How to Charge Accumulators with Nitrogen

By Mike Carney

Dry nitrogen is used to precharge accumulators for several reasons:

1. It is an inert gas. This means it will not react to external conditions such as heat and compression or pressurization. It also does not react readily with other chemicals.
2. Although any inert gas could be used, nitrogen is the cheapest because it is the most readily available. 78% of the earth’s atmosphere is nitrogen, 21% oxygen, and 1% argon and other gases.
3. Never use oxygen or compressed air to precharge an accumulator! As the oxygen is compressed it heats up and can cause a fire or explosion when mixed with the hydraulic oil.

Different manufacturers and styles of accumulator require different gauging/charging assemblies. Before beginning, be sure the style of accumulator and matches the charging assemblies and that they are intended to work together.

 

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Pre-Charge Procedure

1. Install the hose end of the gauging/charging assembly onto the nitrogen gas bottle.
2. Verify the gas chuck is backed all the way out on the gauging assembly.
3. Make sure the bleed valve is closed.
4. Once steps #2 and #3 are verified, begin installing the gas chuck onto the gas valve.
5. Turn the gas chuck in until pressure is indicated on the gauge. Do not turn the gas chuck all the way in, as this will damage the gas valve.
6. Slowly crack open the nitrogen bottle valve; let the accumulator slowly fill until the gauge displays the desired precharge pressure.
7. Shut off the valve on the nitrogen bottle.
8. Let the precharge set for 10 to 15 minutes. If after this time, the precharge is too high, slowly open the bleed valve until desired pressure is reached. Then close the bleed valve.
9. Do NOT reduce precharge pressure by depressing the gas valve core with a foreign object (such as a screwdriver) as this may damage the valve seat. Once the desired pressure is reached, screw the gas chuck out all the way.
10. Open the bleed valve to relieve any pressure trapped in the assembly.

Accumulators should be precharged slowly, as indicated in step #6. This is especially important when filling a bladder style accumulator. Below is a sequence of events outlining a common failure that will occur when a bladder style accumulator is charged too quickly:

• The nitrogen will travel quickly and hit the furthest end of the bladder, extending the bladder longitudinally to the full length of the shell.
• Meanwhile the rest of the bladder expands diametrically and fills out the rest of the shell.
• As the bottom of the bladder reaches the poppet valve, it goes around the poppet valve trying to extrude from the port intact and whole.
• By now, the rest of the bladder is also pushing down on the poppet valve which causes the portion of the bladder that is trying to extrude to get pinched.
• Once pinched, the nitrogen in the bladder escapes rapidly causing either a star burst or a 3/4 circle "C" cut on the bottom of the bladder.

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For more information, or for assistance with your accumulator application, contact the Certified Fluid Power Specialists at Quality Hydraulics & Pneumatics, Inc.