Water pump / accumulator issue

[cutandpolished61]

Good afternoon, had to replace the water pump and accumulator on my boat. The new pump is a shurflo 20psi. what should the pressure in the accumulator be to have the best result? Someone told me to set the pressure to 3psi lower than the pump. Is this correct? Currently set at 10.5 psi and the pump comes on after 2 seconds and continues to run for just 2 seconds after closing the tap.

[Tony Brooks]

26 minutes ago, cutandpolished61 said:

Good afternoon, had to replace the water pump and accumulator on my boat. The new pump is a shurflo 20psi. what should the pressure in the accumulator be to have the best result? Someone told me to set the pressure to 3psi lower than the pump. Is this correct? Currently set at 10.5 psi and the pump comes on after 2 seconds and continues to run for just 2 seconds after closing the tap.

 

NO - not correct, although it is OK for a hot water expansion vessel.

 

Pressurise to the pump's cut IN pressure or roughly half its cut out pressure if easier, so 10PSI in your case.

 

Thanks to another contributor (Tracy). Pressurise it to above the pump cut out pressure (say 25psi in your case), pump on taps close. Wait for the pump to stop and then let the air out until the pump cuts in. Job done.

 

This gives you the maximum pressurised water storage and the higher you set the pressure the less and less volume of water will be stored in the accumulator.

Edited Tuesday at 15:25 by Tony Brooks

[cutandpolished61]

There is no improvement when I set the accumulator to 10 PSI, and I’m finding it difficult to follow Tracy’s method. Could you please send me her step‑by‑step procedure?

[jonesthenuke]

56 minutes ago, Tony Brooks said:

 

This gives you the maximum pressurised water storage and the higher you set the pressure the less and less volume of water will be stored in the accumulator.

No it doesn't. Maximum storage would require setting the accumulator bladder pressure to zero.

[Tony Brooks]

2 minutes ago, jonesthenuke said:

No it doesn't. Maximum storage would require setting the accumulator bladder pressure to zero.

If you set the bladder to zero there will be nothing to provide any pressure when the pump turns off, so the pump will keep cycling. With about half the cut-out pressure the accumulator will store and provide the maximum volume of water when the pump turns off, so the cycling will be slower.

[ditchcrawler]

5 minutes ago, jonesthenuke said:

No it doesn't. Maximum storage would require setting the accumulator bladder pressure to zero.

That would get the maximum into the accumulator bit you wouldn't get much out before the pump cuts in.

[Tony Brooks]

14 minutes ago, cutandpolished61 said:

 

There is no improvement when I set the accumulator to 10 PSI, and I’m finding it difficult to follow Tracy’s method. Could you please send me her step‑by‑step procedure?

 

I just did.

 

How about telling us exactly what the problem is that you are trying to fix.

 

On the basis of the 20psi cut out pressure you state - which I think is very low for a modern pump, more likely 2 to 3 bar, which is  40 to 45 psi.

 

Tap open, pump off.

Pressurise to a few psi above the cut OUT pressure.

Close tap and turn pump on.

When the pump cuts out gradually let air out of the accumulator until the pump runs.

You have now set the accumulator pressure to a tad below the cut in pressure.

[jonesthenuke]

1 hour ago, Tony Brooks said:

If you set the bladder to zero there will be nothing to provide any pressure when the pump turns off, so the pump will keep cycling. With about half the cut-out pressure the accumulator will store and provide the maximum volume of water when the pump turns off, so the cycling will be slower.

When the pump cuts out the bladder will be pressurised to the cut out pressure and will be storing the maximum volume of water practicable in a pump/accumulator system. Increasing the initial accumulator pressure causes reduces the volume of water required from the pump before the cut out pressure is reached..Note that this is what I was responding to with my earlier post. Thereafter the pump will only cycle if there is a leak. When a tap is opened having the maximum practicable water volume store in the accumulator is a benefit. 

1 hour ago, ditchcrawler said:

That would get the maximum into the accumulator bit you wouldn't get much out before the pump cuts in.

Why? 

[Tony Brooks]

1 hour ago, jonesthenuke said:

When the pump cuts out the bladder will be pressurised to the cut out pressure and will be storing the maximum volume of water practicable in a pump/accumulator system. Increasing the initial accumulator pressure causes reduces the volume of water required from the pump before the cut out pressure is reached..Note that this is what I was responding to with my earlier post. Thereafter the pump will only cycle if there is a leak. When a tap is opened having the maximum practicable water volume store in the accumulator is a benefit. 

Why? 

 

OK, have it your own way, but don't try to confuse others.

 

It is accepted in engineering and science that most liquids may be considered incompressible, this includes water in plumbing systems.

