Expansion / Accumulator Tanks

[BroadAmbition]

 

I was recently asked by a forumite - How did I know that the onboard tank was not operating correctly?

This then an idiots guide that most idiots - especially me - can follow

Well first off you have to find out if your boat / Hireboat actually has one fitted, not all do.  A simple look round usually within the engine space adjacent to the calorifier and main water tanks will suffice, you are looking for a steel tank usually painted red or blue with a 'T' piece fitting plumbing fitting on one end and a standard air pressure valve on t'other end

Some boat systems have both an expansion tank and an accumulator tank - Both do the same sort of job.  On 'B.A' we have just the one tank fitted that covers both the hot and cold water supplies, this will suffice for most boat fresh water systems as the calorifier has a safety pressure valve fitted (Or damn well should have)

So if your craft does not have an accumulator tank as soon as you open a tap / shower valve the 12v fresh water pump will immediately burst into life to restore pressure. This is because it has a built in pressure switch and as soon as it senses a pressure drop it will start operating.  (If your craft has a faulty accumulator tank exactly the same thing will happen)

For years this was the acceptable method of supplying fresh water to the boats taps and shower.  It has a couple of drawbacks though.  For the pump itself this leads to premature pump wear as the pump is constantly operating on/off in quick succession (Cycling), especially if a tap / shower is not fully opened.  This leads to a pulsing water supply - annoying and can be noisy with the pipes/delivery.  It also gives a pulsing shower supply which is far from comfortable

Enter the Accumulator (Expansion) tank. They really are the simplest of items with nowt to be frightened of.  There are a few different  types but the most common one is a rubber bladder inside the steel tank itself.  The bladder is similar to that inside a football.  It should be pumped up to a pressure about half that of the fresh water pump (Rule of thumb guide only), usually with a 12v air compressor used for car tyres or a manual footpump.  So for example lets say that the onboard water pump is delivering / maintaining a water pressure of 30psi (Not to be confused with water flow per minute that's a different value).  A 20 x Litre accumulator tank with an air pressure of 15psi will therefore accept around 15 Litres of water compressing the bladder air pressure up to 30psi.  It will lie dormant until a tap is opened.  The bladder pressure forces the water out of the tank supplying the opened tap with the water system constant at 30psi without the pump having to operate.  With a large enough tank, kettles, sinks etc can be filled a few times with out the pump operating at all.  The pump will only start operating when the bladder has sufficiently expanded and the water pressure drops below the 30psi pump rated pressure switch

This then gives a steady constant water pressure and in turn a steady / smooth flow to taps and showers, thereby hugely extending the life of the pump and making water delivery a much more satisfying experience onboard, especially when using the shower

The most common cause of failure is the air pressure inside the bladder is getting too low, just like a car tyre pressure needs topping up now and again, so does the accumulator tank and its a simple job to do too

Hope this helps for those that didn't know - You now do

For those extremely clever forumites - Yes I know I have simplified the system / operation which I make no apologies for

The photo attached is the original accumulator tank in blue with the fresh water pump fastened on top of it aft of the white calorifier tank.  We have since fitted a replacement larger (Red) accumulator tank

Griff

 

 

 

 

[Vaughan]

Very good description, thank you.

In principle, the air pressure in the tank should be 2psi below the cut in pressure (30psi) when the water pressure is zero - taps open and pump turned off.  The type of pump in your photo has a pre-set pressure switch that you cannot adjust but other types come with a separate switch, the "Square D" . This has a big screw for setting the cut-in pressure and a little one for adjusting the "range" of the pressures. In a boat this is usually between 20psi cut in and 40 psi cut off but in a house, it is usually 30 - 60.

If the pump is turning on too often, this is mostly because there is too much pressure in the air tank, rather than too little.  If the system is fitted with a water pressure gauge, this helps a great deal.

[Vaughan]

I have had a thought - is the 30psi that you quote, the cut in or cut off pressure?  30psi seems a bit high, for a boat. 

If it is cut off, then you need to find what the cut in is, before setting the air pressure.

To be honest, it is a bit of a dark art, which we usually do by turning on the shower taps and setting the pump so that it runs constantly with the shower running, but that way, it is a bit "hit and miss"!

[Smoggy]

It is also worth adding to an idiots guide that the red and blue tanks are exactly the same thing except the paint on the outside, they are just set to a different pressure depending which use they are for, either colour will do either job.

Easiest test for bladder leaks (no that type old man, see a doctor) is to depress the air valve a bit, if water comes out the bladder is split, but re-pressurise afterwards if just air comes out.

[OldBerkshireBoy]

May I just add that it is different for those with pressurised water or heating systems at home.

[Bikertov]

1 hour ago, OldBerkshireBoy said:

May I just add that it is different for those with pressurised water or heating systems at home.

I'm sure I have had 'leaking bladder issues' in a domestic boiler expansion tank before - so is it that different to what you can get in some installations in the home ?

[Smoggy]

I've had it a couple of times, I can normally tell by the dripping from the overflow outside when the tank is heating up, mine has expansion tank on it's side which stresses the bladder I think.

If expansion tank is knackered on a boat the calorifier prv will generally dump hot water into the bilge when heating, once up to temp it will stop dripping as the expansion is done and excess got rid of.

