Hello all

I work with deep sea ROVs (remote operated vehicles) and we routinly dive to 10000 feet of water. Below is an email I recently sent to a colleage. I believe at 9400 feet to the surface and a 40 deg C temp change, we should get around a 2% vol increase from pressure trapped alone and another 1.5% change from temp differance.
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I say you can trap oil/water pressure at depth(say 6000 feet for example) in a closed off, check valved circuit (zero leak, check valves, etc) and when you come to the surface, you will bring that trapped high water pressure up with you in the circuit. This is one reason why we have high pressure relief valve in our arm counter balance valves (to bleed off the trapped pressure on the way up).

A technician says that's impossible.

To make the math easy, say we get ½ psi per foot. At 6000 foot the water pressure is 3000 psi relative to the surface. If you open a jam jar and let if fill with water and then close it (no air), you will have 3000 psi in the jar relative to the surface. You will have zero psi relative to the outside water at 6000 feet (3000 psi pushing in, 3000 psi pushing out, 0 psi difference)

When you come up to the surface, the outside water pressure drops off with shallower depth until the jar is on the surface. At that time, you have gone from 0 psi differential pressure (pressure inside compared to outside the jar) at 6000 feet, to 3000 psi difference at the surface (i.e. there will be 3000 psi trying to explode out of the jar).

If you have a one way check valve on the jar and fill it with water at 6000 feet you have the same situation. If you then pressurize the jar more with 3000 psi of oil (using a hot stab) then at depth, you will have a 3000 psi difference of pressure in the jar (relative to the outside water). However you will have a 6000 psi difference relative to the surface (3000 psi of water pressure plus 3000 psi of oil pressure = 6000 psi.) Bring it up and you have a jar filled with 6000 psi fluid relative to the air.

Now substitute a high pressure hose for the jar above (with a check valve that only allows water or oil in, not out). There is alittle stretch on the hose as you come up but you will trap most if not all of the pressure coming up. (There is a little stretch in the hose if you pressurze it on the surface using a pump, this is normal) In fact this trapped pressure is the reason some hoses blow if they don't have the pop off valves in the counter circuit to bleed off the trapped pressure.

The technician says this is only a problem if there is air in the circuit. Trapped fluid alone can't cause a pressure difference.

I disagree, what are your thoughts
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Hello Peter

Thanks for the quick responce. I have seen the formula before. We are using HP hose that has burst around 6000 psi.
There are at least two forces keeping the trapped water pressure contained. One is the outside water pressure and the other is the wall of the HP hose.
When the ambient pressure drops off as we ascend, the differential pressure increases. That is, the trapped water pressure in the hose is no longer being "crushed back" by high pressure outside water pressure. The hose would try to expand but in doing so, the HP construction of the hose causes a back pressure on the trapped water. If the differential pressure in the hose is greater than the burst pressure of the hose, the hose would fail. This is all assuming only water compression/ expansion. There is also water volume increase due to increased tempurature as we ascend from 40 deg F water to 80 deg F water.
With the trapped water under pressure from the hose back tension, I can' see the pressure ever dropping off to zero. This would be even more pronouced if the hose is replaced with SS tubing (which has even less expansion with pressure and the expasion would also create a back pressure on the water).
Lastly, if oil is used instead of water the expansion / pressure rise would be greater. Oil is more compressible and therefore has more expansion, than water.
What are your thoughts?

Thanks for you input

FOR CONVIENCING TECHNICIAN , YOU MAKE AN EQUIVALENT ACCUMULATOR CIRCUIT . IN THIS BLADER WILL REPRESENT YOUR HOSE , LIQUID IN SHELL WILL REPRESENT SEA WATER . USE ACCUMULATOR ISOTHERMAL EXPANSION FORMULA .

Thanks for all the great feedback

The original premise I had was that fluids as well as gases are subject to volume change with pressure (In fact, solids, liquids and gasses are all subject to volume changes with pressure, it is only their relative bulk modulus that differ) Obviously, solids compress much less than liguid and volume changes in liquids and solids are negligible in most hydraulic / mechanical problems.
Newton's laws should help me to easily explain the problem.
A force in one direction is opposed by an equal and opposite force.
If I trap compressed 3000 psi water and bring it to the surface, that pressure will cause a force on the walls of the fluid container (hose, SS tube, etc). The walls, opposing that force (from strong elastic tension) cause the backpressure in the fluid. If I instead inject another 3000 psi of oil into the circuit at depth, I will have trapped 6000 psi of pressure relative to the surface pressure.

