An accumulator in a domestic pumped water system on a boat is installed to reduce pump running time, making use of the taps quieter and smoother. An accumulator is a pressure vessel that is divided into two parts, one filled with nitrogen or air and the other with water, usually separated by an elastomeric membrane. The gas pressure can be adjusted via a Schrader valve. It is necessary to match the gas pressure to the output pressure of the pump to ensure optimum operation.
In use, the water side of the accumulator is connected into the pipework on the discharge side of the pump, preferably as close to it as practicable. When the water pump is initially switched on it may operate to charge the accumulator but not if the gas pressure is already sufficient. Upon opening a tap the gas pressure provides the initial flow until its pressure has decayed sufficiently for the pump to cut in. The pump will run briefly, pumping water to the tap but also compressing the gas in the accumulator until its pressure switch cuts out. Water delivery is then driven by expansion of the gas until the pump pressure switch cuts in again. The pump runs for only a fraction of the time that it would without the presence of an accumulator. Setting the gas pressure to suit the pump pressure switch settings is crucial to optimum operation of the system as a whole.
Suppliers of accumulators and expansion vessels describe methods for setting the gas pressure using gauges. This requires suitable connections to the pump discharge pipework plus, of course, purchase of a gauge or gauges. This method achieves a more accurate setting quickly and simply, with no additional equipment.
Start off by venting the gas entirely from the accumulator through the Schrader valve. Now turn on the pump and open a tap. The pump will run continuously. Begin to pump air into the accumulator, using a bicycle pump, upon which the pump will run increasingly intermittently. Make a note of the start/stop times of the pump as this process continues. Eventually the pump will run non-stop again, when the air side of the bladder is completely full. Now vent gas slowly through the valve until the pump running periods are minimised by reference to the timings noted previously. The accumulator air pressure is now set perfectly.
Some accumulators come pre-filled with dry nitrogen to supposedly reduce the possibility of internal corrosion. I fitted my steel accumulator in about 1998, using this method and have seen no evidence of corrosion since.
Accumulator with Schrader valve at top.
