- Always arrange some method to drain the accumulator at shut down. (At the end of this section, several ways to drain an accumulator automatically are shown. Plus, there is always the old standby, a manual drain.) Never work on a circuit with an accumulator until you are sure it is depressurized.
- Make sure accumulator flow is restricted to a reasonable rate during operation and shut down to avoid damage to the machine or piping. Accumulators will discharge fluid at any rate the exit flow path will allow. Such high flow does not last long, but the damage it causes is done quickly.
- Always isolate the pump from the accumulator with a check valve so fluid cannot back flow into the pump. Without a check valve, accumulator back flow can drive the pump backward -- and overspeed it to destruction in some instances.
- Check the accumulator’s pre-charge pressure at installation and at least once a day for the first week of operation. If there is no noticeable loss of pressure during this time, do the next check a week later. If all is well then, do a routine check every three to six months thereafter. Whenever the accumulator pre-charge drops below nominal pressure, the volume of available fluid is reduced and finally the cycle slows.
One way to check accumulator pre-charge is to turn off the pump, allow the accumulator to empty all oil back to tank, and then connect the items in a charge kit, Figure 16-6. First remove the gas-valve cap and install the charge kit gauge, hose, and tee-handle assembly on the gas valve. Next, turn the tee handle in to open the valve and read gauge pressure. However, every time this operation is performed there is the chance the valve will not reseat and gas will start to leak.
To avoid potential gas leakage, Figure 16-7 illustrates two noninvasive methods to check pre-charge. Both are fast, simple, and can be done almost anytime without a lengthy interruption of production. Either of these ways gives a fast reasonably close check without invading any plumbing. They are not 100% accurate, but will be within ±5% of the gauge reading -- with almost anyone doing them. The method on the left is the least accurate -- especially when using a glycerin-filled gauge.
The Pump Just Starting method on the left shows a jump in pressure after the pump starts then a steady climb to set pressure. This first jump is the pre-charge pressure and the steady climb is during compression of the gas in the bladder or behind the piston. The length of time between the first pressure jump and reaching system pressure depends on the volume of the accumulator and the pump output.
The Pump Shutoff From Full Pressure method is easiest and most accurate, especially if the accumulator dump valve is manually operated. Fluid can be bled off slowly with a manual dump so the gauge reaches pre-charge pressure slowly.
With this method the system must be at pressure and the accumulator charged at least above pre-charge pressure. At system shut down either an automatic or manual drain is opened and pressure starts to fall. Because the gauge is reading oil pressure and the only reason there is pressure is because of trapped gas above it, pressure will fall to a point then suddenly drop to zero. Read the pressure as the gauge suddenly drops to zero to determine gas pre-charge.
This method is the most accurate but is not precise like a gauge reading, so use it for a cursory check as often as necessary to see if the gas charge is holding.
Accumulator pre-charge pressure
Normally, gas-charged accumulators are pre-charged to approximately 85% of the system’s minimum working pressure. This assures that the bladder or piston does not discharge all the fluid during every cycle. If all fluid is evacuated at high rates, bladders can get caught in the poppet valves and pistons can be deformed when metal hits metal.
In certain applications, this 85% figure may be low because minimum system pressure is low. In such a case, use a piston-type accumulator because the piston can move up the bore almost any distance without damage. A bladder accumulator should not be used when pre-charge pressure is less than half the maximum pressure. This avoids compressing the bladder so tightly that rubbing action on itself wears holes in it.
Many applications can use any type accumulator with equally satisfactory results. However, there are some cases where one particular style is more responsive or offers a longer service life. As mentioned in the previous section, the amount of pre-charge pressure is one reason for selecting a bladder or piston accumulator.
Weight-loaded accumulators respond to pressure buildup slowly so they do not work well as shock absorbers. Weight-loaded accumulators will reduce but not stop pressure spikes. Piston accumulators are not as fast as bladder types at responding to fast increases to pressure. So in these situations, the best choice is a bladder-type accumulator.
Some accumulator circuits are installed to dampen high-pressure spikes at the outlet of piston pumps. A piston accumulator in this application cannot respond quickly enough to do the job. Also, the short stroking distance of the piston and seals can cause excessive wear to the bore and seals. A bladder accumulator works best in this type circuit.
Most accumulator suppliers offer information in their literature about sizing accumulators for any of the above circuits. Many offer computer programs that only require the input of system requirements. The program then figures accumulator size and outputs a part number. One company offers a formula and software for use on the Internet.