Hydrotesting Instrument Tubing 5

[DusanAust]

Is anyone aware of a Code requirement to hydrotest stainless steel instrument tubing in gas plants (the compression jointed type)

I am familiar with the normal procedure for hydrotesting pipework prior to commissioning, however the stainless steel instrument tubing appears to not normally be tested. I've reviewed a couple of the piping standards and they appear to neither specifically exclude the tubing or include it. Any help or experiences on this topic would be most appreciated.

Cheers

 

[moltenmetal]

Any component containing process pressure in an ASME B31.3 piping system must be tested with the system unless it has already been tested as part of another standard or is exempted from the code (i.e. pumps, compressors etc. which are tested as sub-assemblies and exempt from ASME B31.3, ASME VIII vessels which have already been hydro'd, etc.). That includes process impulse lines on instruments, and any other tubing used for process service. The owner can elect to test Category D (non haz fluids like air, water etc.) tubing in service rather than by means of a static hydro or pneumatic test.

The choice of hydrostatic or pneumatic testing of the tubing will depend on whether or not a meaningful hydrotest can be accomplished (i.e. if the system can be completely filled with water such that the test is sensitive enough to leakage), and whether or not the instruments in question can survive the test pressure without damage. Pneumatic testing of tubing systems is not without its hazards and must be done under conditions which guarantee the safety of the tester.

Inevitably, you end up with a few joints that cannot be part of the hydrotest, i.e. the connection between the relief device and a vessel etc.). These are tested generally in a pre-start leakage test at the design pressure before operation.

 

[pennpiper]

Now wait a minute, let me see if I understand this.
We spend a lot of money on equipment to filter and dry the air for the instrument air system so there will not be any rust or corrosion in the valve diaphram or cylinder operators which might then cause a malfunction of a critical system.
And now you say you want to hydrotest this system? This sound like a very dumb thing to do.

 

[hacksaw]

Normally you hydrotest up to your spec break which will include an isolation valve on the process. The code is clear on the subject.

Instrument tubing and air lines are rarely hydro'd unless there is a specific need for it. Air receivers are shop hydro'd usually.

 

[dcasto]

Remember, under B31.3, the test is not a strenght test, its a leak test of the mechanical joints. If you tubing contained a cover substance floow Moltenmetal, If it will have acovered sudstance, do a test on it because the joints maybe bad. I had a plant that used 3/4" compression fittings on the liquid propane and two unions failed due to bad torque in the compression fitting. Wish we'd found the proplem with a test back then. (no fire or serious damage, just stained clothes)

 

[moltenmetal]

pennpiper: instrument air piping and tubing etc. is in Category D service. The code permits the owner to specify an "in service" test instead of doing the code-required hydro or pneumatic test- i.e. putting the piping in service with instrument air and test for leaks. No need to hydro- and good reason NOT to hydro because drying small lines after a hydro is a b*tch. Receivers, on the other hand, are usually hydro'd because they will make a spectacular bang if they let go under even 100 psig of compressed air. And the test isn't finished until the receiver is completely dried out afterward!

dcasto: read my original post: what I said is that ALL tubing containing hazardous process fluids (i.e. "general fluid service" under B31.3), including instrument impulse legs, must be tested prior to being put in service if you are to be in compliance with B31.3. The test is for leakage to the external environment, which is indicative of badly-made joints or other flaws in the fabrication. You are correct that it is not truly a proof test- but it's a hell of a lot better than NO test, or a test with live hazardous fluid like liquid propane!

The choice of which test method (hydro or pneumatic) is used for tubing systems is a matter for the engineer to decide based on the risks and benefits of using either method. Hydrotesting of small tubing lines is seldom warranted. A properly designed pneumatic test carried out correctly is safe, gives a more dependable result and results in much less risk of problems to the process equipment later. But if you've ever seen a tubing line let go under significant pressure, you'll gain the respect needed to spend the time to design a proper test that doesn't put the tester in harm's way.

 

[dcasto]

No problem here. I just like to remind the younger people that 31.3 doesn't require a strenght test. 31.8 and a lot of transmission pipelines like to test the strenght by testing all the way to yeild.

 

[DusanAust]

Thanks everyone for your input into my question. The information provided is most appreciated.

Cheers