pressure relief question

[MadReferee]

</font><font color="blue" class="small">( It seems to me that you would want a pressure regulator...or pressure reducing valve (PRV) dedicated to your lower pressure cylinder, tapped off from the main pressure loop. Of course it would have to plumb back in to the return, back to the reservoir. But I could be wrong...I would need to see the circuit.)</font>

On an open center system you cannot "tap off" the main pressure line. All valves must be in series with all except the last valve having power beyond.

A seperate valve with different relief settings for each implement might just work but just the real estate footprint that would be used for multiple spool valves would make this choice unusable.

Now if you want a different and/or adjustable relief for each port on a multi-spool valve, the Prince SV valve supports this configuration. Each spool can have its own adjustable relief built in. The real estate taken up by the valve would not be all that great and you can have 2 spools, each setup with a different relief setting. When using the grapple plug into spool A, when using the shear plug into spool B. This option is well worth investigating.

 

[Henro]

Here is something you could do that would be relatively simple and would work in an open center system...

Given that a double acting cylinder has flow in on the pressure side, and flow out on the other side that is returned to tank by the control valve, you could put a pressure relief valve ACROSS the cylinder, perhaps with a check valve to make sure it was not damaged by pressure coming in reverse when the cylinder is worked in the open direction.

I don't know if you can find a PRV that is an independent item or not. But if you could, what you desire could be accomplished.

The key is to first find out if the PRV would be damaged by feeling pressire against it in the reverse direction. If so, then a check valve might protect it.

Just to be clear, what I am suggesting is connecting a PRV in parallel with the cylinder. The hydraulics external to the cylinder don't care if the return flow is coming out of the other end of the cylinder, or being bypassed by the PRV...and sent back to the control valve throught the same hose the cylinder uses. It is the control valve that sends the fluid back to the tank or to the power beyond outlet.

The actual physical hook up of the PRV would not have to be at the cylinder, it could be done near the control valve if that would work out better.

Just an alternative idea. I don't know if a stand alone PRV is easily found or not.

 

[SPIKER]

henro beat me to it /forums/images/graemlins/frown.gif

he had my same thought, not sure HOW he has hooked up his grapple by 3rd spool using the fel valve, or using a elec diverter valve to switch from tilt to grapple or a dedicated 2nd valve with it's own hoses up front...

anyhow the NEW stand alone relife valve would go between the grapple CLOSE fiting and the infeed line, with the 3rd port of the relife valve going through a check valve and then to the grapel extend hose/line. this like henro explained above let any excess pressure back to the system through the relife valve, the check valve and into the extend hose/line and through the valve that controls the grapple. the check valve stops fluid from getting to the back side of the relife valve when the grapple valve is placed in the open postion...

this contrpation would be mounted on/near the grapple cylinder, this way the QD's are removed wiht the relife valve removed too, this makes the shear full system pressure...

Mark M /forums/images/graemlins/laugh.gif

 

[MadReferee]

Henro and Spiker,

A pressure relief valve is only a sensing device with an inlet to sense pressure and an exhaust to tank which is activated only if the pressure exceeds the spring limits.

No fluid normally flows thru it. Fluid flows against its sensing spring.

It's sort of like a dead end street off the main line with a gate at the end that only opens if you push hard enough. Once the gate opens fluid flows thru it as it is now the easiest way thru.

Look here for an example of a pressure relief valve.

 

[Henro]

<font color="blue"> A pressure relief valve is only a sensing device with an inlet to sense pressure and an exhaust to tank which is activated only if the pressure exceeds the spring limits.

No fluid normally flows thru it. Fluid flows against its sensing spring. </font>

MadRef,

This is certainly is true. Kind of. While a PRV normally does not pass fluid, it will pass fluid when the pressure it senses exceeds its set point. But I think most of us know that.

So I guess I miss the point of your post. Are you saying that shunting a hydraulic cylinder with a PRV would not work as a solution to the problem described above? If so, I doubt you are correct. But maybe you are not saying that at all.

Without question when a PRV activates it shunts fluid through itself..

Otherwise, how could it function?

The reason heat develops as the result of a PRV activating, is that the pressure is dropped across the PRV as the fluid flow is diverted through it.

I believe if you look at any hydraulic control schematic, you will see the PRV is returned to tank in the normal application. This is proof enough that fluid does indeed flow through a activated PRV.

The suggested application is not normal, but the function of the PRV would be the same, that is to activate when the pressure it feels across itself is in excess of the set point.

Just the way things work...

When the pressure exceeds the set point, the spring is overcome and fluid certainly does flow through the PRV, in one side and out the other.

I guess I am confused because you say first "No fluid normally flows through it." but then say "Fluid flows against its sensing spring." Actually probably better to say Fluid presses against the sensing spring. The flow only occurs when the spring force is exceeded...

