Pressure test, N2, He or Compressed air??

[Capntom]

What are the biggest pros and cons about the media when low pressure (225 psig) pressure testing?? We are an OEM and have some cells that use Nitrogen, others use Helium leak detectors while others use the air under water method.

 

[jcfoley]

Capntom,

What you use really depends on your process. If I were leak checking something for liquid service, then I would probably spec out air. Whether the air was dried or not would depend on if the process is moisture tolerant.

For inert gas service, it would depend on how leak free you need it. To me, this would depend on the impurity level of the process. For gases with less than 100 ppm total impurities, I would use helium. Leaks missed with using air can still contaminate the process.

For toxic or flammable gas service I would only use helium period.

It would help if you could explain your process in more detail. The materials of construction can also make a difference.

Helium

Pros: Smallest molecule available. It there is a leak, this will find it. Now how you check for the leak is a totally different matter.

Cons: Expensive

Nitrogen

Pros: Similar properties to that of air. If moisture contamination is a concern, this is better than standard air. If your shop is not set up to handle drying shop air, then this is a good alternative. Even poor grade nitrogen is generally fairly dry.

Cons: More expensive than shop air.

Air:

Pros: Cheap and readily available.
Cons: Moisture contamination.


Hope this helps,
Chris

 

[Capntom]

I am afraid our Engineering department has not spec'd out How we perform integrity tests for welds. They just define the units withstand XXX psig. Our units are pressure transducers, liquid level sensors and fluid valves. Customers purchase our units and can use them in any application from hydraulic oil to brake fluid to freon in an air conditioner unit.
Good comment about the contamination and Dryness. I had not thought of that. How does Mass-flow or pressure decay compare to the others??

 

[jcfoley]

For industrial components where capital costs is minimized by huge volumes, I would use helium with a mass spec. This is not cheap but I used to work for a global supplier of high purity gases. Given the volumes required, I can help you look at an order of magnitude sizing for gas or liguid He supply.

However, your applications are mostly liquid and freon. The freon is relatively low pressure and liquid systems are easy. I would consider dry nitrogen with a pressure decay. If you monitor the temperature of the gas, you can actually calculate a leak rate.

All this being said, it can really all go back to marketing. What value is given to a helium mass spec system in the customer's view? For anything flammable or expensive, I would think that the helium method is required. There are some mass spec units that are capable of measuring leaks down to 10e-9 cc/atm (think the units are correct but I can only rememeber the number for certain!) This is semicondutor purities and levels (impurities less than 1ppm or better). Bubbles can only give you a 10-3 cc/atm.

It seems to me that you need more information from engineering who may, in turn, need more information from the marketing folks.

 

[PZas]

I understand that you are currently looking at leak testing with gasses. However your current items all resemble pieces that would usually be hydro tested with either hydraulic fluid or even water. The fluid used would be totally up to you.

This would be much safer, faster, and cheaper. If contamination on site (oils or corrosion from water use) isn't a problem you may want to look into this.

the use of the word "withstand" makes me believe that this is more of a structural test than looking for 1ppm of leaking. That noted the use of compressed gasses is very dangerous. If any of these items are large in volume you could be looking at a little bomb.

 

[jcfoley]

PZas has brought up a good point. My posts refer specifically to leak checking as it was my understanding that you have already proven the design. This can only happen after a design is proven. If codes require you to do a proof test, that must be done with water.

Leak checks should be done up to the design operating pressure...not 1.5 or 1.3 or even 1.1.

 

[Capntom]

Gotta say, I love this forum. Thanks for all of the help so far. The testing I am doing is all in-process, not design or proof tests. We have a few different kinds of processes. Helium leak check to 10e-6, Compressed air up to 100 psig, Nitrogen up to 225 psig as well as Hydrostatic >225 psig

 

[jcfoley]

Capntom,

I have been working in the high pressure gas industry (read "high pressure" as 3,000 - 7,500 gas compression). It is critical that systems be structurally tested by hydro.

Hydro is not a leak check. Special precautions are always taken when doing pneumatic leak tests. First of all, I normally start with a 50 psig "gross leak" check. This is where you find valves that have been left open. The hydo normally finds any other "gross leak" problems. Then I take the pressure up in 250 psig increments looking for leaks at each stage.

I would reiterate about getting with your engineering dept so that they can determine the appropiate test. It sounds to me like they left something off the print.

 

[robsalv]

Capntom, in my plant, 225psi is not all that low.

I have no idea how large these units are, or what "cells" are, but I wouldn't be pressure testing anything pnuematically if I could avoid it!

If you're only doing sensitive tightness checks, then keep the pressure as low as possible and with air and nitrogen, use soap solution at joints to test for leaks.

With He... well, good luck, those little molecules squeeze through anything. Sniff the joints with the detector.

The stored energy is the prime danger in a pneumatic pressure test. Tread warily.
Cheers

Rob