Pump Curves

Pump Curves

A notable omission from university courses is an understanding of how to read a pump curve, which is an essential requirement to do what we are probably going to do with the head/flow pairs we calculated across the design envelope.

The most frequent use of pump curves is for the selection of centrifugal pumps, as the flow rate of these varies so dramatically with system pressure. Pump curves are used far less frequently for positive displacement pumps.

A basic pump curve plots the relationship between head and flow for a pump at a given supply frequency. On more sophisticated curves, there may be nested curves representing the flow/head relationship at different supply frequencies or rotational speeds, with different impellers, or different fluid densities. The pattern is that curves

for larger impellers or faster rotation lie above smaller impellers or slower rotation, and lower specific gravity above high for centrifugal pumps.

Let’s start with a basic curve (Figure 1):

Figure 1. Basic pump curve. Copyright image reproduced courtesy of Grundfos.

Along the horizontal axis we have increasing flow (Q), and along the vertical axis, increasing pressure (H). The curve shows the measured relationship between these variables, so it is sometimes called a Q/H curve. The intersection of the curve with the vertical axis represents the closed valve head (Shut-off Head) of the pump. These pumps are generated under shop conditions and ideally represent average values for a representative sample of pumps.

We can use our calculated flow/head pairs to plot a system head on the same axes, and see where our system head meets the Q/H curve. This will represent the operating or duty point of the pump.

We will have a system head curve for the expected range of flows at a given system configuration. Throttling the system will give a different system curve. We will need to produce a set of curves which represent expected operating conditions, generating a set of duty points.

That’s it as far as our basic curve is concerned, but it is common to have efficiency and motor rating curves plotted on the same graph (but not the same vertical axes) as in the example in Figure 2.

Figure 2. Intermediate pump curve. Copyright image reproduced courtesy of Grundfos.

So we can see that we can draw a line vertically from the duty point to the efficiency curve, and obtain the pump efficiency at the duty point by reading the vertical axis at the point of intersection. Similarly we can draw a vertical line to the motor duty curve, and obtain a motor power requirement.

Having tackled these basic and intermediate curves, we can look at the common format of professional curves, incorporating efficiency, NPSH, and impeller diameters like this (Figure 3).

Figure 3. Complex pump curve. Copyright image reproduced courtesy of Grundfos

These start to look a bit confusing, but the thing to bear in mind is that, just as with the simpler examples, the common axis is always the horizontal one of flowrate. So the corresponding value on any curve is vertically above or below the duty point.

These more advanced curves usually come with efficiency curves, and it is usually visually obvious that these curves seem to bound a region of highest efficiency. At the center of this region is the best efficiency point or BEP.

We will want to choose a pump which offers good efficiency across the range of expected operating conditions. Note that we are not necessarily concerned with the whole design envelope here—it is not crucial to have high efficiency across all conceivable conditions, just the normal range.

A well-selected pump will have a BEP close to the duty point. If the duty point is way over to the right of a pump curve, well away from the BEP, this is not the right pump for the job. Try another.

These are the basics of centrifugal pump selection. If you are in a position to influence which pump is purchased, any pump supplier’s representative would be happy to talk to you about pump selection for as long as you are willing to listen. Probably buy you lunch too, though obviously that wouldn’t affect your choices.

Even with the most cooperative pump supplier, the curves you want in order to make a pump selection may not be available, as is commonly the case when we want to use an inverter (Variable Frequency Drive (VFD)) to control pump output by speed. We can, in this case, generate the required curves for ourselves using pump affinity relationships. The laws are:

• Flowrate2/Flowrate1 = Impeller diameter2/Impeller diameter1 = Pump Speed2/ Pump Speed1

• Dynamic Head2/Dynamic Head1 = (Impeller diameter2/Impeller diameter1)^2 = (Pump Speed2/Pump Speed1)^2

• PowerRating2/Power Rating1 = (Impeller diameter2/Impeller diameter1)^3 = (Pump Speed2/Pump Speed1)^3

• NPSH2/NPSH1 = (Impeller diameter2/Impeller diameter1)^x = (Pump Speed2/ Pump Speed1)^y

Where subscript 1 designates an initial condition on a known pump curve, and subscript 2 is some new condition.

The NPSH relationship is a lot more approximate than the others. x lies in the range 2.5 to 1.5, and y in 1.5 to 2.5. A worst-case estimate can be established using the maximum quoted x and y figures if impeller speed or diameter is to be increased, and the lowest figures if it is to be decreased.