Fluid Power Journal

Steel Mill Accumulator

figureitout-logoYears ago we built the complete hydraulic system for a continuous steel caster at a steel producer. The HPU had a 300-gallon reservoir with 6 pumps each driven by 300-hp motors, as well as a filter and cooling system. There were several very large manifold stands and two large bag-type accumulator assemblies.

Each accumulator stand consisted of ten 20-gallon bag-type accumulators with Viton bags. The hydraulic piping was all interconnected using 2″ socket-welded fittings and 4-bolt O-ring flanges connected to a common manifold. The stands were fabricated to keep the accumulators mounted in the vertical position.

When the fabrication was completed, the stands were pressurized with hydraulic oil to check for weld leaks. After repairing a couple of leaks, the stands were painted, crated, and shipped directly to the job site. The accumulators would be pre-charged at the job site to 1450 psi since shipping regulations required the units to be shipped without any nitrogen pre-charge. Otherwise special permits would be required.

When the hydraulic system was installed and the accumulators pre-charged, we found that eight of the Viton accumulator bags on one stand were faulty and leaked the nitrogen into the hydraulic system, bubbling out through the reservoir.

Any idea what caused the bags to fail?

See the Solution

Each accumulator stand for the steel mill consisted of ten 20-gallon bag-type accumulators with Viton bags. The hydraulic system was started up and bled of all air. The system was shut down when the Nitrogen tanker showed up to precharge the accumulators. The employees found 8 bags out of the 10 were defective. When they examined one of the defective bags, they found a small hole at the top where the bag was pushed up into the gas-charging passage, punching a hole in the bag. It was an expensive lesson to learn. Never pressurize bag accumulators with hydraulic oil without a Nitrogen pre-charge.

By Robert Sheaf, CFPAI/AJPP, CFPE, CFPS, CFPECS, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPMM, CFC Industrial Training

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