I’ll admit, I’ve fallen prey to one of these “you won’t believe what happens next” taglines floating around Facebook and other social media sites. If you’ve never seen them, hopefully mine suckered you in. Because you won’t believe what happens next.
I’ve actually seen a relief valve used in place of a flow control, and in some regard, it works. This “flow control” was placed after the pump to limit downstream flow, but he didn’t realize why his circuit was behaving oddly. You see, a relief valve is a spring-opposed valve that does not open until pressure rises above the spring setting. As long as there is enough horsepower provided for the pump to create the energy to overcome the relief valve setting, it will open up and pass flow.
In this case, the relief valve worked to control flow, fooling this guy into believing he actually had a flow control. The reason it worked is because he also had a pressure compensated pump. As he cranked up the relief valve pressure, the pump would reduce displacement and limit flow. In fact, it worked perfectly well.
I told you that you wouldn’t believe what happens; and I don’t blame you for your disbelief. Under any other circumstance when using a fixed displacement pump, adding a relief valve directly downstream would do only one thing—create an absolutely massive amount of heat. The pump would not be limited in flow in the least, except perhaps because of increased internal leakage, but that’s minimal. Instead, it would pump just as if it thought the relief valve was a hydraulic motor under continuous load; except no work would be done.
With a fixed pump, this scenario would result in reduced capacity for downstream work. Any load induced pressure would be additive to relief valve pressure, and there is a high likelihood of overloading the pump or the prime mover. And the imminent failure would still occur at full flow, because a relief valve is a pressure control valve and not a flow control valve.
Even though this customer was able to get his “flow control” to work relatively well, it still performed oddly. The tank port of the relief valve was exposed to downstream system pressure, and when load-induced pressured increased, it added its value to the relief valve, essentially changing its pressure setting, and further reducing flow. It was a hot mess, and just goes to show you that we can’t take fluid power education for granted. This guy got lucky, but I can guarantee he won’t be the last one to try; can you guess what will happen next?