Equalising Water Tank Presure on borehole supply

[Roadie]

Hi, I'm looking after a freinds holiday let while they are away. They have a private borehole water supply.

Water is pumped to a tank, c. 50 cm diameter by 1.0 m high. The tank has a pressure valve on top, and a square box to control "cut in/cut out" pressures (currently 80 and 120 psi)

There is also a car tyre type valve on the tank. I think this is to pressure an inner rubber sleeve in the tank to equalise the pressure of the water (this currently trickles and gushes as the pressure fluctuates between 80 and 120 psi).

Any idea what pressure the valve and rubber liner should be at?

Thanks.

 

[Roadie]

..having thought about this for a bit, the pressure between the liner and the tank will be the same as the water, the air acts as a cusion to smooth the water flow. So, just enough air for a cusion. I'll turn the pump off, and drain some water out, and then pump some air in.. Say increase from 80 to 90 psi.. does that sound reasonable?

 

[Onetap]

Any idea what pressure the valve and rubber liner should be at?

70 or 75 psi, probably, so the pump is activated while there is still a little reserve of pressure in the expansion vessel.

80 psi seems excessive to me, 3 bar (45ish psi) from a mains supply would be plenty for all domestic purposes and might need a pressure reducing valve for some applications. I don't see the point of using pump power and electricity to pump it up to 80 to 120, unless it's a water cannon.

It's a pressurisation unit, probably a packaged set, have a look on the internet for a manual for it or for a similar set, all the components are similar.

 

[Onetap]

..having thought about this for a bit, the pressure between the liner and the tank will be the same as the water, the air acts as a cusion to smooth the water flow. So, just enough air for a cusion. I'll turn the pump off, and drain some water out, and then pump some air in.. Say increase from 80 to 90 psi.. does that sound reasonable?

No, read a manual before you mess about with it. You'd turn the pump off, drain all the water, leave the taps open, and then pressurize the air to the recommended pressure; it is probably marked on the expansion vessel.

Increasing to 90 would mean the expansion vessel was emptied and and the flow rate would reduce before the pump starts.

 

[Agile]

I agree that it seems pumping up to 120 psi is far too high a pressure. Thats about 9 Bar !

Many domestic parts like washing machine hoses are not rated for such high pressures and risk failure. The same applies to seals in shower units etc.

It is potentially dangerous to have water at that pressure. If you have a cut and direct water at it at 9 Bar then you will open the cut!

3.5 Bar is a common operating pressure for these pump systems.

The air charge has to be made with the system open to the atmosphere. The manufacturer's instructions should advise but if thats not available then I would suggest an air pressure of about 0.7 times the max ( cutout ) pressure.

Tony Glazier

 

[mysteryman]

75psi is almost exactly 5bar; 90psi is 6.

 

[Boilerman2]

It is normal for a bore hole pump to work at high pressure, with the stored water within the "Accumulator" (it's not an Expansion vessel!!)
this is to reduce the number of Start/Stops per hour to reduce wear and tear on the pump motor, the outlet to the house is probably regulated by a Pressure reducing valve. it sounds like the diaphragm in the Hydronic Accumulator may have failed, Charging with an inert gas (Nitrogen) would be beneficial as unlike compressed air, it is not easiliy absorbed by water

 

[Agile]

Its a senario from hell to leave someone looking after your property who wants to fiddle with everything without knowing what they are doing.

If its not broken then dont try to fix it.

If it is broken then get a professional!

 

[Boilerman2]

I quite agree Tony!!

it makes you wonder how "Looking After" has been defined!!

 

[Onetap]

It is normal for a bore hole pump to work at high pressure, with the stored water within the "Accumulator" (it's not an Expansion vessel!!)
this is to reduce the number of Start/Stops per hour to reduce wear and tear on the pump motor, the outlet to the house is probably regulated by a Pressure reducing valve. it sounds like the diaphragm in the Hydronic Accumulator may have failed,

Why would the initial air charge and the 'pump start' pressure be set so high, 80 psi? The 'pump stop' pressure is a different matter and determines the amount of water pushed into the expansion vessel, but setting the start pressure unnecessarily high serves no useful purpose SFAIK.

Charging with an inert gas (Nitrogen) would be beneficial as unlike compressed air, it is not easiliy absorbed by water

Air is 76% nitrogen. 23% oxygen. The remaining air charge would have a higher proportion of N2, O2 being more soluble in water. Charging with nitrogen is beneficial to those with a nitrogen cylinder, especially if it's filled with an air compressor.

 

[Agile]

I dont see the advantage of using N2. Even if the rubber seperator was permiable enough to allow the O2 through then all it would do is to reduce the previous charge when all the O2 has dissolved.

Tony

 

[Boilerman2]

Sorry but all I have done is stated facts
1) Borehole pumps do run at high pressure
2) Nitrogen in any expansion is best practise (ask any vessel maker)

I spent 5 years working for a major pumping systems Company service
department so i have worked on more than a few of these type of installations - in days gone by the bore would have fed a pressure vessel with no diaphragm, with just a Nitrogen blanket on top of the water! as long as the pump did not fail to keep up with demand the nitrogen charge would last in excess of 6 months, of course if the pump did not keep up with demand, the pressure vessel would empty, and the N2 would be lost through the water pipe work system