pit pumping systems; however have a risk of being flooded. The clients policies and/or preferences should be given careful consideration.

For small manhole type sewage lift stations, submersible pumping system offer advantages over dry-pit systems because they are less expensive to build and they are also available from the manufacturer as a package unit. The manufacturers are Smith and Loveless and Gorman-Rupp. Submersible pump stations can be installed underground, below streets, or in a limited space by the roadside with the motor control panel mounted above ground.

2.3.4. Vertical Turbine Installations Vertical turbine pumps are typically suspended from the structure, with only the pump unit (bowl assembly) submerged below the water surface. The motor is typically installed above a wet well or above ground and supported by the discharge head. Vertical turbine pumps can also be installed inside the can or barrel. Barrel mounted vertical turbine pumps are highly sensitive to the intake hydraulics. The vertical turbine pumps are classified into three types based on their specific speed; bowl assembly and impellers.

Radial and Francis-Vane turbine impeller [Ns: 500 to 4000] enclosed or open type, multi-stage, low flow high head

Mixed flow impeller [Ns: 4500 to 8000] enclosed or open type, maximum two stage, medium flow, medium head

Axial/propeller flow impeller [Ns: 8000 to 15000] open type, maximum single stage, high flow, low head

Figure 2-10: Typical Vertical Turbine Pump Installation

Vertical turbine pumps are sensitive to intake design configuration, including submergence over the 1st stage impeller, spacing between two adjacent pumps or the wall; and spacing from the bottom of

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the pump to the floor. Complying with these requirements mitigates vortices, and a wet well designed to establish uniform flow velocity distribution at the suction bell. Uneven velocity distribution, compounded by insufficient submergence can result in the formation of vortices which may introduce air into the pump suction causing a reduction in capacity, unbalanced impeller loading, rough operation or impeller damage due to cavitation.

For vertical turbine pump intake design guidelines, refer to the Hydraulic Institute Standards. For submergence over the bell requirements, refer to the pump manufacturers published performance curves and pump data.

Design Considerations

When mounting vertical turbine above a wet well, the design engineer shall consider the following requirements:

The top structure of the wet well shall be designed for the maximum down thrust generated from pumping the liquid. The down thrust generated by the pump is absorbed by the motor top bearing and transferred to the structure. The pump thrust information is available from the pump manufacturer. Thrust factor is normally indicated on the pump performance data sheet in terms of pounds per foot of head.

The top deck should be designed so that the natural frequency is at least two times the maximum speed of the pump. The design engineer shall coordinate design with the structural engineer.

Pump pads should be designed integral with the top of the wet well deck. Location of piping and valves for access needs to be coordinated with the tank and nearby area. For pump stations with flow rates in excess of 5,000 gpm, provide isolation baffles between the

pumps.

Provide adequate submergence over the suction bell to prevent vortexing at low water level, create NPSH available greater than what is required by the pump, avoid cycling on/ off pump operation and free board above high water level. Provide NPSH margin of at least 5 feet absolute. Minimum submergence shall be equal to or greater than the pump manufacturers recommendation.

Note to Design Engineer:

The terms minimum submergence and NPSHr refer to two separate items. Minimum submergence is the minimum water level required regardless of NPSHr. It is the Design Engineers responsibility to ensure that requirements are met.

Provide a combination air release and vacuum valve mounted on the discharge pipe located between the pump discharge and the check valve. Size air valve using APCO valve calculator.

Provide opening in deck with adequate clearance to allow removal of pump assembly. Use the maximum diameter of the column pipe flange, bowl assembly or suction bell whichever is the largest.

Wet well shall be provided with an air vent sized to release or admit outside air due to the rise and fall in water levels. The area of vent is usually equal to at least half of the area of inlet pipe. Provide a hatchway for access to the wet well. Hatchway size to be at least 4 ft by 4 ft with appropriately sized safety net or equivalent safety system. Design engineer shall also consider potential debris removal when sizing and locating hatches.

