A manufacturer’s pump performance curves contain data that can help hvac technicians analyze a pumping installation. Pump curves also help identify the system’s operating point, find reasons for a system not performing, and even determine a pump’s impeller size.
After designing a pump, the manufacturer usually produces a number of units for testing. The tests are necessary to establish how the pump will perform. The data collected often includes water flow operating against various system resistances, brake horsepower required, efficiency, and the net positive suction head required for proper operation of the various diameter impellers allowable in the pump volute.

This data is analyzed and then plotted and published as the pump operating characteristics. The pump curve then shows how the pump will perform with varying head or flow requirements. (See Fig. 1).

Figure 1. The pump curve shows how the pump will perform with varying head or flow requirements.

In the real world it isn’t unusual for a pump’s nameplate to be missing. Because the information on a nameplate is so important (it usually includes manufacturer’s name, pump model, size, impeller diameter, head and flow for the duty point), it’s frequently removed for safe keeping. Unfortunately, at times it’s so well safeguarded that it can’t be retrieved. Or, perhaps it’s just painted over. Either way, the information on it isn’t available.
To identify the pump and reestablish the nameplate data, the manufacturer must be determined. Most pumps are fabricated of castings and most of these have casting part numbers and markings on them which identify the pump manufacturer. Once the pump manufacturer is known, the type or model and size can be determined with the help of published literature or a phone call.
Since larger pumps generally have a family of impeller sizes which can be used with a given pump body, at this point the impeller diameter is unknown. A simple procedure using a pressure gauge and the pump’s curves will identify the impeller size in the pump.
Identifying Impeller Size
Close the pump discharge valve and take the suction and discharge pressures. This is the “dead-head” condition. Reopen the discharge valve and reset it to the position it was in prior to closing, if it was used to balance the flow. The algebraic difference between the discharge pressure and the suction pressure is the head being generated by the pump. Convert this to feet of water head and determine the correct impeller from the no flow point on the pump curve. As an example:
P discharge = 20.5 psi
P suction = 4-in. of Hg vacuum (a negative pressure) But, the curves are dimensioned “Head, feet of water” therefore, the gauge pressures must be converted.
To convert pressure in psi to head in feet of water multiply psi by 2.31 and divide by the specific gravity of the fluid being pumped. The specific gravity of water is 1. One inch of Hg is equal to 0.491 psi.
Therefore:
P discharge = 20.5 psi x 2.31 ft. of water
per psi / (divided by) 1 = 47.4 ft. of water
P suction = 4 in. Hg x 0.491 psi per in. Hg = -1.96 psi x 2.31 ft of water per psi/1= -4.5 ft. of water Pump head = P discharge - P suction = 47.4 - (-4.5) ft. of water Algebraically subtracting a minus is a plus, so:
Pump head = 47.4 + 4.5 ft. of water = 51.9 ft. of water.
Locating this head, 52 ft., at 0 gpm flow on the pump curve in Fig. 1, shows the pump impeller diameter to be 7 in.
The system operating point can also be determined by using gauge readings. Take the suction and discharge pressures while the system is operating with the discharge valve in the normal open position. Again convert these into feet of water and subtract (algebraically) the suction pressure from the discharge pressure. This is the head of the pump at the operating flow. Follow the head line from the zero flow axis out to where it intersects the previously identified impeller characteristic curve. The flow at that point is the system’s operating flow.
Example: After determining the pump’s impeller diameter to be 7 in., gauge readings of the pump taken while it operated were:
P discharge = 17.5 psi
P suction = 4 in. Hg (vacuum)
Convert the gauge readings for the fluid being pumped to feet of water: P discharge = 17.5 psi x 2.31/1 = 40.5 ft. of water
P suction = 4 in. Hg x 0.491 psi per in Hg = -1.96 psi x 2.31 ft. of water per psi/1 = -4.5 ft. of water Pump head = 40.5 - (-4.5) ft. of water = 45.0 ft. of water
The pump head of 45.0 ft. intersects the 7 inch diameter characteristic curve at 55 gpm, which then is the system operating flow.
Being able to fully identify a pump, determine the installed impeller size and the system operating point are of great use in troubleshooting.
As an example:

Figure 2. Depicted here is a two horsepower base-mounted pump which regularly trips its circuit breaker. The top graphic depicts a coil configuration and the bottom one, a bypass.