'mix valve' design for lift station 1

[chileheadcraig]

I am doing a upgrade for an exisiting lift station with a siltation problem. It has been suggested to use a 'mix valve' design that will re-suspend the sediment when pumping begins.

Anyone have experience with this design? I've never heard of it.

 

[stressriser]

Sounds like a vortex valve, but they are usually used for flow control.

http://www.usfilter.com/water/ProductDescription.asp?WID=25&PID=261

At least contact them, they might know where to start looking.

 

[FLwater]

It is simply a port in the discharge line that blows off into the storage pit you are pumping out of to re-suspend the solids, ask you pump supplier, Flygt has a discharge head for their submersibles

 

[dicksewerrat]

All you have to do is drill a hole in the discharge line. Tap it out for standard pipe thread and put in an appropriate size pipe that is down in the silt to str it up. You don't have to 'buy' an expensive system. All this can be done by the maint. crew. You will want to size the pipe to less than 1/8 the discharge line or the operating costs may rise too much. Remember your objective is to pump water away from the station.

 

[bhwtam]

I used the Flygt one before on an submerible centrifugal pump.

The valve is not costly by itself but I think it can only be fitted to Flygt pumps. It attached to the pump casing, not at the discharge pipeline. It stirred up the sediment around the pump when the pump starts. It also allows you to set the "flushing" time. It works well in small pump pits and chambers. However, if you have a large tank, you should consider putting a submersible mixer for continuous mixing.

 

[chileheadcraig]

Thanks for the advice. This will give me a head start. Dicksewerrat, is the 1/8 the size of the discharge piping a rule of thumb or is there some sort of guidance regarding this. It would seem, for example, that a 1" pipe off an 8" discharge would soon get clogged being that this is domestic sewage.

 

[chileheadcraig]

bhwtam, was the valve called a 'mix valve' and is flygt the manufacturer? It did not come up in a search on their site.

thanks

 

[bhwtam]

wickedpete,

The manufacturer is ITT Flygt. It is called a 'flush valve' and they put it under the heading "mechanical accessories".

They have an Acrobat File describing the flush valve, try this one:

http://www.flygt.co.uk/19457.pdf

 

[chileheadcraig]

Thanks, that was helpful. I will have to see if there are any other manufaturers which make a similar valve. Still looks like it could be costly and a possibility that solids could keep the ball from getting a full seat after long term use.

 

[BobPE]

wikedpete:

I think Flygt has a patent on the flushing system, and just to add my two cents, it is really horrible in practicle applications....

BobPE

 

[dicksewerrat]

The 1/8 is a rule of thumb, when I was building storm water pump stations, I used a 3/4" on sizes up to 8". then increased the size as the discharge lines got to 16". But I never went over 1 and 1/4 inch. The domestic sewage has already gone through the pump and shouldn't cause much of a problem. Since it is close to the pump, there is a lot of pressure to keep it clean. And the amount of flow is small enought that you shouldn't capture any large particles.

 

[bhwtam]

If you are not using Flygt pumps, then the 'flush valve' from Flygt will not be suitable since it can only be mounted to Flygt pumps.

Perhaps I should also ask where your pumps lie in the process. Are they before the coarse screen, after the fine screen or somewhere in between?

The 1/8 pipe is a good idea but surely will be cocked up pretty soon even your pump is put after the fine screen. However, you can still use this idea but put pressurised water or air (depends what is easily available in your plant) to stir up during the start-up period. The water/air can be simply control by a solenoid valve. Or you can use larger flushing pipe (i.e. 1 1/2" or 2&quot control by a valve and flush with the pumped sewage during the start-up period.

There are lots of ways to tackle this problem and you should compare the cost of different method to make your right choice.

BobPE : What is your horrible experience on the Flygt 'flush valve'? I used that in 0.5% sludge and found that it works quite alright.

 

[BobPE]

bhwtam:

I have had the opportunity to retrofit the pumps into existing wastewater pump stations as well as designed new pump stations. You kinda hinted to the problem that is blockage which in most applications is quite bad since they are primarily used in smaller applications, where screening is often no present or quickly neglected after start up. This coupled with the decreased efficiency of the pump makes them not that good of a choice in my design book.

I wouldn't expect a problem with the pumps in a sludge application other than the efficiency.

BobPE

 

[gtrainor]

Here is also a possible cheap solution; grout steeply in a cone shape around the pump(s) and appurtenances to where the only level ground is directly under the pump intake. Sediment will work its way to the intake if it has no where else to settle. This may take a few yards of grout depending on your lift station diameter, but it may be a lot cheaper than retrofitting or changing a pump.

 

[chileheadcraig]

That could be a possibility, but this is a large rectangular tank. I will investigate that idea though. Thanks.

 

[CB2]

Sloping the wet well floor is very effective.

Also, consider a high pressure air injection into the bottom of the wet well to stir the solids at the begining of the run cycle.

 

[KRSServices]

I love this forum! All the ideas presented so far will work quite well, but, I too had a major liftstation subjected to siltation. The problem encountered was that the suspended silts and sands were dynamite on pump impellers. My solution was to construct and inline sand, silt/sand trap. Essentially, I constructed a large sump outside of the station (mine was a precast deep sump CB barrel), with a flow bypass and valving, on the inlet line where the flow velocity was slowest. The grit and silt settled very quickly in the sump and my annual pump maintenance costs savings paid for the the entore project in less than two years! Call me if you want specific details. KRS Services

http://www.krs-services.com

 

[chileheadcraig]

As with most municipalities, they prefer a no maintenance solution and wish to remove the silt at the wastewater treatment plant. Do you have some sort of cost/benifit analysis that shows that this is not the way to go? Also, there are 3 seperate gravity sewers that feed into this wetwell, so it would require 3 seperate sumps. That would be a good idea for a maintenance consious owner and will keep that in mind for future applications. Thanks.

 

[KRSServices]

In the example I referred to, the cost benefit analysis was a comparison of the historic costs to replace the pump impellers in relation to the capital cost of the sump and servicing with a vac truck periodically. My analysis showed a payback in about three and a half years, but it was realized in about 20 months. My solution was extremely simple, trouble free and did not rely on mechancial devices prone to breakdown or power failures. Prior to sarting my own business, I was a Public Works Director for two municipalities for 10 years. If you already have three inlet lines then I would think there is ample opportunity to facilitate an inexpensive bypass and/or a single large sump. In a cost analysis, I would utilize the historic maintenance records (as a function of flow) to service the pumps and other equipment affected by silt wear compared to the capital cost and longterm vac maintenance. Present value over 20 years (or expected lifecycle) and the numbers will speak for themselves. Size the sump for the total or ulitimate Q. Trust me, if presented properly, your client will quickly dispel any pre-existing notions of location (if that is a great concern) becuse to the surface, all that is seen is a manhole cover, nothing else.
KRS Services

http://www.krs-services.com

 

[chileheadcraig]

If you put it that way.... I will certainly look into it. How would I differenciate maintenance due to silt as opposed to normal wear and tear. I know silt wears on the impeller and the seals, but are there any other indicators that silt is causing premature wear/failure? What would you say the life cycle is of impellers with minimal silt? Was that the 20 years? Thanks for you help.