Contrary to popular opinion, a centrifugal pump is not designed to develop one particular set of operating conditions requested by the pump purchaser. Instead, a pump is designed and produced to supply a whole range of head/capacity conditions as identified on its performance curve. The pump will operate somewhere on that curve.
The Pump Curve
To fully appreciate the nature of a pump curve and the relationship between head and capacity of a centrifugal pump, consider that pump discharging into a straight vertical pipe.
If we can consider the pipe being as high as it needs to be, the liquid being pumped into that pipe will eventually reach a level beyond which it is unable to move. This can be considered as the maximum head the pump can develop. Although the pump will continue to run, it will be unable to push the liquid any higher in the pipe.
Under these conditions, liquid is being churned around in the pump casing, but there is no flow passing through the pump, therefore the capacity is at a maximum and the head is zero.
Expressing pressure as head in this way makes the pump curve applicable to every liquid regardless of its density. If we cut holes in the discharge pipe at progressively lower levels, the head is effectively reduced, and the pump will steadily develop an increasing capacity.
By graphically depicting these results, a typical characteristic pump performance curve can be drawn. This curve is not completed down to zero head. However, as a centrifugal pump does not operate reliably beyond a certain capacity, the curve is usually discontinued.
This curve identifies the capacity, which this pump can develop, and the total head it can add to a system when it is run at a particular speed with a specified impeller diameter. However, the actual conditions on that curve at which the pump will run will be determined by the system in which it operates. In other words, for all practical purposes, the system controls the pump and will operate that pump at whatever conditions it sees fit, regardless of the head and capacity for which it was bought (and designed).
If the centrifugal pump is controlled by the system, we should understand some aspects of a pumping system. For this we look at the system curve, which is created by the combination of factors that resist the flow of liquid from one end of a system to the other. The common factors in all systems are gravity and friction.
To overcome gravity in a typical system, the liquid must be raised through the vertical distance represented by the change in elevation between the originating source, or the liquid, to the final destination. Referred to as the total static head, this distance is measured between the free surface of the liquid in the suction source and the free surface of the liquid in the discharge tank, or to the highest point in the line.
In either case, the total static head is not a variable of the flow rate, and a graph comparing the two would show up as a straight horizontal line.
Friction is the resistance to flow in the system and can usually be calculated from the friction loss tables available from a variety of sources such as The Practical Pumping Handbook. Losses in filters, heat exchangers, etc., can be obtained from the OEM, or from measuring the inlet and outlet pressures on site. As the flow increases, so too does the friction loss, but at a far higher rate.
The only other significant condition we need to take into account are the pressures at the suction source and in the discharge tank. If they are closed vessels under different pressures, the difference in pressure has to be added to the total head required from the pump.
A combination of that differential pressure, the total static head and the friction loss is referred to as the total head. When plotted against the flow rate, this results in a curve that starts at the level of the static head and slowly increases in head as the flow rate increases.
Therefore, when the pump is properly selected, its characteristic performance curve will intersect the system curve at the point at which the pump will operate.
Remember, the system controls the pump, so dont be surprised if the pump doesnt do what you expected it to do. Instead, look at the system and find out what has changed.
Author of The Practical Pumping Handbook and a specialist in pumping reliability, Ross Mackay can be reached at www.practicalpumping.com, or at 800-465-6260.