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Non-Invasive Accumulator Pre-Charge Pressure Check

budt (Industrial)
24 Nov 07 17:35
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Normally it takes a lot of time and equipment to check a hydraulic accumulators gas precharge pressure. Also using the normal way of opening the gas charge valve and reading Gas pressure at the Accumulators Gas Valve can introduce contamination that will allow gas leakage thereafter.

Two non-invasive way of checking precharge pressure very fast and with little or no added equipment is as follows:

Both methods require a KNOWN ACCURATE PRESSURE GAUGE at or near the Accumulator Outlet. A Glycerin Filled Gauge works but a Non-Filled gauge reads more quickly and gives closer results especially to the inexperienced eye.

These methods are not dead accurate but quickly indicate a trend of gas loss and can indicate when an accurate check is required.

1) Observe the Pressure Gauge closely when the hydraulic pump is first started. Since acircuit with an Accumuator normally has all flow blocked when not operating, pressoil at a higher pressure than the Gasure should be seen shortly after starting the pump. Since the accumulator has a Gas filled space there is a void that can be filled by pump oil but that void can only be filled by forcing the oil to a pressure higher than the Gas pressure. So, very soon after the pump starts the pressure gauge at the Accumulator will jump to the pressure of the Gas in the accumulator. After this pressure will continue to increase to system pressure. THE SUDDEN PRESSURE JUMP IS THE PRESSURE OF THE GAS IN THE ACCUMULATOR. The time it takes to build to system pressure depends on the size of the Pump (GPM Flow) and/or the size of the Accumulator or Accumulators.

The above is not real accurate but can be used to get a geneal idea. Following is the most accurate non-invasive way to check for a good Accumulator operation.

(2) When the circuit is running at pressure and the Pressure Gauge described above is in place, turn OFF the Pump and observe the Pressure Gauge. If the installation has an Automatic Drain, as it should, pressure at the gauge will start slowly to quickly dropping since the automatic drain will open at pump shutdown to drain the Stored Energy from the circuit. If an Automatic Drain valve is not installed then use the Manual Drain valve, if a Manual Drain valve is not installed, INSTALL ONE after opening some Bleed Point or manually cycling some actuators to relieve the Stored Energy.

As pressure continues to drop there will be a point when the Gauge suddenly drops to Zero. This pressure drop point is the Accumulators Pre-Charge Pressure and should be as noted on a tag attached to the Accumulator or on the Hydraulic Crcuit Schematic.

Their are three possible scenarios at pump shutdown:

The pressure gauge will act as above and Pre-Charge will be above, Below or very near Optimum Pressure

The Pressure Gauge will drop suddenly at Pump Shut down, indicating no Gas Charge since it was Pre-Charged to a pressure higher than system presuure and the oil cannot enter the chamber.

Or, it was never filled or it has a leak and has lost all Gas.

This second method gives a closer reading especially when using a Manual Drain since flow can be restricted and the point at which all oil is evacuated will not happen before you are ready.

Its's in the books.

Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

hydtools (Mechanical)
24 Nov 07 22:05
It seems to me that simply removing the charge valve cap, screwing on an accumulator tester, opening the charge valve with the gage's valve core actuator and reading the pressure gage is easier, faster, and does not introduce contamination.

It's done all the time on accumulator-assisted hydraulic tools.

By the book method you have to break into the hydraulic system to add a gage and a drain valve and associated fittings.  I would not use a non-filled gage permanently installed on a hydraulic system.  Pressure spikes will ruin the gage and an inexperienced tech may not notice and record an incorrect pressure reading.

Ted

budt (Industrial)
25 Nov 07 11:16
Ted;

You are right, "it is done all the time" using the Acumulator Charging Kit to test for Accumulator Pre-Charge pressure. However, a lot of things are done in diferent ways by different persons and are not necessarily right or wrong, just different.

I did not offer the tip as the only way just another way and possibly a simpler fast check that could be done at any system shutdown.

I have used the shutdown method numerous times to quickly determine an Accumulators Pre-Charge is ot as shown on the schematic or an Accumulator has lost all charge pressure and probably has a Bladder that needs replaced.

One instance I only checked line temperature to the Accumulator and compared it to the Accumulators temperature to determine it was not receiving any oil since its Bladder was shot and all Gas was gone. The tempeature check can sometime show how much oil is in the Accumulator by checking temperature up the length of it.

On a bank of (6} 20 Gallon Accumulators, four were room temperature while the other two had warm oil up the side some distance. Replacing the cold ones got the machine cycle back to speec and at tear down the cold Accumulators all had damaged Bladders.

