Safe operating pressure to work on H2 Line 1

[limaruoo]

HI all,

I work at a chemical plant and we have small hydrogen leak coming out of this long hydrogen line among some fittings.

One of the main issues with H2 besides its wide flammability range is that when you have to purge the line of hydrogen you have to use helium to purge the line before H2 is reapplied. this ensures there is no oxygen that entered the line while the line was down for repairs

my question is what is the lowest pressure the line can be safely worked on?

Right now it has 100 psig of pressure in it. we have multiple gas monitors around the area to ensure the leak doesn't get close to 25% of the LEL. The area is now restricted

We would like to lower the pressure to a safe pressure so we can work on it without having to do a full helium purge.

I tried looking up pressures of gas that can cause injury to personnel and can't find much. the concern is if someone is working on the leak (by tightening fittings) we are worried if the gas pressure were to spray out onto the worker. Obviously, a 600# leak can cause injury but what about 50#? 20#? 10#? I'm aware this depends on multiple factors as well.

any advice?

 

[LittleInch]

The only "safe" thing to do is remove all the Hydrogen from the line.

After that it's all about risk.

What sort of "work"??

The issue is not gas jets, it's fire and explosion. As you say Hydrogen has a huge flammable limit, very low ignition energy and a distinct tendency to explode.

I cannot see any lower pressure limit which would result in an acceptably low risk, especially doing anything which created even the smallest spark by e g. dropping a spanner.

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

 

[limaruoo]

@LittleInch

good points. the leak is coming from some compression fittings. the work scope is to tighten the fittings using non spark tools to see if that creates a good enough seal. If it still can't hold pressure without fixing then the fitting will need to be replaced.

 

[Heaviside1925]

OP,
You are advocating for work to be performed in an explosive environment. The only way I could see this being permitted is if you were able to ensure a non-explosive environment. I think there are ways, but I would not advocate this practice. Consider that if you lower the pressure and personnel tighten the fittings, you have no way of knowing if the leak was repaired or made worse until the line is repressurized. You have not mentioned one of the main reasons for helium being used in a H2 line purge, which is its similar characteristics to H2 in terms of leaking. It also provides ease of detection, not to mention, it's not explosive. Helium purging the line allows for the personnel to tighten the fittings and confirm the leaks are repaired, prior to repressurizing the line with H2. This is good practice and unless there are other more critical factors at play here, outside of just the inconvenience of the helium purge, I would see no reason not to follow it.

 

[Heaviside1925]

I would hesitate to recommend adding more fittings in an H2 design but if this is a problem area, it may help to provide a more localized way of blocking and purging this section for future repairs.

 

[limaruoo]

@Heaviside1925

To be clear, I'm not advocating for this work.

I'm merely asking if it's safe to perform this work at a lower pressure without purging but based on your comments as well as others in this thread it seems a full purge is required to ensure safety.

However, devil's advocate here;

If we are able to tighten the fitting at a lower pressure we are able to see if that worked by monitoring the pressure to see if it drops.

We can also do something less sophisticated and get a piece of tape to see if it moves when held near the fitting.

 

[Heaviside1925]

Devil is also in the details. Line sizes? flow rates? Have you confirmed which fitting is leaking? Indoor / outdoor? existing ventilation? electrical classification? distances from sources of ignition?

Consider with H2 at about any pressure can cause a room to go from safe to unsafe very quickly. Another consideration is that H2 loves to leak. In other words, you may not be able to detect with "a piece of tape". Ultrasonic leak detection may work but in a congested space, it may be impossible to localize.

From just a basic safety standpoint, I will reiterate, you must ensure a non-explosive environment unless there are other more critical driving forces. There are instances where working in a possible explosive environment is required and there are steps to ensure it is done as safely as possible, but this is only after all engineering controls and administrative controls are deemed impractical or unfeasible and all involved are aware if the inherent risks. All that said, there may be ways to ensure a non-explosive environment but without more details, this cannot be determined.

 

[LittleInch]

So just imagine what could occur if the technician "tightening" the compression fitting applies that extra bit to " make sure" and the fitting shears off, strips the thread or destroys the olive?

Risk is probability times consequence.

Probability is medium I would guess, consequence v high...

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

 

[EdStainless]

With Hydrogen non-sparking tools are misleading.
I can still hit two bronze tools together hard enough for a spot to reach the ignition temperature.
Is this indoors or outdoors?
If indoors how confined is the space?


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

 

[shvet]

We would like to lower the pressure to a safe pressure so we can work on it without having to do a full helium purge.

Curious to know why helium, what is wrong with nitrogen? Can you explain please?

