What is the pre-charge pressure of the vessel in an accumulator?

The pre-charge pressure is the air pressure stored within the bladder of the vessel. The air in the vessel is compressed by the water pressure, which pushes the extra volume of water out of the vessel. AccuBoost accumulators require that the pre-charge pressure is 1.5 bar less than the pressure of the incoming mains.

In pumped accumulator installations, the pump boosts pressure of the incoming mains to 3.0 bar and the pre-charge pressure of the vessel is set to 1.5 bar when it leaves our factory, creating a difference of 1.5 bar.

In unpumped accumulator installations, the minimum pressure of the mains must be 2.0 bar so that the vessel can be set to 1.5 bar below (0.5 bar). If the vessel is set to 0 bar then the positive impact of the accumulator is negated.