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.
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?
RE: Would this Valve Move?
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?
Nathan Brink
RE: Would this Valve Move?
RE: Would this Valve Move?
Nathan Brink
RE: Would this Valve Move?
RE: Would this Valve Move?
Nathan Brink
RE: Would this Valve Move?
RE: Would this Valve Move?
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?
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?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
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)."
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?
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?
RE: Would this Valve Move?
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?
hydtools, I think we agree.
RE: Would this Valve Move?
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?
Ted
RE: Would this Valve Move?
RE: Would this Valve Move?
TTFN (ta ta for now)
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RE: Would this Valve Move?
Reaction to change doesn't stop it
RE: Would this Valve Move?
my opinion, the rod moves as pressure inside cylinder < seawater pressure at depth.
RE: Would this Valve Move?
Ted
RE: Would this Valve Move?
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?
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
RE: Would this Valve Move?
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?
"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?
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?
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?
I believe. I've never seen this kind of a homework problem.
Reaction to change doesn't stop it
RE: Would this Valve Move?
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?
This one is a bad question.
http://www.eng-tips.com/viewthread.cfm?qid=419745
RE: Would this Valve Move?
RE: Would this Valve Move?
Reaction to change doesn't stop it