For wastewater pump stations, permanent ladders shall NOT be included due to corrosion and create a potential safety concerns.

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When using a vertical turbine installed in a barrel/can, the design engineer shall consider the following requirements:

The annular velocity between the inside diameter of the barrel and the pump shall be between 3 to 5 fps. The annular velocity shall be calculated using the maximum flow for each pump. This size should be confirmed with the manufacturer requirements for the selected model.

MWHs exception to HI design criteria. The vertical distance from the centerline of the inlet pipe to the barrel to the suction bell shall be at a minimum 3 to 4 times the diameter of the barrel instead of 2 times the diameter of the barrel per HI. MWH exceptions are annotated in Figure 2-12.

Pump foundation or inertia base should be designed with a mass equal to or greater than 4 times the weight of the motor or adequate to support the pump and motor assembly whichever is the largest. Pump foundations shall be isolated from the concrete area or floor of the building. This design will limit transferring vibrations to the slab or building. Any exceptions shall be brought to the Chief Mechanical Engineer attention.

Calculate hydraulic grade line at the pump suction with friction loss based from the maximum pump flow. The hydraulic grade line at the centerline of the pump shall be at least: o One diameter higher than the crown of the inlet pipe o Net Positive Suction Head Available (NPSHa) referenced to the datum of the pump shall be

calculated using the hydraulic grade line inside the barrel, including friction loss through the annular space between the pump and the barrel.

Figure 2-11: Excerpt from Hydraulic Institute Intake Design Various Vertical Turbine Intakes

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Figure 2-12: Excerpt from Hydraulic Institute Intake Design

Vertical Turbine Can/Barrel Pumps

Figure 2-13: Suction and Discharge Piping for

Vertical Turbine Can/Barrel Pumps

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2.3.5. Hydraulic Institute Self-Cleaning Wet Well (Trench Type) The self cleaning wet well design can be used for wastewater or solids bearing fluids with end suction, non-clog pumps. Vertical turbine or submersible type pumps have been used by other consultant and they have been known to have problems with flow distribution and stringy materials depositing around the pump column and slide rails and cables in the case of submersible pumps especially for unscreened and wider screen spacing. Trench type wet wells were developed based on the philosophy that variable speed pumps do not require significant wet well storage volumes. The speed of the pump can be adjusted using a Variable Frequency Drive (VFD) to maintain a constant water level (water entering the wet well equals the water leaving the wet well) thereby minimizing the time the fluid is in the wet well. Over the years, this concept has progressed through much iteration in the design. The most current design shown in Figure 2-12 is obtained from the Hydraulic Institute Intake Design Guidelines. MWH exceptions are annotated within the figure.

Figure 2-14: Open Trench Type Wet Well,

Hydraulic Institute Standards, ANSI/HI 9.8-1998

The two main benefits of the open trench wet well design are minimizing wet well storage volume and the ability to convey solids without the need to have varying water surface elevations for pump control. This feature results in a compact pump station design. With regards to the ability to convey solids, the operational guidelines for this type of wet well includes a cleaning cycle in which the water surface elevation is lowered. The influent flow cascades down the ogee weir at the entrance of the

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wet well. The cascading flow accelerates, with the flow velocity reaching a scouring velocity removing settlement and debris from the bottom of the wet well. The solids become suspended, enter the pump and are conveyed downstream.

Self cleaning wet well design should be provided for sewage pumping stations as indicated within Hydraulic Institute Standards. MWHs previous experience has taken exception to the minimum submergence over the suction bell of 2 times pump bell diameter. The design engineer shall use 4 times pump bell diameter for the minimum submergence. In the three models performed, submergence of 2D or as calculated using Froudes equation, any turbulence in the water surface is carried to the pump suction bell. However using a submergence of 4 times D, when the wet well was drawn down to the operating low water level, the flow entering the suction bell was not affected by any turbulence in the surface.