 

With no air pressure in the accumulator when the pump cuts out the bladder will be squashed against the accumulator case, but with no air pressure behind it to push that accumulator full of water out, so the merest drip will cause the pressure to fall to cut in pressure, so the pump cuts in. As there is no air in the accumulator and the bladder is against the case, the pressure will rise almost instantly to the cut-out pressure so the pump cuts out. This gives you very short period cycling. Put air in the accumulator and that air can force water out when the pump cuts out, and then the pump has to run for a while when it cuts back in to compress the air. This gives you a longer cycling period.

 

This explanation is not for your benefit, but for those your assertion that maximum water volume  (meaning usable volume) is when the accumulator air pressure is zero find it confusing. It is generally accepted that the optimum air pressure for an accumulator is more or less the pump cut out pressure.

 

This, for simplicity's sake, ignores the effect of the air trapped in the water, and system, and any flexibility in the plumbing components.

Edited Tuesday at 19:19 by Tony Brooks

[blackrose]

I always set the accumulator to about 4 psi below the water pump's CUT-IN pressure with the pump switched off and a couple of taps open. That seems as easy a method as any other, but yes you do need a £3.50 tyre pressure gauge.

Edited Tuesday at 20:17 by blackrose

[jonesthenuke]

28 minutes ago, Tony Brooks said:

 

It is accepted in engineering and science that most liquids may be considered incompressible, this includes water in plumbing systems.

With no air pressure in the accumulator when the pump cuts out the bladder will be squashed against the accumulator case, but with no air pressure behind it to push that accumulator full of water out, so the merest drip will cause the pressure to fall to cut in pressure, so the pump cuts in. As there is no air in the accumulator and the bladder is against the case, the pressure will rise almost instantly to the cut-out pressure so the pump cuts out. This gives you very short period cycling. Put air in the accumulator and that air can force water out when the pump cuts out, and then the pump has to run for a while when it cuts back in to compress the air. This gives you a longer cycling period.

 

 

You scenario above is not what I was thinking. I was taking an accumulator with near zero pressure (just above atmospheric) and with the bladder filled with air as the start condition so the accumulator contains all air and no water. Not a bladder with negative pressure such that it has negligible volume which would make the fitting of an accumulator rather pointless.

 

Setting the accumulator air pressure low maximises the volume of water stored in the accumulator when the pump cuts out. When a tap is opened and the pipework is depressurised, water flows from both the accumulator and from the pump; system pressure is likely to be well below the pump cut in pressure and the volume supplied in demand by the accumulator is useful in dealing with the demand transient.

 

Increasing the accumulator pressure to the pump cut in pressure desensitises the system to the effects of small leaks and thus has its uses, but it does this at the  cost of reducing the water storage in the accumulator and negatively affects system flow for short draw-offs. As soon as the water pressure falls to the pump cut-in pressure the accumulator has emptied of water and can be of no further assistance.

 

I would thus advocate setting the accumulator pressure lower than the pump cut in pressure, at say 5-10 psig, unless the system has chronic slow leaks in which case setting the pressure higher will reduce the frequency of the pump cutting in when there is no system demand, noting its better to fix the leaks.

 

I have tried setting the accumulator on our boat using both the high and low pressure approach. Both work. Our plumbing system is leak tight and using the low pressure approach pump cut-in is very infrequent and the response when drawing water off from a tap is better than when the accumulator is more pressurised.

 

PS when I refer to system leaking I am including leaking back through the pump, not just leaks from the pipework.

 

 

 

 

[Tony Brooks]

56 minutes ago, jonesthenuke said:

 

You scenario above is not what I was thinking. I was taking an accumulator with near zero pressure (just above atmospheric) and with the bladder filled with air as the start condition so the accumulator contains all air and no water. Not a bladder with negative pressure such that it has negligible volume which would make the fitting of an accumulator rather pointless.

 

Setting the accumulator air pressure low maximises the volume of water stored in the accumulator when the pump cuts out. When a tap is opened and the pipework is depressurised, water flows from both the accumulator and from the pump; system pressure is likely to be well below the pump cut in pressure and the volume supplied in demand by the accumulator is useful in dealing with the demand transient.

 

Increasing the accumulator pressure to the pump cut in pressure desensitises the system to the effects of small leaks and thus has its uses, but it does this at the  cost of reducing the water storage in the accumulator and negatively affects system flow for short draw-offs. As soon as the water pressure falls to the pump cut-in pressure the accumulator has emptied of water and can be of no further assistance.

 

I would thus advocate setting the accumulator pressure lower than the pump cut in pressure, at say 5-10 psig, unless the system has chronic slow leaks in which case setting the pressure higher will reduce the frequency of the pump cutting in when there is no system demand, noting its better to fix the leaks.

 

I have tried setting the accumulator on our boat using both the high and low pressure approach. Both work. Our plumbing system is leak tight and using the low pressure approach pump cut-in is very infrequent and the response when drawing water off from a tap is better than when the accumulator is more pressurised.