 

[pierreick]

Hi Shvet,
To answer your question:

https://h2tools.org/bestpractices/purging

Pierre

 

[shvet]

@pierreick
thank you very much
This is a chemical plant, this is not a NASA facility. So the question is still on - what reason to purge gaseous hydrogen with helium instead of nitrogen? What's wrong with nitrogen?

@limaruoo
Note that most large chemical plants and refineries handling hydrogen I am familiar with ignore small hydrogen leaks if those are not related to a confined space. Even in cases when those are burning.
What is not ok with this leak other than some attention of a boss? Can you explain?

 

[LittleInch]

Even in cases when those are burning.

Holy shit. Remind not to visit any of your plants....

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

 

[Heaviside1925]

Shvet You can purge with N2 but for leak detection either helium or helium/n2 mix is preferred due to gaseous helium's similarity in molecular size to H2. If only purging is required, I would think N2 would be appropriate but if a subsequent leak checking is required, then helium or a helium mix would be used, and in terms of fixing a leak, I would think purging and leak checking would both be needed. I will validate your observations that I have also seen instances where an H2 leak is if little or no concern but based on OP's description of the situation, fittings, location and without further information, I am leaning towards a more conservative approach.
I have not seen burning leaks though. Out of curiosity, were those instances allowed to burn since burning H2 is a least noticeable as opposed to an unknown/unseen leak? or some sort of brute force leak check?

 

[shvet]

Holy shit. Remind not to visit any of your plants....

Do you mean that at a hypothetical Baton Rouge a manager would stop a hypothetical VGO hydrocracker, naphtha reformer, or steam cracker just because of a small leak of hydrogen? $1 mln losses per day because of someone can't tolerate a finger-sized barely discernible torch? Really? Not convincing, I refuse to believe there exist a such place at this planet.

OK, ok. There is a hydrogen leak, hydrogen is explosive, this is a dangerous situation, safety first and all that bla-bla-bla we know well. And what? Hydrogen leaks. And what? Hydrogen eventually starts burning. And what? Someone noticed it is burning. And what?
Everyone agrees hydrogen is a nasty thing. But what is pushing one to immediately fix it? And how long will it be before a next small leak comes out?

 

[EdStainless]

We used H2 for process in a manufacturing plant, cryogenic 495 purity
We couldn't ignore small leaks that were outdoors because of the oxygen and moisture that diffused back into the lines.

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

 

[georgeverghese]

How did this configuration get through a HAZOP in the first place - you've got a break from piping to compression fitting / tubing with no isolation valve at the break ? How many more like this ?
Even with natural gas, it is common practice to completely depressure and purge down to 50% LEL or lower before breaking a line for repairs.

 

[pierreick]

Hi Shvet,
Have you ever been to Baton Rouge (Louisiana/US)?
Your comment about leaks is very weak, not the first-time regarding Safety and risks by the way.
My opinion only.
Pierre

 

[shvet]

we are worried if the gas pressure were to spray out onto the worker.

You should keep pressure inside of the pipe slightly above atm to ensure oxygen will not come into as a resutlt of a stratification of gases inside (air is ~15 times heavier than H2). Internal explosion is much more severe than cloud one. So yes, for a safety reason this worker shall be sprayed out by some amount of H2 gas.
I personally have never done such in H2 environment so I can't advise. But if you will have to deal with a similar leak of vapors of NH3 or some hot chemical - I have a few stories.

Anyway fixing under ex-environment and O2-depletion simultaneously is not a good idea and should have sound justification other than one's desire. At making decision it is always worth to recall that safety instructions were written by the blood of losers. But this particular worker is not a loser, is he/she?

You can purge with N2 but for leak detection either helium or helium/n2

The topicstarter is going to fix leaking fittings under H2 environment without purging by He. If this is correct then he/she had already passed the leak detection stage or realizes how to find leaks not introducing He. This means the question is still on - why not just purge this pipe with nitrogen and fix safely? Why has this issue become so complicated?

Out of curiosity, were those instances allowed to burn since burning H2 is a least noticeable as opposed to an unknown/unseen leak? or some sort of brute force leak check?

I do not understand. Please ask another way.

Have you ever been to Baton Rouge (Louisiana/US)?

No, I have not.

Your comment about leaks is very weak, not the first-time regarding Safety and risks by the way.

Did not catch. Is it not true what I had been sharing? Should I keep my thoughts to myself? Or is every point valuable for this community?

 

[georgeverghese]

Speak to the compression fittings manufacturer also - they may have some "band aid" type temporary fix to common problems like this.