low pressure sewers

WASTEWATER COLLECTION SYSTEM MODELING AND DESIGN

Authors

Thomas M. Walski

Thomas E. Barnard

Eric Harold

LaVere B. Merritt

Noah Walker

Brian E. Whitman

Contributing Authors

Christine Hill, Gordon McKay, Stan Plante, Barbara A. Schmitz

Peer Review Board

Jonathan Gray (Burns and McDonnell), Ken Kerri (Ret.),

Neil Moody (Moods Consulting Pty, Ltd.), Gary Moore (St. Louis Sewer District),

John Reinhardt (Massachusetts Department of Environmental Protection),

Reggie Rowe (CH2M Hill), Burt Van Duin (Westhoff Engineering Resources)

Click here to visit the Bentley Institute Press Web page for more information

Pressure sewers (also called low-pressure sewers) are similar to force mains (Chapter 4),but differ in that individual sewer pumping systems are located on the property ofeach customer. Low-pressure systems are usually installed where excavation is diffi-cult because of rock, the ground surface is flat or nearly so, or there are long distancesbetween customers. Any of these factors can make gravity sewers difficult and costlyto install.

Low-pressure sewers have both advantages and disadvantages compared to gravitysewers. The primary advantages are

Lower cost of pipe network installation due to smaller piping and loweredexcavation costs

Greater flexibility in routing of piping

Fewer manholes

Reduced potential for infiltration and inflow.

The primary disadvantages include

Higher operation and maintenance costs for power, repair, and replacement ofpumps

Failure during power outages

Maintenance of air-release/vacuum-breaker valves.

Pressure sewers have become a popular solution for areas that lack the populationdensity to support a traditional gravity sewer system and are also unsuitable for sep-tic systems. They are especially attractive as a means of reducing both excavationcosts and infiltration in areas with high water tables.

C H A P T E R

13Low-Pressure Sewers

468 Low-Pressure Sewers Chapter 13

13.1 Description of Pressure Sewers

The first pressure sewers in the United States were installed in Radcliff, Kentucky inthe late 1960s. Although the system was eventually abandoned, it demonstrated thatthe concept could work (Clift, 1968). Fair had earlier proposed a system for hangingpressure sanitary sewers inside large storm or combined sewers (US EnvironmentalProtection Agency, 1991).

The technology in pressure sewers has improved, and they have become a reliablealternative to septic systems and conventional gravity sewers. The primary improve-ment was in the reliability of the small grinder pumps installed at each customer loca-tion. Rezek and Cooper (1985) reported that there were 78 pressure sewer systems inoperation by the mid-1980s. Hundreds of pressure systems are currently in operation,including some that serve more 1000 customers (Feuss, Farrell, and Rynkiewicz, 1994;Farrell and Darrah, 1994).

Pressure sewer systems consist of the following components, which are described inmore detail in the sections that follow:

Pumps move wastewater from the customers premises to the main Storage tanks hold wastewater when the pump is not running and provide

submergence for the pump Service lines connect the grinder pumps to the main Pressure sewer main a larger pipe in the street or right of way Air-release valve releases trapped air from accumulation points in the network Discharge point transition point from pressure to gravity flow at the end of

the pressure system.

All pressure-system pumps collect water from a suction storage tank. The primarydistinction is whether the tank is used for solids removal as well as providing suctionhead. Grinder pumps have a grinder, similar to a garbage disposal, to grind whatever isreceived in the storage tank before it is pumped. General Electric Company developedthe first grinder pump in 1967 (Carcich, Hetling, and Farrell, 1972).

Alternatively, pumps may be preceded by some type of solids-removal process, usu-ally called septic tank effluent pump (STEP) systems. The earliest septic tank effluentpumping systems were documented by Langford (1977).

PumpsDepending on the manufacturer, the pump mechanism in pressure sewer systems iseither centrifugal or progressive cavity. Pumps for pressure sewer systems usuallyhave a grinder reduce the size of solids. Some systems can use solids-handlingpumps, which are designed to pass fairly large solids.

