Pumps & Systems, March 2008

It is common practice for the designer of a new centrifugal pump to model the new pump from an existing pump. Each dimension of the existing pump (except the shaft diameter) is multiplied by a modeling factor "F" to obtain each dimension of the new pump.

I have successfully used the same approach to design a number of new reciprocating power pump liquid ends, and find that it also works for power ends.  Such a procedure may be useful to other designers of power pumps.

The following example illustrates the use of the modeling technique, coupled with valve and spring parameters developed recently. It also reveals the significant increase in NPSHr when larger pumps are run at higher speeds.

## Example

Let's say that a 3-in stroke triplex pump, with 2.5-in diameter plungers, has proven reliable in continuous-duty applications at a discharge pressure of 1000-psi, running at 400-rpm. The continuous-duty rod load rating is 5000-lbs. It is desired to model this pump to a larger pump with a 4-in stroke.

What is the modeling factor? What performance can we expect?

The modeling factor is F = 4/3 = 1.333, so F2 = 1.778, F3 =2.37, F4 = 3.16, F0.5 = 1.155, F1.5 = 1.54, F2.5 = 2.053.

Table 1 features the modeling data.

Table 1

3"-Strk

Multiplier

4"-Strk

Plunger Diameter (inches)

2.5

F

3.33

5000

F2

8900

Displacement/Revolution (gal/rev)

0.191

F3

0.453

Crankshaft Speed (for same accel) (rev/min)

400

1/F0.5

346

Displacement (for same accel) (gal/min)