The accumulator sense, pump unload (ASPU) pilot valve monitors the pressure in an accumulator. When this pressure gets to the setting of the valve it shifts and unloads, diverts, or destrokes the pump. When the pressure drops to the level determined by the ratio of the valve, the valve resets, bringing the pump back on line and recharging the accumulator.
When the pressure at port 1 reaches the valve setting, port 2 connects to port 3 to vent a relief valve and unload the pump. This valve has a 15% ratio between unload setting and reset – when pressure at port 1 falls below 85% of the valve setting, port 2 is blocked from port 3 and the pump will come back online to recharge the accumulator.
The pressure differential between unload and reset is within about 1% of the stated ratio of the valve, with up to an additional 1,7 bar due to dynamic seal friction. The accumulator sensing area is positively sealed. Sun has designed a variety of standard accumulator/pump unload assemblies. Minimum clearances between the spool and sleeve and a seal on the pilot piston diameter significantly reduce the potential for silting. The spool design allows it to maintain a fixed differential ratio because the areas are created by diameters on the spool that will not wear or change with use.
A separate drain line is required to prevent erratic operation caused by tank line pressure fluctuations. It incorporates the Sun floating style construction to minimise the possibility of internal parts binding due to excessive installation torque or cavity/cartridge machining variations.
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