The system head that the pump must overcome to discharge wastewater into the pres-sure sewer main can vary widely from low to high points in the system and fromhigh- to low-flow periods. Installers do not want to keep a large inventory of pumps fordifferent heads. Therefore, it is desirable to use pumps with a fairly steep pump-headcharacteristic curve (see Chapter 4 for a description of pump-head curves).

Progressive-cavity pumps (also referred to as semipositive-displacement pumps) pro-duce roughly the same discharge regardless of the head against which they pump

Section 13.1 Description of Pressure Sewers 469

(i.e., they have very steep head characteristic curves). The head in the main affects thedischarge from centrifugal pumps to a much greater extent. Progressive-cavitypumps can typically function with 1 to 11/2 hp (0.751.2 kW) motors. Centrifugalpumps function best with 2 hp (1.5 kW) motors so that they will work when the headin the main is high. Typical head characteristic curves for progressive-cavity and cen-trifugal pumps are shown in Figure 13.1.

Single-family dwellings and small apartments use a single pump in the pump vault,referred to as a simplex unit. Large apartments, commercial buildings, and trailerparks use duplex units containing two pumps.

200

175

150

125

100

75

50

25

010 20 30 40 50 60

1 - 2 hp Centrifugal STEP Pump2 - 2 hp Centrifugal GP3 - hp Centrifugal STEP4 - 1 hp Progressive Cavity STEP5 - 1 hp Progressive Cavity GP or STEP6 - hp 7-Stage Submersible Water Well

Discharge, gpm

Hea

d, ft

ofW

ater

1

2

3

4 5 6

US EPA, 1991

Figure 13.1 Typical head characteristic curves for various types ofsewer pumps.


Storage Tanks

Storage or holding tanks are usually located just outside the footprint of the structurebeing served, although in some instances they are placed in the basement or crawlspace. The tanks are generally made of HDPE, fiberglass, or treated concrete. A typi-cal home tank has a storage volume of about 47 gal (179 L) below the alarm level (thepoint at which a warning is sent that the tank is getting full). The pump is activatedwhen sewage in the tank reaches a volume of 32 gal (121 L) and is turned off when thevolume reaches 24 gal (91 L). This yields a volume of roughly 8 gal (30 L) to bepumped each cycle, leaving about 24 gal (91 L) in the bottom of the tank (e/one, 2001).This configuration allows for some submergence of the pump intake at the low level,as well as some freeboard to prevent flooding during power outages and malfunc-tions. A typical tank with pump is shown in Figure 13.2. A pump vault in a STEP sys-tem is shown in Figure 13.3.

Service Lines

The service line connecting the pump to the main contains a check valve and a manualisolating valve. For residential customers, the service line is usually 1.25 in. (32 mm)and is made of plastic. The velocities in service lines are typically about 4 ft/s (1.2 m/s).The pressure in the system is usually about 35 psi (240 kPa) (Crites and Tchobano-glous, 1998; US Environmental Protection Agency, 1991). The discharge piping shouldbe at least the same size as the pump outlet.

Courtesy of Environment One Corp.

Figure 13.2 Engineering drawing and cutaway view of a pressure sewer pump system.

Section 13.1 Description of Pressure Sewers 471

Pressure MainsThe pressure sewer main in the street or right-of-way is usually plastic pipe, but,unlike gravity sewers, the slope of the line is not very important. The sizing of thepipe should be based on a hydraulic analysis to ensure that the pipe is large enough tocarry the design flow without excessive head loss, yet small enough to maintainself-cleansing velocities.

Air-Release/Vacuum-Breaker ValvesHigh points along a pressure sewer system can collect pockets of gases, so combina-tion air-release/vacuum-breaker valves should be placed at these locations. A typicalcombination valve is shown in Figure 13.4. Gas pockets can collect not only at highpoints, but also in a downward-sloping closed pipe anywhere the slope increases sig-nificantly. The potential for collecting gas pockets can be estimated usingEquation 12.12 on page 448.

The engineer should look for high points in the system and install combination valves.These valves should also be installed every 2000 ft (610 m), even on lines without highpoints (e/one, 2000). Because these valves can become clogged with grease, mainte-nance is necessary to insure proper function.

Discharge PointsA pressure sewer system usually discharges to a gravity sewer at a manhole located ata high point. Alternatively, the pressure system may discharge to a pump station wetwell or at the wastewater treatment plant.