2.3.6. Forebay / Reservoir Pump Station Wet wells are typically used to create hydraulic breaks between two separate systems. In water distribution or conveyance systems, the wet wells or reservoirs are required to handle large variations in flow so that the pumps can be controlled by level and/or flow. The wet well or reservoir shall be designed with a storage volume for the following:

The storage volume at the bottom of the wet well or reservoir shall provide adequate sub-mergence over the pump suction bell or adequate NPSH margin over the pump datum.

The storage volume at the middle shall provide adequate level or dead band to control the pumps preventing it from cycling motor that the number of starts per hour as recommended by the motor manufacturer.

The top storage volume shall provide free board to prevent the wet well or reservoir from over flowing in the event of abnormal operation. The volume shall also be adequate to allow w all of the pumps to stop during abnormal condition without overflowing.

As a result, it becomes advantageous to store large volumes of water upstream of the pump station. The pump station must rely on a forebay or reservoir as the source of water for the pump station. The forebays or reservoirs are typically above ground concrete structures used for storage of large volumes of water. The forebay/reservoir pump station arrangement can be used with horizontal end suction, split case pumps or vertical turbine pumps. The design engineer shall use MWH Best Practices for designing reservoirs.

FOREBAY PUMP STATION

Figure 2-15: Aurora Forebay/Reservoir and Pump Station

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2.3.7. Industry Standards and Guidelines The following sections identify commonly used design guides in the water/wastewater industry. The design engineer should be familiar with these standards.

2.3.7.1. Hydraulic Institute Intake Design Guide The Hydraulic Institute Standards were established to promote the continued growth and well-being of pump manufacturers and further the interests of the public in the areas of pumping systems. It is a collection of best practices when designing pumping systems. The guide covers topics ranging from pump placement in wet wells to maximum and minimum velocities in the flow stream. The use of the HI Standard is voluntary and the Design Engineer (Engineer of Record) is responsible for his design and therefore shall use his or her engineering judgment in using this Standard. As a minimum, MWH designs shall use the Hydraulic Institute Intake Design Guidelines where ever applicable to the project. The design engineer should be very familiar with the technical content in the guide.

2.3.7.2. Flygt Design Recommendation Guides The design engineer is not always required to develop a wet well design from scratch. Not only are guidelines a useful resource, but under certain circumstances templates are available to aid the design engineer. ITT Flygt is a pump manufacturer who invested a significant amount of time in developing, verifying by Computation Fluid Dynamic (CFD) Models and physical model testing wet well designs. Flygt has developed design guides and templates for large submersible and axial flow pump installations. Although HI has not endorsed the Flygt design guides, these are industry accepted design methods. The HI Standard Committee has agreed to include it as a reference attached to the appendix of the Hydraulic Institute Standards.

MWH has used the Flygt design guide in many successful wastewater installation. The Flygt guide includes a table which relates the individual pump capacities to various dimensions in the wet well. The two templates developed by Flygt are Pump Stations with Large Submersible Centrifugal Pumps and PL Pump Station Design Guidelines.

The following is a link to the Flygt website:

http://www.flygtus.com/

2.3.7.2.1.1. Pumping Stations with Large Submersible Centrifugal Pumps This design guide identifies key dimensions for the wet well, and correlates these dimensions to the individual pump capacity. The design engineer should review footnotes in the design guide concerning appropriateness of use. There are known limitations of the design with regards to overall capacity and number of pumps.

The Flygt pump station design guide utilizes a baffle region in the wet well with ports aligned with each individual pump. The influent line conveys flow into a baffled region, directing it downward along the finished floor towards the pump intakes. The intent on the design is not only to align the flow through the ports with the centerline of the pumps and mitigate short circuiting, but to increase the velocity along the finished floor. The higher velocities along the bottom of the wet well scour solids from the floor.

In some instances, the design may call for the ability to take half of the wet well off line for cleaning or maintenance. In this type of situation, it is necessary to connect two adjacent wet wells using a slide gate. The main advantage of this configuration is to allow one wet well to be taken off line while other station remains fully operational. If the design calls for two wet wells to be joined, the connection point shall be in the baffle area of the wet well or upstream of the baffle area. Positioning the slide gate in the baffle area, or upstream of the baffle, minimizes the extent of cross flow at the pump intake.

