Pumps

Goals• Describe how centrifugal and positive-displacement

pumps operate and common applications.• Calculate system head requirements.• Determine head, pump efficiency, and pump.

horsepower from a typical centrifugal pump curve.• Define net positive suction head (NPSH) and

understand how it relates to cavitation.• Compute NPSH required by a pump.• Determine an appropriate pump (impeller diameter,

efficiency, etc.) for a given required head.• Describe how to modify system to operate on the

appropriate pump curve.

Background

Fluid Moving Equipment

Fluids are moved through flow systems using pumps, fans, blowers, and compressors. Such devices increase the mechanical energy of the fluid. The additional energy can be used to increase

• Velocity (flow rate)• Pressure• Elevation

Background

Pump, fan, blower, and compressor are terms that do not have precise meaning. Generally pumps move liquids while fans, blowers and compressors add energy to gasses.

Pumps and fans do not appreciably affect the density of the fluids that they move and thus incompressible flow theory is applicable.

Centrifugal PumpsMost common type of pumping machinery. There are many types, sizes, and designs from various manufacturers who also publish operating characteristics of each pump in the form of performance (pump) curves. The device pictured on the cover page is a centrifugal pump.

Pump curves describe head delivered, pump efficiency, and net positive suction head (NPSH) for a properly operating specific model pump.

Centrifugal pumps are generally used where high flow rates and moderate head increases are required.

Impeller

Positive Displacement PumpsTo move fluids positive displacement pumps admit a fixed volume of liquid from the inlet into a chamber and eject it into the discharge. Positive displacement pumps are used when higher head increases are required. Generally they do not increase velocity.

Pump Specification

Recall Mechanical Energy Balance

2

42

ˆ22 V

KD

Lf

pzg

VW i

kg

mN

c

icc g

VK

D

Lf

p

g

zg

g

VW

24

2ˆ

22

m

f

lb

lbft

Both equations describe work that must be supplied to system

Pump HeadWhat happens if the MEB is multiplied through by g (gc/g)?

2

42

1ˆ 22 VK

D

Lf

pzg

V

gg

Wi

What are the units (SI)?

2

2

m

s

kg

mN 2

2

2

3

m

s

skg

mkg

m

W/g has units of length and is known as the pump head^

Example

Tank A

2

Tank B

13

Why do we choose point 2 rather than 3 for MEB?

What kind of valve to uses to control flow rate?

Example

Tank A

2

Tank B

13

g

VK

D

Lf

g

pzH i 2

412

Mechanical Energy Balance (in terms of head)

g

VH

2

2

min

Head vs. Flow Rate

g

VHH

2

2

min

gpgzH c

min

g

VK

D

Lf i 241

2

QuadraticIn V or q

System Response

What happens when flow control valve is closed?• Resistance (f) increases• Flow rate decreases• Need more head to recover flow rate

Tank A

2

Tank B

13

System Response

Valve Open

ConstantHead Response

ConstantFlow Response

Valve Closed

Pump CurvesPump manufacturers supply performance curves for each of their pumps. These are normally referred to as ‘pump curves’. These curve are generally developed using water as the reference fluid.

The following can be read directly from a pump curve:• Head vs. flow rate information for any fluid

• Pump efficiency for any fluid

• Pump horsepower for system operating with water

Pump Performance Curves

DevelopedHead

ImpellerDiameter

Efficiency

Flow Rate

NPSH

Horsepower

http://capsicum.me.utexas.edu/ChE354/resources.html

Power Input

For fluids other than water:

W

mPˆ

mins

hps

lbftftlb

galft

mingal

qlb

lbft

gg

H

hpPf

m

m

f

c

60550

48.71

)(3

3

W m

Power Input

Easier Way

fluidwater

fluid

water

fluid GrSpP

P..

Note: A less dense fluid requires less horsepower

ExampleQ = 300 gpm

Di= 10”

Di= 10”

Di= 10”

Head(ft) =

η(%) =

P(hp) =

Head(ft) =

η(%) =

P(hp) =

Head(ft) =

η(%) =

P(hp) =

Goulds Pump CurvesManufacturers provide series of pumps to cover broad ranges of capacities, heads, and suction and discharge piping diameters. Most pumps can be equipped with different diameter impellers and can be operated at different speeds to change capacities.

The curves provided are for a few variations of the Goulds model 3196 process pump. Each curve corresponds to a specific pump and a specific RPM. Pump sizes are denoted with 3 numbers.

