Accumulator-sense, pump-unload valves

hilite_qpvalvesASPU.....doesn't exactly roll off the tongue, does it? Accumulator Sense Pump Unload. How about SSPU, System Sense Pump Unload? Sounds like a snake that smelled something it didn't like. We call them "Q" valves. No reason other that their part letters start with "Q" — QPAA, QCDB. Besides, it sounds like a James Bond gadget. However, in this case "M" did not design the "Q" valve. Rick Grawunde did.

The "Q" valve is a bi-stable, non-modulating, pressure control pilot valve with a fixed percentage differential that when combined with a main section gives you an ASPU circuit. The idea is that the "Q" valve monitors the pressure in an accumulator. When this pressure gets to the setting of the valve, it shifts and unloads, diverts or destrokes the pump. When this pressure drops to the level determined by the ratio of the valve, the "Q" valve resets, brings the pump back on line and recharges the accumulator.

At this point there are two varieties of "Q" valves: QCD* and QPA*. The QCD* incorporates the check valve that is always necessary in these circuits, hence the C. This check valve has the capacity to handle up to a 15 gpm (60 L/min.) pump. The QPA* is only a pilot section, hence the P. It can be used in conjunction with any size check for larger systems. To see the Q valves, navigate to Cartridges - Specialty - Accumulator Sense, Pump Unload, and choose your differential.

Most people that have been in this industry any length of time have ASPU stories to tell and they are almost always horror stories. Bob Koski had said many times that even if we came up with the perfect ASPU valve, we would be better off if we put it in the drawer and did not market it. The application problems would outweigh the benefits. Bob was instrumental in the design concept of the "Q" valve but I am sure that has nothing to do with the fact we are marketing it.

We feel our "Q" valve design is possibly the best design being made today. There are aspects about the design that make it superior. It is probably immune to silting. The area that would silt is very small in relation to operating forces of the valve. It has no seat, per se. Most ASPU valves have seats that wear out in time, presenting a change in ratio as the valve shifts. Eventually the seat widens until the system stalls. The areas in the "Q" valve are determined by diameters and will not change. This product is not easy to manufacture. Concentricities are critical, diameters must be held, and surface finishes are very important. However if we don't make the valve correctly, it won't pass test. If your circuit performs the way you thought it would, it is because of the robustness of the design of the "Q" valve.

We have tried to do our best in the design and manufacture of the "Q" valve. You must pay attention to detail in the design of the circuits and manifolds. Attention to detail is a phrase that is very important to the successful application of this valve. Attention to detail is what Rick has put into this valve. Diameters, spring rates, strokes, gains, timings. When a valve is supposed to function with as low as a 15% differential, attention to detail is what will make it work. Rick has paid attention to detail in the valve but you need to pay attention to detail in circuit design as well as manifold design. If your circuit is working at 600 psi (40 bar) and you are using a QCDA the differential is 90 psi (6 bar). The valve opens at 600 psi (40 bar) and resets at 510 psi (34 bar). Every pressure drop works against you. Everything takes away from the usable differential. Nothing adds to it. 90 psi (6 bar) is what you start with, what is left for the valve to work with in the real world?

Notice I am using I, we, and you. I was taught not to use the word you in a composition as it is considered confrontational. That is exactly why I am using it. We have tried to provide a robust cartridge valve need to provide a robust circuit design and a robust manifold design.

What's the big deal?  What can go wrong?
There are five modes of valve and/or circuit failure;

  • Failure to unload the pump.
  • Failure to bring the pump back on line.
  • Stalling while unloading.
  • Stalling while resetting.
  • Violent, unending oscillation between unloading and resetting.

Given the perfect ASPU valve the last four circuit failures are still a very real possibility.

