Understanding Pump Curves
The Pump Characteristic
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:
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
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
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
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.