Groundwater Pumping for Heatpump/Cooling System

[Jeamon]

Hi,

I am working on an existing system which pumps water at 14.5 l/s from a supply (groundwater) well, through a network of piping into the building, up a 20m vertical shaft to a penthouse, through a condensor, then backdown the vertical drop and back to a return well. The piping is all 100mm diam., the length of forcemain is approx. 270m. As stated earlier, the piping goes through 4 vertical sections of pipe: one 17m section from the supply well, a 20m vertical up to the penthouse where the condensor is, a 20m drop back down to ground level, then an approximate drop of 14m back to the return well. All vertical drops are separated by horizontal sections of pipe approx. 30m long.

The operators of the system complained of loud gargiling and vibration noises (its in a retirement home) when the system was started and stopped. Eventually they elected to leave it running all the time, even though it was only required for intermediate use when cooling. I am working on sizing a new pump for the system and addressing the operational problems.

I have been doing some reading on this site and was considering adding a combination air release/vaccuum breking valve in the penthouse and then making the return drop pipe large enough to be self venting. However, this was too expensive and was not an option. Here are my questions/statements for discussion:

1) Will an air release/vaccuum break valve be beneficial?
2) When I size my pumps, I will make sure they are capable of supplying some flow at a head large enough to overcome elevation head to air release valve, but not necessarily at design flow rate.
3) My design flow static head will be supply well dynamic w/l to return well w/l only.
4) I want to add a Pressure Sustaining valve at the bottom of the downstream vertical drop to maintain a minimum pressure at that point to eleminate the vaccuum that may be created from the vertical drop of water.

Thanks for any help.

Josh

 

[Yorkman]

I've expeirenced a similar water flow problem at The NORAD complex COLO.(hole in the mountain?). The cooling tower and cooling water pumps were located outside the mountain about 100Ft below and 3/4 of a mile away. On power failure a whole lot of water would come rushing down out of the mountain overwhelming the cooling tower.
We went back as an add to install spring return motorized valves to isolate and bypass the loop at shut shutdown turning it into a closed system, while off.
I didn't quite understand items #2, when you size your pump I think it has to be able to deliver design flow at the proper Ft/Hd or you wil not be able to reject the heat from the heat pumps. Item #3 the Ft/Hd the pump will need to come is your max. height above the pump and any pressue drop associated with your piping and condensers at the heat pumps. The elevation drop back to the well is is canceled out.


I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Yorkman]

Sorry after posting this I couldn't understand it myself

"Item #3 the Ft/Hd the pump will need to come is your max. height above the pump and any pressue drop associated with your piping and condensers at the heat pumps. The elevation drop back to the well is is canceled out".
Try this
Item #3 The Ft/Hd pressure drop the pump must over come is the maximum vertical hieght above the outlet of the pump and any pressure drop associated with the piping and condenser(s), the vertical drop back to the well does not figure into your lift requirement.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[BRIS]

You are trying to design a siphon with a negative pressure less than vapour pressure. That will not work and hence you get the vibration and gargling (vacuum) sounds.

You need to maintain the whole system at a pressure above vapour pressure. Say -5.0 m below atmospheric is the maximum you should allow. Your pump static head must be at least the Penthouse top level - well water level - the maximum vacuum pressure of 5.0m. (your point 3 above).

However I doubt that it is worth saving this 5.0m of static head and it may be best to run the whole system at positive pressure. If you run at positive pressure then you can add automatic air release vacuum breakers).

You need to fit your pressure sustaining valve on the outlet so that the outlet pipe remains full and pressurised.

 

[BRIS]

PS Your pressure sustaining valve should be set to maintain positive pressure at design flow (14.5 l/sec). The return pipe will partly drain and refill each cycle. The automatic air release/vacuum valves will prevent vibration and gargeling on drain down and refilling.

Alternatives are to reduce the diameter of the return pipe, fit orifice paltes in the return pipe etc. so that it remains pressurised (at least above vapour pressure) at design flow.

 

[Jeamon]

Thanks for all of the responses. I will size the pump to operate at the design flowrate at a head equal to the friction loss and static elevation difference to the air release valve. One other issue I am trying to wrap my head around is the following:

The existing pump we are replacing was grossly oversized. When it was operating pressure gauges on the line were reading a pressure of around 40psi at a high point on the system near the start of the horizontal piping at that level. However, another pressure gauge near the vertical drop section was reading a negative pressure. My "guess" is that the water in the vertical drop was actually creating a vaccuum because of the drop. Is this assumption correct and how would I calculate/evaluate this?

Thanks

 

[Yorkman]

I think your correct, as BRIS commented :

You are trying to design a siphon with a negative pressure less than vapour pressure. That will not work and hence you get the vibration and gargling (vacuum) sounds. I think his solution was to install a back pressure regulating valve at the end of your return line to maintain a positve pressure on your return. When the pump shuts down the valve should shut off stopping the riser from draining.



I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI