Pumps Can Cause Failures When Ocean Ambient Pressure Testing
Technical Bulletin 103
Positive displacement pumps are sometimes used for conducting ocean ambient pressure testing. Simulating the rise in pressure of the ocean is complicated by the use of pumps. Components used for ocean submersion are generally designed to withstand the ocean depth pressure with a marginal factor of safety. Using pumps to simulate the ocean pressure rise requires greater control of the testing pressure. To prevent pressure surges, the supply pressure should be metered to provide a gradually increasing pressure rise. Without metered pressure control, the damage to components using a positive displacement pump can be severe.
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.
Boost pumps are great for producing high pressures, but improperly used can 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 produce a deadheaded pressure of 30,000 psi. In a closed hydraulic system, such as with ambient pressure testing systems, a potential 30,000 psi pressure pulse is injected into the system. Since the pressure testing system is a closed system, 30,000 psi pressure pulses injected into a 5,000 psi system can produce pressure spikes of extreme proportions, somewhere between 5,000 psi and 30,000 psi., depending on the system configuration
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. 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.
Pressure testing systems should be verified prior to testing components.
Improper pressure testing methods can result is severely damaged components.