Sizing Fire Pumps

Using The Pump Curve

By Roland Huggins, PE

If you need a fire pump to supply 75 psi at 750 gpm, do you simply pick a pump that is rated for 75 psi at 750 gpm or are you taking advantage of the pump’s flow curve? NFPA 20:3-2.1, 1999 edition states "Pumps shall furnish not less than 150 percent of rated capacity at not less than 65 present of total rated head. The shutoff head shall not exceed 140 percent of rated head for any type pump. This is illustrated by Figure 1. Most of us are familiar with this curve, although some are still operating with the older 120% churn. In selecting a pump, it’s important not only to understand the curve but to use the curve in making the best selection. "Best" is intended to be the most cost effective and what is best for the owner.

An excellent example of what is not best for the owner is selecting a pump with a rated pressure near 175 psi and simply installing a relief valve to dump the excess pressure. This is a bad design for two main reasons. One reason is that relief valves are consistent about open at their pressure setting, but they don’t necessarily reset at the same pressure. This could rob water from the sprinkler system. The second reason (and most condemning), is that during the required weekly churn test, a massive amount of water is going to be dumped typically outside; (that is, a 1000 gpm diesel driven pump would dump almost 30,000 gallon). This can cause major problems for the system owner (even in states that are not consider requiring pump discharge to be captured) and likely cause the water department to demand that you cease such activities. This creates a conflict with both NFPA 25 and the fire marshal. Anyone following such a bad practice should expect (and deserves) the lawsuit that will likely occur.

Before discussing how to use the curve, a brief discussion of how a pump works and the impact of the impeller components are warranted. The fire pump converts kinetic energy from the driver (typically electric or diesel into velocity and pressure energy. The heart of the pump is the impeller, which is shown in Figure 2. Water enters the "eye" and the rotation of the impeller forces the water outward between the vanes, transferring energy in the process. The physical arrangement of the impeller determines the shape of the pump flow curve. See Figure 3 for the effect of the impeller configuration. Unfortunately, these are design parameters for a new model and unless you can afford to pay for prototype, are not applicable to ordering a pump. It is useful information, though, for understanding how pumps work.

There is an extensive range for both flow and pressure with listed horizontal centrifugal fire pumps. Available flows are from 25 gpm to 5000 gpm and pressures from 40 psi to 394 psi. The maximum available pressure does vary depending upon the associated flow rate. As stated in NFPA 230:A-3-2, "Listed pumps can have different head capacity curve shapes for a given rating. Shutoff head will range from a minimum of 101 percent to a maximum of 140 percent of rated head. At 150 percent of rated capacity, head will range from a minimum of 65 percent to a maximum of just below rated head."

With the differences between manufacturers and even pump models of the same manufacturer, it’s critical to base you final design on the actual curve of the selected pump. One should particular be aware that even though NFPA 20 now allows the churn to go to 140% of the rated head, very few pumps exceed the old 120% criteria. In fact, one manufacturer’s catalog reviewed for this article did not include any pumps that came close to the 140% allowance. This seems to be the norm for the industry and actually is beneficial in avoiding exceeding the maximum allowed pressure. For example, to avoid exceeding 175 psi on a water supply with a static pressure of 70 psi, the maximum rated pressure for a pump with a 140% churn is 70 psi; whereas, the maximum rated pressure for a pump with a 120% churn is 87.5 psi. For the remainder of this discussion, 175 psi will be used as the maximum allowable pressure.

I want to discuss using the flow curve in selecting pumps for two scenarios: one is to minimize cost by selecting the lowest possible flow rate and the second is to avoid exceeding 175 psi by selecting an increased flow rate. In the examples, the pump curve is assigned as:

  • Churn 120% of rated head
  • 150% of rated flow at 65% of rated head

The 120% churn was selected since that seems most typical. Using the maximum allowance of 65% for reduction of rated head is conservative since it dictates a higher pressure rating for the pump. For example, if 50 psi is required, using the maximum allowance of 65% dictates a rated pressure of 76.9 psi, whereas a flatter curve, say 75%, dictates a rating of only 66.7 psi. Since it dictates the highest possible pressure rating, it is not possible to exceed 175 psi at churn by selecting a flatter curve.

Scenario 1-Lowest Rated Flow

The objective is to use the lowest possible rated flow for a standard pump while staying below 175 psi at churn. This is accomplished by using the overload portion of the pump curve that is beyond the rated flow. This is a two-step process; the rated flow is determined first, then the rated pressure.

