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HomeIntroductionThe Simple TestBack-Engineering?The Most Basic BasicsLet's Talk HydraulicsBack-Engineer This!BattlefieldKaboomLibraryNews ArchiveBla-BlaimerContactsGuestbook |
    One of the first basic components you learn when you enter the world of hydraulics is the RELIEF VALVE. Why? Because most of the times it is the only component that prevents all that other hydraulic stuff from exploding into your face. You learn that its function is important, its correct adjustment is essential, and that fiddling with it is very dangerous.

    Even those who are only remotely connected to hydraulics, have a general idea of what a pressure relief valve is and that its main purpose is to limit system pressure. This is good, because the valves are important, and this is bad, because those fearless fellows, who know no other hydraulic components, but aren't fiddle-shy, will tamper with the relief valves every time their machinery has a malfunction, before handling it over to your work-shop.

    It would be OK, if they left the adjustments as before, but in most cases, the poor valves end up adjusted to ridiculously abnormal values (or even completely blocked), which can seriously jeopardize or render useless any further repairs.

    This is much more common than you might think, and that's why it is very important to verify correct adjustment and function of pressure relief valves whenever you test a hydraulic component equipped with one. Unfortunately, even experienced hydraulic technicians sometimes forget or intentionally omit this simple step.

    An example, something I saw just this morning - a simple test of a simple power-pack, which had gotten a brand new size 1 gear pump. The power-pack was dead simple - electric motor, pump, tank, pressure gauge, manual distributor and a relief valve -  that's it. The technician testing the rig had many years of experience, which was promising a quick and painless test...

     It was the unusual noise the pump made when the power-pack was turned on that caught my attention. As I glanced across the shop, I could see the analogue pressure gauge from a 10 meter distance. I wasn't able to see the reading, but I could see the pointer at the end of the scale, and I knew it was a 400 bar gauge. A thought crossed my mind - 400 bars - gear pump - holy crap! The rig was immediately shut down, and, luckily, the pump withstood the offensive treatment with pride. It did give the technician something to think about, though...

    I have seen this situation (with variations, of course) way too many times - a hydraulic machine "looses pressure", and the "troubleshooting circus" begins - with the first thing to "check" and fiddle being the pressure limiter, of course. A mechanic (who believes a common myth that all hydraulic valves are made on a "turn clockwise = increase" basis) turns the adjusting screw one turn in - nothing changes, another - nothing again, yet another - nothing! (this troubleshooting technique is described here) Then says something like - Well, my work here is done, give the rig to them hydraulics guys, they'll know what to do about it!... Sounds familiar, doesn't it?

     Another similar attitude I get to see way more often than I'd like is the adjustment of pressure relief valves incorporated in closed loop hydraulic pumps (I am referring to the models that have those valves adjustable, of course). Normally, such pumps have two separate pressure limiting systems - one limiting pressure by destroking the pump (like Hydromatik cut-off valve in A4VGs), and one limiting pressure through cross-port relief. As a rule, the first pressure limiting system serves to actually limit the system pressure, and the second one is to cut through spikes and limit pressure when the first one wasn't fast enough. This is why the first system setting is some 30-50 bars lower than the second one, and is the main reason the relief valves "skip" testing, as, unfortunately, often mechanics assume that, as long as their setting is higher than the pressure cut-off system setting, it is "presumably OK".

    Skipping the adjustment of these valves does save time and trouble (sometimes quite a lot of trouble, due to the condition and the position of the adjusting screws, and, oh yes, the temperature) but is a VERY BAD HABBIT due to the exact same reason I described before - it is probable that before sending the malfunctioning pump to your shop someone had fiddled with the pressure relief valves during the "troubleshooting venture", most likely leaving them blocked!

    Besides simply checking a relief valve's setting, it is also very important to check it's adjustability by lowering and increasing its adjustment value and checking its response and repeatability. It may help you discover a malfunctioning valve you wouldn't find otherwise, and will give you an idea of the valve's bars-per-turn ratio, which is a handy future reference.

    Whenever you come across a pressure relief valve in a malfunctioning component, always check both its setting and adjustability. It might take you an extra minute but is a good practice, which in the long run will pay you back. Whenever you are about to re-commission a machine that had gotten a new or overhauled pump, make it a habit of yours to at least identify the main pressure relief to see if it had been tampered with, although the safest practice would still be loosening the adjusting screw and starting the machine or power-pack with a low limiter setting.

    Most importantly, no matter what hydraulic machine, rig, pump or motor you are testing,  do NOT ask yourself whether to test or not to test this or that part of it - THINK OF A WAY to test it! Practice shows that parts labeled "presumably OK" are often the ones to cause the malfunction in the first place, so when I am asked a question "to test or not to test?" I  always respond "claro que TEST IT!"

     Beware of  fiddlers, tamperers, tinkerers and meddlers - they are out there and  are longing for adjustments...

     P.S. There are closed loop pumps that use multifunction vales (like Sauer Danfoss series 90), which perform both of the two functions - pressure relief (cross port) and pressure limiter (destroking), and have a fixed non-adjustable off-set pressure value between the two, which in case of the Series 90 rounds 30 bars, for example. Such valves have only one adjustment for both functions.