 

PS when I refer to system leaking I am including leaking back through the pump, not just leaks from the pipework.

 

 

 

 

 

Yes, both will work, but not for the optimum volume of water delivered with the pump off by air pressure, for two different, but related reasons.

 

Air pressure lower than the pump cut in pressure means that at cut in pressure the diagram will be pushed fully back, yes that seems it would give a greater volume, but as the accumulator delivers water the air pressure will drop more quickly than if the pressure was set at about half the cut in pressure. The water pressure can't fall below cut in pressure because the pump cuts in, so the extra water volume actually can not all be delivered because the pump cuts in.

 

If the air pressure is higher than the cut in pressure the diaphragm will be forced into the water space, even with no pressure in the water. This reduces the water volume that the pump can force into the accumulator before the air and water pressure reaches the cut-out pressure, then the pump cuts in.

 

Either will result in cycling at a higher frequency that optimum, but depending upon how far away from the "about half the cut in pressure"/cut out pressure you set it either way, the greater the difference in cycling speed. The volume flowing from the tap also alters the cycling speed, so it is perfectly possible that you would not perceive the difference.

 

In short, under pressure results in less water being expelled by the accumulator before the pump cuts in, and over pressure results in less water getting into the accumulator (to be expelled when the pump cuts out) before the pump cuts out - thus a lesser volume that can be expelled.

 

 

[jonesthenuke]

14 hours ago, Tony Brooks said:

 

Air pressure lower than the pump cut in pressure means that at cut in pressure the diagram will be pushed fully back, yes that seems it would give a greater volume, but as the accumulator delivers water the air pressure will drop more quickly than if the pressure was set at about half the cut in pressure. The water pressure can't fall below cut in pressure because the pump cuts in, so the extra water volume actually can not all be delivered because the pump cuts in.

 

You seem to misunderstand how an accumulator works or are not explaining it clearly?

 

You say "Air pressure lower than the pump cut in pressure means that at cut in pressure the diagram will be pushed fully back"

 

This is not the case.  At pump cut in pressure the bladder will be somewhat less than fully inflated and not collapsed to zero volume. If you think about it such conditions would require infinite water pressure to squeeze the bladder back.

 

I appreciate you are giving generalised practical advice, however the rationale needs to be correct.

[Tony Brooks]

18 minutes ago, jonesthenuke said:

You seem to misunderstand how an accumulator works or are not explaining it clearly?

 

You say "Air pressure lower than the pump cut in pressure means that at cut in pressure the diagram will be pushed fully back"

 

This is not the case.  At pump cut in pressure the bladder will be somewhat less than fully inflated and not collapsed to zero volume. If you think about it such conditions would require infinite water pressure to squeeze the bladder back.

 

I appreciate you are giving generalised practical advice, however the rationale needs to be correct.

 

I could say the same.

 

I note you decided to talk about negative pressure in one of your earlier posts when I had made no such claim - unless you expect everyone to specify gauge or absolute pressures in their posts when if it is not specified convention says it is gauge pressure that is being referred to. No air pressure is not a negative  (gauge pressure) it is just zero pressure which is not a negative or positive.

 

To address your point (not that I think you will be convinced). In the case of a lower than optimum air pressure in the accumulator and the tap open with the pump off (ignoring any strength in the diaphragm). Such air pressure as there is will push the diaphragm to its fullest extent because there is nothing opposing it. Start the pump and close the tap and the water pressure will force the diaphragm back, the question is how far back. The answer is until the air pressure and water pressure are equal (as per Pascal's law) but as the air pressure was lower to begin with the diaphragm will have to move back further than if the air pressure had been higher to start with. that gives a greater volume of water in the accumulator when the pump cuts out.

 

You said:

 

When the pump cuts out the bladder will be pressurised to the cut out pressure and will be storing the maximum volume of water practicable in a pump/accumulator system. Increasing the initial accumulator pressure causes reduces the volume of water required from the pump before the cut out pressure is reached..Note that this is what I was responding to with my earlier post. Thereafter the pump will only cycle if there is a leak. When a tap is opened having the maximum practicable water volume store in the accumulator is a benefit. 

 

I sated no pressure and if there is no air pressure the slightest water pressure must fully compress the bladder/diaphragm - Pascal's law says that. In that scenario the total water volume in the system will be the maximum BUT there is no air pressure (gauge pressure) push that water back out of the accumulator when the pump cuts out, so the merest leak or contraction of hot water must cause the water pressure to drop all but immediately so the pump cuts in.

 

Now open a tap. The pressure will fall, the pump cut in, but as there is no air pressure the pressure rise will be all but instant, so the pump quickly cuts out, but water flowing from the tap all but instantly causes the pressure to drop to the pump cut in pressure. Hence, the high frequency cycling.

