Setting up a water pressure differential switch.

[RogerS]

Hi chaps. Recently moved into a new place that has it's own private water supply with all the gubbins in the cellar.

There is a pump that is switched on with a pressure differential switch. There is also an expansion tank connected into the water pump at this point. Inside this tank there is a diaphragm with air pumped in on one side to a certain pressure.

So I'm told that it works like this. When you turn on a tap in the house, the water pressure in the pipes drops and that triggers the switch which turns on the pump (let's call this the Low Pressure setting) . When you turn the tap off, the water pressure builds up until it reaches another setting on the differential switch (let's call it the High Pressure setting).

The diaphragm/expansion tank arrangement is to stop water hammers and dampen down the abrupt cut-off when the pump stops.

So...what settings does one set for Low and High Pressure typically and what do you pressure the expansion tank to?

Any ideas ?

TIA

 

[Duncan A]

As well as avoiding water hammer, the accumulator arrangement is designed to prevent cycling of the water pump when small amounts of water are drawn off. Where I have used such systems in the past, they have been run at 3 to 4 bar - but that is for distributing water over 20 metres head so probably not necessary in domestic premises. If your accumulator does not have any identifying info or pressure rating (I would expect the latter, perhaps stamped near the top or bottom and painted over) then I would empty the accumulator of water, charge to 1 bar of air pressure, then switch the pump back on to refill with water and give it a try using the existing differential pressure switch settings.
If it provides useable water pressure without excessively frequent and brief pump operation then it will do.

A similar system uses a simple vertical cylinder with air over water, and with the water level rising and falling between about 40% and 70% full. This may help visualise what is going on inside your accumulator as the diaphragm is there principally to separate the two fluids and protect against corrosion of the pressure vessel. The pressure settings are arranged to provide useable pressure throughout the system and the air within the cylinder is simply topped up as necessary to keep the water level within the cylinder between reasonable limits.
Not sure I've explained it well but hope it helps.
Duncan

 

[RogerS]

Thanks Duncan. My problem is that the switch settings and expansion tank settings have been tweaked (bodged) by others and so the settings aren't what they should be. I've got a new switch and pump (as that was failing) and I think your post will help me get there, thanks.

 

[sunnybob]

My house has a pumped supply.
I have a regulator on the house pipework and set it to 3.5 bar. This allows a reasonably powerful shower without stressing all the joints (which are buried in concrete floors).
If you have a seperate low pressure adjuster I would suggest that at 2.5 bar
If your system includes hot water tank, then the pressure adjuster "the air operated bellows) is there to stop the system exploding when the water heats up and expands. The pressure on the hot side could easily be a couple bar higher when hot than when cold depending on the temp setting. You should have a spring loaded safety valve somewhere. this will have its pressure rating on it. No point pressuring the system to within a decimal place of the relief valve as it will just leak water everytime the tank heats up.