Can flexible hydraulic lines handle external pressure?

[Aconcagua]

I am considering a potential two way hydraulic system for a subsea environment. I should be able to take advantage of the high ambient pressure to drive in either direction, though my flexible hydraulic line must be able to handle external pressure for this to work. I am not experienced designing hydraulic systems. I understand that the reinforcement of the lines is done via wrapping wire around it. This doesn't seem ideal for handling compression, but maybe it can take a little before it collapses. Is this usually something a metric that is tested for in a line? Are there specific types of lines for this? Is there a general percentage of burst pressure I could assume? Or would I just need to test?

Thank you!

 

[3DDave]

Rather than "subsea" is there a particular ambient pressure you intend this system to work with?

 

[IRstuff]

You say "hydraulic," so presumably, not air inside the lines. If it's actually fluid, the fluid, wouldn't if be at about the same pressure?

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[LittleInch]

This sounds like a good idea, but soon runs into problems.

first, as you say, you need to have a lower pressure on the other side of the piston.

That's fine for one operation, but then how do you remove / reduce the pressure from the sea to allow the piston to reverse direction. You then start to need extra valves, pumps,tanks etc etc.

Just do what everyone else does and use a pressurised HPU or supply line. Many subsea hydraulic systems use fluid which can be safely flushed into the sea.

You need to draw a diagram and figure this out.

But flexible lines HATE negative pressure. You would need to pipe this in hard piping which can withstand high differential pressures.

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Also: If you get a response it's polite to respond to it.

 

[Aconcagua]

Thanks everyone for the replies!

Yes, it is an oil filled line. I can use electrical actuation to generate negative relative pressure in my line. Maybe a supply line is the answer, I was just hoping to avoid extra lines.

 

[Aconcagua]

As far as pressure I can work with, it will be generally balanced with ambient pressure (up to 9000 psi). I assume there will be a limit to the relative pressure I can generate and still flow through the line. I am also assuming it will have little to do with absolute pressure.

 

[BEMPE16524]

have been working with subsea ROV hydraulic system using flexible hose and it is totally not a problem.
you have to make sure that the system has a pressure compensator in order to compensate any lose of hydraulic in a particular time so that you can make a safe recovery to the system (i'm talking about my application).
The material must also capable to operate in such condition.
Most of the time, we will be using a wire braided hose to withstand the pressure.
Take a look at the bend limit of the hose to fit your application while choosing an option.
i'm not sure how big is hydraulic hose requirement. but looking at 9000 psi as max, probably you're only working at 3000 psi?


R.Efendy

 

[Compositepro]

I would expect that a sub-sea hydraulic system has a fluid reservoir that is pressure balanced with the ambient pressure. In which case the flexible lines do not see the external pressure. Is this the case?

 

[LittleInch]

"I can use electrical actuation to generate negative relative pressure in my line."

What do you mean exactly?

In theory you could have a single liquid filled line which you presurise to move it one way and then pump out the fluid in a central tank of some sort to reduce the pressure and create a pressure difference.

The issue being that is quite difficult and the differential pressure you can pull through a pump might be one or two orders of magnitude less than what you can create by pressurisation so the forces in the return direction are low.

but flexibles would be your limiting factor I think for how much negative pressure you could withstand.

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

 

[BEMPE16524]

I would expect that a sub-sea hydraulic system has a fluid reservoir that is pressure balanced with the ambient pressure. In which case the flexible lines do not see the external pressure. Is this the case?

you are right. in many cases, the system is normally tested and run on the surface first. remove all air from the system through a vent valve and you are ready to go.
in most condition, the oil is an incompressible fluid, so we are not so worry about the external pressure.
the compensator (reservoir with additional oil) is meant to provide a temporary fill-up to the system in case of leakage until the system is recovered to the surface.
for ROV system, if there is any major leakage, the system is design to allow the operation using seawater as a mean to recover the ROV to the surface only.
after recovery, fix all the leakage and the whole system need to be flushed with fresh water and hydraulic oil until the whole system is clean from salt and debris.

R.Efendy

 

[Compositepro]

I would have thought some flexible diaphragm would be used at the reservoir.

 

[BEMPE16524]

I would have thought some flexible diaphragm would be used at the reservoir.

that is true as well.
there are two common type that we use. first is a fabric type (first photo), second is rubber bellow (the one that is normally used on a truck as suspension unit).

R.Efendy

 

[Compositepro]

Thanks, BEMPE, that is good to know. I have used the Bellofram rolling diaphragm air cylinders where piston friction is a problem. They are a very good product.

 

[akkamaan]

Whether the hoses/tubing can resist (collapsing) the external pressure or not is a matter of the TOTAL pressure differential in the ENTIRE system.
The total collapsing force is proportional to the diameter of the hose. So a larger diameter hose is more likely to collapse at a certain external pressure than a smaller diameter hose.
So with a hose/tube in deep water with an above water delivery system, there will be external pressure, approx 1 bar per 10 meters (33 ft) of depth relative to the atmospheric pressure. But if the reservoir pressure is the same this can't be an issue, which means the entire system is subsea and for that, a bladder reservoir has to be used.

This is a good question because we have the same problem withing the regular systems above the seawater.
The suction hoses are designed to specifically resist external (atmospheric) pressure to prevent collapsing.

 

[Compositepro]

If you have a surface supply of hydraulic fluid the pressure will increase with depth both inside and outside the hose, depending on fluid density. However, I cannot think of too many cases where this would be done.

 

[akkamaan]

If there is surface supply of hydraulic power and a net pressure of 100 bar is needed on 200 meters depth, the absolut at 200 m must be 100+20 bar=120 bar. But the above surface pump pressure just need to be water density/oil density higher than 100 bar ie approx 1029/860×100= 120 bar

 

[Compositepro]

Another option is to use a water based hydraulic fluid so the density is close to water. An even greater reason to go water based is environmental concerns about leaks.

 

[akkamaan]

Agree 100%, my example just wanted to bring that the fluid density is a part of the "deep sea" equation...