3 x 4 - 7

Discharge Diameter

Inches

Suction Diameter

Inches

Casing Diameter Inches

Note: Try to match process piping diameters with the pump discharge and suction diameters.

Pump SelectionGoal is to find a pump whose curve matches the piping system head vs. flow rate curve. We can superimpose the previous head-flow rate curve on the manufacturers pump curves.

To select a specific pump from a product line, find the pump with the highest efficiency that does not require the use of the largest impeller diameter. This will allow for future production expansions.

Suppose that we have a process that requires a flow rate of 300 gpm and has a head requirement of 60 ft. at that flow rate. Can a 3x4-10 model 3196 Goulds pumps be used?

ExampleImpeller Diameter =

For Desired QHead =

How do can you force the system to operate on the pump curve?

Net Positive Suction Head (NPSH)

Associated with each H-Q location on the pump curve is a quantity that can be read called NPSH.

An energy balance on the suction side of the fluid system (point 1 to pump inlet) with pinlet set to the vapor pressure of the fluid being pumped gives a quantity called NPSHA (net positive suction head available).

inletinlet

ivc zz

VK

D

Lf

pp

g

gNPSHA

1

21

24

Net Positive Suction Head

The requirement is that:

NPSHNPSHA Otherwise (if NPSHA < NPSHpump), the pressure at the pump inlet will drop to that of the vapor pressure of the fluid being moved and the fluid will boil.

The resulting gas bubbles will collapse inside the pump as the pressure rises again. These implosions occur at the impeller and can lead to pump damage and decreased efficiency.

Cavitation

NPSH

Do not use NPSH to size or select a pump unless all else fails. Pump selection is governed by H vs. Q requirements of system. When NPSHA is too small, it might be increased by:

• Increasing source pressure (not usually feasible)• Cooling liquid to reduce vapor pressure (not usually

feasible)• Raise elevation of source reservoir• Lower elevation of pump inlet• Raise level of fluid in reservoir

If NPSHA Can’t Be Increased

If the pump must be modified to achieve proper NPSH:

• Larger slower-speed pump

• Double suction impeller

• Larger impeller eye

• Oversized pump with an inducer

Example

1

5 ft

150 ft

2

5 ft

L = 5 ft, 6 inch Sch40

L = 300 ft, 5 inch Sch40

globe valve (open)P1 = 16 psia

P2 = 16.1 psia

P3 = 16 psia

Use Goulds 3x4-10 @3560 RPM

Flow = 600 gpm of benzene 60°F

Data for benzene: PVap = 7.74 psia

= 50.1 lbm /ft3

µ = 0.70 cP

3

Pump Selection from Many Choices of Characteristic Curves

1. Examine pump curves to see which pumps operate near peak efficiency at desired flow rate. This suggests some possible pipe diameters.

2. Compute system head requirement for a few diameters.

3. Compute V for some diameters. For water V in the range of 1 – 10 ft/s is reasonable (see ahead).

4. Re-examine pump curves with computed head and pipe diameters. This may give a couple of choices.

5. Pick pump with highest efficiency.

Selection of Pipe Size

Optimum pipe size depends mainly on the cost of the pipe and fittings and the cost of energy needed for pumping the fluids.

Cost of materials increase at a rate proportional to about D1.5, while power costs for turbulent flow varies as D–4.8. One can find correlations giving optimum pipe diameter as a function of flow rate and fluid density, however the optimum velocity is a better indicator as it is nearly independent of flow rate.

Optimum Pipe SizeFor turbulent flow of liquids in steel pipes larger than 1 in.

36.0

1.012

m

Vopt

3][

][

][

ftlb

slbm

sftV

m

m

opt

Remember

• Maximize pump efficiency

• Power input (hp) should be minimized if possible

• Selected impeller diameter should not be largest or smallest for given pump. If your needs change switching impellers is an economical solution

• NPSH required by the pump must be less than NPSHA

Variable Speed Pumps

Advantage: Lower operating cost

Disadvantage: Higher capital cost

System head requirement(no valve)

Pump curvefor Di

H (ft)

q (gpm)

q* (desired)q produced by pump with no flow control

RPM1

RPM2

Affinity Laws

In some instances complete sets of pump curves are not available. In this instance the pump affinity laws allow the performance of a new pump to be determined from that of a similar model. This can be useful when modifying the operating parameters of an existing pump.

Affinity Laws

1

212 D

Dqq

1

212 RPM

RPMqq

2

1

212

D

DHH

2

1

212

RPM

RPMHH

3

1

212

D

Dhphp

3

1

212

RPM

RPMhphp

51

2

1

1

2

1

1

D

D