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Pumps & Systems, March 2008

The installation of properly sized pulsation dampeners minimize vessel costs while protecting the pump and piping system and improving process efficiency and accuracy.

Defining a Pulsation Dampener

A pulsation dampener reduces or eliminates the variations in pressure and flow produced by reciprocating pumps. In many applications, low frequency pressure waves cause problems within a given piping system and/or process. Eccentric, cam-driven pumps are probably the most commonly applied for services that require pulsation dampening, e.g., metering pumps and reciprocating (power) pumps.

Pulsation dampeners are found in a variety of designs, but for our purposes we will focus on only gas-charged pulsation dampeners, which rely on a calculated volume of compressed gas, usually Nitrogen, which is alternately compressed and expanded in synchronization with the pump plunger to reduce or eliminate pressure pulsations. This gas volume is normally separated from the process fluid by a flexible membrane. Common membrane designs include elastomeric bladders, PTFE diaphragms, PTFE bellows or stainless steel bellows.

Pressure Pulsations from a Reciprocating Pump

Pressure waves or pulses are a consequence of the alternating acceleration and deceleration of fluid velocity corresponding to the travel of the piston or plunger. The pattern and amplitude of these pulses varies with pump configuration, specifically the number and size of pistons, as well as fluid compressibility factors. 

It is precisely the fluid volume above mean on the discharge cycle of each stroke, which induces these pressure pulsations into a piping system. The number of pistons offered by the pump-given that all are of identical diameter and equally phased-displace a known peak volume above mean. These constants may be influenced by fluid compressibility, but for the purpose of this explanation we'll assume none at this point. A pulsation dampener absorbs only that portion of piston displacement above mean flow, and then stores it momentarily before discharging it during the portion of the cycle below mean flow (on the suction stroke).

A simplex pump displaces a volume of fluid above mean that is equal to about 60 percent of total displacement. A duplex pump displaces a lower fluid volume above mean, approximately half that of a simplex pump. Pumps of three or more pistons of equal diameter, stroke length and proportionally phased will always present a very small fluid volume above mean to the piping system. A triplex pump, for example, produces about a 4 percent peak, as long as fluid compressibility factors and pump efficiencies are not at issue.

These smaller fluid volumes are accounted for by the crank angle of each of the cylinders. Triplex pumps are offset by 120-deg. Quadruplex pumps are set apart at 90-deg offsets; quintuplex pumps are offset 72-deg, and so on. It is the resulting overlap in pulses that yield the smaller fluid volumes above mean.

Fluid velocity gradients follow the same mechanical velocity gradients of the eccentric cam that drives the piston(s). Halfway through the piston's forward travel (discharge stroke), fluid velocity between the discharge check valve and the pulsation dampener begins to decay. The corresponding drop in pressure causes the membrane inside the dampener to expand since the internal gas pre-charge pressure is now higher than the line pressure. The (stored) fluid now being displaced by the pulsation dampener maintains velocity downstream of the dampener thereby reducing, if not eliminating, any downstream pulsations.

Note: A pulsation dampener removes pulses only from the line downstream of the dampener-not upstream. That's why it's always recommended that discharge dampeners be installed as close to pump discharge nozzles as possible. In an application of a dampener for suction stabilization (reduction of acceleration head losses), it is the velocity gradient between the supply vessel and the suction nozzle that is minimized.