Obtaining Pump Performance Data

In the preceding post, I shared with you a recent insight I had
about determining where motor speed
and power produced would come into balance with pump speed and
power extracted by the pump it is serving
. In this post, I
thought I would share some of the resources I use to get the data I
need to do the analysis. 

The pump performance data is relatively easy to come by; its
derived from the pump curve similar to the one illustrated
below. 

There are a number of ways to get what you need from the pump
curve. The easiest is to simply apply the pump affinity law
relating pump speed and horsepower to the data for the pump
selection point. Specifically, for the curve above you would
use about 30.9 bhp to deliver 1,600 gpm at 65 ft.w.c.
with the pump running at 1,150 rpm (the rated speed for the pump
curve as indicated in the upper right-hand corner).  Knowing
the brake horsepower and flow, you can solve the
following equation (the pump affinity law relating horsepower and
speed) for a number of data points.

The curve above came from Bell and
Gossett’s ESP plus software
. You can access this software at no
cost via the Internet or via a free CD available from your local
B&G representative. The CD has the advantage of letting you
work with out a web connection and also opening the door to
selection software for a number of other products.

Other manufacturers offer similar tools, usually at no cost and
they are a valuable resource for both the designer and the folks
out in the field working with the implemented design.  For one
thing, experimenting with tools like this represents a training
opportunity.  I can’t tell you how much I’ve learned simply by
downloading a tool and then “playing” with it to gain insight into
the relationships it assesses for a given piece of
equipment. 

For another, operating a system is at its core, applied
design.  If you are doing design, you have identified
performance parameters that you are targeting;  in the case of
a pump a flow that is required to serve a load and the head that it
takes to move that flow through the distribution system.  Your
goal is to find a pump that will perform this service as
efficiently as possible with-in the constraints of your
budget.  In the field, you frequently are confronted with the
exact opposite situation;  you have a known pump and based on
test data, you are trying to figure out what its doing, if its
doing what was intended, and if there are ways to improve its
efficiency.

In the context of the current discussion, having a tool
like this allows you to be a little more accurate if you want to be
regarding the relationship between pump speed and brake
horsepower. Specifically, the tool allows you to look at
the pump performance at different speeds. The curve below is for
the same pump as the previous curve but with the variable speed
option selected.

By reading flow, head and efficiency for different points on the
system curve as it crosses the different speed curves and using
these parameters to solve for pump brake horsepower using the pump
power equation (reproduced below), you can develop the power vs.
speed relationship for the pump based on actual test data vs. the
affinity laws.

Bear in mind that if the manufacturer you are working with projects
all of their curve data for different speeds and impeller sizes
from test data taken at a given speed with a given impeller size
(vs. testing at different speeds and with different impeller sizes)
then the result of going this extra step will be no different than
the affinity law based projection discussed
above.  Affinity law based projections are perfectly fine
and comply with industry standards.  But they assume geometric
similarity and a real pump with different sized impellers operating
in the same size volute will not follow the affinity law based
projections exactly.

So that gives you half the information you need to do the
graphical analysis we have been discussing. In the next post, I’ll
point you at a resource that can provide the motor information plus
a whole lot more.

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