tapflo pump therory

Manufacturer of Industrial Pumps

Pump theory

General and basic pump knowledge

Håkan Ekstrand/Tapflo AB/2012


Pump theory

Pump classification Pump selection Pump theory

Affinity laws System curves Cavitation NPSH



Pump classification

Centrifugal pumps The centrifugal pump

produce a head and a flow by increasing the velocity of the liquid through the machine with the help of a rotating vane impeller.

Positive displacement pumps The

positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid against varying discharge pressure or head.



Pump classification

Centrifugal pumps End suction pump In-line pump Double suction pump Vertical multistage pump Horizontal multistage pump Submersible pumps Self-priming pumps Axial-flow pumps

Positive displacement pumps Reciprocating pumps

Piston pumps Diaphragm pump

Rotating pumps Gear pump Lobe pump Screw pump



Selecting between C or PD Pumps Flow rate and pressure head

The Centrifugal Pump has varying flow depending on the system pressure or head The Positive Displacement Pump has more or less a constant flow regardless of the system pressure or

head. Positive Displacement pumps generally gives more pressure than Centrifugal Pump's.

Capacity and Viscosity In the Centrifugal Pump the flow is reduced when the viscosity is increased

A Centrifugal Pump becomes very inefficient at even modest viscosity. In the Positive Displacement Pump the flow is increased when viscosity is increased

Liquids with high viscosity fills the clearances of a Positive Displacement Pump causing a higher volumetric efficiency and a Positive Displacement Pump is better suited for high viscosity applications.

Mechanical Efficiency Changing the system pressure or head has little or no effect on the flow rate in the Positive Displacement

Pump Changing the system pressure or head has a dramatic effect on the flow rate in the Centrifugal Pump

Net Positive Suction Head - NPSH In a Centrifugal Pump, NPSH varies as a function of flow determined by pressure In a Positive Displacement Pump, NPSH varies as a function of flow determined by speed. Reducing the

speed of the Positive Displacement Pump pump, reduces the NPSH



Centrifugal pumps A centrifugal pump converts the input power to kinetic energy in the liquid by accelerating the

liquid by a revolving device - an impeller. The energy created by the pump is kinetic energy according the Bernoulli Equation. The energy transferred

to the liquid corresponds to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, the higher will the velocity of the liquid energy transferred to the liquid be. This is described by the Affinity Laws.

Pressure and Head If the discharge of a centrifugal pump is pointed straight up into the air the fluid will pumped to a

certain height - or head - called the shut off head.

A pump does not create pressure, it only creates flow. Pressure is a measurement of the resistance to flow.

In Newtonian fluids (non-viscous liquids like water or gasoline) the term head is used to measure the kinetic energy which a pump creates.

Head is a measurement of the height of the liquid column the pump creates from the kinetic energy the pump gives to the liquid.

The main reason for using head instead of pressure to measure a centrifugal pump's energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not.

The pump's performance on any Newtonian fluid can always be described by using the term head.



Centrifugal pumpsDifferent Types of Pump Head Total Static Head - Total head when the pump is not running Total Dynamic Head (Total System Head) - Total head when the pump is running Static Suction Head - Head on the suction side, with pump off, if the head is higher

than the pump impeller Static Suction Lift - Head on the suction side, with pump off, if the head is lower than

the pump impeller Static Discharge Head - Head on discharge side of pump with the pump off Dynamic Suction Head/Lift - Head on suction side of pump with pump on Dynamic Discharge Head - Head on discharge side of pump with pump on

The head is measured in either feet or meters and can be converted to common units for pressure as psi or bar.

It is important to understand that the pump will pump all fluids to the same height if the shaft is turning at the same rpm.

The only difference between the fluids is the amount of power it takes to get the shaft to the proper rpm. The higher the specific gravity of the fluid the more power is required.



