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Would this Valve Move?

Would this Valve Move?

Would this Valve Move?

(OP)
Hi there,

I was hoping someone may be able to help with a query that has been dividing opinions of the people I ask to comment on it.

If I were to place a dual actuator valve filled with a compressible fluid into a compression chamber (or drop it to the bottom of the sea), with the open and close ports connected (as per sketch in attachment), would the valve move?

img https://res.cloudinary.com/engineering-com/image/up...]

If we consider the valve in the picture is (almost) open, it is noted that the close actuator has a smaller volume due to the valve shaft passing through it.

Therefore, given that the volume in the open side (RHS) is greater than that of the close side (LHS), and given the difference in volume and surface area of the two chambers; would the volumetric change of the compressible fluid from hydrostatic head, drive the piston to move at all to equalise?

Thank you ever so much for any advice you could provide.

RE: Would this Valve Move?

The extreme pressure at the bottom of the sea would tend to compress the valve actuator housing and force the piston to move. The housing will collapse more on the right side. So yes.

RE: Would this Valve Move?

Absolutely nothing would happen.

You've connected the two ports together so any change in pressure would be the same on both sides of the piston so there is no differential force and no movement.

Volume is totally irrelevant - its differential pressure that moves things.

also there is no path for external pressure to influence anything in the piston.

Now if the port on the right was open to the sea and the port on the left sealed, then there would be a very small movement to the left, but it would be very small if this whole thing was filled with liquid.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Would this Valve Move?

LI, do you not have to consider the area of the stem exposed to higher external pressure? Would think it would open...

Nathan Brink

RE: Would this Valve Move?

If you have dual actuator cylinders opposing each other they should not move. A single cylinder will move because the cylinder rod area will apply more pressure to the gas in the cylinder and compress it. But with a dual cylinders ambient pressure will have no effect if the cylinders are identical.

RE: Would this Valve Move?

I'm not sure I follow... Why would the piston not move all the way to the right? The piston may as well not exist (since the chambers can communicate with each other). Differential pressure across the stem should move it? No?

Nathan Brink

RE: Would this Valve Move?

For a single cylinder it will move. How far it moves would depend on how much the ambient pressure rises and what is the pressure in the cylinder to start with. As the piston rod enters the cylinder the gas volume in the cylinder must decrease (the gas gets compressed).

RE: Would this Valve Move?

Right, but since they're communicated, it'll behave just like a single cylinder.

Nathan Brink

RE: Would this Valve Move?

It will move until the forces equalize.

RE: Would this Valve Move?

"Right, but since they're communicated, it'll behave just like a single cylinder."

I don't understand your point. Please elaborate. But what ever way you mean this, I think you are wrong. If you elaborate you may see why.

RE: Would this Valve Move?

Alright, here goes. With the equalization line between both chambers, the piston may as well not be there (excepting of course a small amount of additional friction).

Go ahead and picture removing the piston (since it's not doing anything). All you have is a rod going through some packing into a chamber of compressible fluid, effectively creating a single acting piston. Now, if there's a pressure difference across that rod, it'll move, no?

Not my bailiwick, but I can't see it any other way. Why isn't this moving?

RE: Would this Valve Move?

The diagram isn't great so it's not easy, However once you connect both sides of a piston like that together its not going anywhere.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Would this Valve Move?

Nbrink, my previous post addressed your concerns.

"For a single cylinder it will move. How far it moves would depend on how much the ambient pressure rises and what is the pressure in the cylinder to start with. As the piston rod enters the cylinder the gas volume in the cylinder must decrease (the gas gets compressed)."

Yes, the piston in the cylinder acts as though it is not there except for friction. So the rod will move until the pressure in the cylinder equals the pressure out side. The rod moving into the cylinder will increase the air pressure in the cylinder.

LittleInch, I have not seen the diagram but we are talking about an air cylinder not a hydraulic one, so it will move.

RE: Would this Valve Move?

Sea water pressure acts on the piston rod area.
Internal pressure acts on the annular area on the left of the piston.
Internal pressure acts on the total area of the piston on the right of the piston.

Charge the cylinder with some mass amount of compressed air. The pressure will be P = nRT/V
V doesn't change.
R gas constant doesn't chagne.
T might change, in which case P might change, however if T remains constant and

the charge's pressure is,

1) less than sea water pressure, the piston moves all the way right.
2) is equal to sea water pressure, the piston moves not.
3) greater than sea water pressure, the piston moves all the way left.

