Water pump accumulator/air tank pressure should be what?

[Emerald Fox]

Next to our Flojet 'Quiet Quad' (model R403-500) water pump is a roughly football-sized grey tank which apparently holds air at a pressure for keeping air out of the water system. There's a Schrader valve - as on a car - and I presume air can be pumped in using a bicycle pump.

What should the pressure be? What happens if it's too low / too high?

As I remember one boatyard told us 20 psi (which is 1.4 bar) - at that time I had measured our pressure and it was 20 psi. Does this sound right? (me not knowing nuffin about such things).

 

Our water pump sucks from the water tank and delivers to 1) shower 2) kitchen taps 3) Morco instant water heater. 2 weeks ago I opened the 16 bolts to the water tank hatch/cover, pumped all the water away and for the first time saw the little pipe that magically delivers our water. I dabbed the sand/rust from the bottom of the tank so it's all nice and clean now. The water pump has a plastic filter which had gathered some bits, but it just unscrews and you tap the debris out of it.

 

Sorry, as usual I digress... question is what should that tank's pressure be? Taking bicycle pump + Scrader adaptor + pressure guage next visit to the boat.

[Paul's Nulife4-2]

Hi ya,

Although mine is a different make to yours,Does your set up look anything like this -

https://www.dropbox.com/s/pux0fzijleausem/IMAG0273.jpg

 

https://www.dropbox.com/s/muhs6lebuseqxcw/IMAG0271.jpg

 

Mine cuts out at approximately 2.4Bar ish.& yes you pump the internal Bladder up with a Foot, or Hand pump, (I use a mini 12v car Booster/Compresser).

But it may be worth checking the Cut in & Cut out pressure's with you particular manufacture on there web site.

Edited August 19, 2014 by Paul's Nulife4-2

[Paul's Nulife4-2]

BTW I don't Think it works quite how you suggest,(Well mine doesn't anyway). Basically the Bladder pressurises the system, it then uses this 'Stored Power' to allow a smoother water flow, The pump only needs to cut in & cut out, to maintain pressure rather than run all the time to deliver water in pulses.

Others might explain it better, (I know what I mean).

Edited August 19, 2014 by Paul's Nulife4-2

[NMEA]

A really good reason for having an accumulator (and the most important in my mind) is that the pump switches on and off far fewer times than without, this means less contact arcing and consequently increased switch life. There are a few good descriptions on how to match the accumulator to the actual pump activity on this forum, it is far better to use those than the manufacturers numbers as theirs tend to be nominal.

[trackman]

Accumulator pressure depends on water pump cut off pressure. It's not to keep air out of the system it's there to smooth water delivery and stop the pump from cutting in and out at short intervals (cycling).

It does this by accumulating pressure (hence the name) in gas (usually just air) trapped behind a rubber membrane that separates it from the water in the system. The gas pressure helps keep up the pressure in the water system as water is drawn off, meaning that the pump doesn't have to kick in so soon.

If the gas pressure is close to the pump cut off pressure then the pump, when it runs, won't be able to increase it much. In that case there would be little point in having an accumulator!

So the trick is to check the accumulator pressure with the pump off, just after the pump has been running. Then run a tap until no more water comes out and recheck the pressure.

The first value will be the pump cut off pressure. The second will be the current accumulator pressure. Different people have different ideas about what the difference should be, but with a 30psi pump I reckon somewhere about 20psi works OK.

If yours is lower, pump it up to about that figure. If it's higher then I'd let air out until there's a difference of 10psi or so below the pump cut off value. Make sure you do this before turning the water pump back on or it will pressurise the accumulator and you won't get a true result.

You can play with the pressure setting a bit if you want to, to get the best results for your system and requirements.

[Tony Brooks]

The position suggests the grey tank is an accumulator and an accumulator is pressurised to around the pump cut in pressure or it may be easier to pressurise it to about half the pump cut out pressure. If your pump has a cut out pressure of about 2.8 to 3 bar than 20psi in the tank will do.