For any deviations from the design guide, no matter how minor, the design engineer shall consult the MWH Chief Mechanical Engineer. The concern is small changes (which may seem minor), could potentially have hydraulic ramifications to the design. For example, the port openings at the base of the baffle should have a velocity for 7 to 10 ft/sec and, subsequently, large head losses. If the design

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engineer is concerned the velocity is too high, he or she may incorporate larger openings. To the design engineer, it may be a small change, however hydraulically the high head loss is beneficial and helps to ensure even flow distribution between the ports. Increasing the size of the ports is actually detrimental to the pump station.

MWH has in-house capabilities to perform Computational Fluid Dynamic (CFD) simulations. Any wet well design which deviates from a recognized wet well design guideline shall be verified using CFD Modeling. The design engineer shall seek advice from the Chief Mechanical Engineer and Hydraulic Specialist if a CFD Model is appropriate.

Figure 2-16. Includes images of the various configurations included in the Flygt design guide.

Figure 2-16: Excerpt from Flygt Design Guide

Pump Stations with Large Submersible Centrifugal Pumps

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Figure 2-17: Excerpt from Flygt Design Guide

Pump Stations with Large Submersible Centrifugal Pumps

2.3.7.2.2. Propeller (PL) Pump Station Design General Principles In addition to the design guide for large submersible pumps, Flygt also publishes a design guide for submersible propeller pumps. The submersible propeller pump is a high flow low head vertically suspended pump. The PL designation is a Flygt sales code for:

P: Multi-blade propeller pump with bowl assembly, bell-mouth and outlet cone, for large capacity pumping of clean liquids.

L: Semi-permanent vertical installation in large diameter discharge column made of steel or concrete.

Typically this type of arrangement is used for flood control applications. The pump and motor are inserted into a pipe or caisson and submerged below the water surface. The discharge from the propeller is conveyed over the exterior of the motor, and then vertically to the discharge fitting at the surface.

Figure 2-18: Submersible Propeller Pump

Propeller pumps are more sensitive to inlet flow disturbances relative to other pumps. Ideally the flow at the pump inlet should be uniform and steady without swirl, vortices or entrained air. To aid the design engineer in creating ideal hydraulics at the suction of the pump, Flygt developed the PL Pump Station Design General Principles. The recommendations presented in the design guide utilize established principles of hydraulic design obtained from the Hydraulic Institute Standards. The guide

The radial type wet well shown to the left has been used in the past in wastewater systems. To our knowledge, the pump station worked well. The main complaint was that this design was difficult to isolation individual flow to one side. If this configuration is to be used, a dual wet well is recommended.

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also includes design information based on model and full scale tests, specific to propeller pumps, conducted by Flygt. The guide provides the design engineer a reliable hydraulic configuration for this type of pump.

CAUTION: The design engineer shall request Flygt to review the design. If possible, request Flygt to verify by CFD simulation at no or minimal cost. CFD Model can also be prepared by our MWH specialist. The Flygt design is typically suited for their pumps. If an or equal is required on the project, the design engineer must indicate on the drawing that the pump manufacturer shall verify the intake design configuration and recommend to the engineer any modifications required to suit the or equal pump.

The Flygt guide is a combination of narratives describing the various wet well configurations and dimensional drawings. The following text and images were obtained from the Flygt guide. The exact dimensions for each configuration are not included. The design engineer shall refer to the Flygt PL Pump Station Design General Principles for more information.

The designs are divided into three configurations A, B and C.

Configuration A Standard Open Sump

This configuration is the simplest to build and is often the first alternative considered. However, as it requires more submergence and possibly a longer approach than other configurations, the total cost of the station may be higher than other options.

