(Ref. No: MSB-003)

What's this Maintenance Safety Bulletin about? A number of manufacturers, including Caterpillar® and New Holland®, have recommended, in their respective service manuals, that certain relief valves on their equipment can be set (or the setting can be checked) with the aid of a porta-power.
Does this present a problem? Maybe! We at the FPSI™ are assuming that the manufacturers who make these recommendations in their service literature have taken the necessary precautions to make sure that these procedures are safe and sound. However, we do take issue with the fact that none of these manufacturers provide appropriate warnings and/or cautions about conducting these procedures on valves other than their own.
Tell me more! When setting (or checking the setting of) a pressure relief valve with the aid of a porta-power, there are definite precautions that one has to take, otherwise an extraordinarily hazardous condition can be inadvertently created. This problem usually occurs when people who work on and around hydraulics do not have an intimate understanding of how hydraulic components work - in this instance, a pressure relief valve. A pressure relief valve is a rather simple device. Its main purpose is to protect a hydraulic system from an over-pressure condition which will occur if an actuator is overloaded, or in the case of a cylinder, it is "dead-headed." Without a pressure relief valve, catastrophic failure is inevitable. The failure usually occurs at the weakest link. There are two pressure relief valve designs: direct operated, and pilot-operated. A direct-operated relief valve incorporates the simplest design. It consists of a body with a moveable spool. The moveable spool is held against a seat in the body by a bias spring. The spool closes the inlet port off from the outlet port. The amount of spring tension determines the pressure at which the valve opens. When the system pressure increases to the value of the spring setting, the spool compresses the spring and moves away from the seat. When the spool moves, it immediately joins the inlet port to the outlet port and allows the pump flow to pass through the valve and flow to the tank. A direct-operated relief valve has a distinct disadvantage; the pressure varies with a change in flow. This pressure variation is called "valve override."
This is how it occurs: Let's assume that the valve, in this example, is rated for a flow rate of 10 GPM. We are going to set the valve at 1000 PSI (69 bar). In illustration "A" the valve is at rest and the spool closes off the ports.
In illustration "B" the pump is running. The spool has moved out of the way to allow the pump flow (10 GPM) to pass through. Now let's set the valve at 1000 PSI (69 bar). We accomplish this by rotating the adjusting screw and increasing the tension on the spring. NOTE: If we decrease the tension on the spring, the pressure will decrease. Now watch what happens when we decrease the pump flow to 5 GPM (illustration "C"). Because there is not as much oil flowing through the valve, the opening does not have to be as large. Consequently, the spool will drop to compensate for the flow decrease. If the spool drops, the spring tension will become less, and the pressure will decrease. In other words, the spool in a direct-acting relief valve positions itself to accommodate the amount of oil that flows through the valve at any given time. Thus any variation in flow rate, will cause the pressure to vary.
Now let’s study this phenomenon in reverse: Let’s set the same valve with a hand-operated porta-power (illustration "D"). The volume of oil that flows from a porta-power when the handle is pumped is generally miniscule. How far do you think the spool will move to permit the amount of oil that is being pumped out of a porta-power to pass through the direct-operated relief valve? Barely - you guessed it! Now while we are pumping oil through the valve with the porta-power, let’s set the valve at 1000 PSI (69 bar). Now, after setting the valve at 1000 PSI (69 bar) with a porta-power, let’s pass 10 GPM through the valve. Based upon what we learned about how a decrease in flow causes a pressure decrease, we can now see that a flow increase causes the pressure to increase. This phenomenon is called "valve override," and while it occurs on both pilot-operated and direct-operated relief valves, it has a more dramatic effect on a direct-operated relief valve.
Back to the point of this bulletin! We at the FPSI™ assume that the manufacturers who recommend setting a pressure relief valve, regardless of type, with the aid of a porta-power, have taken all of the aforementioned dynamics into account. However, we are more concerned about the fact that manufacturers do not include cautions and/or warnings in their respective literature about using "their" tried and tested procedures on other manufacturer's equipment and components. We have received a great deal of feedback from maintenance personnel who say that they use this method to set all relief valves they have difficulty "loading," regardless of who manufactured it. And where did they get the idea from? The manufacturers who, in our opinion, failed to provide adequate warnings about the dangers of replicating their respective service procedures.
What immediate action must I take? Never set a pressure relief valve with a porta-power without the specific written instructions from the manufacturer. Further terstimony that this is an unacceptable practice: as part of an accident investigation, the FPSI™ conducted a test whereby we set a 35 GPM pressure relief at 1000 PSI (69 bar), with a porta-power. We then subjected the pressure relief valve to its rated flow of 35 GPM - it opened at 3500 PSI (241 bar)! This particular pressure relief valve was rated for a maximum operating pressure of 1500 PSI (103 bar)! WARNING: This test was conducted in a controlled environment - do not attempt it! YOU HAVE BEEN WARNED!
Conclusion - Once again, we are focusing on a safety issue. You can see how easily life-threatening accidents can occur if you are uninformed. Please share this safety bulletin with your friends and colleagues.

---