A characteristic of pumps that is sometimes misunderstood is the fact that fire pumps do not really produce flow; instead, they are useless if the water supply is unable to deliver the necessary flow. A fire pump can only raise the water supply’s pressure. the net pressure (differential pressure) at rated flow and rated speed as indicated on the manufacturer nameplate, as defined by NFPA 20. Each fire pump will have a pressure at a certain flow, and much like the specified flow, the specified pressure serves as a benchmark. The fire pump’s flow and pressure are intimately correlated; as flow increases, pressure lowers, and as flow decreases, pressure rises.

Shutoff or churn, also known as the fire pump pressure, when there is no flow, is limited to no more than 140% of the rated fire pump pressure. The highest pressure that a fire pump is permitted to discharge, for instance, is 182 psi when no water is flowing, and the pump is rated for 130 psi at 500 gpm. While the maximum pressure for all pumps is 140 percent, the maximum for centrifugal fire pumps was formerly 120 percent. As a result, the majority of centrifugal fire pumps on the market today still do not surpass 120 percent of the rated pressure at churn.

NFPA 20 specifies the lowest pressure that a fire pump is authorised to produce in addition to restricting the churn pressure. Pumps must provide at least 150 percent of the rated capacity at at least 65 percent of the entire rated head, according to NFPA 20 Section 6.2.1. This indicates that the pressure must be at least 65% of the rated pressure at 150 percent of the fire pump’s rated flow. For instance, a pump that is rated for 130 psi at 500 gpm must be able to generate at least 84.5 psi at 750 gpm. In this article’s figure, the pressure constraints are shown

The size of the fire pump itself has been the main emphasis up to this point, although table 4.28 of NFPA 20 provides minimum specifications for the components. Based on the fire pump’s rated capacity, this chart lists the minimum sizes for pump suction, discharge, relief valves, flow meters, and test headers. When the fire pump is operating at 150 percent of its rated capacity, a flow velocity of 15 feet per second is used to determine the magnitude of the suction. While the size of the supply line for the hose header is listed in the table, if the supply line is longer than 15 feet, it must be a pipe size greater than the diameter listed in the table, or the length of the line can be determined hydraulically.

The first part of the fight is choosing a fire pump that can enhance supply pressure sufficiently to fulfil the demands of the fire suppression system. The maximum pressure that a fire suppression system can withstand will always exist. Although there are high pressure connections, the most typical maximum pressure rating for components on a fire suppression system is 175 psi. Nonetheless, there are situations when this maximum pressure must be avoided. For instance, ESFR sprinklers are only offered with a 175 psi maximum pressure rating. The churn pressure plus the static pressure of the water supply must not exceed the maximum pressure rating of the system components, and the fire pump must boost the pressure adequately to fulfil demand.

This can be difficult depending on the circumstances since certain supplies are insufficient because of a steep curve, which means that the static pressure is high and the residual pressure is low. It might not always be able to employ a typical, constant speed fire pump due to a combination of a steep supply curve and a high demand from the suppression system. (Courtesy Jeff Dunkel)

It could be feasible to utilize a variable speed fire pump if a constant speed fire pump is not an option. Using a variable speed fire pump, the pump speed may be changed to lower pressures at lower flows, create a “flatter” curve, and employ a larger pump without having to worry about high static pressure. There are drawbacks to a variable speed fire pump, one of which being size. A variable frequency drive (VFD), which can be extremely big, must be installed in the controller for electric variable speed fire pumps. Variable speed fire pumps have a much higher price tag in addition to a greater area. The size and cost of a diesel variable speed pump are less affected. A pressure limiting device (PLD) is used by a diesel pump to change the speed, and while this does affect cost, it does so to a lesser extent than a variable frequency drive (VFD).

Courtesy Jeff Dunkel, P.E., the National Fire Sprinkler Association’s fire protection engineer, wrote this issue of Technotes.