Accumulator tanks - help

[bountyboat]

The pump on my domestic water system has died when I fit a replacement I am wondering if I should also fit an accumulator tank. Questions:
1 Do they work ? are the results as claimed. What size to fit?
2 Some seem to come with a 'membrane' - what is that all about.
3 One I found had a small note - needs regular drain down ! as mine will be 'tucked away' a regular drain down would be a pain.

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[HenryB]

They work by maintaining a reservoir of air under pressure in your pressurised water system. This results in your pressure pump needing to switch on and off less often, especially while drawing small amounts of water from the system, so less wear on the pump and switch. The membrane forms a seal between the air and the water. Without it the air tends to be absorbed by the water over time so you may need to periodically drain the accumulator tank to refill it with air.
IMHO you are better off fitting one.



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[Chris_Stannard]

Yes an accumulator is worth it. If you fit Jabsco I do not remember seeing a requirement to drain it down, but see below.
As to the size you should fit, it depends on the space you have available, and the size of your boat. I have a 9 litre tank which holds about 4 litres of water, so when the system is working the pump does not come on too often. You can also get a smaller tank, about 1 litre. It might be that the instruction you refer to draining down is that when I leave the boat I switch the water pump off and then open a tap to relieve the pressure on the tank.
As to the bladder the way the thing works is that inside the tank there is a balloon or bladder. The water goes into the bladder so the air pressure in the tank, but outside the bladder, rises as it is compressed. When you turn the tap on the air pressure forces the water out until the pressure falls below the pump cut in pressure, the pump then cuts in a repressurises the system. Obviously if you are having a shower the pump will run continuously, but if you are just filling the kettle the pump may not cut in at all.
At the top of the accumulator there is a valve, ordinary car tyre valve usually and you set the pressure in the tank according to the pump switch. There are two ratings for the pump, either 10-20 psi or 20-40 psi and it makes a difference to the internal pressure setting for the accumulator. If you have a fairly large boat with long runs to showers and hand basins a high pressure system is obviously desireable.
If you are fitting an accumulator try to fit it where you can get at it as the bladder may rupture after a while and you will need to fit a replacement tank.

<hr width=100% size=1>Chris Stannard

 

[MainlySteam]

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.

John

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[AndrewJ]

everyone seems to be mentioning a membrane in their accumulator. My accumulator recently went bad, (leaking at the fitting) and I purchased a new one. Just for my own satisfaction, I opened up the old one (a jabsco) and found it was just a hollow tank, no membrane, with just one fitting. apparently the water would come in fill up the tank with a air pocket someplace and be pressurized. I don't know about my new accumulator, but if this goes bad, I'm going to try to make one with a piece of 4 inch PVC and glue some caps and fittings on it and see if that will work.

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[gus]

Several years ago I had a problem with my water system and in my ignorance purchased a new accumulator tank as I suspected it had a fault. Then I found that the tank should be pressurised. I fitted the new tank (pressurised) and cured my fault. I thus have a perfectly good tank available if anyone wants it. It is an 8ltr tank as supplied by ASAP and is in 'as new condition'. Make an offer???

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[Oldhand]

I hope you do some calcualtions on the strength of your homemade accumulator. Remember, for safety it should be able to withstand twice the working pressure before failure. Relying on glue in this situatuation will require very detailed information from the adhesive manufacturers and strict compliance with the instructions for use if there is to be any certainty on the integrity of the assembly.

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[HeadMistress]

This, from Jabsco may help...

Accumulator Tank Info - from Jabsco ITT

WHAT ACTUALLY IS AN ACCUMULATOR TANK (AND WHAT DOES IT DO)?
Most people know that a water pressure system relies on the presence of an accumulator tank to function properly but few actually understand what it does. They function on the principle that air will compress under pressure whilst fluids will not. An accumulator is an air filled tank that half fills with water as the pump operates. When the pump is turned off the system has a store of water in the tank which is under pressure from the compressed air. This means that, depending on the size of the tank, you can run off a quantity of water without the pump needing to cut in, particularly handy if the pump is installed immediately below a bunk occupied by a tired crew member!

Accumulators will also release their store of pressurised water if demand on the system exceeds pump capacity, the larger the tank the more water in reserve.

Finally, the air acts as a cushion, absorbing pressure fluctuations and vibrations. This quietens the whole system and allows the pump's pressure switch to reach cut-off pressure cleanly, lengthening pump, motor and switch life.

Where a boat has a simple tank without a bladder to separate the air and water do remember to drain the tank down at least once a season, the air tends to get scoured out and eventually the tank will be full of water and serving no useful function at all!


TROUBLESHOOTING A WATER PRESSURE SYSTEM
Marine water systems are now more technically advanced than ever and should give many years of trouble-free life. However, problems can occur and the following guide to some common complaints may help:

FAILURE TO PRIME can be caused by an air leak in the intake or discharge line, a blockage in the inlet pipework, punctured diaphragm, debris under the valves or a crack in the pump housing.

PUMP FAILING TO TURN ON can be caused by poor wiring or loose connections, blown fuse, or defective pressure switch or motor. Check that there is full voltage available at the pump before investigating other causes.

A PUMP FAILING TO TURN OFF is normally because the switch cannot reach its cut-off pressure. This can be caused by simply having an empty water tank but other causes can be a punctured diaphragm, debris under the valves, leaking pipework, insufficient voltage or a defective pressure switch. Clean inlet filters and check valves and diaphragms, if they are worn now is the time to replace all wearing parts.

PUMP CYCLING ON AND OFF WHEN NOT IN USE is most commonly caused by small leaks in the pipework or a tap left dripping allowing the pressure to slowly reduce to the pump's cut in point. Tighten all loose connections after racing the source of the leak.

PUMP CYCLING ON AND OFF DURING USE is normally caused by a restricted delivery, check and clean filters and all pipework for restrictions to flow including undersized pipe. Fit an accumulator if one is not fitted.

LOW FLOW AND PRESSURE may be just that the pump is undersized for the system. Check the manufacturer's recommendations. However, deteriorating pump performance can be caused by an air leak at the pump inlet, accumulation of debris or scale within the pump and pipework, worn valves and diaphragm, low voltage or defective motor.

Low voltage due to inadequate cable size being used is a common cause of pump problems, always follow the cable size recommended in your data sheet. Checking that correct voltage is available can only be done by measuring voltage at the pump when it is running.

INCREASE THE LIFE OF YOUR WATER PRESSURE SYSTEM
Most boat owners leave their boats at the end of the weekend and sensibly turn off power to the pump. However, pump life can be increased by also opening the taps to relieve pressure in the system once power is off. This depressurises the system saving wear and tear on diaphragms and valves when not in use.





<hr width=100% size=1>Peggie Hall
Specializing in marine sanitation since 1987