Re: Accumulator tanks - help
Not sure if you are talking about a system on a 20 footer or 120 footer, but in general to your questions -
Firstly, just in case, the accumulator provides a compressive air cushion that provides a reservoir of energy to maintain flow when the pump stops on its pressure switch reducing cycling and smoothing the operation of the pump. It also provides an absorber of energy so that when the pump starts, especially if positive displacement as they normally are, there is not a sudden pulse of water out the faucet and so that water hammer (pressure gradient travelling down the pipe from the pump start up and stopping) is absorbed. The size of the accumulator is dependant on the size of the system, but generally on yachts of typical forum member size the accumulator can be quite small (around 3 litres is common). In a small boat with soft flexible plastic pipework, the pipework itself provides some of these accumulator functions.
1. Some freshwater pumps for boat domestic supply use claim not to need an accumulator and they say one should not be fitted (but in the instances I have come across an accumulator does not disadvantage the pump's operation). However, it has been my experience that while some pumps are claimed not to need one, it does reduce random running of the pump, when no normal draw off, when the pressure slowly drops in the pipework while standing. For example, as the water in the pipework cools during the night, the pump may run to make up the loss in pressure due to contraction of the water. Similarly there may be pressure loss through leakage through the relief valve on a hot water cylinder. This cycling may or may not be important to you, but it generally nice not to have it if one spends many nights aboard. I have also found that hammer in things like gas fueled calorifiers may be noisy (and perhaps damaging?) when the pump starts and stops, even with a pump not normally needing an accumulator.
2. The membrane in the accumulator seperates the water from the air cushion so that the air does not get absorbed into the water. Occasionally, and in my experience very occasionally (like years on our own boat) the air side needs to be pumped up to replace air leakage out the pumping up valve and gaskets, usually using a car or bicycle pump. If there is no membrane the accumulator has to be pumped up (or drained) more regularly - see 3.
3. The only reason that I know why one would drain an accumulator down is in the case of no membrane the air cushion can be replenished by draining water out of it. There may be other issues associated with the particular accumulator you mention, requiring it to be drained, but I do not know what they could be. Stagnation of water and the resultant health risk may be one of those but that would be unusual I would have thought as regular use of the water system will replace some of the water in the accumulator on each cycle of the pump.
<hr width=100% size=1>