It was the technician's premise that ONLY gas could cause these volume / pressure problems.

I am planning on taking down a series of "experiments" and video tape them with the ROV at depth. One is a glass tube connected to a valve and a gauge. I would fill the tube with water on the way down and at 9000 feet. I will insure all the air is out, lock it off and ascend watching the gauge and checking for trapped gas expanding in the tube. This should give me some data to put this issue to bed. I am guessing the tube won't survive the entire trip to the surface.

Thanks

rovman,
You are correct in your thinking, I design vibros that go as deep as your depth, and you had better equalize the pressure before raising the load.

Peter thinks hydraulic hose acts like “Sponge Bob”, and expands much more than it actually does. He is caught up in formulas and facts that he does not know or have regarding actual expansion rates of hydraulic hose.

There are no known fixed facts regarding expansion rates for rubber hydraulic hose, and that just blows Peter’s mind because his formulas give him monkey math answers, not real world answers. Too many variables to consider when looking at hydraulic hose expansion rates, do yourself a favor, decompress when coming up from 9400 feet, save breaking lines, we do.

Speaking of hose,you might be interested in this article:
http://www.hydraulicspneumatics.com/200/TechZone/Hydrau...Zone-HydraulicHoseTu
It appears Hose not only EXPANDS but can change in LENGTH when pressurized. As much as 2-4%.

This article was in H&P magaznea and came from a Google search for "spiral wound+braided+hydraulic+hose." There were over 3,000 hits from the above Google search so you might get the information you want from some of these.

Aeroquip puts on classes for their distributors and also for customers on a regular basis. You might check with your local distributor to see when one will be held. I'm sure other hose manufacturers have similar training sessions.

Froma Aeroquip training class way back, Braided re-enforced wound hose is subject to shortening due to hose expansion at pressure which causes the Braids to act in reverse of the Chinese Finger Lock device. I would say there is some lenth that Diameter growth would be cancelled by Length reduction??????

quote:

it doesn't say whether the volume increases.

At least you may have some information that might help get your questions answered by some knowledgeable person at Aeroquip . Worth a try.

Peter,

Sure turned your tail, which was fun. Look my little electron buddy, there is no secret information they are keeping from Peter the Great, no CIA plot against you and no effort to fail your wonder formulas. I really liked the part about you can feel the hose breathe, only could the little electron like you would come up with that one, its great I will have use it someday.

What we have here is an EE on steroids with MathCAD and is out of control. God save us all, or better yet maybe Congress can bail us out.

Gee, let me see here:
All rubber is made from the same company in the world and used in hydraulic hose by everyone, so it has the same exact properties.

All steel wire is also made by one company in the world, so it has the same properties.

All hose manufactures stretch, pull and wrap all hydraulic hose exactly the same way with the exact same machine (they share).

All hydraulic hose is ALWAYS cut to the same length no matter what.

All hydraulic hose is used at a constant temperature, no matter where in the world the product is used.

All hose manufactures know these secrets and make little electron people crazy, just for fun.

All rubber hose is tested for secret information on expansion in just one size, because it is forbidden to use different size hydraulic hose.

Jack Johnson invented hydraulics and his writings date back to Greek Times.

Quote – “And yes it blows my mind that either the manufacturers are fools and don't have this data or they think we are fools and can't use the data”.

I would never ever call my old electron friend a fool.

Quote – “I am sure that any top notch manufacturer of hose has had heard these questions before and has the answers. It is just a matter if they will share it. I bet they even have the formulas”.

Of course, we have them, but it would not be a secret if we shared them with you.

Peter, you make a motion controller, as does several manufactures, yours is not the Holy Grail of motion controllers, nor are we keeping secrets from you.

Buds quote – “It appears Hose not only EXPANDS but can change in LENGTH when pressurized”.

Wow, no kidding Bud, we have known this since the 1500 hundreds, talk about coming up to speed very fast on a complex subject.

quote:

Buds quote – “It appears Hose not only EXPANDS but can change in LENGTH when pressurized”.

Wow, no kidding Bud, we have known this since the 1500 hundreds, talk about coming up to speed very fast on a complex subject.

Gary: Now I know you can see the need for TRAINED/DEDICATED Fluid Power persons after only 5 Centuries of muddling along without me or others knowing about Hose installation situations that can cause problems on everyday designs much less Motion Control circuits.