Figure 2-19: Configuration A with Plain Intake or Vortex Cone and Swirl Plate

Figure 2-20: Configuration A with Intake Modifications for Asymmetrical Flows

Configuration B Compact Closed Intake

This type of configuration is typically constructed of concrete or steel. The geometric features, like the curvature of the front wall, the corner fillets and the benching at the back wall, have been developed to allow smooth acceleration and turning as the flow enters the pump.

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Figure 2-21: Configuration B With Concrete Construction

Configuration C A Closed Intake of the Draft-Tube Type

This type of configuration utilizes a draft-tube intake, (also called a formed suction intake) and can be constructed of either steel or concrete. The intake reduces inconsistencies and swirl in the approaching flow. This intake is more effective than Configuration B because the sloping front wall is designed to minimize stagnation of the surface flow. The geometrical features of this intake provide for smooth acceleration and turning as the flow enters the pump. The minimum inlet submergence should not be less than 1xD.

Figure 2-22: Configuration C Closed Intake with Draft Tube

2.3.8. Hydraulic Intake Design Intakes are considered to be an integral part of pumping facilities. Intakes vary in design, requiring special knowledge and expertise. A comprehensive discussion of intake layouts is beyond the scope of this Guide. This section is meant only to introduce the topic to the design engineer.

Intakes include necessary structures located within a lake, reservoir, river, stream, canal or other body of water. The structures are inlet points for the pump station, and serve to minimize hydraulic disturbance and/or attempt to prohibit debris from entering the suction pipe. Pumping equipment may be mounted directly on the intake structure or may be located some distance away on dry land. When intakes provide a supply to remotely located pumps, the conveyance between the intake and pumps must be designed for gravity flow.

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In large bodies of water the actual intake may be located some distance from the shore. The intake should be located at a sufficient distance from shore (or edge of the reservoir) to assure adequate submergence under all anticipated seasonal and cyclic conditions. Frequently water quality considerations or ice influence the depth and/or distance from the shore. Potential damage of submerged intakes (by vandalism and anchors) needs to be considered if the area is used for recreational activities. The simplest intake consists of a screened pipe or concrete box, which supplies a pipeline extending to the pumping stations. Where the body of water is deep (more than 30 ft), and may experience seasonal turnover, a tower may be constructed with valve or gated intake ports to enable selection of the best quality water. The tower may also serve as the suction sump for vertical pumps. The latter requires a bridge, floats or other means for the discharge piping and electrical equipment to reach shore. In some lake intakes, fish screens are required. If fish screens are required, consult our fish screens expert in the MWH US Seattle Office. Example of a lake intake design at Lake Meade, Lake Powell and Morse Lake is shown in Figure 2-23.

Figure 2-23: Example Lake Intake Design

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2.4. Preliminary Design, Equipment and Piping Layout

The design engineer shall follow the design and quality review requirement in the MWH Delivery Framework. In summary, the delivery framework consists of a list of activities and tasks required to implement a project from the preliminary design phase through detailed design and construction document development. The delivery framework requires the design engineer to follow the steps in chronological order to enhance efficiency, accuracy, avoid rework to meet quality, budget and schedule.

Once the Client and MWH have selected a specific pump station configuration, the design engineer shall develop a preliminary piping layout. The preliminary piping layout shall be thoroughly developed as it directs further work by Structural, Architectural and Civil disciplines with respect to the overall building size. Future changes in the piping layout could result in a ripple effect through other disciplines forcing hours of additional rework. As a minimum, the design engineer shall comply with the piping layout guidelines identified in the Hydraulic Institute Standards and within this guide.

The first step in defining the piping layout is to determine the total number of pumps. The number of pumping units and operating range of each pump is partially defined by the minimum and maximum capacities required by the pump station. The combination of pumps in the pump system must be capable of covering the entire range of required operational conditions. The Engineer shall identify the minimum pump station flow and the maximum flow at full build out. Providing a system which covers this range is a critical component of the design criteria. If the lower flow and head range is beyond the operating range of the main pump(s), a small jockey pump may be required. The jockey pump would operate at low flow and low head conditions. The design point of the jockey pump should be determined such that the flow and head range will slightly overlap with the operational envelope of the first large system pump.

