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Technical Discussions
Bimal
Bimal
22:12 Aug-27-2009
MSLD

In my job, a need for leak detection has come & MSLD (Mass Spectroscopic Leak Detection) with Helium is being proposed. As Helium is very expensive, a suggestion has come that only a certain minimum percentage of Helium is required to do the test (say 20% Helium & 80% Nitrogen).
That is, since the volume in which test is to be done is large, we can fill it with 80% Nitrogen & 20% Helium.
But I want to know the reliability in doing the test in this manner. Will this test give satisfactory results as comparable to use of 100% Helium for the test?
Please help me by giving a satisfactory answer.
If anyone come across a document connected with this subject, please let me know about that also.

Regards,
Bimal :-)

    
 
 
Roger Duwe
NDT Inspector, API-510, 570, 653
MISTRAS, USA, Joined Jan 2009, 148

Roger Duwe

NDT Inspector, API-510, 570, 653
MISTRAS,
USA,
Joined Jan 2009
148
17:51 Aug-28-2009
Re: MSLD
In Reply to Bimal at 22:12 Aug-27-2009 (Opening).

Yes, it is common to use a diluted helium gas. If leak detection is the goal, this is very sensitive. Sometimes too sensitive, as any leakage from the supply hose will be detected too. On heat exchangers, we usually tape a plastic sheet over the area of interest, and ensure that the quantity of He detected under the sheet is less than the ambient He level.

It your specification requires quantification of the leakage rate, you will either have to use pure He or compensate the rate calculation by using the Partial Pressure of He, not the total pressure of the mixture. This is self-defeating, so use pure He. Will take the asme quantity.

    
 
 
Bimal
Bimal
21:28 Aug-28-2009
Re: MSLD
In Reply to Roger Duwe at 17:51 Aug-28-2009 .

Thanks for the fast response, Roger...

Yeah, as you told, we want to have a quantification of the leakage rate with the use of the Helium detector, it'll show the value in its dial. So, as you said, if we use 100% Helium, we will know the leak rate directly, that's fine...

But if we use 20% Helium & 80% Nitrogen, will it be sufficient to use the partial pressure calculation? Nitrogen molecule is having much bigger size & so, can we tell that nitrogen will leak through all places in which Helium leaks? Will that leakage be also be in the same ratio as partial pressures?

& I would be really grateful if you provide me with any document support in this regard, as I'm not the single person to decide on this matter :)

    
 
 
Roger Duwe
NDT Inspector, API-510, 570, 653
MISTRAS, USA, Joined Jan 2009, 148

Roger Duwe

NDT Inspector, API-510, 570, 653
MISTRAS,
USA,
Joined Jan 2009
148
20:23 Aug-30-2009
Re: MSLD
In Reply to Bimal at 21:28 Aug-28-2009 .

You will get reliable qualitative results using mixed gas. The only way to get good quantitative results is to use pure He. The leak rate of N2 versus He (1) will be greatly lower. Don't know if anyone has quantified the difference, when in a mixed gas.

You will need about the same total volume of He in either case. Just evacuate the item being tested to about 1 torr, and fill with He to about 3 atmospheres. This will only require 4 volumes of He.

Personally, I'd go with a mixed gas and achieve a zero leak rate, assuming there are no rotary joints in the item. With welded major items and fittings, and swagelok instrument connections, a 'tight' system is achieveable.

    
 
 
Muthu
Director, - -
Metalcare Group Inc., Canada, Joined Oct 2008, 15

Muthu

Director, - -
Metalcare Group Inc.,
Canada,
Joined Oct 2008
15
21:46 Aug-30-2009
Re: MSLD
In Reply to Roger Duwe at 20:23 Aug-30-2009 .

Is this a vessel or piping? I assume the previous dicussions referring to "Inside-out" technique where you require lot of helium to fill-up. Usually, Inside-out technique is considered comparatively less sensitive even using 100% He. The leaking He tend to get diluted with atmosphere before reaching the detector. Do ever considered "Outside-in" technique?; where you creat a negative pressure inside the vessel(inside is connected to MSLD) and spray all the joints with He.

