In the use of the graph for high-temperature liquids, particularly with water, due consideration must be given to the susceptibility of the suction system to transient changes in temperature and absolute pressure which might require additional NPSH to provide a margin of safety, far exceeding the reduction otherwise permitted for steady-state operation. (5)ĭo not use the graph for liquids having entrained air or other noncondensable gases which may be released as the absolute pressure is lowered at the entrance to the impeller, in which case additional NPSH may be required for satisfactory operation. Vapor pressure for the liquid should be determined by the bubble point method-do not use the Reid vapor pressure. In the absence of test data demonstrating NPSH reductions > 10 ft (3 m) the graph has been limited to that extent and extrapolation beyond that point is not recommended. No NPSH reduction should exceed 50% of the NPSH required for cold water or 10 feet (3 m) whichever is smaller. ![]() The NPSH reductions are based on laboratory test data at steady-state suction conditions and on gas-free pure hydrocarbon liquids shown its application to other liquids must be considered experimental and is not recommended. NPSH reduction for pumps handling hydrocarbon liquids and high-temperature water. Every 1000 feet of elevation reduces NPSHA, the equivalent of 1.1 feet of water head.įig. ![]() High elevation reduces atmospheric pressure and NPSHA. Inlet screening devices, if used, must not have high pressure drops. When this is impossible, the use of rotating vanes before the elbow can solve problems. Place pipe elbows at least 5–10 pipe diameters from the pump inlet, with the lower ratio applicable to large pipe diameters. Pump inlet velocities should be below 10 ft/s. Suction piping should be at least twice the diameter of the pump inlet nozzle. Good design practices on the inlet piping help insure an adequate NPSH. ![]() It does not increase this fast at flow rates much below the rated flow rate. NPSH increases as the square of the pump flow rate near and above the design flow rate of a pump. NPSHA is the net positive suction head available and must be greater than NPSHR, the net positive suction head required. NPSH is the absolute suction head minus the vapor pressure of the liquid being pumped. Pratima Bajpai, in Biermann's Handbook of Pulp and Paper (Third Edition), 2018 NPSH, NPSHA, NPSHR
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