Process Gas Lines and Fuel Oil Line in Same Trench?

[michael333]

Hello,

Got a project where the client is asking us advice about running several lines in a trench.
Advise them on what to do. The lines include 2" Nitrogen, 2" helium, 2" low pressure argon, 3/4" high pressure argon, and 4" fuel oil line.

Questions:
1) Can the process gas lines and the fuel oil line be run in the same trench?? Some trench boxes offer dividers.
So we need to advise on separate trenches, trench with divider, or trench without divider.
2) What are requirements for spill containment for the fuel oil? Our only experience is fuel stations. We used direct-buried piping rather than a trench.

What's the main codes to begin research on this? I've already looked at NFPA 30, 30A.

Thanks,
MV

 

[LittleInch]

Here's my take.

1) Can the process gas lines and the fuel oil line be run in the same trench?? Some trench boxes offer dividers.
Yes. No reason not to. I've never seen any code or standard which prevents this.
By "trench" do you mean buried or ?? Trench boxes are the worst of both worlds - just fill up with water / gunk / dead animals / snakes, cost more and offer no appreciable benefits. Just bury them.

So we need to advise on separate trenches, trench with divider, or trench without divider.
Define what you mean by trench, but see above.

The only thing you could apply is to calcualte the size of the crater if one of those gas lines suffered a full bore rupture. The distance though is likely to run into metres of separation, which is probably impractical. Dividers would, IMHO, be useless in practice from any such event.

2) What are requirements for spill containment for the fuel oil? Our only experience is fuel stations. We used direct-buried piping rather than a trench.
No particular requirements. Design it and build it so it doesn't leak, or not within a very low probability. Conduct HAZID and HAZOP studies to eliminate or reduce risk of loss of containment.
Buried, fully welded, coated and with CP applied and the risk of leakage is very low.

What's the main codes to begin research on this? I've already looked at NFPA 30, 30A.
Your piping code? Usually ASME B31.3 if there is n local code, but surely you have a piping design code agreed with the client?

But basically it comes down to acceptable risk. The risks you can reduce to as low as practical, but unless you separate them by 5m or more, you can't reduce the risk that failure of one won't impact the other. If you have a large desert then you have the space, but most people just accept the risk and do what they can to minimise it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[michael333]

Hello LittleInch,

Thanks for your response!

To answer your questions:
Surface trench with grating, or trench box, so the client can visually inspect the length of the pipe.
No, the client does not have an agreed upon piping design code. Your question makes perfect sense, but this client does not provide any design standards to us.

What does "CP" mean under 2)? Forgive me for the stupid question.

My understanding is that the nitrogen, helium, and argon are all non-flammable inert gases per the MSDS sheets.
Should I still worry about a rupture of one of them?

Thanks for all your input. Makes sense to me to direct bury, etc. the piping.

Regards,
Michael

 

[EdStainless]

CP is cathodic protection, used on buried pipe.
With high purity gas lines they need to be fully inspectable.
I presume that the gas lines will all be SS?
How high is the HP Ar line?

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P.E. Metallurgy, consulting work welcomed

 

[michael333]

Hello EdStainless,

Thank you on clarifying CP.
Is there a code reference to look up that states high purity gas lines need to be fully inspectable???

Yes, SS for the gas lines.

Good question! My guess is the low pressure Argon is in the range of 850psi or below. High pressure Argon is 1900psi or above.

Thanks again,
Michael

 

[1503-44]

Visual inspection of underground, coated, CP, pipelines external surfaces is not usually required, is generally unnecessary nor is it practical. They also usually do not need to be SS, unless of course they carry corrosive products.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Michael,

Your client sounds like a right doozy.

No piping code is bad enough, but then "Surface trench with grating, or trench box, so the client can visually inspect the length of the pipe."

Total rubbish. No one, but no one visually inspects the length of the pipe. What are they looking for?

After about a month, the trench will either be full of water, dirt, leaves, dead animals who have fallen in and can't get out / dead / alive insects and half the grates will be damaged, missing or you can't get them off. So any inspection will not be possible.

I had a project where the client, being French, would simply not see sense and just because some lunatic who didn't know better, had specified these in the "basic Engineering" phase, they would not listen to reason and insisted on installing them across undulating countryside.

Total and utter waste of time and money. And just leads to more problems as the pipes squirm about as the heat up and cool down in the sun / weather, rub on the trench side, and liable to increased corrosion, even in Stl Stl.

If you need high purity gases then yes Stl Stl is recommended. Never heard of a requirement that they are "fully inspectable" before.

Either put them above ground properly on a rack or bury them.

Can you tell I don't like surface trench boxes?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

If the pipes are 6" or larger you probably have to drain the trenches to somewhere, or clamp them to supports, or weight coat them for buoyant protection.

Visual inspection isn't much use in a trench box, because you only see the top and corrosion on the pipes will in general be on the underside, probably at the supports.

Direct burial is far better. Otherwise above grade on pipe supports.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[EdStainless]

We took all of our gas lines out of trenches and underground to run them all above ground.
Even very slight pitting or crevice corrosion can allow the diffusion of impurities into the gas and ruin the process.
Our lines only worked up to about 350psi and they were fully welded 316L tubing.
SS was for gas purity reasons, not external ones.

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P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

The way SS is made these days, think you may have to apply a 3coat paint on those SS lines, particularly if they are in some trench that is likely to flood up. Else switch out to 22Cr duplex SS since delta cost is well worth it. Would suggest keeping these SS / duplex SS lines some distance away from the carbon steel fuel oil line to cater for line replacement due to external CS pipe corrosion in this soggy trench.

 

[LittleInch]

"Even very slight pitting or crevice corrosion can allow the diffusion of impurities into the gas and ruin the process."

Really?

How does that work for external corrosion on a pressurised pipe?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

You can get pitting in SS that is so small that you will not be able to measure the leakage rate using normal indicators.
But with any leak there will also be diffusion back into the piping.
Since the pure gasses are <0.001% O2 there is a huge diffusion driving force.
We could lightly loosen a compression fitting and not see a leak with real bubble solution, and yet withing seconds pick up the spike in O2 300' down the line.

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P.E. Metallurgy, consulting work welcomed

 

[LittleInch]

Wow.

I don't deal with that level of contamination but there is some reverse flow.

Even more resson not to put it in a trench....


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Are you building chips?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[EdStainless]

Nope, we were a high-volume welded SS tube mill.
Nothing ruins your day faster than O2 in the wrong place.

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P.E. Metallurgy, consulting work welcomed

 

[1503-44]

H₂S can be kind of like that at times.

Interesting. I didn't know this. Do you have to make it under a N₂ atmosphere, or what? I didn't know that Ti can't be made under under N₂. Is O₂ a problem during manufacturing only, or afterwards too? What is the biggest use of these tubes?


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[EdStainless]

Weld gases were Ar based with additions of He and/or N2 and/or N2.
Furnace gas was H2 with N2 dilution sometimes.
High speed semiautomatic welding does not like any condition changes.
Most was for heat exchangers but also aerospace and some mechanical applications.
It was all SS and Ni alloys.
Ti is easy, pure Ar is the only option.
This applies to welding and HT.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed