18 Pump Selection

8/10/2019 18 Pump Selection

1/13

ENGR 120 Using Pump Curves to Select Pumps

Lets close the loop.

How does the pump testing that you have

done connect to real-world engineering?


2/13

MotorPumps come in a variety

.

Here are a few examples

Drive shaft connectedPump

.

Pump

to tractor PTO


3/13

This is a submersible pump consisting of

4 stages connected in series. Each stage

Water out

is a centrifugal pump in itself. The entire

pump is submerged under water.

Pum sta es

In this submersible pump water flows from

one stage to the next. The pressure (or head)

increases as water moves through the pump,

but the discharge remains constant.

This is analogous to batteries connected

in series. The total voltage is the sum of

the voltage from each battery, but the current

remains constant.

Water in


4/13

Procedure for Selecting a Pump

(2) Select the design discharge for the pump

(3) Check pump manufacturers catalogs and select a pump that

will operate at maximum efficiency near the design discharge.


5/13

System Head Curve

(1) Static lift vertical distance between the static water surface

and the ground surface

(2) Static discharge vertical distance between the ground surfaceand the ultimate point of use

(3) Well drawdown decrease in water level in the well in response

to pumping

flows through the pipe, valves, bends, etc.

(5) Operating head pressure (or head) required at the point of use.

For example, irr igation sprinklers require a certain amount ofpressure to operate correctly; a well system for a home typically

pumps water into a pressure tank which then supplies water to the

.


6/13

These are all dynamic they increase with

increasing discharge from the pump.

System Head Curve

H(

ft) Operating Head

tal

Head,

Friction Loss

T

Static Discharge

Static Lif t

Discharge, Q (gpm)


7/13

Operating head is determined by whatever is required atthe ultimate point of use (sprinkler, pressure tank, etc.)

Friction loss, hf, can be calculated from the Darcy-Weisbach equation

where

Well drawdown, s, can be calculated from the Jacob equation

where


8/13

Select a submersible pump that will deliver 900 gpm to a water tank.

Static lift = 20 ft

SE

a c sc arge =

Dynamic head is discussed on the next slide.

Static

Discharge

motor

Static lift

Drawdown

pump


9/13

Lets assume we are able to calculate the frict ion loss, hf, from the

Darcy-Weisbach equation. Note that hf varies with velocity of the water.

This means it also varies with pump discharge, Q.

, , .

this also varies with pump discharge, Q.


10/13

We calculate and plot the total system head curve asthe sum of static lift, static discharge, well drawdown,

.

curve will be a function of pump discharge.

Lets assume we have done these calculations and

plotted the system head curve as shown of the following

graph.


11/13

System

Head Curve

Now we superimpose this curve on the pumpcurves obtained from the pump manufacturer.

Static L ift + Static Discharge

The pump curves from the manufacturer were determined in

essentially the same way you determined your pump curves.


12/13

System

Static Lift + Static Discharge

Operating Point

If we operate this pump at 1600 RPM, it will deliver 900 GPM

against a total system head of 120 ft.Efficiency is 71% (slight ly less than peak effic iency of 72%.)


13/13

System

Static L ift + Static Discharge

What wil l happen if we operate the pump at 1200 RPM ?New Operating Point

The total system head drops to 75 ft, and the pump willdeliver only 550 GPM. The effic iency drops to 69%.

Documents

18 Pump Selection