Cover

Inlet

High Water Line

Low Water Line

Mercury Float Switch

Vault

Check ValvePump Off Level

Pump On LevelAlarm Level

FlexibleDischarge Hose

Quick Coupling

Discharge

Gate Valve

CoverJunction Box

Septic TankUS EPA, 1991

Figure 13.3 Septic tank and pump vault in a STEP system.


13.2 Estimating FlowsThe rate of generation of wastewater in pressure sewer systems (e.g., 100 gpcd, 380Lpcd) (GLUMRB, 1997) should be the same as in gravity-flow systems. However,because of the lower potential for I/I due to positive sewer pressures, some engineersuse lower values, such as 5070 gpcd (190260 Lpcd), to model pressure sewer sys-tems (Thrasher, 1988). Although these lower average inflow rates may be used, theratio of peak inflow to average inflow will typically be higher in pressure systemssince these systems tend to serve fewer customers.

Cover

Optional Back Flushing Hose

Approved Prefabricated MetalFiber or Concrete

1 in.BlowOff Valve

Granular Material(8 in. Minimum Depth)

Pressure Sewer Mainor Collector

Threaded Teeto Suit Inlet

2 in. Minimum Clearance

2 in. Shut Off Valve

Sewage AirRelease Valve

Slope

Valve Box Arrangement

Valve Box Arrangement

Valve Box ArrangementAir Release Arrangement

2 in. Valve, in. HoseConnection Nipple Arrangement

Locking Cover Valve BoxArrangement

ApprovedMeter Box

Standard ValveBox or PVC Riser

PVC RiserFull PortedGate Valve ofAppropriate Size

Valve Box DetailValve Box Detail

Air Release Detail

Pressure Mainor Collector

Standard Wye Fitting

45 Ell

18 to 24in. Varies with Sewer

Depth

Granular Material

US EPA 1991

Figure 13.4 Typical air-release valve installations.

Section 13.2 Estimating Flows 473

Flows change gradually in gravity systems; however, the discharge from an individ-ual customer in a pressure sewer system is zero until the water in the customers stor-age tank reaches a preset level, causing the pump to turn on. The customer thendischarges to the sewer at a peak (or near-peak) flow rate for a short time, the tankdrains, and the discharge drops back to zero.

Much of the challenge of estimating flows in pressure sewers relates to predictinghow many customers will be discharging to the system simultaneously (that is, howmany pumps will be on at the same time). Research on contributions to pressurewastewater systems from individual homes has been published in numerous sources(Bennett, Lindstedt, and Felton, 1974; Jones, 1974; Watson, Farrell, and Anderson,1967). Some more-sophisticated methods based on statistical analysis of water-usepatterns from individual homes are given by Buchberger and Wu (1995) and Buch-berger and Wells (1996).

Although there is very little opportunity for infiltration and inflow to enter the systemalong the mains and service lines, pump vaults (tanks) do provide a potential avenuefor flow increases during wet weather. These need to be inspected during installationto prevent connections from downspouts and French drains. Directly metering theflow from customers is difficult and expensive. If customers are suspected of dis-charging wet-weather flow to the system, a simple run-time meter can be placed on thepump to determine if the pumped volume increases significantly during wet weather.

Empirical Approaches

A detailed analysis of pressure-sewer flows is based on the probability that a givennumber of pumps out of the population are running at any one time. Therefore, peak-ing-factor formulas commonly used with gravity sewers, such as those of Babbitt(1953) or Harmon (1918), do not apply to pressure sewers. A simple rule of thumb forcomputing design flow in a pressure sewer uses equivalent dwelling units (determinedbased on the average load from a typical residence in the area of interest) and is givenby (US Environmental Protection Agency, 1991)

Q = 0.5 N + 20 (13.1)

where Q = design flow (gpm)

N = equivalent dwelling units upstream

(For flow in L/s, replace the constants 0.5 and 20 in Equation 13.1 with 0.032 and 1.3,respectively.)

Alternatively, various sources have provided representative curves for determiningdesign flow. Several such curves are plotted in Figure 13.5.

