Leak Test Method for Piping in N2 Service

[DGrayPPD]

All,

I am in the process of trying to gather as much information as I can on the subject. I would like to know some different viewpoints and opinions on the topic of which type of leak test should be performed on piping used in N2 service.

The piping is all 2" NPS and smaller, and I believe falls under the ASME B31.3 Cat D fluid service as the pressure is not over 150 psig. IMO and from the information gathered from the code and online, this piping is acceptable to be given a service test in lieu of a hydrostatic or pneumatic test.

But if the pressure were to exceed 150 psig and the piping were to require a leak test, which one should be used? Hydro or pneumatic? So far I have gotten varying opinions from individuals with different backgrounds and I am seeking more opinions. If hydro is performed, are there concerns that the water and nitrogen may react if the piping is not properly blown down and dried out prior to start-up, which could lead to corrosion? Is it better to avoid the risk by performing the pneumatic test?

Thanks for any help on the topic.

 

[EdStainless]

I have never hydro-ed pure gas lines, I didn't feel that we could reliably dry them out.
Our N2 service has a DP <-40 at the point of usage. (He, Ar, and H2 are all <-60F)
We leak test every joint with bubble solution (purchased, certified, Cl free).
Don't make your own. You won't know the sensitivity and very liquid soap is loaded with salt.


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P.E. Metallurgy, Plymouth Tube

 

[DGrayPPD]

Thank you EdStainless. What are the implications to worry about if they were to hydro the line and not reliably dry them out?

 

[bimr]

Piping leak testing for specialty gas piping is typically accomplished with a pneumatic test.

The piping is typically tested to say 50 psig above the system operating pressure, but you have not provided the system operating pressure.

Of course, pneumatic testing is considered to be a more hazardous operation than hydrostatic testing, but sometimes there are not alternatives. There are specialists that do this type of testing.

 

[DGrayPPD]

Thank you bimr.

There are 2 nitrogen lines in question. One is operating at 50 psig and one at 140 psig. Both have a max operating pressure of 150 psig, which is why I say they should fall under Cat D and only a simple service test is required. But if the lines were to operate at say 175 psig, then I'm just curious which test is preferred or better practice. If a pneumatic test is preferred, but a hydrotest is specified, what implications could this have?

These questions are just a hypothetical scenario to gain a better understanding, so that I may be on the lookout for future discrepancies.

 

[EdStainless]

This depends on the purity targets of the service.
While hydro can be used to apply stress to the system, it is not a leak test.
At least not if this for high purity service.
We had some high pressure lines (200psi) that we vacuum pumped for a week before we He tested every connection.
How much care was used in fabrication?
Was all of the tubing purged and capped when you received it?
If the system is dirty then a hydro may be a good idea.
But getting all of the water out afterward is a big task.


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P.E. Metallurgy, Plymouth Tube

 

[bimr]

Here are some piping testing links to review:

Link 1

Link 2

 

[DGrayPPD]

Thanks bimr for the links.

I've heard that assuring that all the water is out of the line afterward is a big challenge. Which is why instrument air piping is typically only done with dry, clean air. I'm assuming that the same method should be done for the specialty gas piping such as N2, but I'm getting push back from other engineers that are saying it needs to be hydro tested and not service tested.

 

[LittleInch]

Well a good read of B31.3 (if this is your code) section 345 is required by you and the "other engineers".

Cat D fluids are quite specific in allowing a service test, but for others there is still a wide variety of allowable practices, but all at the option or with the consent of the "owner".

The clear default case is a hydrostatic test, but a number of other options are presented including pneumatic testing, extensive / 100% weld testing and bubble or sensitive leak testing ( Helium etc)

I can't see any issues in terms of Nitrogen or water combining to do anything bad other than increase the dew point of the system.

Now there are many ways to de-water and dry your piping system and only you have any idea how complex your system is, how many branches, dead legs, drains or lack thereof there are which would complicate the de-watering system.

Or given N2 is often very dry, especially if it comes from a liquid N2 supply, so just blow a load of N2 through it until the dewpoint gets to what you want.

The key in B 31.3 is the view of the owner.

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

 

[EdStainless]

If this is for high purity N2, and they want to hydro, then it will require a solvent flush and long term purge. If the system has been designed with no dead legs or blinds then there might be a chance.
If you finish by purging with N2 that has a -80F dew point you might purge for many days to get the exhaust gas down to -40.
I should also ask, what is the MoC and fabrication method? Brazed heavy gauge Cu? welded stainless?

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P.E. Metallurgy, Plymouth Tube

 

[DGrayPPD]

Yes, the design code is B31.3. The 2 lines in question are both 2" CS SW piping. More details below:

2" Nitrogen conforming to 150# class spec
CS SW piping
Normal Op. Press: 140 psig
Normal Op. Temp: 80 deg. F
Max Op. Press: 150 psig
Max Op. Temp: 100 deg. F
Design Press: 250 psig
Design Temp: -20/130 deg. F
Test Method: Hydro
Test Press: 450 psig

2" Hot Nitrogen conforming to 300# class spec
CS SW piping
Normal Op. Press: 50 psig
Normal Op. Temp: 250 deg. F
Max Op. Press: 150 psig
Max Op. Temp: 300 deg. F
Design Press: 3000 psig
Design Temp: -20/300 deg. F
Test Method: Hydro
Test Press: 1125 psig

 

[EdStainless]

Since these are socket welded you will likely never dry them out after hydro.
You do need to find out if they care about gas purity (low O2 and DP).
I am guessing that since they used CS they don't have a very demanding application.

The other issue is if they want to do a leak test (bubble or He) it needs to be done before any hydro.
Water from the hydro will plug small leaks and make them impossible to find.

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P.E. Metallurgy, Plymouth Tube

 

[Gator]

Is this utility N2?

 

[DGrayPPD]

Gator,

Yes.

 

[DekDee]

Guys,
A bit confused here.
Cat D applicability is based on design pressure, not maximum operating pressure.
Both of the examples supplied exceed 150 psi design pressure.
Am I missing something here ?
Cheers,
Shane

 

[KevinNZ]

Check what is your system for valves etc. These class 150/300 items my limit the test pressure. 450 psi seems high for a class 150 system.

 

[GD2]

And have a look at your project design basis (DBM). Check if the system is designed for hydrotest load in terms of piping supoorts. You will find valuable info in the IFC ISOs. It should give you the test type and the test pressure.

Ganga D. Deka, P. Eng
Canada

 

[DGrayPPD]

DekDee,

At the time of posting the question, the two lines were still under review and the thought was that the design pressures and temperatures would be lowered. Now, it seems that may not be the case and that the conditions may stay the same, which would mean they probably wouldn't qualify for Cat D.

KevinNZ & GD2,

The test types and pressures are under review currently. The pressures seemed high, and for the utilities, I questioned why a simple service test would not be sufficient. I also have questioned some of the test methods as well, as even on instrument air lines the line list shows hydrostatic testing at 450 psig, when it has been my understanding that instrument air should always be tested with clean, dry air.

 

[DekDee]

DGrayPPD,
This is where I was confused.
"Both have a max operating pressure of 150 psig, which is why I say they should fall under Cat D and only a simple service test is required".
Regards,
Shane

 

[DGrayPPD]

Yes, I was confused. My apologies. After going back and rereading the code, I see that I was mistaken.