Affinity laws The Affinity Laws of centrifugal pumps or fans can be used to express the influence on volume capacity, head

(pressure) or power consumption of a pump or fan due to change in speed of wheel - revolutions per minute (rpm) geometrically similarity - change in impeller diameter

Volume Capacity The volume capacity of a fan or pump can be expressed like

q1 / q2 = (n1 / n2)(d1 / d2)3 (1)whereq = volume flow capacity (m3/s, gpm, cfm, ..)n = wheel velocity - revolution per minute - (rpm)d = wheel diameter

Head or Pressure The head or pressure of a fan or pump can be expressed like

dp1 / dp2 = (n1 / n2)2 (d1 / d2)2 (2)wheredp = head or pressure (m, ft, Pa, psi, ..)

Power The power consumption of a fan or pump can be expressed as

P1 / P2 = (n1 / n2)3 (d1 / d2)5 (3)whereP = power (W, bhp, ..)



Cavitation What is Cavitation?

Cavitation may occur when the local static pressure in a fluid reach a level below the vapor pressure of the liquid at the actual temperature.

According to the Bernoulli Equation this may happen when the fluid accelerates in a control valve or around a pump impeller.

The vaporization itself does not cause the damage - the damage happens when the vapor almost immediately collapses after evaporation when the velocity is decreased and pressure increased.

Avoiding Cavitation Cavitation can in general be avoided by

increasing the distance between the actual local static pressure in the fluid - and the vapor pressure of the fluid at the actual temperature

This can be done by: reengineering components initiating high speed velocities and low static

pressures increasing the total or local static pressure in the system reducing the temperature of the fluid



CavitationReengineering of Components Initiating High Speed Velocity and Low Static Pressure Cavitation and damage can be avoided by using special components designed for the actual

rough conditions. Conditions as huge pressure drops can - with limitations - be handled by

Multi Stage Control Valves Difficult pumping conditions - with fluid temperatures close to the vaporization temperature - can

be handled with a special pump - working after an other principle than the centrifugal pump.

Increasing the Total or Local Pressure in the System By increasing the total or local pressure in the system, the distance between the static pressure

and the vaporization pressure is increased and vaporization and cavitation may be avoided. The ratio between static pressure and the vaporization pressure, an indication of the possibility of

vaporization, is often expressed by the Cavitation Number. Unfortunately it may not always be possible to increase the total static pressure due to system

classifications or other limitations. Local static pressure in the component may then be increased by lowering the component in the system. Control valves and pumps should in general be positioned in the lowest part of the systems to maximize the static head.

Reducing the Temperature of the Fluid The vaporization pressure is highly dependable of the fluid temperature



System curve

A fluid flow system can in general be characterized with the System Curve



Pump performance curve

The pump curve describes the relation between flowrate and head for the actual pump.

Other important information for proper pump selection is also included – efficiency curves, NPSHr curve, pump curves for several impeller diameters

and different speeds, and power consumption. Håkan Ekstrand/Tapflo AB/2012


Selection of Pump

A pump can be selected by combining the System Curve and the Pump Curve:



NPSH - Net Positive Suction Head

Low pressure at the suction side of a pump can encounter the fluid to start boiling with reduced efficiency cavitation damage of the pump as a result.

To characterize the potential for boiling and cavitation, the difference between the total head on the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual temperature may be used

The Net Positive Suction Head - NPSH - can be expressed as the difference between the Suction Head and the Liquids Vapor Head



NPSHRequired NPSH - NPSHr The required NPSH - NPSHr - is the NPSH that must be exceeded

to avoid vaporization and cavitation in the impellers eye. The NPSHr is always higher than the theoretical NPSH due to head loss in the suction pipe and the pump casing, and local velocity acceleration and pressure decrease on the impeller surface. NPSHr is in general determined experimentally by the pump

manufacturer and a part of the pump performance curves documentation.

The required NPSHr increases with the square of increased capacity.

Available NPSH - NPSHa The available NPSH - HPSHa - is the NPSH available for a

particular system must be determined during design and construction of the system, or determined experimentally on the actual physical system.



NPSH



Pumps in Parallel or Serial Connection

Pumps in Serial

Heads AddedWhen two (or more) pumps are arranged in serial, their resulting pump performance curve is obtained by adding heads at the same flowrate.

Pumps in Parallel

Flow Rate Added

When two or more pumps are arranged in parallel, their resulting performance curve is obtained by adding their flowrates at the same head.



Pumps in Parallel or Serial Connection



Density, Specific Weight, Specific Gravity Density Density is defined as an objects mass per unit volume. Mass is a property.

