Hydraulically-controlled clamping system losing pressure in steel mill

Appeared in print as "Clamping system losing pressure"

A steel mill has four identical hydraulic systems that perform the same function. When the cylinders extend to a clamp position, one system seems to slow down with a drop in pressure for a short time, and then the pressure climbs back to minimum pressure needed to do the work. The other three systems stroke quickly without the pressure dip experienced by the one problem unit.

Maintenance checked the gas precharge on all four piston accumulators and confirmed they were all close to 500 psi. They made sure all the accumulator bleed down valves were open and had drained all the hydraulic pressure from the systems.

When running and when idle, all four systems would cycle on and off as you would expect with accumulator unloading circuits as shown in the circuit. When pressure reached 1100 psi, the unloading valve would open completely, allowing the pump flow to free-flow back to tank with very little backpressure.

When the accumulator’s hydraulic pressure dropped about 15% to 17%, the unloaded pressure control would snap closed. This forced the pump flow to enter the accumulator circuit until pressure reached 1100 psi, then the pressure control would unload again. The problem circuit seemed to cycle more frequently than the others.

All four cylinders were isolated from the system and checked for internal leaks. One of the cylinders that worked well had a small by-pass, but it did not affect the needed cycle time. They installed flow meters and all four pumps checked out as being in good condition.

Any ideas about the cause of the problem?

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