Using data from a study in Albany, NY, e/one (2001) developed a table relating thenumber of pumps running at peak times to the total number of pumps. These valuesare listed in Table 13.1.


This table can be used to estimate the peak flow at any point in a system by countingthe number of pumps upstream, looking up the number that are likely to run at anyone time, and multiplying that value by the flow from a pump.

Walski (2002) adapted these data to account for population and number of users perresidence to derive the following expression for peaking factor:

Table 13.1 Number of pumps operating during peak conditions (e/one, 2001).

Number of Pumps in System

Max. Number of Pumps Operating Simultaneously

Number of Pumps in System

Max. Number of Pumps Operating Simultaneously

1 1 114149 9

23 2 150179 10

49 3 180212 11

1018 4 213245 12

1930 5 246278 13

3150 6 279311 14

5180 7 312344 15

81113 8

200

100

0

Equivalent Dwelling Units

Flo

w, g

pm

300

100 200 300 400 500

EnvironmentOne

Battelle

ASCENortheasternU.S.

ASCE California

F.E. Myers

0.5 + 20N

Barnes

US EPA, 1991

Figure 13.5 Various curves for estimating sanitary design flows.


(13.2)

where PF = peaking factorq = average discharge from a single pump (gpm, L/min)P = populationn = number of persons per pumpu = average demand per capita (gpcm, Lpcm)

For a relation for peaking factor that is independent of units, Equation 13.2 can berewritten as

(13.3)

where a = q/u

and

(13.4)

Values for q, u, and n vary depending on the system. Typical ranges are q = 915 gpm(3560 L/min), u = 0.0350.056 gpm (0.130.21 L/min), and n = 24 persons per pump.The value for a ranges between 200 and 500 and b ranges between 6000 and 20,000.The value of q is lower for systems with centrifugal pumps rather than positive-dis-placement pumps. During peak flow times, the pressure in the system increases;therefore, the flow from individual centrifugal pumps decreases accordingly. The plotin Figure 13.6, calculated using some typical values for the parameters, compares thepeaking factors obtained from Equation 13.2 with those obtained by methods dis-cussed in Chapter 6 for traditional gravity systems.

Poisson Distribution to Estimate Loads

One method commonly used in Europe to estimate the number of pumps running isbased on the fact that the probability that any number of pumps are running simulta-neously can be determined with the Poisson statistical distribution (Soderlund, Jons-son, and Nilsson, 1994). Given the probability that any one pump will run, thenumber of pumps in the system, and the an assumed number of pumps running, theprobability that exactly that number of pumps is running is given by

(13.5)

where PP = probability that exactly Nr pumps are running simultaneouslyduring the wastewater generation period

N = number of pumps in the systemNr = number of pumps running simultaneouslyPr = probability that a single pump is running

PFq 0.71q P

n--- 1 0.53+

uP--------------------------------------------------=

PF a bP 0.53+

P-----------------------------=

b0.71 q

u---

1.88

n----------------------------=

PP N!Nr! N Nr !--------------------------------- 1 Pr

N Nr PrNr=


The probability that any single pump is running at a given time during the wastewa-ter generation period is a ratio of the flow into the sump divided by the rate at whichthe wastewater is pumped out when the pump is running, which is given by

(13.6)

where Qin = effective inflow rate to the pump facility (gpm, L/s)Qp = pump discharge rate (gpm, L/s)

A typical single-family pump discharge rate Qp is approximately 1015 gpm (0.61.0L/s), depending on the type of pump and the location. The effective inflow rate Qindepends on the number of users at the location, the per capita wastewater flow pro-duction, and the number of wastewater-producing hours. It is given by

(13.7)

where Vw = volume of wastewater per inhabitant (gpcd, Lpcd)Nu = number of inhabitants per pump

cf = unit conversion factor (60 for gpm, 3600 for L/s)nh = number of wastewater-producing hours per day, usually 812

(hr/d)

For typical values, such as three inhabitants per pump, 80 gpcd (300 Lcpd), and 10wastewater-producing hours per day, the inflow rate to a single-residence pump maybe 0.4 gpm (0.025 L/s). For a 15 gpm (1 L/s) pump, the probability that a single pumpis running is approximately 0.025 during the 10-hour wastewater generation period,which corresponds to a pump running roughly 15 minutes per day.

