The Old Timer Part 9: Pump Torture Test

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What happens when flow from a pump exceeds flow into the control valve?

The Old Timer of Royal Oak, Mich., was a regular contributor to H&P years before we ever even heard of the internet. But most of his advice is just as ueful — and interesting — today.

So rather than leave his wisdom printed on pages archived in our storage room, I pulled out issues from the late 1980s and early 1990s and have been reproducing relevant entries in this blog. Here is my ninth entry, which was originally published in the June 1988 issue:

Pump arrangement turns into torture test

If you take a 90-gpm, variable-displacement, pressure-compensated vane pump that has a 100-ms recovery rate, short-couple it with heavy-duty pipe to a fast-acting (20 to 50 msec) blocked-center 4-way valve, then run it on a high-cycle application, you can get some unhappy results. When we tried to operate some equipment with such a combination, we suffered poor performance, heavy leakage, cracked and blown line fittings, a sheared coupling, warped bearings, chewed-up vanes and rotors, a distorted pump case, and short pump life.

Obviously, we had a problem, and a little thought identified it. When the control valve centered while the pump was running full tilt, we could have about 0.15 gal [(90 gpm)/(60 sec/min) (0.0 sec)] of high-pressure oil that had to go somewhere, but there was no place in the circuit for it to go.

To take care of this fluid, we found a fancy, nitrogen-charged, bladder-type surge suppressor that could handle an almost-instant inrush of 0.5 gal of oil. This suppressor then slowly bled the fluid back during the next cycle. With the suppressor teed into the line between pump and valve, pump life went from weeks to years on the half-dozen machines with the original problem.

Then the suppressor manufacturer merged into another company and dropped the product from their line. We needed a replacement, and a bladder-type accumulator with a flow control valve looked like a good candidate. Unfortunately, we discovered that we couldn’t keep the flow control adjusted properly. We eventually cured this situation by substituting a simple ball-check valve with an orifice drilled through the ball for the flow control. Sometimes it took several tries to get the orifice just right, but once we had it, it never got out of adjustment.

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Alan Hitchcox

Alan joined Hydraulics & Pneumatics in 1987 with experience as a technical magazine editor and in industrial sales. He graduated with a BS in engineering technology from Franklin University and...
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