Pumps Can Cause Failures in Hydraulic Systems
Technical Bulletin 102
Positive displacement pumps produce pressure pulsations essentially because fluid is chopped up in discreet quantities by the action of the pistons. With a triplex pump, the flow variation is about 14% of the maximum flow, and the pulsations occur six times per revolution. Flow from the three pistons overlap to give combined flow peaks in between the three individual peaks of the pistons, resulting in six flow peaks. Operating at 1200 rpm, a triplex pump will produce a pulse frequency of 120 cps.
High-pressure pulsations, a form of wasted or lost energy, create noise. Oscillating noise is representative of oscillating pressure. Pressure oscillations from 1,000 psi to 5,000 psi or more is possible in a 3,000 psi system. After one hour with oscillations of 120 cps, there will be 432,000 stress cycles. Since pressure oscillations translate into stress oscillations, the resultant damage can be fatigue cracks.
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.
The mere presence of an accumulator, if not properly analysed, may not be effective in damping pulsations. Pulsation problems are often pseudo solved in systems by strengthening the components. Pulsation damping can be accomplished with proper accumulator sizing analysis.
Boost pumps are great for producing high pressures, but improperly used can produce overpressure and result in component damage. When the boost pump strokes, a fluid volume is displaced at the rated pressure, for example, a 30,000 psi pump having a displacement of 0.10 cubic inch. With the outlet of the pump capped off, a stroke of the boost pump will instantly produce 30,000 psi. This is known as deadheaded pressure. A boost pump, separated by a long length of tubing to an accumulator, in a closed hydraulic system, a stroke of the boost pump will develop an instantaneous pressure pulse of 30,000 psi at the pump outlet. Theoretically then, the pressure distribution in the tubing will range between 30,000 psi and the accumulator pressure. The components located between the pump and the accumulator will be subjected to the same range of pressures, depending on the system configuration. In this example, the accumulator is improperly used. Properly used accumulators and snubbers should be used to reduce over pressure of the system and its components.
(See also Technical Bulletin TB-101.)
A noisy hydraulic system spells problems.
Pressure pulsations must be considered in hydraulic system design.
Other causes of damage from pressure pulsations include cavitation and water hammer.
Pressure testing systems should be verified prior to testing components.
Improper pressure testing methods can result is severely damaged components.