The first piece of information provided by a pump curve is the flow that the pump will develop at any given operating head. The curve that provides this information is called the pump performance curve. Some pump curves only provide a single pump performance curve, but most will provide the maximum performance the pump is capable of achieving with a full-trim impeller, the minimum performance the pump is capable of achieving with a minimum-trim impeller, and the performance provided by the design-trim impeller.
What is a pump curve?
Manufacturers publish curves for all of their pumps, reflecting thorough and rigorous testing in the laboratory or factory. This testing allows manufacturers to communicate how a pump curve will perform under a given set of conditions and which pumps will be the most efficient solution for any given application.
Head Vs Flow Curve (H-Q)
In this curve, the Head is on the vertical axis and Flow rate on the horizontal axis. With an increase in pump flow rate, the head developed by the pump start decreases. The pump develop maximum head at zero flow it is called shutoff point or shutoff head and the corresponding pressure is called shutoff pressure. Normally the shutoff pressure is 1.25 time of pump discharge pressure.
How to read a pump performance curve
In order to help people understand how to read a pump performance curve, Xylem has created a short white paper that explains the basics. Impeller recently spoke with Larry Konopacz, manager of training and education for the Bell & Gossett Little Red Schoolhouse in Morton Grove, Illinois, to learn more about the white paper and why this knowledge is important.
Identifying the best pump for your application starts with examining the pump curve, which indicates how a pump will perform against specific rates of pressure head and flow. The proper interpretation of this data is the only way to make informed decisions on the choice of pump, motor sizing, power consumption strategies and other factors. Before reading a pump curve, the following information must be gathered for a given system:
How to read a pump curve
The flow or capacity (Q) is measured in m3-second according to the international standards but usually you can find it expressed in m3-h, l-min or gpm (in the US). It is the volume of liquid moved in an amount of time. On the curve below (Picture 1) you can see the flow marked in red on the horizontal axis.
Normally, pump curves in data booklets only cover the pump part. Because of this, power consumption, the P2-value, listed in data booklets, only covers power going into the pump (see figure 1). The same goes for the efficiency value, which only covers the pump part (η = ηP). In some pump types with integrated motor and perhaps also an integrated frequency converter, e.g., canned motor pumps, the power consumption curve and the η-curve cover both the motor and the pump. In this case it is the P1-value that has to be taken into account.
Zodiac’s Steve Gutai has spent his career educating builders and servicers on the fine points of hydraulics, system design and equipment selection. As he points out here, the first step down the path toward energy efficiency is learning how to read charts that define a pump’s performance characteristics.