Proportional Valve With Spool Feedback Oscillates Badly on Start-up
By Robert Sheaf, CFPAI/AJPP, CFPE, CFPS, CFPECS, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPMM, CFC Industrial Training.
A dam out west had a hydraulic system installed to replace an old system that was just giving them too many maintenance problems.
The new system was very similar to the old unit except its directional valve was ungraded to a proportional pilot operated valve with spool feedback on the main spool. The unit would open gates allowing flow to all the downstream rivers during the dry season to keep them from totally drying up and causing large fish kills.
Every time they would start it up, which could be about every 2 to 3 months, the proportional valve would stutter and oscillate making it difficult to trust and worried that it could fail at a critical time, possibly spilling too much water. The oscillation would only last 30 to 50 seconds and then smooth out and work fine.
The unit was built with the reservoir mounted over the pump motor units and the valving and filters were mounted on top above the reservoir to conserve floor space. Replacing the expensive valve did not solve the problem.
See Solution
When we encounter this type of problem, we install a tank line check valve with a 5 to 10 PSI spring to solve this problem. Proportional valves with feedback are very sensitive to air bubbles that could collect in the coils. The height of the valve above the reservoir to the pumps was over 7 feet allowing the weight of the oil pulling down to cause about a 5” hg vacuum and allowing air to enter the system. Air can enter a system where oil will not leak out if there is a vacuum present.
Robert Sheaf has more than 45 years troubleshooting, training, and consulting in the fluid power field. Email rjsheaf@cfc-solar.com or visit his website at www.cfcindustrialtraining.com. Visit fluidpowerjournal.com/figure-it-out to view previous problems.
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