Pressure Spike in a Hydraulic System

[BEMPE16524]

Hi Guys,

Just wonder if anyone ever encountered same problem as mine.
I have a system running in sea water (1400 meter depth). It is actually an ROV system.
The system pressure is 2600 psi and to run 7 propellers.
We encountered hose damage several times where in fact the hose rating pressure is 3500 psi (max. test pressure normally 3 to 4 times).
Normally it was a pin hole damage. After changing the supplier for few times, it still happened.
The last thought that i have is a pressure spike that could probably cause by a trapped air/bubble? But how big can a pressure spike be (considering the max test pressure of the hose is really high)?
Any idea or suggestion?

 

[gruntguru]

If you have 1400+ metres of hose and the flow is suddenly reduced at the downstream end, you will experience water hammer which can produce extreme pressure spikes. If your failures are occurring near the downstream end, this is the likely cause. The solution is either a relief valve or an accumulator at the downstream end.

je suis charlie

 

[BEMPE16524]

GG,

the whole hydraulic system is operating in subsea. It was a remotely operated underwater vehicle (ROV).
The hoses lenght is about 1 to 2 mtrs from the valve pack.

 

[gruntguru]

"The hoses lenght is about 1 to 2 mtrs from the valve pack."

So all control is performed with valves at the surface end of the high pressure line(s)? If so, I assume you have multiple pressure lines - how many? Is the failure always the same line? Only one return line?

je suis charlie

 

[BEMPE16524]

GG,

Control is done from surface. All valves, HPU (in subsea) are pilot operated. We have one pressure coming out from the pump and divided into 2 valve packs (both have their own return line to same tank).
One valve pack has 3 servo valve, and the other one has 4 servo valve.
Failure occurred at different pressure line. I mean it happened after the servo valve to the propeller/thruster hydraulic motor.

 

[hydtools]

Is the failure at the same place along the length of the hose? Do the vendors make the hose assemblies or are they assembled in-house? What is the hose structure or specification? Do the hoses need to flex?

Ted

 

[BEMPE16524]

HT,

Most of the time failure occurred near the end crimp fitting.
Hose assembly were done in our workshop before the system was sent to offshore.
When failure happened, the changing was done by offshore crew.
Yes the hose need to be flex and our hose is a high rated flexible hose.
Initially we used 1/2" PIRTEK CLASS 35 C35-08 Series 3, 5000psi hose (after our 3500psi rated hoses always burst).
By changing to this type, we managed to reduce the burst frequency.

Now we change all hoses (total 14 hoses) to 1/2" Transfer Oil Offshore Master 5K 5000psi hose.
We haven't had chance to test it in offshore environment yet. Just praying no major failure again.

 

[gruntguru]

It shouldn't be an issue, but at 1400M depth the "ambient" pressure is over 2,000 psi so your hose is effectively cycling between -2000 and +600 psi. If the hydraulics are also operated with the ROV at the surface the range is of course 0 to 2600 psi.

je suis charlie

 

[BEMPE16524]

GG,

Yes, the HPU will start before the ROV dive. So, it already have 2600psi pressure before diving. And normally all thrusters are tested before they jump into the sea.

 

[gruntguru]

So the hose cycles between -2000 and +2600 psi.

je suis charlie

 

[HPost]

Hoses are not designed to withstand large external pressures. That is not to say that they wont work, but the level of performance will suffer greatly.

All hydraulic system have some sort of inherent ripple and on land, where a system running at 25mPa with a +/- 1mPa ripple for example, the external pressure acting on the hose will only ever be 0.1mPa.

At 1400m, the external pressure on the hose will be 14mPa. With a constant external pressure of 14mPa and a fluctuating internal pressure of 17.9mPa, that will likely dip below the 14mPa, the hose will be cycled through its neutral axis and will wear out.

In effect, where the hose is designed to return to a neutral state, it's actually coming under external stress and so the cyclic loading is doubled. The winding of the braid or wires in the hose are exposed to excess strain.

I have see low pressure hose that was exposed to a very slight vacuum caused by a hydraulic hammer return flow. That failed very quickly and the subject matter expert confirmed that hoses don't like external pressure. So, following that logic, applying 14mPa will most likely be the cause of this repeated failure.

 

[BEMPE16524]

HP,

Are you saying that even the pressure line is constantly filled with oil, that doesn't help at all?

 

[hydtools]

Is the hydraulic system not pressure compensated such that it operates at and above ambient pressure? So there would be no high external pressure collapsing the hose at operating depth.

Ted

 

[BEMPE16524]

Ted,

It is pressure compensated.

 

[hydtools]

Perhaps in flexing the hose is also twisting. A swivel fitting would solve the problem.

Depending on the structure of the hose reinforcement layers, the hose will shorten when pressurized. This would tend to pull the hose out of the fitting. There should be excess hose length to prevent this.

Ted

 

[Compositepro]

Is the hydraulic power unit on the surface or on the ROV? Minus 2000 psi on the hose will collapse the hose and crimp the hose wall, causing it to fail. The oil reservoir needs to be at the ambient pressure of the ROV (use a bladder tank).

 

[BEMPE16524]

No twist on the hose because it's an A Lok fitting, so fastening can be controlled.
HPU is on the ROV. The tank is also pressure compensated.

The external pressure is 14MPa (at 1400mtr) and the internal pressure is 18MPa (2600psi). I think that is still ok, additionally it is an incompressible fluid? Correct me if i'm wrong.
The only cause that is possible now is the air/bubble. So, should i add a releive valve at each pressure line?

Or is there any other options?