Hydrostatic pressure testing of pressure vessels 4

[CengMBA]

Many thanks to all the pressure vessel experts who have been kind enough to reply and contribute to my question.Judging from the diverse range of replies it seems that we have not bottomed this one out yet.It seems to me that the questions I have posed in the first part of my note are valid. Also, I think the second part of my question was slightly misunderstood.However, that is my fault and I apologies. I should have phrased the question better.

The question should have read as follows:

Once the vessel has been in operation and requires revalidation, what is the benefit of rehydrotesting and are there better ways of going about the revalidation process than simply pressurising the structure upto the original pressure test value.

 

[eyec]

hydros usually are run @ 1.5 X the design pressure. this substantiates a safety factor and aides in the discovery of any leaks due to insufficient welds or materials weakness in the vessel.

the only other test i can think of and is used in some cases is a helium test, but i don't think you would want to go that route.

besides, the cost of a hydro is insignificant. if a hydro causes delays in a critical schedule then it is not the fault of the hydro requirement (and the delay in painting), it indicates that a re-evaluation of your vendor surveillance program is in order to prevent previous (prior to hydro) slips in schedules.

 

[metengr]

Yes, absolutely. I have been heavily involved with the NBIC in downplaying the use of hydrotesting in-service vessels – especially boilers. Their are other methods to assure the integrity of an in-service pressure containing item other than subjecting the vessel to an ASME B&PV code hydrotest (1.3 X or 1.5X MAWP).

You have proven the design and integrity of the component with the first hydrotest. After the component has been in service, their are other means to "re-validate" integrity. What this all boils down to for in-service vessels is knowing the past operating history, reviewing the inspection logs, documenting previous repairs and understanding the damage mechanisms. If the answers are known to these items above, a hydrotest is not required and non-destructive testing (UT, RT, field metallography, etc) can be utilized to assure vessel integrity.

Performing hydrotests on in-service vessels can result in damage to supporting equipment or structure that was not coupled to the component or was added after the original hydrotest of the vessel. MOST IMPORTANT - a hydrotest does NOT indicate any level of remaining service life for the vessel! It only indicates at this time, the component does not leak water. I have supported an operating pressure squeeze in cases where the vessel cannot be totally evaluated by NDT or there was no supporting documentation on the past operating history of the vessel.

 

[eyec]

for in-service hydros, what has been accepted by a State and the owner's AI is to perform a working pressure hydro
AFTER 100% x-ray on any welds to internals repairs or replacements.

this will prevent having to jump thru a lot of hoops in re-adjusting or gagging SRVs, or taking the chance on causing downstream problems ro other components that should never see the 1.5X pressure.

 

[athomas236]

About 10 years ago I was involved with some extensive boilers repairs in the middle east on an expanded tube two drum boiler operating at about 90bar. There was nothing in the way of construction or operating records.

Following a number of simultaneous expansion joint failures a large section of the evaporator bank was repalced. There was then the issue of what pressure to carry out the hydro test. Some argued for a working pressure test others for a repeat of the original 1.5 times WP test. In the end the local statutory inspector insisted on the 1.5 times test.

Did we damage the boiler or not.

athomas236

 

[metengr]

Based on your information, I would say that you probably did no damage to the pressure parts of the boiler. If the statutory inspector (whom I presumed witnessed the hydrotest) had observed no gross deformation to any of the boiler components during or after the hydrotest, you should be ok. The concern that I really have is the hanger support system for the boiler reheater and superheater steam circuits. This could be your source of trouble after an in-service hydrotest. If the hanger supports have degraded over time they may not be able to handle the dead weight of the water during a boiler hydrotest.

Under what standard was the boiler designed and fabricated? The bottom line is what the statutory inspector recommended.

 

[athomas236]

The boiler was on a 30MWe unit without reheat and designed/constructed to BS1113

athomas236

 

[eyec]

a hanger walkdown & inspection prior to taking a boiler down should be performed and settings logged so that re-adjustments can be made while the unit is down & cool.

if this is not done prior to coming down, still do the walkdown and inspection while the unit is off.

in addition to the external hanger inspection. a thorough internal visual inspection, as soon as a unit is safe to enter, should be done as early as possible to provide time to address any concerns, such as broken re-heat/super-heat or economizer clips or shields.

the visual conditions should also include a description of any ash deposits/build up and the color of the deposits and any misaligned tubes/panels.