Update on
June 1, 2010
Something to Think About
A hydraulic fuse provides protection to a hydraulic circuit by detecting a broken fluid line and closing off the circuit. It is a check valve that closes when a predetermined signal has occurred — generally, a sudden change in pressure or flow. Make no mistake, a hydraulic fuse is an “after the fact event”, a circuit must experience a loss of flow or pressure before any hydraulic fuse will act. I disagree with the idea that a hydraulic fuse is used to prevent serious equipment damage or injury, it simply “limits” the damage or injuries, not prevent them.
Hydraulic designers, at least the good ones, will go through a “what if” analysis of any hydraulic system to determine different situations where failure may cause serious equipment or injuries. The idea behind this analysis is to “prevent” not “limit” the damage or injuries. It involves an in depth review of the hydraulic circuit as to what is controlling the actuators, will a component failure become a factor, does it require automation, or actual operator control and whether a circuit can continue to operate should the operator become disabled.
A good example would be a hydraulic cylinder used to lift a load. A hydraulic fuse would allow the load to move unexpectedly should a hydraulic hose break (loss of fluid or pressure), and this movement may cause damage or injury well before the hydraulic fuse reacts to the hose break. Another problem with hydraulic fuses are they will be affected by oil temperature and oil surges frequently found in hydraulic circuits, both of which will cause premature (false trips) or delayed reaction by the hydraulic fuse.
An in-depth review of “what if” analysis must be done on all hydraulic circuits on a case by case basis and specific designs incorporated to address failure modes to “prevent”, equipment damage or injury, not just “limit” the failure. Safety factors of components must also be part of this review for hydraulic components, hose, piping and structures of the entire hydraulic equipment and operation must be reviewed.
In our example above with a hydraulic cylinder holding a load, (assuming the blind side of the cylinder is doing the holding), the best solution would be to have a pilot operated check valve machined right into the blind side of the cylinder. A pilot operated check valve provides a positive hydraulic lock on the load by blocking all hydraulic flow until an opposite positive pressure on the rod side of the cylinder is seen to unlock the check and allow flow out of the blind side of the hydraulic cylinder.
If a machined pilot check valve located in the cylinder is not an option, then we go to the next best solution, which is to mount as close as possible to the blind side of the hydraulic cylinder a steel block which houses the pilot check valve. In addition, the piping or tubing connecting the pilot check block to the cylinder ports must be keep as short as possible and have a safety factor of 4-5:1 ratio. We never use hose in this area if at all possible, and if we were forced to, the safety factor would increase to 5-6:1 ratio. And the hose would be guarded from outside mechanical forces from damaging the hose. It goes without saying that cylinder rod buckling and burst pressure would be reviewed as well.
Good hydraulic design dictates we try to “prevent” hydraulic failures, not just try to “limit” them. Then the designer can move on to the analysis of “preventing the failure did not work”, now what happens next? This design analysis is absolutely critical to good design, without this analysis simply exposes people and equipment to very dangerous situations and a lot of ligation by Lawyers.
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