Once the quantity of pumps is defined, the design engineer shall proceed with designing the piping layout. The design engineer shall determine the following as part of the preliminary piping layout.

Flow, TDH, horsepower, speed System head curve and pump curve operating envelope Type and number of pumps Define characteristics of fluids, temperature, specific gravity, and fluids constituents Identify corrosivity of fluid Pump, valves and piping material selection The system boundary for the piping layout. Size of the suction and discharge piping. Pressure rating of the piping and valves Pipe materials of construction Types of valves Select ancillary equipment such as air compressors, HVAC, electrical rooms

2.4.1. Hydraulics Before commencing on hydraulic calculations, basic criteria should be summarized. The design engineer must know the full range of conditions that will be expected of the pumping facility. The following criteria should be obtained and summarized;

Maximum design capacity including the estimated seasonal, daily (or hourly rates) and other aspects of flow patterns.

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The minimum required plant capacity including whether or not such flows need to be intermittent or continuous and the estimated percentage of the time that minimum or low flows may persist.

Future and/or estimated ultimate flows. Where applicable, determine the pump station flow for which the pumps will operate most of the

time or exceedence flow. For example, a sewage pump station in a collection system is to be designed for 200 MGD, but the 90% exceedence flow would be approximately 24 MGD.

The range of pressure or water levels that may be available on the suction side of the facility The discharge conditions including the range of pressure and/or levels, that may exist at

discharge locations

Details of existing (or proposed) discharge piping including materials, pressure rating, age, condition, diameters, layout lengths, valves and fittings.

Figure 2-24: Example Exceedance Curve

In most cases, the degree of accuracy required for computation of friction losses (and static heads) in pumping systems needs to be in the range of plus or minus 2 percent. If losses thru valves and fittings are not computed, include an allowance for minor losses and velocity head in the range of 2 to 4 percent of calculated pipeline friction, depending on the complexity of the discharge piping. For low lift applications (total pumping heads of less than 60 ft) minor losses (including velocity heads) may be a significant portion of total pumping head and need to be individually calculated. If information on discharge system dynamic losses is available from actual field tests and/or other hydraulic analyses, it should be scrutinized and if representative utilized in preference to computed data.

It is important to note, that total dynamic head estimates are frequency over-estimated. A rare or extreme condition should not be the basis of design. There is a tendency to use conservative friction factors in computing dynamic losses in pumping systems, particularly transmission mains. Overly conservative assumptions should be avoided because they may result in misapplication of pumping equipment. Always use realistic and applicable assumptions and confirm with in-house advice. Use hydraulic data that you are familiar with. Hazen-Williams, Manning and Darcy-Weisbach/Colebrook formulas are all applicable. For water and wastewater friction loss calculation, use Hazen-Williams equation with C-factor per AWWA M-11. For viscous fluids such as chemical solutions, water and wastewater with velocity greater than 10 fps, use Darcy-Weisbach/Colebrook equation.

2.4.1.1. Pump Station Capacity Considerations The design capacity of pumping facilities should be based on detailed investigations and projections. For most municipal applications, the station infrastructure such as buildings, embedded pipes and

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electrical conduits will have a design life of more than 50 to 75 years and equipment can be replaced every 25 years. It may be appropriate to design the facility so that it could be expanded to meet anticipated demands of a 50 to 75-year planning period.

Wastewater pumping stations and certain water applications may need to be designed to meet projected peak hour demands. Other water pump stations may be designed to meet estimated maximum-day demands. It is necessary to be familiar with the rationale of how the proposed plant capacity was determined and to be sure that it is appropriate and objective. It is also necessary to know what changes may be proposed on the discharge side of the pumping station in the future. Future pipeline improvements within a distribution system, for example, can significantly influence head (and capacity) of a pump station.

