Stainless Steel Buried Pipe

[Lan123]

Has anyone used stainless steel (Type 316) buried pipes for process air services? I have to design a 1000 mm SS pipe buried 1.5 m below grade for air services. The total length of the pipe is about 150 m. The temperature can vary between 0 ~ 100 C. If so what is the best way to provide expansion joints?If the pipe was located above grade I would have provided metal bellows with a combination of fixed and sliding supports. Can metal bellows still be used for the underground services?

Thanks in advance.

 

[europipe]

Why bellows, it,s a bad idea.
Cover the undergr pipe with densotape or equal and you're finished.

 

[LittleInch]

A 1m diam stl stl pipe is some size as is your temperature range. At 150m, you probably won't get to locked in status, but your end points will move a fair bit. It is much better to allow the pipe to move rather than lock it in place. You need to do a stress analysis of your pipe and the end connections. One thing to watch out for is the allowable stress decrease due to temperature for stl stl.

What wall thickness do you have and what does the end connection look like as it comes out of the ground?

Bellows for buried pipe are a bad idea.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Lan123]

LittleInch,

The pipe thickness is 6.25 mm. Actually the ends have vertical risers. In between the two ends, there are two 90 bends with thrust blocks and therefore anchored. So the expansion joints will have to be at placed between the anchor points.

 

[bimr]

Would this service be air blower discharge with low pressures? You should not be seeing the temperature increase along the entire pipe with the maximum at the blower discharge being around 80 C

Note that the thrust blocks are not necessary with restrained pipe. In addition, lightweight materials are typically used for this application. Consider the use of fiberglass pipe.

 

[LittleInch]

1000mm diam and 6.5mm thickness is very thin for a buried pipe. You need to check out the buckling from external soil pressure. Normally anything greater than a D/t of 90 is considered very thin in buried pipe usage. Also like bimr, I don't understand why you have thrust blocks unless your jointing system is not welded?

I think you really need to give us the full info here - pressure rsting, pipe and material details, flow rate, min / max temperature, soil temp, jointing system proposed sketch of the profile etc. Have you worked out what the temp losses are from one end to the other ( if your flow velocity is quite low this could be considerable)

With two thrust blocks and buried pipe you are fully restrained. You need to analyse the pipe to see if this is overstressed or not - see ANSI B 31.8 for an idea of what you need to look at and how to analyse it r use something like Ceasar. Even if you somehow get some flexibility there will be considerable forces / stress in those sections furthest away due to pipe/soil friction. If all you have experience of is surface piping then you need to understand you can't just use the same system below ground.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[EdStainless]

And to add one more, external corrosion on buried stainless pipe is a serious concern.
You really need to know the soils and moisture to know if you can get away with it.
I like the sound of fiberglass (and I make stainless!).

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[rconner]

As I suspect you have found air lines and particularly significant lengths of welded air lines can undergo significant thermal movement and/or stressing from initial assembly, to service (due to quasi-adiabatic effects, with compressors or blowers attached) and so on. With seasonal, diurnal and/or shut-down etc. variations as well, the movements or stressing can also be repetitive and re-bating in nature.
While I would not ordinarily make such response, I will due to where this thread has headed and mention that push-on joint ductile iron piping has traditionally provided satisfactory performance for some low and moderate pressure buried air piping services (you don’t note here the specific process nor pressure involved). I believe for major wastewater plants in the USA it is most common to see either unlined ductile iron and/or stainless steel piping employed off the blowers of low pressure aeration systems, and I have seen particularly ductile iron (and sizes I believe at least up to 54") used for much buried service. Along with greater strength than the quarter inch steel you are talking about here, another advantage to a rubber-gasketed ductile iron system is when the expansion/contraction is by design broken up into discrete e.g. 6m (or 20 feet) lengths, the amount of thermal movement of even quite large delta-T’s plus and minus from whatever initial installation temperature becomes very small in such applications (you really don't need bellows or expansion joints in most cases). You can see the effect of this e.g. in the Table No. 17-12 on page 17-24 of the file at

http://www.american-usa.com/system/assets/151/original/section 17_RB.pdf

. Such small amount of movement can basically be accommodated, most likely in this case just by some very slight and gradual flexing of the individual sizeable gaskets involved, in the pipe/size system you have described.
One thing for sure to note however is the rubber gasket formulation must be chosen to be suitable for the maximum temperature and any other e.g. fluid etc. exposures that may be applied. If e.g. 80 degrees C. (176 degrees) is actually to be applied to the buried piping, AWWA standard SBR gaskets would not be acceptable per Table 2-1 pg 2-7 of the file e.g. at

http://www.american-usa.com/system/assets/140/original/Section 02_RB.pdf,

though special EPDM push-on gaskets are suitable and available for operation at 200 degrees F. Everyone have a good weekend.