RE: Would this Valve Move?

Type of valve is not indicated, but I'm not seeing that sea water pressure is acting on the piston rod area. Unless it can, then the piston won't move with the ports connected like that. I suppose if the external pressure was sufficient to collapse the cylinder, the piston may move a little to the left, but what's the point of that?

RE: Would this Valve Move?

The compressible fluid (gas) acts as a spring on the piston rod. The force of the gas spring is the product of the gas pressure times the area of the rod. With no force acting on the left end of the rod, the rod will extend. If some force is added to the end of the rod, it will push the rod back into the cylinder if the applied force is greater than the internal force pushing the rod to the left. Since the internal and external pressures are acting on the same rod area, the rod will not move if both internal and external pressures are equal. If the external pressure resulting from submergence is higher than the internal pressure, the rod will be pushed into the cylinder until the resulting volume reduction increases the internal pressure to equal the external pressure.

With both cylinder volumes connected together, the net operating area of the piston is the area of the rod.

Ted

RE: Would this Valve Move?

JohnGP the rod is exposed to the ambient pressure outside of the cylinder which will force the piston into the cylinder until (if ever) internal pressure x (piston_area-annulus_area) equalizes with the seawater_pressure x piston_rod_area. That might happen before the piston reaches the end of the cylinder.

hydtools, I think we agree.

RE: Would this Valve Move?

Ok, it was abit of friday afternoon response and I missed a couple of things.

SO

The connection of the cylinder ports in essence makes this a rod being inserted into a cylinder of fixed volume Vc. - As pressure acts on the remaining area in equal measure.

The force on the rod is therefore Pc x Ar = Pressure in the cylinder times area of the rod

If the fluid is compressible (something I missed) then as the rod enters the cylinder, the pressure inside the cylinder will change due to the volume of the rod reducing the volume available for the fluid. We don't know the relative volumes Vc and Vr, but unless Vr was close to Vc then the pressure rise wouldn't be big. If however this was virtually incompressible fluid then the pressure rise would be much much greater.

Hence if the pressure acting on the end of the rod Pr, is less than Pc, or force is leass than Pc x Ar then the piston/rod will move left in the diagram. If pressure Pr > Pc( a free end) or force on end of rod > Pc x Ar, then it will move right until it either hits the end of the cylinder or PC becomes equal to Pr or the force is equal to Pc x Ar.

SO if it is just the actuator cylinder dropped into the compression tank then yes, it will move right.

BUT, if the end of the rod is either fixed to something or is not exposed to the external pressure then it won't move. You also need to factor in friction and the forces on the end of the rod (an unknown unknown).

The question was "will this valve move", not will a rod move. I actually don't know because there are too many unknowns.

I note the OP calls this actuator cylinder thing a "dual actuator valve". I don't see any valve anywhere so it's all a bit confusing....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Would this Valve Move?

To answer the OP question, it will move depending on the balance or imbalance of forces on the rod.

Ted

RE: Would this Valve Move?

My take on it was that the piston rod end may not be exposed to external pressure. If it is then I agree the piston could move.

RE: Would this Valve Move?

I'm tempted to think that the piston will move. If we separate the area of the piston face between the non-shaft part and the shaft part, we can see that the forces balance out on the non-shaft area. But there is an imbalance in forces in the shaft area, internal pressure on the right side, and external pressure on the left side. The external pressure attempts to shrink the exposed volume of the piston, and everywhere but the shaft, there is mechanical resistance against shrinkage. The shaft, however, is freely able to move, and therefore, it will, because of the external pressure component that's longitudinally applied.

TTFN (ta ta for now)
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RE: Would this Valve Move?

LI you only get one try.

Reaction to change doesn't stop it smile

RE: Would this Valve Move?

thinking outside the box, perhaps conducting a test by placing the actuator inside a pressure vessel, submerge actuator with seawater, and apply air pressure equivalent to seawater depth for installation. perhaps a smaller size actuator or set-up can be configured for the test. if conducting this test, be smart and safe.
my opinion, the rod moves as pressure inside cylinder < seawater pressure at depth.

RE: Would this Valve Move?

Really just do the numbers. It is pretty straight forward.

Ted

RE: Would this Valve Move?

(OP)
Hi there, OP here. Thank you all for your input, it has been enlightening seeing the varied replies and debate going on.

