Pressure Loss Through Discharge Valves
A sub-committee of the Fire Apparatus Manufacturers’ Association, under the direction of H. W. Yocum, recently ran tests to determine pressure loss in pump discharge valves at different rates of flow.
Tests were made on three types of 2 1/2 fire pump discharge valves. The three types tested were: (1) a plug type valve with rectangular waterway through the plug; (2) a ball type valve with a full 2 1/2″ diameter opening through the ball; and (3) a venturi type ball valve with a 2″ diameter opening through the ball.
The valves were set up in a 2 1/2″ hose line, and all joints between the 2 1/2″ fittings were aligned as closely as possible and any sharp edges between the fittings due to misalignment were smoothly gnound off. The rubber washers in the hose couplings were flush with the inside surface. In addition, a stream straightener was placed in the tube about ten inches in front of the valve being tested, to reduce turbulence of flow. The drop in pressure across the valve was determined with a differential mercury manometer. These readings were converted to P.S.l. to plot the curves (Fig. 1). Rate of flow was determined with standard fire stream nozzles and a pitot gauge.
In the plug valve, the 2 1/2″ diameter circular section at the entrance gradually blends into a rectangular section of the same area as the entrance and then back to circular at the outlet. As indicated by the curves, this gradual change in stream shape, without appreciable increase in velocity, results in a pressure drop nearly as low as that in the valve with the full 2 1/2″ diameter circular passage. The pressure drop in the venturi type valve for rates of flow above 50 G.P.M. is higher than the drop m the other two types. This higher loss is attributable in part to the increase and decrease in velocity of the stream through the venturi section. Another part of this higher loss is logically due to turbulence caused by the 3/4″ pipe tap necessarily located in the diverging nozzle at a point of higher velocity than in the case of valves having full area opening.