Machinery's Handbook, 31st Edition
HEAD 2799 Static Head: This term is used to include static effects like elevation and pressure head, while excluding dynamic effects like friction and velocity head. Dynamic or Velocity Head: The head imparted by the specific kinetic energy of the fluid. This value is proportional to the square of the fluid’s velocity. Velocity head is calculated using the following equation: h v v 2 2 g = --- Acceleration Head: This head is critical when evaluating the suction side of reciprocat ing positive displacement pumps and other pumps with pulsating flow. It is also important to consider during startup of a kinetic pump. Acceleration head is subtracted from the NPSHA , which is covered in the section NPSHR and NPSHA starting on page 2804. Acceleration head is the head required to accelerate the fluid in the line. It must be con sidered when evaluating cavitation risk at the inlet and peak loads on the pump compo nents at discharge. It is calculated using the following equation and Table 32. h a LVNC gk = -------- where h a is acceleration head in feet, L is the length of the suction line in feet, V is the aver age liquid velocity in the suction line (ft/sec), N is the speed of the pump in RPM, g is the gravitational constant, and C and k are given in Table 32. Table 32. Acceleration Head Constants Pump Type Constant C Fluid Type Constant k Single Acting Simplex 0.4 Non-Compressible (degassed) 1.4 Single Acting Duplex 0.2 Most Liquids 1.5 Double Acting Duplex 0.115 Compressible 2.5 Triplex 0.066 Quintuplex 0.04 Septuplex 0.028 Friction Head: The head required to overcome friction effects in the line due to surface roughness and restrictions. Restrictions can include fittings, valves, and other devices. Velocity of the fluid is directly proportional to friction head. Friction head is calculated as the sum of all energy losses incurred in the pipe system and fittings. These equations were given in section Energy Loss in Pipes on page 2779, and following pages. Vapor Pressure Head: This head term ( h vp ) is used on the suction line to evaluate the potential for cavitation. It is the head equal to the vapor pressure of the liquid being used, and is calculated using the equation for pressure head. The vapor pressure is equal to the saturation pressure at a given temperature. The vapor pressure head of water at 68°F (20°C) is 0.84 ft (2.338 kPa). When the pressure of a liquid drops below its vapor pressure, it boils. Discharge from Tanks.— To calculate the velocity of discharge of liquid from a tank the following equations can be used: For a non-pressurized tank, v o C V 2 gh = where v o is velocity through the discharge orifice, h is the height of the free surface of liq uid in the tank above the orifice, and C V is coefficient of velocity for the discharge orifice (see Table 33).
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