Just to clarify a few points:

1. It is hydraulic fluid and not air in the simplified ‘system’.
2. The hydraulic fluid is compressible but only slightly (density of ~1072 kg/m³, bulk modulus ~3.15 x 109 Nm-2).
3. This dual actuator’s rod is connected to a gate valve (apologies for confusion).

In summary, it would seem that most believe that movement will depend on whether the ‘other end’ of the rod is exposed to external pressure or not. The consensus seems to be if it was exposed to external pressure then the rod would move, but if not, then probably not.

Would I be correct in assuming that if there was a small amount of air inclusion in the RHS chamber (air being significantly more compressible than the hydraulic fluid), then the rod would almost definitely sweep to the right?

RE: Would this Valve Move?

With "incompressible" hydraulic fluid filling the cylinder, NO movement you can measure.
A small amount of air? If it is truely small, NO measureable movement.
More air may result in some net volume compression of the fluid mixture within the cylinder, so a tiny bit of movement to the right might occur, but only when the exterior cylinder pressure is greater than the pressure of the little bit of air inside the cylinder.

Reaction to change doesn't stop it smile

RE: Would this Valve Move?

Hydraulic fluid in most applications with pressures less than 5000psi is considered incompressible. In that case the piston will not move, the fluid can neither lose nor gain volume any significant volume.

A slight entrainment of air will allow a slight movement. The amount of movement will depend on the volume of trapped air. Trapped air is what makes hydraulic systems spongy or springy.

Problem clarity early on would have been very helpful. Just sayin'.

Ted

RE: Would this Valve Move?

Details matter....


"dual actuator valve filled with a compressible fluid " does not equal "It is hydraulic fluid and not air in the simplified ‘system’" !

"a small amount of air inclusion in the RHS chamber " - how small is small?

A small amount will allow some compression of the hydraulic fluid, but depending on the relative size of rod versus cylinder, it might not go all the way across (sweep), but it would move to the right if the rod end is exposed to the external pressure and not connected to anything.

If the end area of the rod is not exposed to the external pressure it won't go anywhere. If it's "connected to a gate valve " it's all academic.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Would this Valve Move?

Wow!
What a bloody waste of time. The Original Post here appears to be homework for some hypothetical problem in Fluid Mechanics related class, but nothing to do with pipeline or piping. Bottom line, we should not be doing peoples homework.

Sometimes its possible to do all the right things and still get bad results

RE: Would this Valve Move?

(OP)
I am sorry to have caused offense. This is certainly not homework. Instead it is an issue I have observed at work in an oil and gas application.

I simplified the model in a hope to get responses that would help me (and hopefully others) to better understand the fundamental principles and influences rather than focus on a unique application.

The late inclusion of the air element was not a hypothetical scenario, more a means to check that I fully understood what was going on.

Again, apologies if you think I have wasted time.

Admittedly, I thought the original question I asked resulted in an interesting exchange in opinions. OP

RE: Would this Valve Move?

It was a little interesting for a minute or two.
I believe. I've never seen this kind of a homework problem.

Reaction to change doesn't stop it smile

RE: Would this Valve Move?

Most of the exchanges were due by a poorly worded question that caused confusion. You called your device a dual actuator valve when it was in fact a double acting hydraulic cylinder (a single actuator). You said compressible fluid when in fact it was hydraulic fluid, which most engineers immediately recognize as incompressible for most practical purposes.

The rod of a single hydraulic cylinder will always see external pressure on its end. It does not matter what the geometry is of any mechanism attached to the rod. The only way to avoid this is to connect it to an opposing cylinder. It does not matter what the geometry of this mechanical connection looks like.

In the case of a gate vale, the pressure in the valve will exert a variable force on the cylinder rod given by valve pressure times valve stem area. Ambient pressure will exert a force on the cylinder rod area minus the valve stem area. This seems complicated but it is really very simple once you understand the basics of hydraulics.

RE: Would this Valve Move?

I don't think it was that bad a question, given there was a diagram. After you'd seen the diagram the rest of the words didn't matter much either way.

This one is a bad question.
http://www.eng-tips.com/viewthread.cfm?qid=419745

RE: Would this Valve Move?

The main question was "will the valve move at the bottom of the sea" Well really it depends on the seabed and the extent of the sea currents. I would expect it would move at some time due to movement of the seabed subjected to sea currents (LOL)

RE: Would this Valve Move?

Right. Maybe just down into the mud. LLOL

Reaction to change doesn't stop it smile

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