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A brief A to Z of pulsation and what to do about it – “don’t excite the system so much”
Causes of pulsation, more details. An approach to excitation reduction and damping the residual.
Modulating devices cause pressure pulsation:
Relief valves bouncing on their seats
Flow controllers in spill back loops, opening a tad, closing a bit, constantly changing the
resistance to flow and so changing pressure
Check valves clacking, “back pressure” valves hunting for position
Pipes that are shaken cause liquid pressure pulsation:
This is the highest pressure and lowest frequency when shaken along the pipe axis.
Lower pressure pulsation, but at higher frequencies, come from shaking side to side and
shaking up and down. The characteristics are greatly affected by the distance between pipe
hangers, clamps, saddles and other forms of support.
Pumps on the other hand, make flow.
When the flow fluctuates because:
The pump shaft deflects, impeller blades pass an opening, vanes cut off a flow slot, lobes nest between each other, worms take new gulps from their supply, diaphragms hesitate between strokes, peristaltic tubes and hose are squashed or rolls lift off them, gear teeth mesh, and the check valves from plunger and piston chambers flutter.
The resistance of the system turns the flow fluctuation into acceleration head pressure change. This is the instigator, or exciter of pulsation. It is like the plectrum that plucks the string.
This excitation is very easy to reduce (Note we did not say prevent):
Simply provide a soft compressible place, close to the pump so that the pump can suck and blow without having to accelerate the whole of the mass in the pipes.
Unless the soft cushion near the pump is infinitely large, there will be some residual acceleration head excitation. How much pressure pulsation this becomes depends on the system response, whether it resonates whether the system amplifies it.
To address the resulting residual pressure pulsation, however small it may be, because it is now pressure traveling up and down the pipe at 1400 meters per second and not flow at a 300th of that velocity, it has to be intercepted. Fortunately, a cushion chamber that is also an in line through flow, in and out device, is smaller and costs less than purely a cushion or “accumulator”.
The pressure waves can enter the through flow dampener and are not reflected away by elbows or by orifices neither resonance nor amplification will occur.
Decoupling, isolating, exciters from system response with true dampeners is simple logic. Pulsation is usually a valve problem, occasionally pump related and least often caused by a reciprocating pump (because that is so simple to resolve).