PrQinQp---------=

QinVwNucfnh---------------=

Population

0

5

10

15

20

25

30

35

40

45

0 100 200 300 400 500 600

High

Low

Babbitt

GLUMB

High

Low

Babbitt

GLUMRB

40

35

30

25

20

15

10

5

0

0 100 200 300 400 500 600

Population

Pea

king

Fac

tor

Figure 13.6 Peaking factors for pressure sewers.


Figure 13.7 shows the probability of a given number of pumps running simulta-neously for systems with 10 through 160 pumps for Pr = 0.2. Soderlund, Jonsson, andNilsson (1994) recommend using the number of pumps running at a 10-percent prob-ability for the design flow. A more useful value for design may be the cumulativeprobability of at least that number of pumps running, as shown in Figure 13.8.

Overall, this method gives somewhat lower estimates of the number of pumps run-ning than methods based on the Albany, NY, data (e/one, 2001).

10

30

100

160

100

90

80

70

60

50

40

30

20

10

0

0 1 2 3 4 5 6 7 8 9 10

Number of Pumps Running ( ) ifNr

Pro

babi

lity

ofR

unni

ngN

r

Probability of Operation = 0.2

Number ofpumps insystem

Figure 13.7 Probability of Nr pumps running at one time.

100

90

80

70

60

50

40

30

20

10

0

0 1 2 3 4 5 6 7 8 9 10

Number of Pumps Running (N )r

Pro

babi

lity

ofor

Gre

ater

Run

ning

Nr

Probability of operation = 0.2

10

30

100

160

Number ofpumps insystem

Figure 13.8 Cumulative probability or Nr or more pumpsrunning.


13.3 Pressure Sewer Design Considerations

The design of a pressure sewer requires a layout of the piping, which should prefera-bly be done on a CAD- or GIS-based map, much like any other sewer design. The useof an electronic base map makes it much easier to import the geometry and distancesinto the model for hydraulic calculations. If necessary, the design can be based onpaper maps or drawings, but then the representation of the system in the model willnot be exactly to scale; therefore, the modeler must input pipe lengths manually.

The key design parameter for pipe sizing is the velocity. For the system to work effec-tively, the velocity at peak flow should be kept below 5 ft/s (1.5 m/s) (HydromaticPumps, 2001), although there are some situations, such as short runs of pressure pipe,in which higher velocities can be tolerated. More important, velocity should begreater than 2 ft/s (0.6 m/s) in all pipes for at least some part of the day to prevent sol-ids deposition.

The European Standard for pressure sewers, EN 1671, states that a minimum velocityof 0.7 m/s (2.2 ft/s) must be achieved at least once every 24 hours (CEN, 1996). Periodicflushing is recommended when this velocity cannot be maintained. The pipe boremust be the same size as the pump outlet or larger. The standard also states thatdetention time in the system should be less than 8 hours.

Oversizing of pressure sewers (e.g., due to assuming that all pumps are running atonce) is discouraged, as it has been documented that grease and fibrous material canthen block the pipe (Carcich, Hetling, and Farrell, 1972).

The sizing of individual pumps is based primarily on the peak flow from that cus-tomer or group of customers. Individual home pumps are usually sized for 1015gpm (0.71.0 L/s). The methods described in Chapter 6 and the fixture unit methoddescribed by American Water Works Association (2003) can be used to determinepeak flow in commercial and industrial buildings.

With positive-displacement pumps, standard pumps can operate with line pressuresas high as 60 psi (475 kPa) at low points, and standard pumps will still operate. How-ever, the pressure in the main must be considered when selecting centrifugal pumps.High-horsepower pumps with large impellers may be required at low points. There isno minimum pressure that must be maintained in the system.

If the hydraulic model predicts negative pressures (see Modeling Pressure Sewerson page 479), this is an indication that a combination valve may be needed at thatlocation to prevent air blockage or pipe collapse. Many of these situations are a resultof pumping downhill and can be avoided if the pressure sewer terminates at a highpoint. These issues are discussed in Chapter 12 with respect to force mains, and in thisregard, pressure sewers act as force mains.

