pressure relief valves

[oneintheeye]

anyone ever used pressure relief valves as a means of relieving floatation forces from groundwater in sewage treatment tanks. I know of they're use in swimming pools but can find no evidence of they're use in treatment works. Anyone any info?

 

[SteveWag]

I don't like to use them, but when needed, the Clow F-1492 works OK. I use 1 PSI as the cracking pressure, Clow says 9" of water. The number of valves, 4" dia. only, depends on how fast you expect external water to rise. In permanetly high ground water conditions you can never dewater a tank, so painting, blasting and welding operations cannot be performed.

Steve Wagner

 

[oneintheeye]

so by what you are saying they are not excluded from this application as I had feared. In the majority of cases we are looking at a condition of a temporary high water table (up to ground level in a lot of cases). At present we either design a toe to take into utilize some soil weight or specify mass concrete below the base. I am trying to find ways of reducing the cost of excavations and the sometimes high amounts of concrete required. It would be unlikely that the tank is empty for any significant period as its in a sewage treatment works, only for cleaning etc. as you stated. So Guess from what you are saying is that you have in the past used them for this kind of structure?

 

[JedClampett]

They're not reliable. They tend to get stuck. Plus I've heard of contractors putting rocks on them during construction to keep dry. The best way to resist flotation is to provide adequate mass to counteract it. Alternatively, you could put duckbill (Tideflex) valves in the walls near the slab. These are more reliable than slab mounted valves.
There are valves made just for this application by Clow, Neenah and others. I don't think a strict PRV is appropriate.

 

[oneintheeye]

thanks for the advise, we would normally just provide mass to overcome the uplift as you suggest but sometimes this can prove costly. We had a project recently that took delivery of the local concrete suppliers daily output for x number of days just to do the mass concrete pour which obviously proved costly and risky should output be interupted for any reason. I am attempting to find a more cost effective solution in the future as we are working on a number of these schemes in a coastal region of the UK where we have a temporary high water table at periods throughout the year.

 

[SteveWag]

Yes, Hampton, I have used them, but they are not my first choice. I know of one very large design firm that flatly refuses to use them because of a bad experience. If mass concrete with extended slab is too much, or if site size is limited, I use soil or rock anchors. A lot more design AND INSTALLATION expertise is required. Not a lot of contractors can do a good job of installing anchors.

Steve Wagner

 

[JedClampett]

We had a similar situation with "perched" groundwater. We suggested an underdrain and sump pump design. It worked very well. Ask whoever did your groundwater investigation if you could pump out the groundwater fast enough to eliminate the buoyancy.

 

[rcooper]

I've seen flap valves used in the side of a storm water tank. The only problem was they had all stuck open so the water in the tank leaked out into the ground water. We replaced them all with plastic coated flat valves, but I have always used the weight of the tank to stop flotation on a new design. We did also look at duckbill valves for this application.

 

[oneintheeye]

thanks for all your posts, looks like I'm back to the old fasioned methods of resisting the uplift. I would find it unlikely our client will go with the valves if there is any doubt as to their effectiveness and judging by the responses they would seem to be plenty.