ENGR 120 – Using Pump Curves to Select Pumps

Let’s “close the loop.” How does the pump testing that you have done connect to “real-world” engineering?

Pumps come in a variety of configurations. configurations

Motor

Here are a few examples of centrifugal pumps pumps. Pump

Pump

Drive shaft connected to tractor PTO

Water out This is a submersible pump consisting of 4 “stages” connected in series. Each stage is a centrifugal pump in itself. The entire pump is submerged under water. Pump p stages g 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 i series. in i Th The totall voltage l iis the h sum off the voltage from each battery, but the current remains constant.

Water in

Procedure for Selecting a Pump

(1) Calculate the system head curve (2) Select the design discharge for the pump (3) Check pump manufacturer’s catalogs and select a pump that will operate at maximum efficiency near the design discharge.

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 surface and the ultimate point of use (3) Well drawdown – decrease in water level in the well in response to pumping (4) Friction loss in the system – head loss due to friction as water flows through the pipe, valves, bends, etc. ((5)) Operating p g head – p pressure (or ( head)) required q at the point p of use. For example, irrigation sprinklers require a certain amount of pressure to operate correctly; a well system for a home typically pumps water into a pressure tank which then supplies water to the house. house

These are all dynamic – they increase with increasing discharge from the pump.

To otal Head,, H (ft)

System Head Curve

Operating Head

Friction Loss

Well Drawdown

Static Discharge

Static Lift

Discharge, Q (gpm)

Operating head is determined by whatever is required at the 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

Select a submersible pump that will deliver 900 gpm to a water tank. Static lift = 20 ft St ti discharge Static di h = 30 ft Dynamic head is discussed on the SE next slide. motor

Static Discharge

Static lift

Drawdown

pump

Let’s assume we are able to calculate the friction 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. where

We can also calculate drawdown, drawdown s, s from the Jacob equation. equation Note that this also varies with pump discharge, Q. where

We calculate and plot the total system head curve as the sum of static lift, static discharge, well drawdown, friction loss and operating pressure pressure. The system head curve will be a function of pump discharge.

Let’s assume we have done these calculations and plotted the system head curve as shown of the following graph.

System Head Curve

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

Static Lift + Static Discharge

The pump curves from the manufacturer were determined in essentially the same way you determined your pump curves.

System Head Curve

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% (slightly less than peak efficiency of 72%.)

System Head Curve

Static Lift + Static Discharge

What will happen if we operate the pump at 1200 RPM ? The total system head drops to 75 ft, and the pump will deliver only 550 GPM. The efficiency drops to 69%.

New Operating Point