The density can be expressed asρ = m / V (1)Whereρ = density (kg/m3)m = mass (kg)V = volume (m3)

The SI units for density are kg/m3. The imperial (BG) units are lb/ft3 (slugs/ft3).

The higher the density, the tighter the particles are packed inside the substance.

Density is a physical property constant at a given temperature and density can help to identify a substance.



Density, Specific Weight, Specific GravitySpecific Weight

Specific Weight is defined as weight per unit volume. Weight is a force.

Specific Weight can be expressed asγ = ρ g (2)

whereγ = specific weight (kN/m3)g = acceleration of gravity (m/s2)

The SI-units of specific weight are kN/m3. The imperial units are lb/ft3. The local acceleration g is under normal conditions 9.807 m/s2 in SI-units and 32.174 ft/s2 in imperial units.



Density, Specific Weight, Specific GravitySpecific Gravity The Specific Gravity - SG - is a dimensionless unit defined as the ratio of density of the material

to the density of water at a specified temperature. Specific Gravity can be expressed as

SG = = ρ / ρH2O (3)whereSG = specific gravityρ = density of fluid (kg/m3)ρH2O = density of water (kg/m3)

It is common to use the density of water at 4 oC (39oF) as reference - at this point the density of water is at the highest.

Thermal Properties of Water Density, Freezing temperature, Boiling temperature, Latent heat of melting, Latent heat of evaporation, Critical temperature ...

Since Specific Weight is dimensionless it has the same value in the metric SI system as in the imperial English system (BG). At the reference point the Specific Gravity has same numerically value as density.



Dynamic, Absolute and Kinematic Viscosity The viscosity of a fluid is an important property in the analysis of liquid behavior and fluid

motion near solid boundaries.

The viscosity is the fluid resistance to shear or flow and is a measure of the adhesive/cohesive or frictional fluid property.

The resistance is caused by intermolecular friction exerted when layers of fluids attempts to slide by an other.

The knowledge of viscosity is needed for proper design of required temperatures for storage, pumping or injection of fluids.

Common used units for viscosity are CentiPoises (cp) = CentiStokes (cSt) x Density SSU1 = Centistokes (cSt) * 4.55 Degree Engler1 * 7.45 = Centistokes (cSt) Seconds Redwood1 - 4.05 = Centistokes (cSt)



Dynamic, Absolute and Kinematic ViscosityThere are two related measures of fluid viscosity - known as dynamic (or absolute) and kinematic viscosity.

Dynamic (absolute) Viscosity is the tangential force per unit area required to move one

horizontal plane with respect to the other at unit velocity when maintained a unit distance apart by the fluid.

Kinematic Viscosity is the ratio of absolute or dynamic viscosity to density - a

quantity in which no force is involved



Liquid behaviourNewtonian Fluids Fluids for which the shearing stress is linearly related to the rate of shearing strain are designated as

Newtonian Fluids. Newtonian materials are referred to as true liquids since their viscosity or consistency is not affected by shear

such as agitation or pumping at a constant temperature. Fortunately most common fluids, both liquids and gases, are Newtonian. Water and oils are examples of Newtonian liquids.

Thixotropic Fluids Shear Thinning Fluids or Thixotropic Fluids reduce their viscosity as agitation or pressure is

increased at a constant temperature. Ketchup and mayonnaise are examples of thixotropic materials. They appear thick or viscous but are possible to pump quite easily.

Dilatant Fluids Shear Thickening Fluids or Dilatant Fluids increase their viscosity with agitation. Some of these

liquids can become almost solid within a pump or pipe line. With agitation, cream becomes butter and Candy compounds, clay slurries and similar heavily filled liquids do the same thing.

Bingham Plastic Fluids Bingham Plastic Fluids have a yield value which must be exceeded before it will start to flow like a

fluid. From that point the viscosity will decrease with increase of agitation. Toothpaste, mayonnaise and tomato catsup are examples of such products.


tapflo pump therory

Engineering

pd pumps flow rate

system pressure

constant flow

function of flow

liquid energy

varying discharge pressure

constant volume of liquid

given volume of liquid