What does that mean? A good circuit design is one that works as you designed it to. A robust circuit design will work the way the user expects it to. Even if that is beyond what you imagined. What if he can't get the pump he told you he was going to use and puts in one with 50% more displacement? What if he never changes his filter? What if he has to move the accumulator to the next room? What if he changes the fluid he is using? What if the next installation is in Fairbanks, Alaska? He doesnt want excuses, he just wants the machine to work. That is what he paid for. High flows and low operating pressures. These are the things to watch out for. The "Q" valve is a direct operating valve so it likes higher pressures. Low operating pressures mean low differential pressures. 15% of 600 psi is only 90 psi whereas in a 3000 psi system the valve has 450 psi to work with. High flow means higher pressure drops. These subtract from the differential that the valve has to work with. A combination of the two can get you into trouble and make us look bad. Attention to detail. Robust designs are possible even with the combination of high flow and low pressure if you allow for them.

We offer 15%, 20%, 30%, and even 50% reset differentials. The smaller the reset %, the cheaper the system, or so I am told. It must have something to do with the cost of accumulators. However the smaller the reset %, the more critical everything becomes. I would suggest designing for 20% but allowing for 30%. If 20% works then try 15%. If that works you are a hero. If 20% doesnt work, drop down to 30% and you are still a hero. If you start out designing for 15% and it doesn't work you are a schmuck and we look bad too. The 50% range was designed for demand type circuits where the accumulator is the hose between the pump and the directional valve. Sounds kind of interesting doesnt it?

"B" range is from 400 psi to 1500 psi. "A" range is from 1000 psi to 4000 psi. If you want to operate at 1200 or even 1500 psi you are best off with the "B" range. This has to do with spring rates and gain. It is the nature of the beast.

When I say plumbing I mean both in the manifold and out. Probably the most critical area is the passage between where the "Q" valve is sensing and the accumulator. All the drop because of flow into the system and/or the accumulator is subtracted from the differential that the "Q" valve has to work with. See circuit A. The other critical area is the drain. If the drain is connected to the tank line of the main section it will see the pressure drop of the pump flow when the system unloads. This will subtract from the differential.

When operating pressures are low even impingement pressures become a factor. If the flow from the unloaded pump impinges on the drain, substantial pressure can be subtracted from the desired differential.


Do not skimp on the plumbing between the "Q" valve and the accumulator. Do not connect the drain of the "Q" valve to the tank line of the main section. Don't pick too small of a differential %. I dont want to hear that the OEMs brakes are going to fail if system pressure drops below 2500 psi and the accumulator you picked is rated at 3000 psi and the circuit you designed with an 15% valve is short cycling. Don't call me if the machine works fine in July but cycles like a metronome in December. You did not put large enough holes in the manifold.

The series 1 "Q" valve is meant to be used as a pilot valve in conjunction with a Sun valve as a main section. Unless you have a specific problem you can ignore this paragraph. At this time we only offer the "Q" valve in the series 1 size. It has a finite capacity as a pilot section. If you push the capacity of the valve as a pilot section you subtract from the differential percentage plus you may not unload to as low of a pressure as one would expect.

Please, no. Not only is the "Q" valve application sensitive it is application specific. The sensing port and the switched port work together to give the differential function. Before the valve shifts they are the same and when it shifts the switched port drops in pressure as the main section unloads. This is where the differential comes from. If you fully understand why and how the valve works then maybe you can apply it to a different circuit.

The "Q" valve like most of our valves will do what it is supposed to do. If correctly applied it will probably outperform other products. It will not accommodate misapplication any better than the worst ASPU valve ever made. You can take almost any relief valve we make and stick it into almost any cross port relief application and stop an actuator from self destructing. Any of our flow controls set at 5 GPM stuck into the inlet of a directional valve will limit the flow into that circuit to 5 GPM. The "Q" valve requires thought on the part of the person applying it.

I do not mean to scare anyone with this essay. We have been in production of these valves since 10 AUG 90. We have thousands of valves in the field in both critical mobile applications such as brake circuits and in demanding high cycle industrial applications. Several of the early users were very critical and did a lot of testing. Our product outperformed the competition. The valves work like a charm. None of the valves have failed. We do not even know of a possible failure mode other than seals. We have learned things since we started making them to make them work even better. There have been application failures. Not all of them can be blamed on the circuit designers. We have been remiss in supplying information on these valves and this is an attempt to rectify that.