    
 
 
Bimal
Bimal
21:09 Aug-31-2009
Re: MSLD
In Reply to Muthu at 21:46 Aug-30-2009 .

The area of application is a tank, & that too of somewhat huge size, that's why filling with full volume of Helium is under a question mark. Anyway, it will be really useful if you give me the link to any document showing the leakage in terms of partial pressure if a gas mixture is used. Let us assume that Helium & Nitrogen molecules will come through all types of pores :)

& Sri Muthu, this tankage has got some plumbing lines also, which need to be checked. However, there is no feasibility to do the outside-in technique as the MSLD detection unit cannot be taken inside due to accessibility problems. So, if any document support regarding the inside-out technique is with you, you may kindly inform me.

Regards,
Bimal :-)

    
 
 
Bimal
Bimal
07:55 Sep-04-2009
Re: MSLD
In Reply to Bimal at 21:09 Aug-31-2009 .

Now, another doubt has come to me regarding the leak check. In all types of leak check documents & data, we can see terms like "working pressure", "operating pressure" etc. So, what does this exactly mean? Is it the pressure at which we are keeping the sniffer (atmospheric)? Or, is it the pressure at which the component to be leak tested is pressurised?

So, another question regarding this is: Suppose we pressurise a bottle to 10 bar of Helium & do the sniffer test. We will get one leak rate. Then, let us pressurise the bottle to 20 bar & do the leak check. Of course, we may get a different reading in MSLD, I hope. So, is there any way to include the differential pressure (pressure difference between inside & outside bottle) to account for this? How will we fix the specification for leak rate when our actual operating pressures may be different from the pressures at which we are doing leak check with MSLD.

Please do help...

    
 
 
ALBERTO
Engineering, Leak Detection Service - Thermography Service -
ETS Sistemi Industriali, Italy, Joined Aug 2002, 17

ALBERTO

Engineering, Leak Detection Service - Thermography Service -
ETS Sistemi Industriali,
Italy,
Joined Aug 2002
17
14:51 Sep-04-2009
Re: MSLD
In Reply to Bimal at 07:55 Sep-04-2009 .

Dear Mr. Bimal,
Thank you for your appreciated questions and for helps and answers by the collegues.
I thing that the matter of doubts that you have cannot be solved simply via mail. The final results of your tests is that the manufact don't have a leak or there is a leak that must be repaired and certified under normatives and state of the art calculation and conversion to the real leak. From level of your doubt I intend that you haven't done Helium Leak Test until now (I don't seen your profile in ndt.net, so I don't know your activities).
Because there are more LT Experts that have invest many years of studies, experiences and money for instrumentations, courses and certifications around the world to obtain the answers that you need in a few mails, I suggest you to inform the forum about your real activities and Company and to contact an expert in your Country to follow and training your for this particular inspection.
Sorry for my direct but realistic answer.

    
 
 
Muthu
Director, - -
Metalcare Group Inc., Canada, Joined Oct 2008, 15

Muthu

Director, - -
Metalcare Group Inc.,
Canada,
Joined Oct 2008
15
18:35 Sep-05-2009
Re: MSLD
In Reply to ALBERTO at 14:51 Sep-04-2009 .

Dear Mr.Bimal,
Thanks for the information that you have provided. I agree with Mr. Alberto that Helium leak testing is a specialized area where it requires extensive experience in addition all the classroom and hands-on training. You may identify a specialist or service provider in your region and providing them the specifications/drawings or any other specification documents you have and they would come up with test procedure for your review. On your review, you can clarify with them any questions your may have. I assume when the owner has budget in building such tank and they specify leak test to assure the integrity, they should have a sufficient budget for such service to test the tank to ensure the integrity of the tank before putting into service.
Best wishes,
Muthu

    
 
 

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