Health of a Hydraulic Accumulator 1

[Haycord]

Is there a way to determine the condition of a bladder in a Hydraulic Accumulator without taking it off-line? If not, how would you know if it were ruptured until the Accumulator is needed... and it's "not there"? Thanks

 

[Montemayor]

Haycord:

Have you looked at your hydraulic accumulator recently? It normally should have a Nitrogen pressure gauge monitoring the gas side of the bladder. This pressure will indicate if you have any inert gas on that side in order to "absorb" the pressure shocks - or if you've lost that gas due to leaks or ruptured bladder (diaphragm).

Art Montemayor
Spring, TX

 

[aviat]

Further to what Art said, if the accumulator is charged and with syetem down you slowly open the charge valve and let air out. If you get hydraulic fluid mixed with the air,coming out of the valve, it is usually an indication of a ruptured diagram. You will get some fluid mixed with air depending on size of accumulator but it should stop and only air should come out.

When the accumulator looses its charge or ruptures the pressure usually starts to fluctuate, unless it is installed as a standby unit.

 

[Haycord]

Thanks, for the replies, but please help me understand a little better.

If the bladder is ruptured the gauge will still read the "process pressure" correct? Thereby leaving the impression all is okay.

If it does rupture, the nitrogen charge will eventually "work its way out" as its absorbed or entrained in the process fluid, correct? Leaving the Accumulator full of fluid.

 

[kcj]

Bleeding off some nitrogen may not tell you anything, as the entrained oil may all be in the bottom of the bag. N2 will come still come out.

A quick method to check the N2 precharge is to watch the oil side pressure when the system is shut down. It will slowly decrease as the oil is bled off, until the precharge setting is reached. At that point, as the last drop of oil comes out of the accumulator, the bag hits the steel, and the oil side gauge will drop abruptly to zero.

This assumes you can somehow remove the system load, gravity loads, etc from devices held up by the hydraulic system. Otherwise, the pressure will drop to that of the heaviest static load being held up by anything still connected to system. It will hold that pressure as the load slowly lowers, then the pressure will start decresing again.

On systems that keep the accumualtor on line, but slowly bleed it off, you will have to know what loads are on, or possibly cap off the system and just have the accumulator and pump and bleed valve in the circuit.

On systems that take the accumulator totally off line at shutdown, then bleed it down separtely to tank, it is very easy to see this abrupt drop.

In either case:
1. Get a gauge right near the accumulator, before any bleed down or safety valves.
2. Power up the system. Oil pressure on this new gauge should be equal to system pressure.
3. Shut down the pump. Accum is now bleeding off to tank.
4. If oil pressure drops immediately, the N2 is all gone, the accumulator is a big steel tank full of oil.
5. If pressure decays slowly, but abruptly drops to zero at some value, say 900 psi, then that value is the N2 precharge pressure.

This is a quick field test, not to be relied upon as an exact pressure. i.e. if reading 900, I could actually have 750 or 1200 psi, but at least I know there is N2, and approximately its precharge value.

We try to get the operator to keep an eye on the system preessure gauge each time it shuts down, to catch problems early.

kcj

 

[budt]

I have diagnosed damaged accumulators by checking the temperature of the system oil and comparing it to the temperature of the accumulator. A good accumulator will be cooler where the gas is, usually the upper 1/3 to 1/2 of the container. Bad accumulators show almost the same temperatur from inlet to gas valve. Find the temperatures on a newly installed accumulator and use these findings to compare to later readings.

kcj gave a fast way to check and it even works on running circuits if there is and isolation valve at the accumulator inlet. After discharging the oil make sure to upen the isolation valve slowly until the accumulator gets up to system pressure so there will not be a big pressure drop on refilling.

I have a demonstration of this fast, non-invasive pre-charge check on my web site. It is a Power Point presentation and though it runs slow on the web it runs like a movie from a hard drive.

http://www.fluidpower1.us

Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[hydrae]

Haycord

A method that I have used (from the surge tank vendor) is the installation of load cells on the tank, the tank will need expansion joints on the piping and two load cells (on a four legged tank). With the bladder in a known good condition, operate the system through its entire range and note normal operating weights, if the bladder fails, the tank will get heavier as the gas is replaced by the liquid this increase in weight will be gradual as the assumed failure mode of the bladder is a pin hole or small tear. Load cell monitoring can be done in any number of methods, electronic, connection to SCADA, or simple dial indicators with markings of normal operation. The load cell will also tell if there is too much precharge, in that the weight will not change as much as normal operation.

Hydrae

 

[13392017]

Bravo kcj,
You make very good explanation on determining accumulator condition.