Machinery's Handbook, 31st Edition
FLUID CONDITIONING AND STABILIZATION 2797 Pulsation Dampeners & Snubbers: Pulsating flow can wreak havoc with hydraulic de - vices in a circuit. A pulsation dampener is a type of accumulator that absorbs these pres sure variations and stabilizes the downstream flow velocity. They absorb only the portion of fluid accelerated above the mean flow rate, store the excess fluid momentarily, and then discharge it during the portion of the cycle where flow rate drops below mean. These devices are less effective at higher frequencies. For best results, dampeners are placed as close as possible to a pump or source of pulsation. Snubbers are generally placed before gauges. Suction stabilizing dampeners are placed soon after the supply reservoir to pro- tect a pump from acceleration head variations. The diaphragm and bladder types of pulsation dampener are extremely common. The diaphragm or bladder is pressurized on one side with a gas (nitrogen is common) to about 70 to 80 percent of system peak pressure. A pulsation dampener can be as simple as a section of elastic tubing in a circuit that can expand and contract as pressure builds and subsides. Sometimes pulsations can be dampened by placing a restriction in the line to limit fluid flow. This method is less preferred because dangerous levels of pressure may occur behind the restriction. Gauge snubbers vary in their method of dampening. Less expensive ones often restrict flow using a porous disc, while more expensive snubbers use a piston mechanism to dampen pulsations. The most common application for a pulsation dampener involves a reciprocating piston pump system. Since the dampener must absorb and store the volume of fluid above the mean discharge volume of the pump, this excess fluid volume is the sizing criterion. A simplex pump (one piston) displaces a volume of fluid above mean equal to about 60 per - cent of total displacement. A duplex pump (two pistons) displaces a fluid volume above mean equal to about 30 percent of total displacement. A triplex pump displaces a fluid volume above mean equal to about 4 percent of total displacement. Piston displacement is calculated by multiplying face area by stroke. The following equation is used to calculate the volume of gas required for the pulsation dampener. V g V p p g XC p = -------- where V g is the volume of gas required, V p is the piston displacement volume, p g is the gas precharge percent (as a decimal), X is the level of dampening desired (as a decimal), and C p is the pump constant. The level of dampening is the residual peak to peak pulsations, such that lower values correspond to more damping. The precharge percentage of system pressure is typically 70 to 80 percent, as mentioned earlier, so as a decimal this would be expressed as 0.7 to 0.8. The level of dampening de- sired is expressed as the percentage of residual peak to peak pulsations. So, for 5 percent residual pulsations, this would be expressed in decimal form as 0.05. Conservative values for the pump constant are 1.5 (Simplex), 2 (Duplex), and 7 (Triplex). Snubbers, on the other hand, are sized primarily based on the pressure range they are going to experience in operation. The pressure spikes must be quantified when selecting a snubber. Head.— The energy added to a liquid by a pump or by gravity is often quantified in terms of head. Head has units of length, usually feet or meters. Pressure can be calculated from head, and vice-versa, through use of the following equations: p = 0.434 × h × SG where h is head in feet, p is pressure in psi p = 0.0981 × h × SG Where h is head in meters, p is pressure in bar h = 2.31 p × SG where h is head in feet, p is pressure in psi h = 10.197 p ÷ SG where h is head in meters, p is pressure in bar
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