Nothing flows through the PRV up to the point where the PRV set point is exceeded. Then fluid would flow through the PRV. And once the PRV tripped, at that point the pressure applied to a cylinder hooked in parallel with it would be limited to the set point of the PRV.

So...are you in agreement, or disputing what I said and Spiker agreed with?

In a nut shell, I am /forums/images/graemlins/confused.gif by your post... /forums/images/graemlins/crazy.gif

 

[jinman]

</font><font color="blue" class="small">( The problem with putting a pressure relief valve, like the one you suggested, on a work port circuit is that you still have to have a return to tank hose since when the relief pops the fluid has to go somewhere, ie. to the tank.
)</font>

Mad, if the relief valve is placed between the "close" and "open" side of the cylinder, then the relief pressure will return to the tank. On a double acting cylinder, one side is always connected to return when pressure is applied to the opposite side.

In the case of the grapple, all the pressure is on the "close" side, so the inlet to the relief valve would be from that side of the cylinder. The outlet of the relief valve would connect to the "open" side of the grapple cylinder. The grapple has almost no resistance when opening, so very little pressure would be felt in that direction, but if needed a checkvalve could be installed so the relief valve is removed from the circuit when the grapple is being opened.

What am I missing? /forums/images/graemlins/confused.gif I'd mount that relief valve right on the side of my grapple cylinder and give it a try. If there are two cylinders, they will be in parallel, so it still only takes one relief valve.

Edit: Henro, I replied before reading your posts. I'm in complete agreement with you. I just don't see why the relief valve would not work just as you and I think it does.

 

[RickB]

I still think the obvious solution to the problem is a smaller diameter cylinder. Most of what is proposed here is needless overcomplication of a simple problem. If a generic cylinder was used at first, another smaller cylinder is probably cheaper than other "fixes".

 

[MadReferee]

A relief valve senses only the pressure applied to its inlet. Fluid "presses" (a better term /forums/images/graemlins/smile.gif) against the spring and if the pressure setting is exceeded the gate is opened and fluid will flow thru the valve which then normally goes to tank.

Henro, I said what you said in 25 words, more or less /forums/images/graemlins/cool.gif /forums/images/graemlins/cool.gif. Just read my entire post. I am not as wordy as you are /forums/images/graemlins/blush.gif.

For a double acting cylinder you would need a PRV on both sides of the cylinder since the pressure on the exhaust side of the cylinder is not the same as the pressure on the inlet side and the inlet/outlet sides are reversed depending on which direction the cylinder is moving. Thats why you are not supposed to dump exhaust fluid back into the pressure line.

As I tried to explain earlier, you cannot connect a normal PRV in series with a pressure line. You must tap/tee into the pressure line so the PRV inlet can sense the pressure of the flow. PRV's typically have only an inlet (used for sensing) and an outlet for return to tank if the relief pops.

Your earlier example of a PRV in parallel with the cylinder will not work unless I misunderstand you definition of parallel.

I have attached a diagram of how PRV's would have to be connected in a work port circuit to allow for checking on either opening or closing of the cylinder.

re: Jinman

</font><font color="blue" class="small">( In the case of the grapple, all the pressure is on the "close" side, so the inlet to the relief valve would be from that side of the cylinder. The outlet of the relief valve would connect to the "open" side of the grapple cylinder. The grapple has almost no resistance when opening, so very little pressure would be felt in that direction, but if needed a checkvalve could be installed so the relief valve is removed from the circuit when the grapple is being opened.
)</font>

Now for BigAl's application, a PRV would only be needed on one side of the cylinder, the side that closes the grapple. He shouldn't care about pressure while opening the jaw. This is the better solution as Jinman pointed out. The check valve would be required so as not to damage the relief valve.


As to RickB's comment about using a smaller cylinder, what do you mean by a smaller cylinder? A smaller bore probably. Well, there are only so many choices available for this type of application and going from say a 3" bore to a 2" bore will most likely make very little difference, if any. You still have the same pressure and flow issues to deal with.

 

[jinman]

Mad, since the spool valve will connect the opposite side of the cylinder to the reservoir tank, why wouldn't the attached modification to your hydraulic circuit work? The pressure relief valve would then become a differential pressure relief between the "close" side of the grapple cylinder and the "open" side, even if the return line has some backpressure while extending the cylinder.

 

[MadReferee]

Should be no problem. That is the way you suggested in an earlier post. The diagram I supplied assumed the user cared about the open and close pressures.

As long as a check valve is present to protect the PRV tank outlet against high pressure when going in the opposite direction you should be ok. A couple of fittings and a short section of hose is the most that should be required to plumb it up.

Alternatively, if you were using a Prince SV valve, a spool section with a built-in PRV could be used. Much cleaner installation and it would cover both directions.