 

Now do the same with a little air pressure in the accumulator. The volume of water contained will be a little less that with zero pressure, but when the tap is opened the air pressure will force a small volume of water out of the accumulator with the pressures dropping more slowly than when starting with no air pressure, so the cycling will be at a lower frequency. 

 

Look at the volume of water the air pressure can expel when the tap is opened and before the pump cuts in.  A low initial air pressure must give a lower volume of expelled water before the pressure of both air and water drops to cut in pressure.

 

Unless you grasp that liquids are deemed incompressible and that any flow from the accumulator comes from a combination of the air pressure and air volume, you won't understand the system. A low air pressure means a low air volume which in turns means a low water volume the air is able to expel from the accumulator.

 

 

The bottom line is that Physics, not me, not you, and lots of practical real life experience says the optimum accumulator air pressure is around the cut in pressure

 

 

[jonesthenuke]

Tony, re your statement "I sated no pressure and if there is no air pressure the slightest water pressure must fully compress the bladder/diaphragm"

 

Again you miss my point. I was taking a zero (gauge pressure) accumulator bladder as one which where the internal bladder is filled with air at atmospheric pressure. Specifically an accumulator with all the internal volume occupied by the air filled bladder and no water.

 

Then for any water pressure above atmospheric, the bladder volume will be decreased, but it cannot go to zero (fully compressed) as you are suggesting unless the water pressure is extremely high.  For the water pressures that apply in the cases being considered the bladder will be compressed by a small amount.

 

Reading your post I am starting to think that you do not grasp that  the accumulators have an internal bladder fixed only at the end.

 

PS I have not made any suggestion that water is not incompressible, I do not know why you are saying that.

[Tony Brooks]

18 minutes ago, jonesthenuke said:

Tony, re your statement "I sated no pressure and if there is no air pressure the slightest water pressure must fully compress the bladder/diaphragm"

 

Again you miss my point. I was taking a zero (gauge pressure) accumulator bladder as one which where the internal bladder is filled with air at atmospheric pressure. Specifically an accumulator with all the internal volume occupied by the air filled bladder and no water.

 

Then for any water pressure above atmospheric, the bladder volume will be decreased, but it cannot go to zero (fully compressed) as you are suggesting unless the water pressure is extremely high.  For the water pressures that apply in the cases being considered the bladder will be compressed by a small amount.

 

Reading your post I am starting to think that you do not grasp that  the accumulators have an internal bladder fixed only at the end.

 

PS I have not made any suggestion that water is not incompressible, I do not know why you are saying that.

 

So how much air pressure is there is the bladder or behind the diaphragm of an accumulator that has punctured and been collapsed by the water pressure. 

 

I agree that if the bladder still has air contained in it then that will be compressed until it reaches the water pressure, but the question is what air volume does that represent between cut out and cut in pressure. It is basically the air volume between cut in and cut out pressure that determines how much water that air can expel into the system when the pump is off.

 

Low pressure = lower volume = lower water volume the diaphragm/bladder can expel before the falling pressure reaches the cut in pressure. You are assuming that at zero pressure the bladder or diaphragm is still "inflated" to the same extent as if it had a higher pressure in it. Put a higher pressure in  t with zero water pressure and that air pressure will expand the bladder or diaphragm or compress the spring in a spring accumulator. That sounds like less available volume for water and indeed it is, but the pressure will not drop so fast when the tap is opened because of the higher air volume to start with so more of the available water can be expelled from the accumulator before the pump cuts in. Any initial air pressure below the pump cut in pressure can play no part in expelling water before the pump cuts back in, which is why accumulators are used - to modify the pump cycling frequency. It would however continue to expel water if the tap was opened and the pump turned off, but that is not the reason they are fitted.

 

You say that you do not notice a difference between an over and under pressurised accumulator, but that depends upon the pump delivery volume and the flow volume from open outlets. A higher flow from the outlet than from the pump and you would not notice any difference because the pumps runs all the time the outlet is open. A higher flow from the pump than from the outlet and the pump must cycle and the accumulator is supposed to modify that cycling frequency or for small volumes of water delivered prevent the system pressure reaching cut in pressure, so the pump does not run at all.

[ditchcrawler]

https://www.pentair.com/content/dam/extranet/web/nam/shurflo/manuals/911-731-series-182-accumulator-tank-iom.pdf an interesting read

[jonesthenuke]

11 minutes ago, Tony Brooks said:

 

So how much air pressure is there is the bladder or behind the diaphragm of an accumulator that has punctured and been collapsed by the water pressure. 

 

 

 

I think you are going off track.

 

1. Why consider a punctured bladder? That's irrelevant, its a system with a broken component.

 

2. An accumulator bladder cannot collapse under water pressure in the systems we are talking about. The water pressure is orders of magnitude smaller than would be required.