Air test pressure on boilers 1

[sb3]

Is there a maximum test pressure allowed for pressurizing a boiler with air to look for leaks in power boilers? I don't see one in the boiler code. We try to limit pressure due to stored energy.

 

[metengr]

The National Board Inspection Code (NBIC) requires a pressure test after weld repairs are performed to assure the integrity of the weld repair (see RC-2051). The pressure test can be conducted using liquid or gas (pneumatic). In addition, other tests can be performed in conjunction with a pressure test. The limit for the pneumatic test is a minimum pressure necessary to verify leak tightness of the repair.

For conducting in-service leak checks if no weld repairs were performed, there is no Code guidance. However, you are wise to use a cap on air pressure - we limit air squeezes to 25 psig.

 

[briand2]

I don't understand why you want to use an inherently dangerous pneumatic test in something that's eventually going to contain water anyway. Why not just stick with a hydraulic test?

 

[metengr]

In some of the steam circuits in a power boiler, for example, reheaters or low temp superheaters you cannot and do NOT want to use water to backfill and squeeze for leaks.

 

[briand2]

metengr,

It may be that we're talking about very different boilers here. I'm in the UK, and I normally only deal with gas / oil fired smoke-tube boilers producing about 17000 to 30000 1b/hr steam at around 50 to 120 pounds/in2. I have regularly pressure tested them with water, under instructions from our insurer / engineer surveyor. I seem to recall doing the same thing with ship-board boilers when I was an apprentice (thirty years ago!).

Perhaps you're talking about relative large (presumably water tube) power station boilers? In any case, I'd be interested to know (for my own education) why a hydraulic / water pressure test is not acceptable in the circumstances your'e thinking of.

Thanks,

Brian

 

[metengr]

Yes indeed, I am speaking of a large (>1.5M lb/hr) power boiler for an Electric Generating Station in the US. In some large power boiler designs, the reheater and other steam circuits do not fill with water like a waterwall tube circuit during a boiler fill. In addition, most of our boilers are over 40 years in age, and the original hanger systems are also aged. Filling the steam circuit with water for a squeeze results in tremendous static weight and could damage these boiler hanger support systems.

Last, but equally important, some of the steam circuit heavy wall components take literally days to cool. The introduction of water for a pressure test increases thermal gradients and stresses in these heavy wall components that can accelerate fatigue damage – like ligament cracking in the bores of superheater and reheater outlet headers, and welded nozzle attachments.

 

[briand2]

Thank you very much for the explanation, but I'm still a little puzzled. There is a difference (in the UK) between "soundness testing" (testing for leaks) and "pressure withstand testing" (testing to see if the vessel / pipework is strong enough. Soundness testing, as in the case of gas pipework, can be done at a relatively low pressure. Pressure withstand testing, on the other hand, is typically done hydraulically to 1.5 times the normal working pressure of the vessel / pipework. Assuming you want to limit the energy stored in pneumatic testing by keeping the pressure relatively low, then how do you know that the vessel / pipework will be OK at normal pressure unless you also do a (hydraulic) withstand test.

Please excuse my ignorance as the boilers I work with are babies compared to those with which you are familiar!

 

[metengr]

The 1.5X WP (an example of a proof test) hydrotest that you speak of is normally not performed unless their is specific reason to question the integrity of a boiler due to age or prior service. A 1.5X MAWP hydrostatic test is done after construction of the boiler to assure two things - adequacy of boiler component design and no gross or missed material defects during fabrication. In addition, the 1.5X MAWP provides a benefit on new material where local yielding of new material can blunt pre-existing small flaws (a major benefit).

Once a boiler is placed in-service a new set of Code rules applies in most if not all US Jurisdictions. These repair code rules deal with in-service inspection and weld repairs. Two basic concerns to assure safe operation.

For the in-service inspection any number of tests up to and including a pressure test can be performed to check for leaks. The adequacy of design was already established so there is no need to revisit this issue.

As part of an in-service inspection of a boiler to continue operation, visual and crawl-thru inspections are conducted, a review of previous repairs and operating history. The pressure test should only performed to check for missed boiler tube leaks not to re-assure integrity of the boiler or to take the boiler component to failure. There are plenty of approaches to evaluate the condition of boiler components without the need for subjecting them to elevated pressure at ambient temperature.

We have specific requirements to assure safe operation of boilers and pressure vessels and these requirements are mandated by local authorities. Pressure testing is sometimes used by several Jurisdictions when a boiler reaches a specific age. Again, I look at this as a false sense of security and can do more harm than good. As a boiler ages, understanding the failure mechanism(s) that cause tube leaks and annual or semi-annual inspections by competent personnel will avoid the necessity for a proof test.

 

[davefitz]

Don't forget the real need to ensure that ALL metal parts of the tested component must be above 70F for any sort of pressure test, and this is espescially important on pnuematic tests.

In particular, the newer alloys ( P91) are very brittle atlower temperatures. Many catastrophic failures during pnuematic tests can be traced to nil ductility, and this can be expected at low temps in eld zones.

 

[unclesyd]

Not mentioned above, there is no need for any testing of an inservice boiler exceeding 1.1 x operating. Most of the old gals don’t like to be over stressed after a year or so on the job. One other thing that used to be done was a hybrid test with N over boiler feed water.
After a wetting incident, me, during testing in the winter I prefer air or N or N + He.

 

[sb3]

Thanks for your replies, particularly Metengr. He is correct that we do lower pressure air tests to look for leaks when a boiler is offline for several reasons. The reheater sections are dry and it is extremely inconvenient to fill them with water due to isolation and pipe loading constraints. The boiler may also be checked with air after fixing a major leak that may have forced the unit offline to look for other existing leaks in superheater sections which would also normally be dry.

I didn't think there was a code limit for these low pressure checks since obviously the boiler will be expected to carry its rated pressure with steam or water.

Metengr, you stated that you limit pressure checks to 25 lbs. Considering that the boiler may soon be carrying 2700 psi drum pressure or maybe 550 psi reheat is the 25 psi limit even required for safety or just to limit time to pressurize and depressurize the boiler.

 

[221175]

Pressure testing through air is not recomended in boilers due to the oxidtion that reduced the life of tube.You can use N or HE. Is your boiler is leaking or you want to check as precautionary measure? I think hydraulic pressure test is suitable and cheaper test to find out any leakage and this is a standard procedure. That why code donot specy pneumatic test.

 

[metengr]

The limit of 25 psig for air is strictly safety.