Effect of drilling though compensating pad??

[MadDog88]

We have a refractory lined cylindrical pressure vessel which is exhibiting some hot spots due to gas tracking through fine cracks. The vessel ID is too small to repair from the inside so I wanted to weld some 1/4"- 1/2" nozzles onto the vessel and inject liquid refractory from the outside. The nozzles would be plugged after use.

One of the hotspots is in the junction of the vessel and a nozzle, incorporating a compensating pad. Am I able to weld a nozzle to the compensating pad, drill though the pad and underlying material and inject refractory through the hole? How would drilling through the com pad affect vessel strength?

The vessel OD is 28" and the attached nozzle is 20" OD. Materials is carbon steel.

Thanks for any help.

 

[chicopee]

ARE YOU DESCRIBING A DIRECT FIRED STEAM GENERATOR SUCH AS THOSE USED IN CAST CONCRETE PLANTS?

 

[bvi]

A couple off the cuff comments. First, welding your nozzle to the compensation pad makes the compensation pad the pressure boundary of the vessel, which is unacceptable for a number of reasons.

To answer the specific question, putting a hole in a compensation pad reduces the metal available for reinforcement (reduces its strength).

The nozzles that you are adding should be designed per the code rules for nozzles. If they are closely spaced, they should follow the rules for closely spaced nozzles.

 

[prex]

Adding to cb4 remarks, that I confirm:
-a larger hole in the pad could be acceptable, especially if it is not on the same longitudinal line as the manhole; you'll need to revise the compensation calculations accordingly
-it is likely that the new opening and the manhole will have their limits of reinforcement overlapping, and this also will require additional calculations: however pad compensated openings have often more added metal than the minimum required, so I think you have quite a few chances
-of course additional welding onto the vessel need be checked against any other requirement per code (PWHT, NDT,...).

prex

http://www.xcalcs.com

Online tools for structural design

See

http://www.xcalcs.com/docs/symbolinfo.htm

if you want to use symbols on these fora

 

[unclesyd]

There is a way to accomplish this and that is open up a new hole just large enough to accommodate the pipe coupling (normally 1/4&quot and associated filet weld. The lost of pad area is insignificant as we have accomplished this many times. A lot of times we have just opened up the tell-tell hole in the pad. Analysis shows no significant loss of reinforcement. The only problem would come if as mentioned that the hole straddles the nozzle/vessel centerline in a cyclic service.

The only thing I would recommend is that you have someone well versed in the art kill the "stingers".

Describe the problem area a little better and can you access the area of refractory in question while the refractory is still hot.

 

[MadDog88]

Thanks for your help – you have given me some food for thought. The vessel is a reformed gas outlet header (hydrogen service) operating at about 2000kPa and 800 deg C. Although the production team would probably like us to carry out this repair when online and hot, we intend to cool it down and isolate the vessel to eliminate the risk of explosion.

 

[unclesyd]

This is the way to go with your situations. Take back anything said about a online repair. The outlet header/tails gas header is not a place to fool around. There are several firms that make their living repairing meltdowns from these header failures. I've seen three fires in this area and none were pretty.

A word of advice is to get someone to make the refractory repairs who has experience, as this is no place for the low bidder or someone learning. If the designer or the refractory people recommend that the refractory be replaced, take this suggestion seriously.

Another trick is when you go back is put a heat sensitive paint on the header. This will give you a little extra time the next time the refractory starts to fail.

Good luck with your repair.

 

[GenB]

If you drill and weld a 1/4-1/2" makes not much impact, you
would have to groove-weld the nozzles all the way to the vessel isntead just a fillet to the pad. If this is an ASME stamped, your inspector has the last word.
ER