Insane Hydraulics

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Defeating That Back-Pressure

A fellow tech asked me a question about a valve he had to "tweak" the other day:

"How can that sequence valve be not affected by back pressure in the C port when its spring is wet?"

What was meant by "wet " was the fact that the spring of the said valve was constructively in constant contact with the C port oil, unlike its atmospherically-referenced siblings with their isolated "dry" springs.

That was a good question because it brought me back to the time when anything that had these opposing dotted pilot lines on the switching element in the symbol seemed "sorcery" to me:

I liked the question so much that I decided to make this short post about the design of these relief, sequence, and over-center valves, that are built in a way that the back-pressure in the return line does not affect their setting.

I still remember how, as I was learning about all the "wonders" a fluctuating return line pressure can do to a pressure-control valve, I discovered that one could build a "back-pressure-proof" valve by either placing the spring in a chamber that is open to the atmosphere, or by adding a 3d (drain) line that would provide a stable (tank) pressure reference - and "life was good" until I ran into an inline sequence valve (exactly like the one I was asked about) that absolutely could not have its spring placed in an open chamber - and yet it behaved exactly like any "good" sequence valve would - completely "ignoring" the back-pressure in the "C" port, only... it didn't have a drain port, and the spring was clearly "bathing" in the C-port oil.

This does not seem like much today, but it definitely felt like a big deal when I finally figured out that you don't need a spring to be physically placed "in the atmosphere" to be atmospherically referenced, all you need is "something move-able" to be exposed to the atmospheric pressure. Here's an illustration:

Pretty cool, isn't it? As you can see, while the "dry spring" design makes it very easy to add an adjusting mechanism to the spring, the "wet spring" design will require "jumping through a couple of additional hoops" for that, but still - both of the designs will make sure that the poppet-lifting pressure depends only on the spring setting, and nothing else (aside from the negligible and very stable atmospheric pressure, naturally - I am including this phrase for scientific purposes only).

And what about the inline sequence valve that confused the "young and aspiring" hydraulic technician? Well, it was designed like this (they all are):

And, as you can see, the moving part - the spool with unequal areas, is built very differently from the wet-poppet from the picture above, but it follows exactly the same principle - it has a spool-pushing area exposed to the V(P) port, and a completely balanced moving part "bathing" inside the C port.

By the way - I took the image from the Fluid Press catalog (they seemed to have the best cutaway drawing I could find), but I don't think the symbol they used is the best choice, because it kind of suggests the dry spring design. I think that placing a small triangle on the side of the square would be a better fit - but then, once again, this shows how arbitrary hydraulic symbols in catalogs are.

But... Why does a tech need to know about them atmospherically referenced designs? And, who uses them sequence valves these days anyway?

Well - you're right in saying that sequence valves are less used today than they used to be, at least for movement sequencing, due to the omnipresence of PLCs, but a technician still needs to know that such valves exist, he needs to be able to recognize them when he sees them (look for that very obvious vent orifice covering o-ring or a groove that has one missing), and know their biggest weakness - their increased sensitivity to the environment, especially moisture (even for the "wet spring" variants).

Here's a good example for you - this sequence valve (wet spring design, by the way, just like the one I showed in last week's post) looks perfectly fine on the outside, the venting orifice o-ring intact and all, and yet - look at what's inside the "atmospheric-reference chamber":

Yes - water (or whatever corrosive fluid there may be around) can get in that orifice and do all kinds of bad things. The valve will run just fine as long as it is constantly cycling, but let it "sit still" for a couple of weeks, and you may suddenly discover that it stopped opening for some reason.

For the record - I never liked atmospherically referenced valves, and I will always choose a "properly drained" valve over an atmospheric one whenever I can.