The design criteria for the pump station should be shown on the design drawings. The criteria should show initial and future pump station capacity, number of units at each stage of development, key piping diameters, nominal velocity in suction and discharge piping, maximum operating pressure and motor ratings.

For storm water pumping stations there are significant risks and exposure. All assumptions that were made to estimate peak flows need to be confirmed and indicated on a design criteria sheet that is part of the contract drawings. Key criteria should be listed such as:

Design storm frequency Rainfall intensity Watershed area Runoff coefficient Percentage impervious area Storage basin capacity Basin operating levels If there is uncertainty due to such variables as rainfall intensity or demand forecasting assumptions, such uncertainties should be exposed and discussed with the client. Frequently the agreed solution is to provide for staged construction. The design criteria and other data on the drawings should clearly indicate if future stages were assumed. The criteria sheet should be signed by a responsible person representing the client.

Be cautious in adopting past designs and criteria, for both wastewater and treated (potable) water. In the past, temporary overflows from wet wells or forebays, due to short term power out-ages may have been acceptable. Currently in the US, however, releases of untreated wastewater (or even disinfected potable water) due to negligence, including deficient designs can result in legal action and criminal charges.

It is imperative that all foreseeable conditions and events should be considered to ensure that inadequate station capacity or design deficiencies will not result in releases.

Once the design capacity of a pump station is established, subsequent steps should ensure that the capacity can be delivered by the proposed pump station with at least one pumping unit off line.

For water pumping stations that directly serve a distribution area, the capacity determination (on whether the pump station capacity should be based on peak hour or maximum day projections) is related to the capacity of on-line service reservoirs and/or elevated storage. It will be necessary to confirm that there is sufficient storage to provide for peak hour demands and fire requirements.

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2.4.1.2. Total Dynamic Head Total dynamic head under a given flow is the sum of

Static lift Friction head Velocity head Static head is the difference in elevation between the water level (or equivalent hydraulic grade line) on the suction side of a pump and the level on the discharge side. It is a measured or estimated under zero flow (static) conditions.

Friction head is the sum of losses due to friction in suction and discharge piping and valves and fittings. For most applications it needs only be computed for one flow condition and at other flow conditions it can be estimated by assuming that total friction is proportional to flow squared.

Velocity head is the energy required to move liquid. It is equivalent to V2/2g. It usually is a minor consideration but can be significant in low-lift pumping applications that are why velocity head should always be included in the calculation. Normally kinetic energy (or velocity head) is lost when flows are discharged into a larger pipe or into a reservoir. The kinetic energy can be recovered with gradual velocity reduction. If the velocity head is a significant factor (more than say 5 percent of TDH) which may be the case in low lift applications, particular attention should be given to hydraulic computations and all minor losses should be computed.

TDH is the steady state head that will need to be developed by pumping equipment under operating conditions. TDH does not include temporary surges or increased head conditions (due to starting and stopping and associated valve operation) that may prevail for less than 30 seconds.

In the US, TDH is normally quoted in feet of water of head. For SI units, the correct term for head (pressure) is kilo Pascal (kPa) but in practice it is often quoted in meters. (1 meter of head is equivalent to ten kPa)

One reason that head is quoted in linear units (feet or meter) instead of pressure units is that head produced by a pump may be quoted linear units without correction for specific gravity. Thus, head is interchangeable for pump performance at different specific gravities. Whenever there is an application using sea water (or liquids other than fresh water), the head developed by the pump must be multiplied by specific gravity in order to compute pressure and energy requirements.

2.4.1.3. System Head Curves In order to establish typical pump operating conditions, it is advisable to graphically represent the total range of pumping conditions with system head curves. Once such curves are prepared, they can be overlaid with pump performance curves. See Section 5.1 for methodology for creating a system curve.

