One of our newsletter members wrote to me recently about the following problem:
"We have a hydraulic power unit that runs a calendar roll on a paper machine. The calendar roll has multiple zones inside it to vary the pressure on the paper to maintain uniform thickness. Moog servo-valves control the adjustments to the different zones. The problem we have is high vibration in the pumps, lines and tank. Lines have broken and we had to disconnect one bank of filters from the tank to keep it from breaking off. This has been going on for some time. Do you have any ideas?"
As always, there are a number of possibilities and issues to consider. Some background reading on vibration and noise in hydraulic systems is available in this tutorial on hydraulic system noise. One possible explanation that jumps to mind is water hammer.
Coining a phrase
Water hammer is the term used to describe the effect that occurs when the velocity of the fluid moving through a pipe suddenly changes. Sudden change in fluid velocity causes a pressure wave to propagate within the pipe. Under certain conditions, this pressure wave can create a banging noise, similar to that you would expect to hear when beating a pipe with a hammer. Hence the phrase. Not surprisingly, common symptoms of this problem are high noise levels, vibration and broken pipes.
Hitting the wall
When a moving column of fluid hits a solid boundary - when a directional control valve closes suddenly for example, its velocity drops to zero and the fluid column deforms, within the rigid cross-sectional area of the pipe, to absorb the (kinetic) energy associated with its motion - similar to a car hitting a concrete wall. However unlike a car, the fluid is almost incompressible so the deformation is small and a store of energy accumulates in the fluid - similar to compression of a spring. The magnitude of the pressure rise that results from the subsequent release of this stored energy can be expressed mathematically as follows:
Where: P is initial pressure, u and p are initial fluid velocity and density respectively and c is the speed of sound through the fluid.
In our reader's application, uniform paper thickness is dependent on the constant adjustment of the calendar roll zones by the servo-valves. Under certain conditions, rapid switching of these valves could result in something that resembles peening a pipeline with a thousand hammers.
Accumulators and other damping devices are sometimes installed in an effort to deal with this problem. However, the significance of the pressure rise equation shown above is that fluid velocity is the only variable that can be altered to address the root cause. Put simply, reducing the velocity of the fluid column that hits the solid boundary, reduces the magnitude of the subsequent pressure rise. Returning to the traffic crash analogy - the slower the car is travelling when it hits the wall, the less damage is caused.
In hydraulics, the easiest way to do this - on paper at least, is to increase the diameter of the pipe. This reduces fluid velocity for a given flow rate. The other alternative is to control deceleration of the fluid column by choking valve switching time to the point where the pump's pressure compensator and/or system relief valve reacts fast enough to reduce flow rate through the pipe and therefore velocity of the fluid.
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