stainless steel pipework underground 2

[eski1]

Hi
we are planning to put a 14" sch 10 x 42m long pipe underground , it will be waste water going thru this to an effulent plant under gravity feed . my question is what type of welding join should we do , at present we are thinking of doing a butt weld full penetration & then weld a band over the top of this for extra strength. This would be to stop any potential future ground movement causing cracks in the weld etc . does anyone know the correct procedure ?

regards
chris

 

[bimr]

One would think that it is important to know what loadings the pipe will experience before you select the pipe materials.

 

[eski1]

hi

Thanks for the reply's so far , i have some answers and more info on the job :-


304L Stainless was chosen by our customer so would assume they have done this for good reason .

The product going thru the pipe is 100 degree c C.I.P wash from a food production tank so it will be diluted caustic & nitric acid this won't be a constant flow , only every half hour when tank clean is required .

The pipe will be burried about 1250mm underground with pea gravel back filled ontop then a road surface ontop . There will be lorry's travelling over this on a day to day basics .

As far as joining the pipe we are looking purge weld most joints ,and also flange a couple where required on these what gasket material would you recommend ?

With regards any chlorides in the soil i must admit not knowing the answer to this but i will look into this as i assume it would eat away the stainless steel .

hope this helps

best regards chris

 

[bimr]

Sch. 10 stainless steel is not going to be suitable for use under truck loading. Sch 10 is usually used above grade in food plants, not in wastewater applications. You would probably have to use sch. 40 stainless steel.

Welling with stainless steel is usually butt weld using the GTAW process.

Note that the chemicals should be neutralized prior to combining the chemicals in the piping. One would suspect that the chemicals are neutralized quickly after the tank washing in a separate sump.

You should get the actual temperature and chemical concentrations and review with the piping manufacturer.

You would be best served if you recommended an FRP pipe system. The FRP should stand up to the caustic and nitric acid. FRP will also be less expensive than SS.

Bondstrand makes a good FRP product.

http://www.ameronfpd.com/FP875.pdf

 

[stanier]

I think you are courageous using 304 ss in a buried situation. Check out

http://www.nidi.org.

There are extensive studies done on the cvorrosion of stainless steels in buried situations. Generally they need wrapping to rpevent pitting corrosion.

The welding will require pickling and passivating to prevent corrosion occurring from the process fluid. Check out ASTM A380.

Any reason you are not using thermoplastic piping? This overcomes the corrosion issues and also as a flewxible pipe does not take the external loads. Check out books by Moser or Watkins. You will also have AWWA standards you could consider.

 

[Pulpeng]

The reason they chose stainless is obviously the heat (212F) and corrosion - but I agree a sched 10s pipe underground is pretty thin. I also think if they are going to spend the dough on stainless they might as well protect it from the ground corrosion.

I would propose an alternative approach if it was my plant.

Figure out

1) if 212 F is accurate.
2) if there is anyway to regain some of that heat through an exchanger to lower the temperature (energy savings)
3) if there is a way to dump some of the heat through a contact cooling arrangement (i.e dump some cold water with it)

If they can get the temperature down just a smidgen it would be well worth looking into CPVC or some other thermoplastic to save money and extend the life of an in ground pipe.

Check out the following for some info.

http://www.plasticpipe.org/pdf/tn-11_temperatue_limits_for_thermoplastic_non_pressure.pdf

 

[stanier]

Go for GRP as this will withstand the temperature with the correct design and overcome corrosion problems inground.

 

[SteveWag]

I have inherited a client who discharges 8000 gallon kettles of boiling water a minimum of 6 times a day, average is 12 times a day. pH varies from 4.2 to 11.5. All below grade piping is PVC. The line from the plant, some 6-inchers, and the line connecting to the sewer is PVC, 8-inch.They run delivery vehicles over the 8-inch main. All of this was installed 15-20 years ago. All of the pipes surcharge for 5 to 10 minutes two or three times a day. I have inspected the piping, televised the inside and “tested” the exposed end with a digging bar after a submergence incident. The pipe is as good as new! Manhole inverts show attack, with concrete being removed. PVC will save a lot of money and, at least in my clients case, provide years of service.
Steve