2.4.1.4. Major Pump Stations For major pump stations (in excess of 10,000 HP connected loads) factors which may influence the number or units include:

Limitations of size for commercially available pumps. Large pumps and motor are available up to 30,000 horsepower

Impact on power grid of starting large motors based on motor starting current Matching power demands with generation capacity and other pumping stations. A review of major large pump stations with capacities to 5,000 cfs for the US Bureau of Reclamation, California Aqueduct Pump Stations by State Water Resources Board, Central Arizona Water Project, Southern Nevada Water Authority and MWD of Southern California, indicates that the number of pumps ranges from 2 to nine units.

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2.4.1.5. Medium Size Pump Stations For medium size plans, say 500 to 10000 HP there may not be size limitations on commercially available equipment but starting electrical loads may influence the maximum motor size. Other factors which may influence the number of units include:

Starting and stopping frequency due to forebay size or delivery side storage limitations Clients standards, or requirements for more than one standby unit Provisions for future expansion Delivery to more than one water system pressure zone The unit costs of design and construction are increased when more than one-size (of pumping unit) is specified. Not only are pump layouts and specifications affected by more than one size of pumps but mechanical appurtenances (including valves and pipe fittings) and electrical (motors, starters ad controls) details are also more complicated. The clients requirements for stocking spare parts and other operating considerations, such as maintenance manuals, servicing and records, may be increased. In the long run, the perceived advantage of providing smaller pumps for in-between flows, may be lost. Unless there are specific limitations on electrical loads or frequency of stopping and starting (due to inlet or discharge considerations) the one size layout is preferable.

2.4.1.6. Small Pump Stations Pumping stations with total installed capacity of less than 500 HP are the most numerous applications for water and sewage. For sewage pumping plants, there is a need to be more conservative than for most water applications. It is common practice to provide two standby pumps for sewage pumps and it also may be the clients preference to provide two sizes of units (one size for normal dry weather flows and one size for peak wet weather flows). Thus, the minimum number of units for sewage plants may be three (or four if the client so directs)

For water applications, it is necessary to consider if the facility that is being proposed is the sole source of water for the area being served or if there are other pumping or gravity sources available. If the facility is the sole source of water and it serves an area with limited storage (less than 6 hours) or high fire risk (industrial and/or heavy commercial) then two standby units could be provided. For stations servicing residential areas and areas with adequate storage (capable of furnishing peak hour demands, fire flow and emergency provisions) a simple 2-unit stations (one duty and one standby unit) may be applicable.

2.4.1.7. Standby Units and Standby Power Supply The number of standby pumping units is usually based from the Clients preference or as mandated by the EPA or other applicable government agency to meet the reliability design criteria. Standby unit is defined as the additional pump unit(s) in addition to the minimum number of unit required to deliver the maximum pump station capacity with one largest unit out of service. The capacity of the standby pump shall be equal to the largest pump unit.

Several factors enter into an analysis of the risk of power interruption (vulnerability) associated with the pumping design. The risk factors that need to be evaluated include: equipment reliability, electrical demand patterns, power interruption frequency and available forebay storage or emergency storage. It is seldom possible to compute and accurate or comparable risk factor. Historical data is generally not representative of future interruptions because the causes of past interruptions may have been partially corrected. Also future conditions may not have been experienced in the past. Unless it is a contract requirement, risk analysis should be avoided and the rationale for standby units (and power supplies) should be evaluated based on the following considerations.

In the case of water systems, most short-durations power interruptions are not a serious concern because of the provision of system storage. Most water pumping stations therefore, do not include standby power provisions but do include an electrically-driven standby pumping unit with

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a capacity equivalent to the largest unit. Generally, the standby unit is identical and interchangeable with other units and there are provisions in the control system for any pump to be designated as the lead pump and any unit to be designated the standby unit.

Power interruption is more of a concern with sewage pumping plants because of accidental sewage overflow to the nearby river or ocean. Also the risks associated with maintenance on pumps are significantly higher with sewage pumps, as compared to water pumps. If the client has adopted policies with respect to standby units and/or standby power, they should be accepted, unless it is apparent that they are not adequate. In general, two standby pumps should be considered for sewage pumping stations.

2.4.1.8. Net Positive Suction Head (NPSH) The NPSH available (NPSHa