Understanding Pump Curves
The Pump Characteristic
The basic pump curve is the Head Capacity curve, showing the flow rate delivered by the pump at a given resistance. A complete Pump Characteristic however includes 4 pieces of information:
Head Capacity
NPSHR
Efficiency
Power
Since pumps are roto-dynamic machines used in dynamic systems, pump performance must be reported to the user as a continuous line covering the range of operating conditions under which the pump operates.
Graphs are an elegant method of presenting complex dynamic pump performance data. By plotting a few data points on a graph and then accurately extending a continuous line through the data points, the point values become continuous along this line allowing the user to quickly obtain specific values at any given condition. The values are read by means of calibrated scales located on the graph. The lines, especially the Head Capacity line, are often curves, thus the phrase Pump Curve or Performance Curve.
Without curve extension lines, the values for Head, Flow, Power, NPSHR, and Efficiency would have to be reported in rows of long columns of numbers. These columns would have hundreds of entries, would be difficult to read, difficult to use, and could not give visual clues about the complex behaviors pumps exhibit.
Graphs enable us to visually remember the complex relationships between pump characteristics in dynamic systems. Once we memorize the shape or geometry of these curves and their relationship to each other, we are better able to understand, predict, and control pump and system behavior under variable conditions.
The graph below is a Pump Characteristic
The 4 Pump Characteristics shown in the graph above are:
Head Capacity - Black Curve, Scales are gpm & ft
Power Capacity - Blue Curve, Scales are bhp & gpm
Efficiency Capacity - Green Curve, Scales are %eff & gpm
NPSHR Capacity - Red Curve, Scales are ft & gpm
Information Not Shown but Assumed
The information for the graph is obtained from tests done under "Standard Conditions". Standard Conditions for pump tests are specified by various organizations such as Trade Associations, Standards Organizations, or possibly the end user of the pump. In the United States (and other countries also), Standard Conditions are most often defined by the Hydraulic Institute (HI), American Petroleum Institute (API), and the National Standards Institute (ANSI), and sometimes these three organizations issue joint standards.
Curve Geometry
Curve Geometry describes the shape of the Head Flow Curve. The most common terms used to describe the shape of these geometries are: Stable Curve, Steep Curve, Flat Curve, Unstable Curve, and Drooping Curve. Curve geometries are useful for at least the following 3 purposes:
Pump control schemes must account for the curve geometry. Robust and refined control methods are either invulnerable or less vulnerable to differences in pump curve geometries. Many pump control schemes however encounter severe problems especially concerning flat curve and unstable curve pumps.
Pump curve geometry may be used to determine if a pump is suitable for a specific application or project. One frequently seen error results when a steep curve pump is selected to boost city water pressure. The pump delivers correct pressure at the required flow rate, but when flow is reduced city water pressure rises to maximum, and when combined with the steep pressure rise from the steep curve pump at shut-off, the result can be high pressures capable of damaging system components instantly. Control devices and alarms can be used to protect the system in case a malfunction occurs, but any device can fail.
Correct curve geometry can result in less expensive, simpler, more reliable, and more efficient systems. For instance a flat curve pump might be used to eliminate the need for a pressure relief valve and a pressure control valve on a system where a steep curve pump might require both of those valves. Or a flat curve pump might be used to eliminate the need for a Variable Speed Drive, thereby resulting in a less expensive and more reliable system. And if the flat curve pump has a low Specific Speed, then that simple system may consume less power than a variable speed drive system. Steep curve pumps also have their proper applications.
The links below take you to pages focusing on types of information provided in the Pump Characteristic and more, including graphs and explanations.