 

If you want to check the cut in or out pressure then with the pump off and a tap open pressurise the grey thing to about 10 psi. Then put your tyre pressure gauge on the valve, close the tap and turn the pump on. The pressure showing as the pump cuts out is the pump cut out pressure. Then open a tap a little and watch the gauge. The pressure at which the pump runs again is the pump cut in pressure. Turn the pump off, open a tap and re-pressurise the grey thing to the "correct" pressure.

 

An accumulator will extend the pump on-off frequency when drawing water but more to the point it will also keep the pump off for longer when the taps are closed. IN some systems it might also reduce the amount of water expelled by the PRV on the calorifier but this all depends upon where any non-return valves are located.

[Nickhlx]

When I supplied RO systems for houses the accumulator pressure was specified as 7 psi.

 

All that is needed is some pressure around the bladder (which fills from the pump / RO membrane) and this needs to be sufficient pressure to squeeze the bladder until there is (almost) no water left in it. It is derived from a combination of the head needed from the accumulator to the outlet tap / faucet ( probably about 4 feet max or (4/33 x 15) psi = 2psi approx, plus a bit for pipe losses ( say 2 psi) and a bit for "leakage" with time say 3 psi, which is how the figure of 7 psi was arrived at. If it is any more than is needed, you are reducing the cycle time for the pump, although you are gaining " smoother flow or more consistent pressure from less differential between the pump cut in and cut out pressures.

 

I run mine at around 7-10 psi, only because I am lazy don't remember to check it very often. There's not really a "right or wrong" pressure, as long as its "enough" and its approximately optimised to reduce pump cycling consistent with smoothness of flow

 

Nick

[nicknorman]

Accumulator pressure depends on water pump cut off pressure. It's not to keep air out of the system it's there to smooth water delivery and stop the pump from cutting in and out at short intervals (cycling).

It does this by accumulating pressure (hence the name) in gas (usually just air) trapped behind a rubber membrane that separates it from the water in the system. The gas pressure helps keep up the pressure in the water system as water is drawn off, meaning that the pump doesn't have to kick in so soon.

If the gas pressure is close to the pump cut off pressure then the pump, when it runs, won't be able to increase it much. In that case there would be little point in having an accumulator!

So the trick is to check the accumulator pressure with the pump off, just after the pump has been running. Then run a tap until no more water comes out and recheck the pressure.

The first value will be the pump cut off pressure. The second will be the current accumulator pressure. Different people have different ideas about what the difference should be, but with a 30psi pump I reckon somewhere about 20psi works OK.

If yours is lower, pump it up to about that figure. If it's higher then I'd let air out until there's a difference of 10psi or so below the pump cut off value. Make sure you do this before turning the water pump back on or it will pressurise the accumulator and you won't get a true result.

You can play with the pressure setting a bit if you want to, to get the best results for your system and requirements.

The pump cut out pressure is not relevant to the desired accumulator pressure mr T! The accumulator pressure should be just below pump cut-in pressure. Sure you can measure cut out pressure and then subtract something for mother, but why not measure the correct thing in the first place!

 

Anyway, to set the pressure correctly (really ought to cut and paste from one of the other threads)

 

Put pressure gauge on valve (try not to lose any air). Partially open a tap and watch the pressure changing up and down as the pump cuts in and out (the air pressure and water pressure must be the same unless the accumulator is completely empty. This will give the pump cut out and cut in pressures. Now turn off the pump, open the taps until no more water flows. Check the pressure again, it should be just a few psi below the cut-in pressure. Adjust as required. If the pressure was the same as the cut in pressure, let out a good bit of air and repeat the whole process (because if the pressure is initially too high, you will get a false impression of pump cut-in pressure because the accumulator will completely empty).

[Emerald Fox]

Thanks all. I was told the idea of the accumulator-thingy was to keep air out of the system, to stop the taps going splutter! splutter!

All seems a bit technical for me but I'm happy to see that 20psi looks about right. I have a car tyre pressure guage. I might give a few pushes with the bicycle pump 'just to make sure'!

When taps open the water pump seems to just run non-stop. When taps are off and water pump is 'live' it occasionally makes rumbling noises so we've been keeping the switch off when not needing water.

