Would this Valve Move? 1

[jm-darcy]

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/upload/v1484926474/tips/Untitled_ckn62m.png

]

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.

 

[IRstuff]

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)
I can do absolutely anything. I'm an expert!

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[BigInch]

LI you only get one try.

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[pmover]

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.

 

[hydtools]

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

Ted

 

[jm-darcy]

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?

 

[BigInch]

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

 

[hydtools]

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

 

[LittleInch]

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.

 

[pennpiper]

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

 

[jm-darcy]

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

 

[BigInch]

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

 

[Compositepro]

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.

 

[BigInch]

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

 

[DSB123]

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)

 

[BigInch]

Right. Maybe just down into the mud. LLOL

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