EN 14359 Gas-Loaded Accumulators — Scope, and Where Gas-Law Sizing Fits

A lot of accumulator calculators use the ideal gas law. Between 90 and 400 bar, nitrogen doesn't behave that way. It's stiffer.

The ideal law overstates the oil you get by 4 to 58 percent, and it's on the unsafe side every time.

This is the MechanixCalc accumulator calculator. It sizes bladder, piston, diaphragm and bellows accumulators on real nitrogen, and shows every check with its source. So let's size one properly, and see what changes.

Here's the job. A press needs 10 litres of oil, between 150 and 250 bar. The shop runs from 10 °C in winter to 50 °C beside the power pack in summer.

A bladder accumulator, nitrile bladder, hydraulic oil. The usual choice.

First question. Where does the precharge go? For energy storage, the makers' rule is no more than 90 % of the minimum pressure, at the hottest temperature. Here, 135 bar at 50 °C.

Why the hot end? Gas pressure rises with temperature. Set it on a cool day, and by summer the precharge is above your minimum pressure. Then the bladder closes on the poppet at the bottom of every stroke.

Nobody fills an accumulator at 50 °C, though. So the tool gives you the workshop number. At 20 °C, with the oil side drained, fill to 119.4 bar. And there's a gauge table for whatever day you check it.

The other side of that coin. On a 10 °C morning the same charge reads 114 bar, and the accumulator gives less oil. So the size is taken at the cold end.

Next question. How fast does it run?

If the oil came and went slowly, the gas would stay at room temperature. That's the slow, isothermal case. 36.5 litres of gas.

Cycle it fast, back to back, and the gas heats on the way in and cools on the way out. It's a stiffer spring. 62 litres.

But a press does neither. It charges, then sits there, and the gas settles back to room temperature. Then the cylinder takes the oil in one fast stroke, and the gas expands and chills. 72 litres.

Same machine. Same 10 litres. 36 or 72, just from how fast it runs. That's double the vessel.

If you know your real cycle times, the timed model sits in between. About 65 litres with these times. When in doubt, take slow charge, fast discharge. It's the conservative one.

And for comparison, the catalogue short-cut, a fast compression straight from the precharge, says 48 litres. For a press that sits charged and fires, that's a third too small.

Now the checks. Precharge, fine. Pressure ratio, 2.2 to 1 against a limit of 4. Fine.

But one says CHECK.

After the press has sat charged, a fast discharge drops the gas to −28 °C. A standard nitrile bladder is good to −15 °C.

The bladder wall won't follow the gas all the way down. But do that every stroke, and nitrile goes brittle and cracks. And it's the expansion that does it, not the weather.

The tool gives you the ways out. A lower pressure ratio, a slower discharge, or a low-temperature compound. And it says to confirm with the maker. Which is what you'd do anyway.

So let's take one. Low-temperature nitrile, good to −35 °C.

And the check clears.

72 litres of gas. So you order an 80-litre accumulator. And you'd be short.

The number on the nameplate isn't the gas volume. The bladder and the poppet take up room, and a datasheet can list as little as 85 % of it as gas. 80 litres can mean as little as 68 litres of gas. So the tool recommends the next size up, 100. And the datasheet's effective gas volume is the number that counts.

Which brings up the question you get on site. There's an 80-litre unit already on the shelf. Will it do?

If this press cycled back to back, yes. 11 litres.

But it sits charged and fires. Taking 85 % as gas, it gives 9.4. About half a litre short at the cold end. Put in the datasheet's real gas volume before you write it off.

The 100-litre unit gives 11.8. With the low-temperature bladder, that's the one to buy.

Before you sign it off, the safety step. On the gas side this is a PED category IV vessel. The test pressure has to be at least 393 bar.

Now a trap. Pressurise it on a cool morning, valve it off, and let the plant warm up to 50 °C. The trapped gas and oil climb to about 319 bar. The system relief can't see it, it's on the other side of the valve. And this vessel is set up for 275.

So it sits on a safety block. An isolating valve, a relief between that valve and the accumulator, and an unloading valve that dumps the oil at shut-down.

Through a 1.5 mm bleed orifice, that takes about four minutes. Then the oil side reads zero. The gas side doesn't. It still holds its precharge. Never take the accumulator off until the nitrogen is let down through the charging kit.

It also covers who inspects it and how often, and the nitrogen bottles to charge it.

Working in psi and gallons? One click, and every number follows.

All of it goes into the PDF. The method, every check with its source, the fill table, and that hot-day FAIL, in red, until you deal with it.

What it doesn't do: it doesn't design the shell. That's the vessel maker's job, under the construction standard, and the tool says so.

Real nitrogen. The right cycle. The cold end. The nameplate read properly. And the cold-gas check and the hot-day trap dealt with. Those are numbers you can put your name to. Size your next accumulator on MechanixCalc.com.