Speaking of Gauges though, all Pressure Gauges should have a "Push to Read valve" installed, that drains the Gauge to Tank when deactivated, to keep it from operating continually and damaging the sensitive mechanism theat allows them to accurately read system pressure.

On top of that, any well designed circut should have "Pressure Test Ports" liberally placed thoroughout the circuit so pressure can be checked at strategic points without invading the circuit and having the advantage of using a "Test Gauge" that is regularly tested for accuracy.

Where do you put these Pressure Test Ports? IIt's up to the circuit designer but I have never found a circuit with enough of them. Electricians don't have a problem with test points sine flow and pressure can be read without invading the wiring.

The "Pressure Test Ports" should have a screw on cover that seals the port until it is needed so it is clean and ready for use for Trouble Shooting. I know from my experience that these Test Ports are a great way to reduce downtime when a circuit stops working as planned.

Then there is often the need to knw flow when Trouble Shooting. Flow Meters are much more time consuming to install and add a lot of cost if installed at strategic oints originally. I still specify one at all Pump outlets and lately have used a Check Valve and a Three-Way Ball Valve so the Flow Meter can be put in a side stream arrangement. Like a Gauge Isolator, the Flow Meter only sees oil flow when activated during Trouble Shooting. This means it is ready for action an accurate any time it is necessary.

Maybe others have Tips on circuit design that would make Trouble Shooting simpler and faster.

Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

hydtools (Mechanical)
25 Nov 07 17:47
I agree it is another way of doing the test.  Checking temperatures is an interesting way to quickly isolate problem accumulators.

I also agree that test ports should be used in designing circuits.

Having a flow meter makes sense, too.  I have used a flow and pressure tester many times to check the health of a hydraulic system.

Ted

hydromech (Mechanical)
26 Nov 07 3:48
Ted...Bud,

Just a couple of comments, based on my experience.

Pressure test points are always a very useful tool when trying to troubleshoot sytems. The metric ones we use in Europe also fit with condition monitors and flow meters, so it makes it very easy to draw fluid from working systems to get a good idea of the cleanliness or to check pump performance.

Having said that, I found it very difficult to get customers to accept them. Most people just saw them as optional extras that should be removed to make the system price more competitive.

Also, the comment about checking the temperature of an accumulator to get a rough idea of the pre-charge???

Obviously a stone cold accumulator is clear indication the bladder has burst, however, as bladders shrink into the centre of the accumualator shell, I cannot see how checking along the length of the accumulator shell will give an accurate indication of the precharge. The bladders will contract uniformly along its entire length. In my experience, the temperature change along the length of an acc' has nothing to do with the pre-charge pressure.

What you said is true with a piston accumulator to some extent as the piston will move upwards as the pressure rises. But to roughly calculate the pre-charge pressure, one would need to know the pre-charge volume and correct working pressure and the system pressure.

Again, in my experience, the only way to reliably and safely measure nitrogen pre-charge pressure is to use a pre-charge test gauge. And only when the system is off and there is no hydraulic pressure in the accumulator.

I am not questioning anyone's methods, I am just offering my experience to give some balance.

Regards

Adrian

 
budt (Industrial)
26 Nov 07 8:55
Adrian wrote:
"Again, in my experience, the only way to reliably and safely measure nitrogen pre-charge pressure is to use a pre-charge test gauge. And only when the system is off and there is no hydraulic pressure in the accumulator."

I totlly agree Adrian, that is why I qualified my post with this statement

"These methods are not dead accurate but quickly indicate a trend of gas loss and can indicate when an accurate check is required."

Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

kcj (Mechanical)
26 Nov 07 9:38
If I understand correctly, using temperature is not to check for level of precharge, but to check for total loss of precharge? Makes good sense.

The biggest advantges I see of these methods is to train the operators to watch for those signs, without requiring any extra testing or equipment or time.

Prior to plc control, we used to have the operators watch the cab gauges upon shutdown for exactly that situation, the sudden drop of pressure on shutoff and bleed down. Took only a couple seconds, and gave them daily monitoring. Every couple months they would check with gas gauge, but that bleeds of a small amount of N@ each time. No one would do that daily, and eventually they have to call out a service truckwith N2 bottles. (mobile equipment)   But by watching the normal, daily signature as it shut down, problems were averted.  Operators would do it, because if loss of accumlator, the sequencing of picking things up of road crossings was affected. With the million pounds moving at 8 mph, if the carriages come up slow and snag a road crossing, they are history. So the Ops would rather check the gauge each time than spend a shift replacing carriages.

kcj

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