Waste disposal & treatment 4

[arj00]

Is it possible to provide air compressor in a seawge pumping station in order to pump compressed air into the wet well or this can cause serious troubles to the pumps since air bubbles may enter the pumps.

 

[RWF7437]

Why do you want to do this ? What are you trying to accomplish ?

 

[Ussuri]

Possibly as an agitator to keep solids in suspension in the wet well?

I would be concerned about cavitation and damage to the pump impeller.

 

[bimr]

Probably not a good idea as entrained air will reduce the capacity and discharge head of a centrifugal pump. Actual results vary from pump to pump depending on pump design, speed, pump size, suction pressure, etc. Entrained air may also damage a pump, but not by cavitation. Cavitation occurs when the absolute pressure of the inlet drops below the vapor pressure of the pumped fluid.

 

[Ussuri]

bimr

I stand corrected, thanks. I did a little digging so can you confirm if this makes sense. The vapour pressure of water is 49 lbf/ft^2 (which is 2345 N/m^2 in SI units, I think). Atmospheric pressure is 100000 N/m^2 hence cavitation can't occur if open to atmosphere??? Is that right or is it not as simple as that?

 

[nackra]

http://www.flygtus.com/19457.pdf

one of my clients likes the "flush valve" w/ Flygt

good luck
nackra

 

[dicksewerrat]

If you are trying to agitate solids, just tap the discharge line on the side and put a 1 inch pipe back down to the bottom of the wet well. When the pump turns on, you get some flow through the pipe and agitate.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[stanier]

ABS and Flygt have flush valves available as an option on their submersible pumps.

Injection of air into systems is a viable way of actually reducing the effects of damage from cavitation. Cavitation damage occurs when vapour bubbles collapse. Microscopic exmanination of the collapse of the bubbles shows a phenomenon like a sphere buckling. One side collapses in until it forms a funnel into what was the sphere. At the point of the funnel the surface tension is broken and a high speed jet fires across the vapour bubble, through the other side and impacts on the surface of the pump impeller, pump case or whatever. Thios is thge gravelly noise one can often here when cavitation occurs. This high speed jet causes impact damage. The introduction of air into the system impedes the jet.

Air is also a viable means of reducing celerity in a system subject to high surge presures. A small amount of air (0.5%) can reduce the celerity by 50% (Streeter & Wylie Fluid Transients in Systems)

 

[bimr]

Ussuri,

The pump type has not been specified. If the pump is a submersible, then it is probably impossible for cavitation to occur. However, when you use a different type of pump that is mounted above the water, cavitation is possible because the pump has a suction lift.

http://www.cheresources.com/centrifugalpumps2b.shtml

 

[stanier]

Pumps used on Sewage applications with a suction lift are not common. Those that are used like the Hidrostal have a screw ompeller to guide the solids into the pump volute and have extremely good NPSH characteristics.

Some self priming pumps such as Varisco are used on sewage applications with suction lift and these have good NPSH characteristics.

But air in the system does not equate to cavitation. One concerns air and the other vapour. Two entirely different phenomena.

Removal of air from a sewage main can be achieved using the Ventomat RGx automatic air valve

http://wwww.ventomat.com.

It should be installed on a tee at least half the line size. One could be installed immediately downstream of the pump, or for a submersible where the pipe reaches the surface.

 

[arj00]

Thanks for all replies ,I like to add to my question the following:
1-The pumps are submersible pumps,they are large pumps 6 No. @1000GPM each.
2-I need to agitate the solids since if the pumps do not turn for 1 hour the sludge becomes very hard & the pumps will not work.
3-Actually I am thinking of 3 options:
1st: to use flush valve,but I am afraid of maintenance
problems,so I need an advice from some one that used them for long time.
2nd: I have used mixers in another submersible pump station & they are working fine ,& I tend to use them again
3rd: I am thinking of using compressors since they are almost maintenance free,but I am afraid of the air bubbles problem.

 

[dalcazar]

How about a small submersible pump beside the main pump? you could get a pretty decent flow out of it and that would create enough movement to keep the sludge from settling. A venturi nozzle sucking air on the pump's outlet would only help things more.

An air compresor with a diffuser plate would probably work just as well but you do have the chance of air getting into the lines, this isn't going to give your big pumps any trouble, as has been said, being submersible pumps they won't have any cavitation. you might want to rig the circuit in order to turn off the compressor while the pump is running if you choose that path.

I guess it comes down to which one is cheaper and more reliable: a pump, an air compressor or a mechanical mixer.

 

[stanier]

I have been involved in the upgrade program of 300 Sydney Water Pumping Stations. The experience has shown that the choice for pumps of this size would be a separate mixer. Flygt or ABS. Both have software available for design of size and location of the mixer.

Flush valves do work and are reliable however they will not stir up a large sump containing six pumps. You will have dead ares where the sewage will go septic.

Use of air in this application should be avoided as it will create an odour problem in the pumpstation. This will also add to corrosion if in building. I dont think your problems would be in the pump itself although corrosion may be accelerated in the pump impeller and bowl just because there is more oxygen available.