Oh Well Someday Maybe??????????????????????

Hello All

I did not intend to stir up a hornet's nest with my question. With all the comments and discussion, I have not heard a definitive answer yet (although I believe most are in agreement).

Just to be clear, below is summary of what I believe is happening. I welcome any discussion, comment or information either pro or con

1) All materials experience some volume changes when subjected to pressure (Bulk Modulus)

2) Gravity and mass produce the high water pressures found at depth

3) These high water pressures are compared to surface pressure (i.e. 3000 psi means 3000 psi water pressure compared to surface air)

4) If a portion of this high water pressure is trapped in a container and brought to the surface, the pressure on the container walls will start at a zero differential pressure (at the depth it is captured at) and rise towards a pressure equal to the pressure diffenece found at depth compared to the surface.

5) Bulk Modulus and temperature changes both combine to try and increase the volume of the trapped fluid in the container as you ascend to the surface.

6)if the volume cannont change (perfectly rigid container wall), then the pressure in the fluid must rise

7)If the volume does increase (container wall flexes alittle), then the back tension from the stretched walls causes pressure increase in the fluid.

8) All the above point to the need for pressure relief circuits to be incorporated into any hydraulic circuit that has the potential to trap high pressure fluid in a container that must be recovered to the surface. Without these protection circuits, damage to the circuit can occur i.e. blown seals, burst hose, etc. In fact we have numerous pressure relief circuits in our manipulator arm joints that perform just these functions.

I welcome any constructive comments pro or con on the above points. Specifically is there anything wrong with the above statements?

Thanks in advance

quote:

I did not intend to stir up a hornet's nest with my question.

rovman;
This nest has been in a stirred up condition for almost as long as the Internet Fluid Power Forums have been up and running. Peter is a late comer though.

But, what would you expect from a discipline that gets all it's training from Fluid Power manufacturers or from the College of hard Knocks. Everyone has an opinion, based on personal experience, and you better not poke holes in any of it. There may be a lot of Cut and Dried information on Hydraulic and Pneumatic eqipment operation, but all the nebulus stuff is up for grabs and there are some wierd ideas on a lot of it. I know, I have some.

I would agree that any container that is empty and sent to depth in a body of water had better be strong enough not to be crushed. Then if that containner is opened and filled with that Pressurized, Cold Liquid, in relation to Atmospheric Pressure and Temperature, and allowed to return to Atmosphere, it better be strong enough to not split a seam as its exterior pressure decreases and its Liquid heats up and adds to the internal pressure. Of course your thought on some type of pressure relief system will take care of the potential problem also.

Pascal, Boyle and Charles wrote Laws on Pressure that all take effect in your scenario.

rovman : 7)if the volume does increase ,( container wall flexes little ) , then the back tensions from stretched walls causes pressure increasein the fluid .-------- I THINK IT IS WRONG . AS THE VOLUME INCREASES , THE PRESURE DECREASES DUE TO EXPANSION OF CONTAINER . SO CONTAINER WILL NOT CAUSE PRESSURE INCREASE IN FLUID .

Hello mbsboo

In an ideal situation where the container wall has no back force, perhaps. But in real world situations, the walls must produce a back force. Even ballons which streach easily, where the walls increase dramatically with pressure cause a back pressure on the trapped air (Just let a ballon go and see it fly around from the released pressure).

I am talking here about containers (from HP hose to rigid steel) that are designed to resist volume increases and closely contain high pressures. Even these are not perfect but in streaching alittle, the back pressure force does not just disappear. The elastic tension in the HP hose or SS tubing resists the attempted volume increase by causing pressure to increase. The failure mode of these hoses and tubes are not to blow up in volume like a ballon but to streach very little then fail. The pressure in these hoses and tubes rise quickly as very small amounts of fluid are forced in past the "fully filled" point.

One last point. Even if bulk modulus and thermal expansion are ignored (which they can't be). The trapped water brought up from depth would still causes forces on the tube / hose wall. (i.e. say we when to 20,000 feet and trapped the ~ 10000 psi water in the circuit. Ignore expansion from bulk modulus and thermal, you still would have a 10000 psi pressure difference trying to burst the hose when you reach the surface. If you now concider the real expansion from bulk modulus and thermal, the pressure would rise even more.
Any thoughts?

rovman