If accumulator tank is knackered see Griff's description, you won't have an accumulator at home as mains supplies the pressure and you won't drain the local water tower brushing your teeth (Rob Beckett might...).

[BroadAmbition]

To be honest, it is a bit of a dark art,

That's my take on it.  I have before now tried to find out if there is any written guidance as to what pressure to charge the tanks with but that depends on the capabilities of the onboard pump and how many outlets

The water pump onboard, I have no idea what pressure the pump is set to, nor the cut in pressure either.  I do have a photo of it somewhere which should give me that information but stuffed if I can find it at present

I'm not even sure right now what pressure I have our tank set to but do know it will be lower than what the pump is rated at otherwise it would be pointless having it in the system.  Too low a pressure in the tank will cause the pump kick in far too early.  As has been stated its a bit if a black art, suck it and see and play about with the air pressure for the best results worked for us

Griff

[annv]

Hi They are called Accumulators if fitted on cold side to stop cycling and should be pressured at just below cut in pressure of pump often 12 to 13 psi if used on hot side now called a pressure expansion tank and should be at just below pressure of release valve pressure. John

[Smoggy]

Surely just above pump cut off pressure but below prv pressure otherwise it would leak out the prv before filling the expansion tank.

[annv]

Hi Yes sorry just below pressure release valve pressure , trying to do two things at once. John

[Regulo]

If anyone is thinking of updating/changing their water system, I can recommend a variable pressure pump. They're VERY expensive, but there's continuous pressure assured at all times (no pulsing, very good for thermostatic mixer valves), they're quiet enough that you won't hear them in the night if someone wants a drink, and there's no need for an accumulator tank. Not a project to do for the sake of it, but worth considering if you need to rip your old stuff out anyway!

[OldBerkshireBoy]

2 hours ago, Bikertov said:

I'm sure I have had 'leaking bladder issues' in a domestic boiler expansion tank before - so is it that different to what you can get in some installations in the home ?

No, but how you deal with expansion vessels in homes is.

[OldBerkshireBoy]

2 hours ago, Smoggy said:

I've had it a couple of times, I can normally tell by the dripping from the overflow outside when the tank is heating up, mine has expansion tank on it's side which stresses the bladder I think.

you won't have an accumulator at home as mains supplies the pressure and you won't drain the local water tower brushing your teeth (Rob Beckett might...).

Expansion vessels in homes should be sited vertically, being the correct colour for the application and I`ll wager that there are far more accumulator tanks around than many imagine.

[OldBerkshireBoy]

37 minutes ago, Regulo said:

If anyone is thinking of updating/changing their water system, I can recommend a variable pressure pump. They're VERY expensive, but there's continuous pressure assured at all times (no pulsing, very good for thermostatic mixer valves), they're quiet enough that you won't hear them in the night if someone wants a drink, and there's no need for an accumulator tank. Not a project to do for the sake of it, but worth considering if you need to rip your old stuff out anyway!

This is for houses/ flats - yes?

[BroadAmbition]

Hmmmm,

This thread is getting somewhat confusing between boat and house systems 

Griff

[Vaughan]

2 hours ago, Smoggy said:

Surely just above pump cut off pressure but below prv pressure otherwise it would leak out the prv before filling the expansion tank.

You would be surprised how often this happens, with pumps that have their own pre-set pressure switches.

I have also known modern water pumps to inflate the calorifier tank, if a PRV has not been fitted.

Which leads me to suspect that a lot of water systems on boats run at a much higher pressure than they actually need to.

Edited to add : I was talking about 2psi below cut in pressure, not cut off. If the air pressure is set at, or higher than the pump cut off, the tank would not work.

[Smoggy]

My tank basically prolapsed when I upgraded my pump, it flipped the concave base outwards, I keep expecting it to appears on the list of expense and despair but it's not leaked yet, I'll replace when it does, about 4 years so far.

[Regulo]

8 hours ago, OldBerkshireBoy said:

This is for houses/ flats - yes?

No, boat. Jabsco V-Flo. Should have said constant pressure, variable speed pump in my post. The pump made a world of difference compared to the pulsing and therefore varying pressure from the old pump/accumulator set up we had. 

[Vaughan]

5 hours ago, Regulo said:

No, boat. Jabsco V-Flo. Should have said constant pressure, variable speed pump in my post.

Yes, I have seen these on hire boats.  They seem to work well but I don't have much experience of them.  Only trouble is that if there is a small leak in the system, they will make occasional little grunting noises in the night.

Rather like someone I know well . . . . 

[Regulo]

Better than the raucous clatter of a pressure pump at full whack in the middle of the night! 

[annv]

HI always follow manufacture's instructions, house hold ones are normally set at one bar =15lb approx .John

[Vaughan]

27 minutes ago, annv said:

HI always follow manufacture's instructions, house hold ones are normally set at one bar =15lb approx .John

Thanks John, I was going to suggest, as a rule of thumb, set it at one bar and then adjust the pump accordingly.

Remember though, that this means one bar when the water pressure is zero. Taps open and the pump turned off.

[Islander]

this may help. 
 

Colin

[WherryNice]

Further to Islanders post, here is a pretty decent guide from Jones Boatyard website...

http://www.jonesboatyard.co.uk/water-systems/