Potable Water Pressure

[Graham376]

Have had a problem with fresh water pump kicking in immediately when tap turned on as opposed to after a cupful. No tap on, no pumping so, not a leak situation. Checked the (membrane) pressure vessel, no pressure so pumped it up again. After a couple of days, no pressure again so, changed the schrader valve, which mad no difference so, have just replaced it with same type Zilmet Z200. This comes pre-charged to 3.5 bar, Flojet water pump is 2.2 bar. What pressure should the vessel be charged to? Easy to adjust via valve and bike pump.

 

[Catalina36]

From the Surejust site:

"The tanks are pre set at our factory for most systems, however the pressure is worth checking as some installations are set up with different pump pressures. In general the required pressure setting for an expansion bottle is 5psi above your pump pressure. If you do not know your pump pressure then set the expansion tank to 30 psi

Surejust > Expansion Tanks

 

[VicS]

From the Surejust site:

"The tanks are pre set at our factory for most systems, however the pressure is worth checking as some installations are set up with different pump pressures. In general the required pressure setting for an expansion bottle is 5psi above your pump pressure. If you do not know your pump pressure then set the expansion tank to 30 psi

Surejust > Expansion Tanks

The question relates to an accumulator tank ( rather than an expansion tank)

The pressure recommended by Cleghorn Waring for an accumulator tank is 2 to 3 psi below the pump cut in pressure

See https://www.jabscoshop.com/files/Accumulator and Expansion Tank Instructions ZPWL4 doc595.pdf
for detailed information on pressurised water systems

 

[Graham376]

The question relates to an accumulator tank ( rather than an expansion tank)

The pressure recommended by Cleghorn Waring for an accumulator tank is 2 to 3 psi below the pump cut in pressure

See https://www.jabscoshop.com/files/Accumulator and Expansion Tank Instructions ZPWL4 doc595.pdf
for detailed information on pressurised water systems

Thanks for that, although a pressure vessel, its is as you say used as an accumulator. Setting its pressure above the pump wouldn't work.

2 to 3 psi sounds reasonable, will have to check the link you've given to see if that's its stored pressure with system depressurised. edit - yes it is

 

[Catalina36]

The question relates to an accumulator tank ( rather than an expansion tank)

The pressure recommended by Cleghorn Waring for an accumulator tank is 2 to 3 psi below the pump cut in pressure

See https://www.jabscoshop.com/files/Accumulator and Expansion Tank Instructions ZPWL4 doc595.pdf
for detailed information on pressurised water systems

For my education, could you explain the difference between an expansion tank and an accumulator tank? Pressure is built up by a pump, stored somewhere and released though a tap. The ambient pressure in the tank versus the pump pressure surely just decides where the diaphragm will settle?

 

[VicS]

For my education, could you explain the difference between an expansion tank and an accumulator tank? Pressure is built up by a pump, stored somewhere and released though a tap. The ambient pressure in the tank versus the pump pressure surely just decides where the diaphragm will settle?

There is no physical difference between them . They are just used for different purposes. The accumulator tank acts as a buffer to prevent rapid on off cycling of the water pump and to maintain a pressure reserve .

The expansion tank absorbs the expansion of the water in the calorifier as it is heated. Without an expansion tank the expanding hot water is likely to be pushed back into the cold water system or, if this is prevented by a non return valve, lift the pressure relief valve and dribble water into the bilges .

Initial pressures are different but not carved in tablets of stone. You will find some variation between manufacturers.

Read the link I posted. It explains it all . That's why I posted it.

 

[Graham376]

There is no physical difference between them . They are just used for different purposes. The accumulator tank acts as a buffer to prevent rapid on off cycling of the water pump and to maintain a pressure reserve .

An accumulator tank acts in a similar way to a compressor air receiver. Without it, as soon as you press the trigger on the spray gun, the compressor would start, whereas the receiver provides a reserve of air. Likewise, on a boat water system, the accumulator tank provides the reserve and to a large extent, stops the water from pulsing out of taps.

 

[Graham_Wright]

As an aside, if the accumulator is fitted with the connection at the top (i.e. pressure vessel beneath), is the bladder then redundant?

I've always wondered.

 

[Graham376]

As an aside, if the accumulator is fitted with the connection at the top (i.e. pressure vessel beneath), is the bladder then redundant?

I've always wondered.

No.

 

[myquest]

Whilst there is no difference in the way these vessels operate, there is a difference in their characteristics. Pressure vessels (normally used with pumped systems) are used for potable water and internally do not allow water to come in contact with the steel casing. Expansion vessels are used to allow for the increase in volume of the heated fluid in a sealed system (central heating) and in some the fluid is in contact with the steel casing.

 

[Graham_Wright]

No.

Would you care to add a few more words?

 

[Graham376]

Would you care to add a few more words?

A container with the opening at the top and no bladder, isn't a pressure vessel and would as useful as a Coke bottle in the same position.

 

[rogerthebodger]

As an aside, if the accumulator is fitted with the connection at the top (i.e. pressure vessel beneath), is the bladder then redundant?

I've always wondered.

This shows a typical hydraulic accumulators. This is the same as a pressure water accumulator

 

[[163233]]

As an aside, if the accumulator is fitted with the connection at the top (i.e. pressure vessel beneath), is the bladder then redundant?

I've always wondered.

It does work a bit, you can indeed use a coke bottle upside down. But you can't precharge it with more than the volume of the bottle or the excess air leaks out through the tap.

 

[Graham_Wright]

A container with the opening at the top and no bladder, isn't a pressure vessel and would as useful as a Coke bottle in the same position.

As others have implied, the restriction is that the container cannot be pressurised which , of course, is wrong. The pressure can be increased to the cut out pressure of the pump. All fine and dandy but as soon as that external pressure drops (e.g. when water is drawn) and the water in the container expelled, the air in the container will chase the water out and probably cause an air lock. A bladder filled container will expel water until the pressure drops to zero. The bladder remains trapped in the container.

There - answered my own question - with steers from others.