A 200 bar press circuit kept spiking at 260 bar every time the main cylinder hit the workpiece. The design called for a 20 litre bladder accumulator to absorb the shock. It was installed, precharged to 90 bar, and the pressure spikes were worse than before. The engineer re-read the precharge instructions, set it again to 90 bar, and the spikes stayed. The problem wasn’t the accumulator. It was the precharge being checked while the system was running. The gauge showed 90 bar on a system that was sitting at 200 bar, which compresses the bladder and reads the gas pressure, not the true precharge. The accumulator was effectively dead.
The three jobs, and why each needs a different number
An accumulator does three things, and the precharge logic differs for each.
Energy storage. The accumulator stores oil during the pump-on phase and releases it during peak demand. Typical use: a press that needs high flow for 5 seconds every 30 seconds. The pump can be sized for average flow instead of peak. Precharge for this job: 90% of the minimum system pressure.
Shock absorption. The accumulator absorbs pressure spikes when a valve slams shut or a cylinder stops against a hard stop. Precharge: 60-70% of the system pressure. If the precharge is too high, the bladder can’t compress, and the accumulator passes the shock straight through.
Leakage compensation. A small accumulator (0.5-2 litre) holds a circuit at pressure while valves and cylinders leak. Precharge: about 90% of holding pressure, and the volume is small because the leakage is small.
The failed press accumulator was doing shock absorption with a 90 bar precharge on a 200 bar system. That’s an energy-storage precharge on a shock-absorption job. The bladder was already bottomed out when the spike arrived.
The gas law and the polytropic exponent
The gas in the accumulator follows pV^n = constant. n is 1.0 for slow, isothermal compression (energy storage, where the gas has time to exchange heat) and about 1.4 for fast, adiabatic compression (shock absorption, where the event lasts milliseconds).
For the 20 litre accumulator at 200 bar with a 70 bar precharge, using n = 1.4 for the shock case:
Effective gas volume at 200 bar: V1 = 20 x (70/200)^(1/1.4) = 20 x 0.46 = 9.2 litres.
If a shock pushes the pressure to 260 bar, the gas compresses to V2 = 20 x (70/260)^(1/1.4) = 20 x 0.38 = 7.6 litres. The accumulator absorbed 1.6 litres of oil. If the shock needs 3 litres of absorption, the 20 litre accumulator is undersized for this precharge.
With a 90 bar precharge: V1 = 20 x (90/200)^(1/1.4) = 20 x 0.53 = 10.6 litres. At 260 bar, V2 = 20 x (90/260)^(1/1.4) = 8.8 litres. Only 1.8 litres absorbed. Worse.
The fix: precharge to 65 bar, and the accumulator absorbs about 3 litres through the spike range. The spikes dropped to 215 bar.
How to check precharge without lying to yourself
Isolate the accumulator with the shut-off valve. Bleed the oil side to atmosphere. Then check the gas pressure with the charging kit. The gas pressure on a fully empty oil side equals the precharge. Check it while the system is running and you’re measuring gas pressure plus oil pressure, which is a useless number.
Do this quarterly. The bladder loses gas through the valve and through permeation at about 1-2 bar per year. A 90 bar precharge that has drifted to 80 bar changes the absorbed volume by 20%. Nobody sees it until the spike happens.
Bladder, piston, diaphragm
Bladder type is the standard for energy storage and shock absorption up to about 50 litres. Piston type handles bigger volumes and higher pressures, but the piston seal leaks a little and it reacts slower to fast transients. Diaphragm type is small and cheap for leakage compensation.
For a shock absorption duty below 20 litres, bladder is right. Above that, look at piston. The selection isn’t exotic. It’s about volume, pressure, and response speed.
The charging kit you need
The $50 charging kit (gauge, hose, and a Schrader-style connector) pays for itself the first time it finds a bladder that has lost 10 bar. The press circuit stopped spiking, the pump didn’t cycle as often, and the maintenance log now has a quarterly precharge column.
Precharge rules: 90% of minimum pressure for energy storage, 60-70% for shock absorption, 90% of holding pressure for leakage compensation. Check the precharge with the oil side bled to atmosphere, and check it quarterly. The 260 bar spikes weren’t a bad accumulator. They were a 90 bar precharge doing a 65 bar job.
