Pump Selection

• Most pumps fall into three categories:- positive displacement pumps- centrifugal pumps- axial flow pumps

• Positive displacement pumps are used in the following situations- low flow rate- high pressure rise- pump flow rate must be accurate (metering)

• Centrifugal pumps are used in the following situations- medium-high flow rates- low-medium pressure rise

• Axial flow pumps are used in the following situations- high flow rates- very low pressure rise

Types of Positive Displacement Pump

Lobe Pump

Gear Pump

Peristaltic Pump

Types of Positive Displacement Pump

Pressure

head

(

†

DP rg)ge

nerated

bythepu

mp

Flow rate (m3 s-1)

something breaksin the pump

constant flowindependentof ∆P

Positive displacement pumps are self priming - they can be started up dry

Centrifugal Pump

Inlet(suction)

outlet(discharge)

pump impeller gives theliquid a high rotationalspeed

volute converts kinetic energyof the liquid as it leaves theimpeller into pressure energy

By far the most common design in the chemical and materials processing industries

single pump stage(many stages used in series for higher ∆P)

Axial Flow Pump

outlet

inlet

• Very low pressure rise.

• Used to increase the kinetic energy of aliquid/gas.

• Similar characteristics to a centrifugalpump.

Centrifugal Pump

Pressure

head

(

†

DP rg)ge

nerated

bythepu

mp

Flow rate (m3 s-1)

The pressure generated by the pump changes as the flow rate through the pump changes

z1= 4 m

P1 = 101300 Pa

d = 0.05 m, hydraulically smooth(

†

f = 0.0791Re-0.25 ). r = 950 kg m-3 m = 0.003 Pa sSuction side: 10 m of pipe, 6 standard 90˚ bends, gate valve fully open.Discharge side: 50 m of pipe, 15 standard 90˚ bends, gate valve fully open,

A plug disc globe valve will be used to control the flow (initially 60% open, KL=29)

P2 = 600000 Pa

z2= 10 m

Centrifugal Pump - Example

flow rate500 litres min-1

• The flow rate (500 litres/min) and WS/g (118 m) are plotted on the graph.• This point lies below the pump curve.• The pump provides more power than is required by the system and the flow increases

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700 800

flow rate (litres/min)

pre

ssure

hea

d (

m)

pump curve

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700 800

flow rate (litres/min)

pre

ssure

hea

d (

m)

• Increase the frictional losses by closing the valve until the point for the system lies on the pump curve.• In this case the valve will have to be between 20% and 40% open.

10% open

20% open

40% open

60% open

Pump Performance Charto r

Pump Composite PerformanceChart

separate curves fordifferent impeller diameters

best efficiencypoint

Try and operate a pump asclose to its best efficiency point

as possible

Pump Efficiency

• For a 100% efficient pump, the energy balance for a pump is:

†

WS,100 = ˙ m DPr

change in enthalpy of liquid being pumped

†

DH100 =WS,100

˙ m =

DPr

• If the pump is not efficient then as well as being pressurised the liquid is heated

†

WS =WS,100

hin this case the enthalpy change is given by

†

DH =WS

˙ m =

DPr

+CPDT

Pump Efficiency - ExampleIn a pumping system work required from a 100% efficient pump is 16.3 kW. The mass flowof liquid is 7.9 kg s-1 and the pressure change in the pump is 1 960 000 Pa (19.6 bar). Forthe liquid being pumped CP = 2800 J kg-1 ˚C-1 and r = 950 kg m-3.

If the pump is 65% efficient calculate the temperature rise in the fluid:

For a 65% efficient pump: WS = 16300/0.65 = 25076 WThis is equivalent to an enthalpy change: ∆H = 25089/7.9 = 3174 J kg-1

For a pump that isn’t 100% efficient

†

DH = 3174 =DPr

+CP DT =1960000

950+ 2500DT ∆T = 0.4 ˚C

Not a large temperature increase. If the liquid is being recycled it can be

Avoid throttling back a pump too much (using a valve on the outlet and a recycle). WHY?

Pump Design1. Given mass flow of fluid and the pipe work design

2. Calculate the head loss in the system and the shaft work for the pump.

3. Find out what materials are compatible with the fluid being pumped.

4. Select a pump (and impeller) that are compatible with the fluid being pumped.

5. Using the pump curve, make sure that the pump is capable of supplying the required pressurehead and flow rate.

6. Calculate the opening of the control valve required to fix the operating point at the requiredpressure head and flow rate.

7. Get the pump efficiency and required NPSH from the pump chart.

8. Check that the NPSH of the system is larger than the required NPSH.

9. Use the efficiency to calculate the size of electric motor required and the fluid temperature rise.

Example

6m

Octane at 87˚C is pumped through a heat exchanger where it is cooled to 30˚C before flowing into astorage tank as shown above. The properties of the octane are given below. The flow rate of octane is3.6 kg s-1 and the pipe diameter is 0.06 m. The pipe is hydraulically smooth.1. The heat exchanger is a 2 pass shell and tube exchanger consisting of a shell 0.4m in diameter and 25 U-tubes that

are 5 m long and 0.012m in diameter.2. On the suction side of the pump there are 30 m of straight pipe and a gate value.3. Prior to the exchanger there are 8 m of straight pipe, a gate valve and a plug disc globe vqlve.4. After the exchanger there are 200 m of pipe, a gate valve and 8 standard radius 90˚ bends.

Propane Properties:At 30˚C r = 704 kg m-3 m = 0.00050 Pa sAt 87˚C r = 651 kg m-3 m = 0.00028 Pa s

At 87˚C

†

P sat = 30000 PaUsing the pump curve given on the next page, determine the valve opening required, he powerrequired for pumping and check the NPSH.

150 kPa

101.3 kPa

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150 200 250 300 350 400

Fluid Flow (litres/min)

Head

(m

)

0

1

2

3

4

NP

SH

(m

)

Head (m) NPSH(req.)

pump curve

NPSH curve