It is usually desirable to draw a profile of the ground and pipe. The profile views fromcomputer models can provide insight into pressures along the line. It may be better touse gravity sewers in areas with long stretches of downhill slope and only use pres-sure sewers in the portions of the system where the terrain undulates. Although thegravity section may have a higher construction cost, it should be easier to operate anddoes not require replacement and repair of pumps. An example of a mixed gravityand pressure system uses pressure sewers on one side of a drainage divide, butswitches to a gravity sewer on the other side to flow down to the plant, as shown in

Section 13.4 Modeling Pressure Sewers 479

Figure 13.9. Additional information about pressure sewer design can be found inThrasher (1985) and Flanigan and Cadmik (1979), as well as in manuals provided bypump manufacturers (e/one, 2000; Hydromatic Pumps, 2001).

13.4 Modeling Pressure SewersThe fundamental hydraulic concepts involved in modeling pressure sewers are thesame as those of force mains and pump stations. The primary difference between thetwo types of systems is that, with a pressure sewer, each customer has their own stor-age tank and pump; thus, the pressure system pump stations are smaller and muchmore numerous. While a manual or spreadsheet hydraulic analysis may be sufficient for a system withonly a few customers, a hydraulic analysis model becomes necessary as the number ofcustomers and the complexity of the system increase. Of course, if the pressure sewersare integrated into a system with gravity sewers, then a hydraulic sewer model thatcan handle both types of flow is necessary if all sewers are to be analyzed simulta-neously. A typical model of a pressure sewer discharging into a gravity sewer at ahigh point is shown in Figure 13.10.

Pressure sewers can be modeled using different levels of detail, depending onwhether the modeler is interested only in the overall sizing of mains or in pumpcycling and the unsteady nature of the flows. Most design work can be done with asteady-state model, whereas operational studies typically require extended-periodsimulations. Three basic modeling options, in order of increasing complexity, aredescribed on the following pages.

ToTreatment

Gravity SewerSouth of Divide

DrainageDivide

Pressure SewerNorth of Divide

Gravity Pipe

Pressure Pipe

Figure 13.9 Mixed gravity and pressure sewer system.


In all three of the options described, the modeler should look for situations where thepressures at low elevations are too high during peak flow conditions. Excessivelyhigh pressures may be unavoidable at very low elevations. In such cases, pipe with ahigher pressure rating and/or a pump with a higher head may be required in somelocations. If the high pressures are due to a steep system head curve, larger pipingmay be needed downstream. Model runs should also be checked to insure that veloci-ties of at least 2 ft/s (0.7 m/s) are achieved at least once daily.

Modeling to Size Pressure Mains If the modeler is simply trying to size the pressure mains as part of a system design, asteady-state simulation that omits individual customer pumps is often sufficient. Peaksystem loads are modeled by allocating multiple users to model junction nodes, muchas multiple users are placed at junction nodes in water distribution models or at man-holes in gravity sewer models. The corresponding peak loads are entered as knowninflows.

Model nodes need only be placed at pipe intersections and changes in diameter, andat system low points and high points (which correspond to combination-valve loca-tions) to check pressures. The modeler reviews the results to evaluate head losses andvelocities, and sizes the pipes accordingly.

If local high points exist along the pressure sewer, the modeler must be aware that thepressure calculations may not be accurate. The model may report an unrealistic nega-tive pressure (HGL dropping below the pipe) when, in actuality, a combination valveprevents the negative pressure from occurring. If a negative pressure is computed, thecombination valve at this location cannot be modeled as a simple junction node.Instead, the pipe on the upstream side of the valve may be modeled as terminating ina reservoir open to atmospheric pressure and set to a hydraulic grade elevation equal

Gravity sewer

Pressure sewer

Figure 13.10 Pressure sewer discharging to a gravity sewer.


to the elevation of the valve. An inflow equal to the discharge into the reservoir is thenplaced on the junction node representing the downstream side of the valve. For alter-native modeling techniques that can be applied in this situation, Modeling a Pipelinewith Multiple High Points on page 444.