(same with the gas pilot light, we have that off unless hot water needed - seems a waste of gas otherwise...)

[Paul's Nulife4-2]

That's sounds as if all is ok though !.

The pump should Rumble on momentarily to 'Recharge' the Bladder. Then when the pump stops, turn the tap on and you should get a steady splitter free stream for a while before the pump kicks in again.

[nicknorman]

Having the pressure too high is much worse than having it too low, and 20psi is probably around the upper limit for what you want (20psi with pump off and taps open, that is) so I would be disinclined to add any more.

[Paul's Nulife4-2]

What you could do is,

With the system charged, But Pump off, Open a tap and measure the total amount of water given in a bowl. (Mine is Jabsco par-Max3 that cuts in at Approximately 10psi cuts out at 25psi & the Accumulator tank gives approximately 4.5 Litres, But this will entirely be depending on size of your tank fitted).

Then turn the pump back on.

If happy with the volume given, Comparison / corresponding results, to your Accumulator Tank, then all is good.

Edited August 20, 2014 by Paul's Nulife4-2

[bottle]

Emerald Fox

 

I see that Nick has mentioned that 20 psi may be a little high and I agree.

 

Just to run through what should happen when you use your water and the accumulator is correctly pressurised.,

 

Turn on tap, water will flow, pump will not run.

 

After a short while pump will turn on, the time from tap on to pump on will vary by installation (size of accumulator)

 

We can get approximately 0.5 ltr of water before pump starts.

 

Turn tap off pump will run until accumulator is re-filled (pressurised).

 

to stop the taps going splutter! splutter!

 

In a way it does, if you only part;ly turn on a tap the pump can supply more water than the tap is allowing out so, pressure builds up, pump stops, water stops, pressure drops pump starts, hence splutter, sputter.

 

An accumulator smooths this process so no sputter,sputter.

[bottle]

and just to make sure.

 

The process for setting up the accumulator correctly.

 

One tap open.

 

Pump turned off , electrically.

 

Set pressure in accumulator, your choice.

 

Turn on pump

 

Turn off tap when water flows. (pump will run on for short time)

[Sabcat]

turn on the water heater and the shower. Does the shower keep a constant temperature? If yes, don't mess.

[Emerald Fox]

Have made a note 20psi = max!

Yes, nothing wrong with our system, just wanted to check what the pressure should be.

Gradually getting to know our boat = will be an expert this time next year

[dannyboy]

Hi all

on a similar note I was wondering if anyone can help me....for some reason my pump ( a Johnson WPS 2.9 ) randomly decides to pulsate like mad, expelling water from the PRV I have checked the pressure in the accumulator, that's fine. I have replaced the PRV so that's fine. It doesn't seem to matter if I'm drawing hot or cold water and it may go a couple of weeks with no problem and then on a whim off she goes !! any ideas folks ??

[Murflynn]

Have made a note 20psi = max!

Yes, nothing wrong with our system, just wanted to check what the pressure should be.

Gradually getting to know our boat = will be an expert this time next year

But you may be missing the point. The pressure will vary depending on whether the accumulator is full of air or (nearly) full of water. You really need to empty it of water by switching the pump off and opening the taps. The pressure will then be at its lowest. Check the pressure and adjust.

 

 

We can get approximately 0.5 ltr of water before pump starts.

 

 

That seems to be working incorrectly. The volume of water should be at least half the volume of the accumulator if the pressure is set correctly. How small is your accumulator?

[nicknorman]

The volume of water should be at least half the volume of the accumulator if the pressure is set correctly. How small is your accumulator?

That will depend on the difference between pump cut in and cut out pressure, as well as having the correct pre-charge pressure. I recently tried to help a friend set up his accumulator properly since the pump seemed to have a high cycle rate, but it turned out the problem was a cut in pressure of 17psi and a cut out pressure of 21, meaning not much movement of water in and out of the accumulator.

Edited August 24, 2014 by nicknorman

[Paul's Nulife4-2]

Spuds !,

 

Have you tried something like suggested in post 12 yet,just to give an indication ?.