Varem PLUSVAREM Accumulator High Pressure Vessels 16 Bar. Expansion vessels or also known as pressure vessels are to provide a reserve of pressurised water to limit the number of on/off cycling of water boosting pumps. Within the vessel is a membrane which on one side is pressurised air and the other side is water (inside the membrane). When the pump system is started and a valve is closed water is pushed into the membrane, reaching the set pressure by the pressure switch. The system then holds at pressure. When water is drawn the pressurised air forces the water out until the pressure falls below the set level of the pressure switch and the pump will then re-start.