Representing All Service Connections as Nodes

At an intermediate level of detail, the modeler can represent each service line (e.g.,house connection) as a junction node. This network configuration can be used with asteady-state run to identify the pump head required at any service connection. Deter-mining the head at service nodes is not a major issue for positive-displacement pumps(which have steep performance curves) if the maximum pressure required is belowthe pumps threshold, but it can be important for centrifugal pumps.

In the steady-state model, nodes representing services with pumps running are assignedfixed inflows. By varying the number of pumps running at one time (i.e., the number ofservices with inflows), the modeler can determine the pressure range for any number ofpumps and select the needed pump head and horsepower. Table 13.1 provides guidanceon the number of pumps assumed to be running during a peak condition.

A model with all service connections represented as nodes can also be used withextended-period simulation runs to provide information on unsteady flows and howthe system performs over time. With an extended period simulation, inflows anddimensionless flow patterns should be assigned to reflect the changing on/off statusof the pumps.


Because the pumps themselves are not included, an extended-period simulation withthis level of detail can only be used to assess system operations insofar as the pump-ing rates can be assumed to be relatively unaffected by fluctuations in tank level anddischarge pressure. This assumption generally works well for positive-displacementpumps, but is less accurate for centrifugal pumps.

In addition to simulating normal operations, the modeler can examine extreme situa-tions such as all pumps turning on simultaneously following an extended power out-age. When the system is being initially filled, transients can damage piping, sovelocities should be kept below 1 ft/s (0.3 m/s) during filling to prevent water hammer(Hydromatic Pumps, 2001).

Detailed ModelsAt their most complex, models can include each individual pump and tank and can beanalyzed with an extended-period simulation. Runs that simulate each pump provideinsight into the range of conditions that the system will experience, including the riseand fall of water in the storage tanks, changing pump status, and fluctuations in pres-sure and flow. This type of model is also useful for designs with centrifugal pumpsbecause their discharges may be significantly affected by fluctuating system conditions.

A detailed model requires significantly more information, including Base loads and patterns for loading at storage tanks Volume and elevation data for individual storage tanks (may be modeled as

small wet wells) Individual pump performance curves Junction node and pipe connecting the pump discharge to the pressure main

for each service connection.

Figure 13.11 is a model view of a small pressure sewer system with 10 pumps. Thedischarge point of this network is on the far right side. The pipes are annotated withthe flow rate in gpm, and the nodes are annotated with the pressure in psi. The waterlevel and pump status for every pump are explicitly considered in a model of thisdetail. Figure 13.12 shows the discharge at the downstream end of this system over a

J-4

J-9

Transition to Gravity Flow

LateralPressure Main

Pump

Switch

Direction of flow

Customer

Sump

Nodes

Figure 13.11 Pressure sewer model with a pump and tank at each residence.


3-hour period. Note that, with such a small system, there is no flow in the pipes formuch of the time. Figure 13.13 shows the pressure at nodes J-4 and J-9. J-9 is furtherdownstream and at a higher elevation, so it has a lower pressure and less pressurevariation. J-4 is further upstream and has a lower elevation, so the pressure is consis-tently higher than that of J-9. In general, the most-upstream nodes experience thegreatest pressure variations.

Time (min)

0

5

10

15

20

25

30

35

40

45

50

0 30 60 90 120 150 180

Time, min

Flow

,gp

m

Figure 13.12 Plot of discharge versus time in a sewer model.

0

5

10

15

20

25

30

35

40

45

0 30 60 90 120 150 180

Time, min

Pres

sure

,ps

i

J-9

J-4

Time, min

Pre

ssur

e, p

si

Figure 13.13 Plot of pressure versus time in a pressure sewermodel.


References

American Water Works Association. 2003. Sizing Water Service Lines and Meters.Publication M-22. Denver, CO: American Water Works Association.

Babbitt, H. E. 1953. Sewerage and Sewage Treatment. 7th ed. New York: John Wiley andSons.

Bennett, E., K. Linstedt, and J. Felton. 1974. Rural home wastewater characteristics.Proceedings of the National Home Sewage Symposium, American Society ofAgricultural Engineers.

Buchberger, S. G. and L. Wu. 1995. A model for instantaneous residential waterdemands. Journal of Hydraulic Engineering 121, no. 3: 232.

