Questions for air pressure vessel in water system

[MartinLe]

For a small wwtp we need to plan a water supply (non-potable water). The client wants an air-pressure vessel downstream of the pumps as they don't trust soleyly controlling the pressure in the system via FI.

So far the function of the vessel is clear to me: pre-pressurized air cushion, pumps turn on when pressure is below (say) 4 bar and swith of when system pressure is 6 bar. With FI controlled pumps, we would control for constant system pressure and allow some deadband to minimize start/stop cycles. Correct?

The max flow is 20-30 m³/h

Another firm already did some design work for this system and wrote a tender and this included a compressor, controlled by a pressure switch, with the pressure vessel. This confuses me (But I can't ask them): How are the pumps and the compressor coordinated, both can't be controlled by system pressure (= air pressure). The tender at least included no level measurement for the pressure vessel.

Other questions:

At this flow rate and a 3m³ vessel, how often approximately would I need to replenish the air cushion? Worth it to automate?

No description of water systems with pressure vessel I found so far featured an automatic replenishing of the air cushion, except for very small installation via the pump (specialised pump for that). In my specific installation, pressurized air is available. How would I manage this control-wise? The easist way I could think of would be to empty the vessel and isolate it from the water system, then fill from the pressurized air system with a pressure regulator.

 

[itsmoked]

They are using a check valve into a pressurized vessel. Then they keep an air space in that vessel charged by the compressor.

To admit any water into that vessel thru the check valve they have to supply the pumped water at a slightly higher pressure than the maintained air pressure + the check-valve head.

If the pump fails or stops due to a power outage and while the backup generator is coming on line the air reservoir will keep the water supply unchanged.

This is how just about every domestic well system works except instead of a compressor they let the pump compress the air behind a rubber membrane to store the energy. It stores pressure at the pump's delivery pressure minus the check-valve. As water is used the bladder-backed pressure continues to drive the flow out of the tank. Eventually the pump comes back on after sufficient pressure delta has passed to reasonably not short-cycle the pump.

The difference with using a compressor is that you can charge the compressor to a far higher pressure than the tank uses then regulate the pressure into the tank mechanically. This means the compressor can be powerless for an extended period too. The compressor method also keeps the outlet pressure rigidly constant whereas the rubber bladder system always reduces pressure as water is used.

Up side of the rubber bladder version is no maintenance headache or power wastage of a compressor and no contamination by the delivered air. Down-side, pressure will droop as water is used.

Don't forget that a pressure relief valve would be required using the compressor method as a failed regulator could explode the tank.

Very, VERY, few applications could not use the bladder method. Little notable pressure drop would occur in the time a generator would take to start.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[nackra]

hydropneumatic tank with operating comments above:

http://www.johnwood.com/wp-content/uploads/2014/02/Technical-Bulletin.Form-002.002.pdf

 

[Valvecrazy]

I agree a diaphragm style tank would be best. But if you want to use the old hydro pneumatic type tank look into a White Water compressor system. They come with a pressure switch for the compressor and a probe that goes in the tank to maintain an air level.

 

[MartinLe]

I found this description of the White water system:

http://www.whitewatermfg.net/air_rite_brochure-10-31-08.pdf

... and this clears up my confusion re. pressure vessels somewhat.

Is there a reason to use a pressure vessel when the water pumps are all equipped with VFD and controlled for pressure? I have no feeling for how fast or not the pumps would react to a consumer coming online or beeing turned off. Danfos or KSB claim that there's no need for a pressure vessel with VFD controlled pumps.

 

[Valvecrazy]

It depends on the conditions and which VFD you have. I think Danfoss makes the PID for Pentair. That one most certainly needs a pressure vessel as it is more a glorified pressure switch than a VFD. Some others may operate fast enough that in a really large system you might get away without a pressure vessel. But a lot of the time the pressure vessel is needed if for nothing else but to cushion the transducer.

 

[itsmoked]

MartinLe; Give this thread a read, especially starting about 1/3 the way down. thread1528-419466

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MartinLe]

Danfoss was a typo (though they supply VFD pumps I think), I meant Grundfos with their pre-packeged booster pump stations.

itsmoked, that thread is helpful.

My thinking is to install VFD pumps and provide space to retrofit a membrane vessel, if the pressure is too unstable.