FIRE PUMP CAPACITY AND HEAD RATING

 

The capacity and pressure ratings of fire pumps must be adequate to meet flow and pressure demands consistent with water supply requirements for the property in question. Fire pumps are designed to provide their rated capacity with a safety factor built in (150 percent of rated capacity at 65 percent of rated pressure) to provide some protection in case of greater than-expected demand at the time of a fire.

STANDARD HEAD DISCHARGE CURVES

The shape of the standard head discharge curve of a fire pump is determined by three limiting points: the shutoff, the rating, and the overload.

Shutoff

With the pump operating at rated speed and no flow, the total head of a horizontal centrifugal pump, vertical turbine pump, or an end-suction pump at shutoff must be between 100 and 140 percent of the rated head pressure at the 100 percent flow capacity.
The shutoff point represents the maximum allowable total head pressure. Otherwise, the pump would have a rising or convex characteristic curve. Such pumps are not listed. With a convex curve, there could be two flow points for one pressure.

Rating

The curve should pass through or above the point of rated capacity and head.

 

Overload

At 150 percent of rated flow capacity, the total head pressure should not be less than 65 percent of the rated total head. The curve should pass through or above the overload point. Most fire pumps have curves with a small margin above the theoretical overload, and some models have a cavitation or “break” point in the curve just beyond overload.

Source : NFPA 20

what the benefit of relief valve in fire pump ?

 

relief valve

These are required on the pump discharge line when the operation of the pump can result in excess pressure that would exceed the pressure rating of the fire protection system. The design of this device is very critical to the proper operation of the fire protection water supply. If the pump is oversized for flow and pressure, the main relief valve should not be used to limit the pressure and discharge excess quantities of water during the normal weekly run test and annual flow test. This is of particular importance in climates where there is freezing weather for a portion of the year. In these situations the designer should look further at other pump choices and optional drivers for the pump that will limit the discharge head. A good example is a variable-speed diesel engine or electric motor listed system.