Buchberger, S. G., and G. J. Wells. 1996. Intensity, duration and frequency ofresidential water demands. Journal of Water Resources Planning and Management122, no. 1: 11.

Carcich, I. G., L. J. Hetling, and R. P. Farrell. 1972. Pressure Sewer Demonstration. USEPA, R2-72-091. Washington, DC: US Environmental Protection Agency.

Clift, M. A. 1968. Experiences with pressure sewage. ASCE Sanitary EngineeringDivision 94, no. 5: 865. Alexandria, VA: American Society of Civil Engineers.

Comit Europen de Normalisation (CEN). 1996. Pressure Sewer Systems, OutsideBuildings. EN 1671. Brussels: Comit Europen de Normalisation.

Crites, R., and G. Tchobanoglous. 1998. Small and Decentralized Wastewater TreatmentSystems. New York: McGraw-Hill.

e/one. 2000. Low Pressure Sewer Systems Using Environment One Grinder Pumps.Niskayuna, NY: e/one Corporation.

e/one. 2001. Specifications for GP 2010. Niskayuna, NY: e/one Corporation.

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Problems 485

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Problems

13.1 List three advantages and three disadvantages of pressure sewers.

13.2 Of the two pump head curves in the figure, which is more likely to be a centrifu-gal pump and which is more likely to be a semipositive displacement pump?

13.3 Using the Babbitt formula and the Great Lakes Upper Mississippi River Boardformulas for peaking factors, determine the peaking factors they predict for thepopulations given in the following tables and compare with Equation 13.3 inthis chapter. Use average typical values for q, n, and u.

Pump A

Pump B

Flow, L/s

Hea

d, m


Given an average daily flow of 70 gal/capita, what is the peak flow rate in gpm?

Why are the peaking factors for pressure sewers from Equation 13.3 higher thanthose from the other two methods?

13.4 Consider the pressure sewer being shown in the figure, which is to be installedin an industrial area with insufficient slope for a gravity sewer. When operating,each pump discharges the flow listed in the table. The discharge manhole (R-1)water level is at an elevation of 16 ft.

a. Select a diameter for each pipe segment; possible diameters are 2, 2.5, 3, 4, 6,and 8 in. Assume a C-factor of 130 for each pipe. Find the velocity in the pipesand the pressure and HGL at the junctions indicated when

All pumps are running.

Only the pumps at J-1 and J-4 are running.

The velocity should be about 4 ft/s when all pumps are running. The pressureshould be less than 40 psi at all times. Complete the following tables.

Population

Peaking Factor

Babbitt GLUMRB Equation 13.3

20

100

500

Population

Peaking Flow, gpm

Babbitt GLUMRB Equation 13.3

20

100

500

J-1

J-7 P-7

P-1P-2

J-8 P-8

J-3

J-2

P-3

J-4

P-4J-5

P-5J-6

P-6

R-1

Problems 487

b. Prepare a profile drawing of the piping from J-1 to R-1 showing pipe eleva-tion, ground elevation, and the two HGLs.

13.5 A pressure sewer serves 45 homes (one pump per home) with an average dailyload of 200 L/home, 10 hours of production per day, and a typical pump dis-charge of 0.8 L/s when the pump is running.

a. Determine the probability that any pump is running.b. Determine the probabilities that zero, one, two, three, or four pumps are

running.c. Determine the cumulative probability that zero, one, two, three, or four

pumps are running..

d. What is the maximum number of pumps running, based on Table 13.1?

JunctionGround Elev. ft

PipeElev., ft

Inflow, gpm

All Pumps Pumps J-1 and J-4

Pressure, psi HGL, ft

Pressure, psi HGL, ft

J-1 12 7 30

J-2 26 19 20

J-3 20 15 15

J-4 23 18 45

J-5 19 14 15

J-6 17 12 15

J-7 10 5 35

J-8 12 7 15

LabelLength,

ftDiameter,

in

Velocity, All Pumps,

ft/s

Velocity 2 Pumps,

ft/s

P-1 225

P-2 165

P-3 350

P-4 425

P-5 180

P-6 270

P-7 605

P-8 420

Number of Pumps

Probability of Running

CumulativeProbability

0

1

2

3

4

low pressure sewers

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