To select the rated flow, divide the required flow for your system by 150% (1.5) then round up to the first standard pump size. For example, I have a system demand of 1100 gpm at 85 psi. My water supply is 70 psi (static) and 34 psi at 1100 gpm (residual). To determine the rated flow:

  • 1100 gpm divided by 1.5 = 733.3 gpm
  • The next standard pump size is 750

To select the rated pressure, start by determining the pressure to be provided by the pump at the demand flow. This is found by subtracting the pressure provided by the water supply at the demand flow from the pressure demanded by the system. Then, determine your percentage of overload flow capacity by dividing the system demand by the pump rating. Using this percentage of rated capacity, find the percentage of rated head using the 120% in Figure 1. The pressure rating of the pump is then determined by dividing the pressure provided by the pump at the demand flow by the percentage of head. (Always round the pressure up the next whole number). After determining the pump rating, verify the churn pressure does not exceed 175 psi.

Continuing the above example, the pump needs to provide 51 psi (85 psi - 34 psi = 51 psi).

  • The percent of overload flow is: 1100 gpm divided by 750 gpm = 147 (14%)
  • From Figure 1: 147% of rated capacity produces 67% (0.67) of rated head
  • Rated Head: 51 psi divided by 0.67 = 76.2 psi. The minimum pump rating is 750 gpm at 77 psi
  • Churn pressure is: (77 psi x 1.2) plus 70 psi = 162.4 psi

If one had simply selected the next standard size pump, based on the system demand, it would be rated at 1250 gpm at 85 psi. This size pump, including the electric driver and controller, costs about $14,000. The 750-gpm pump assemble costs about $10,500. Part of this cost difference is due to the larger pump requires a 100 HP motor, whereas the smaller pump requires only a 50 HP motor.

Scenario 2 - Avoiding Pressures Beyond 175 psi

Sometimes, such as with high-rise buildings, pressures beyond 175 psi cannot be avoided, driving the need for pressure regulating valves. But, the more mechanical devices a system contains the higher the maintenance and the lower the reliability. For systems with high demands, such as ESFR sprinklers, excessive pressures can be avoided by taking advantage of the side of the pump curve below the rated flow (underload flow). This is accomplished by selecting a rated flow greater than the demand.

The objective is to stay below 175 at churn by selecting a pump with a lower rated pressure. The process starts with selecting a pump with the next largest rated flow. Determine the percentage of underload flow capacity by dividing the system demand by the rated flow. Select from Figure 1 the associated percentage increase in rated head. Then, divide the pressure required from the pump (demand minus water supply) by the increased percentage of head. This provides the minimum rated head, which must be rounded up to the next whole number. The process may have to be repeated, using an even larger rated flow if the churn still exceeds 175 psi.

Our example will use the same water supply as before (70 psi and 34 psi at 1100 gpm) but the required pressure for the system will be increased to 125 psi, with the same flow demand of 1100 gpm.

  • The next larger pump size is: 1250 gpm
  • The underload flow percentage is : 1100 divided by 1250 = 0.88 (88%)
  • The percentage increase of rated head is: 107% (1.07)
  • The pressure required of pump: 125 - 34 = 91 psi
  • The minimum rated head is: 91 divided by 1.07 = 85.0 psi
  • The pump rating is 1250 gpm at 85.0 psi
  • The churn pressure is: (85 X 1.2) + 70 = 172 psi

Selecting the best pump is not a simple matter. The approach described above is pretty straightforward, but there are some issues worth briefly discussing.

The water supply obviously has a significant impact on the pump selection. The steeper the water supply curve, the bigger the impact on the churn pressure and on the horsepower requirements for the motor (which affects cost). In the first example, if the residual pressure at the demand flow was 50 psi instead of 34 psi, the 16 psi increase would have dropped the pump rating 24 psi and the horsepower requirement from 50 to 40. This example is at 147% of rated flow and the further out on the curve the greater the impact.

Another water supply issue is the necessity of knowing the "actual" water supply values. Some jurisdictions and design firms will artificially lower the water supply to create a safety factor. For standard sprinkler systems, this practice has a beneficial effect. On pump design, though, it can be detrimental since it can cause the churn pressure of an otherwise acceptable pump selection to exceed the allowed maximum.

By selecting a larger pump size, the cost is slightly increased. An electric pump rated at 1250 gpm at 85-psi costs about $14,0000. A small 750-gpm pump could also supply this systems demand, but would require a rated pressure of 136 psi with a churn of 233.2 psi. This pump would cost about $13,000. The cost is close because both pumps require a 100-hp motor. It seems $1,000 is a small price for reducing the maintenance while increasing the reliability.

By using the overload portion of the pump curve, the cost can be reduced by selecting a smaller pump with a lower rated flow. By using the underload portion of the pump curve, maintenance can be reduced and reliability increased by avoiding excessive pressures, which eliminates additional mechanical equipment. The only way to select the "best" pump rating is to know and use the pump curves.

About the author:

Roland Huggins is the Director of Technical Services for the American Fire Sprinkler Association. He is a graduate of the University of Maryland and is registered in Fire Protection Engineering. He is a member of the SFPE and a member on multiple NFPA technical committees, including NFPA 13 Correlating Committee and NFPA 13 Discharge Criteria.

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