Machinery's Handbook, 31st Edition
2804 PUMPS where h i is impeller efficiency (typically 0.85 – 0.95), v is tangential impeller velocity at a given radius, and v f is the tangential velocity of the fluid. NPSHR and NPSHA: Net Positive Suction Head Required is the minimum fluid energy required at the pump inlet for normal operation. NPSHR is measured when the pump total head is reduced by 3 percent due to cavitation. NPSHR depends on flow rate as well as the characteristics of the pump and suction line. This value is normally provided by pump manufacturers for one or more flow rates. To some extent, NPSHR depends on the charac- teristics of the valves used on the pump inlet and discharge ports. If NPSHR for one flow rate is known, it can be calculated for another flow rate using the equation:
Q 2 Q 1 --- 2
NPSHR 2 NPSHR 1 ------------
=
Net Positive Suction Head Available is the actual fluid energy at the pump inlet, which can be calculated for the suction line as the difference between total suction head and the vapor pressure head using the following equation. For reciprocating pumps, acceleration head ( h a ) must be accounted for as well.
NPSHA h z h p h v h f – h vp – h a – = + +
where all terms are taken with respect to the suction line. Cavitation: Cavitation occurs when the pressure of the fluid in the inlet of a pump drops below its vapor pressure for a given temperature. A fluid whose pressure drops below its vapor pressure boils, or vaporizes, degrading pumping performance. The symptoms of cavitation include noise, vibration, and pitting of pump surfaces. In extreme cases, struc tural damage can occur. If NPSHA < NPSHR , the fluid will cavitate excessively and the pump will not run as expected. An additional margin is recommended, with recommenda tions ranging from 10 to 200 percent depending on application. If the liquid has been ex- posed to gas other than its own vapor, additional margin of NPSHR over NPSHA is needed to prevent cavitation. If NPSHR is not yet known, the designer must aim for a NPSHA as high as reasonably possible, and certainly above zero. Cavitation can be caused by a variety of conditions, including: high pump speeds, high fluid temperatures, low suction head, and fluids with high vapor pressures. When working at high altitudes, the effect of altitude on vapor pressure must be taken into account when calculating suction head. If NPSHA is zero or negative, the suction head is by definition below the fluid’s vapor pressure. To increase NPSHA and prevent cavitation, the inlet pressure can be increased through a variety of methods: pressurizing the fluid source or using a booster pump, raising the fluid source, reducing the flow rate, cooling the fluid, shortening and/or widening the suction line, and reducing suction line losses due to fric- tion or turns. Additional strategies include adding a suction line stabilizer (pulsation dampener) to the suction line, or adding a section of highly flexible hose to the suction line to allow some dampening. Hydraulic (Pump) Power: Hydraulic power, or pump power, is a function of mass flow rate and total dynamic head (added head). This is the energy imparted to the fluid per unit time by the pump. Typical units for power are horsepower or kilowatts. The equa- tions for hydraulic power are given below. These equations assume the density of water is 62.4 lbm/ft 3 (1000 kg/m 3 ).
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