Pressure Pulsation Can Cause Failures in Hydraulic Systems
Technical Bulletin 101
A noisy hydraulic system spells problems.
Failures in piping, valves and other components are the effects of hydraulic shock in fluid lines, caused by unsteady flow of incompressible fluids in pipes. Maximum noise is generated because of turbulence and sonic shock waves.
Cavitation is the result of the collapse of vapor bubbles that have been formed by an unstable pressure condition in the system. The most typical case is where a flow restriction increases the velocity and decreases the static pressure. When the static pressure falls below the vapor pressure, bubbles form. When pressure increases to a certain point, bubbles collapse to cause pressure fluctuations. Cavitation in fluid systems can be a major source of noise and fatigue failure damage. Restrictions in fluid passages can produce pressure pulsations large enough to cause equipment failure.
When a valve is suddenly closed, the liquid is brought to rest progressively along the pipe with a wave action called “water hammer”, the term used to express the resulting shock caused by the sudden velocity decrease of the fluid. This type of surge pressure initially peaks, then oscillates about some mean pressure until viscous damping reduces the oscillations to zero. In a 3,000 psi system, the pressure can rise to 6,000 psi or more.
Pumps: Positive displacement pumps produce pressure pulsations because the fluid is chopped up in discrete quantities by the action of the pistons. A 7-piston pump operating at 1200 rpm will produce a ripple frequency of 140 cps. A 3-piston pump operating at 1200 rpm will produce 60 cps. Fluid compression and decompression at the pump outlet set up a pressure ripple, a minor shock wave. A high speed pump with high piston speeds produces higher pressure pulsations than a pump with low piston speeds. If valve and porting diameters are small compared to piston diameters, pressure pulsations increase. Pump pressure pulsations are amplified if the pump is cavitating. High cycling rate combined with reversing pressures will promote premature fatigue failures.
Accumulators: Accumulators generally are not in the line, but are set off as a branch of the system and isolated by the impedance of the connection. Seldom considered is the inertia of the fluid in the line. The effect of fluid inertia can be many times greater than that of the fluid in the accumulator, which is often neglected, or the mass of the piston, which usually is the only factor considered.
Pressure fluctuations caused by unsteady flow can result in fatigue failures. Fatigue failure is failure brought about by the repeated reversal, removal or fluctuation of the applied load. Fatigue failures involves three phases: crack initiation, crack propagation, and final (brittle) rupture.