steam drum repairs
steam drum repairs
(OP)
A couple of recent tube leaks have turned into a larger headache. The leaking tubes were plugged, only to show they were still leaking in between the steam drum and tube. They could not be re-rolled due to the plugs, so it was decided to seal-weld around the tube. When pressuring up with air, before doing a hydro, the leaks had become worse. It seams as though their is a problem with the steam drum itself. I'm looking for information/guidance regarding repairs to the steam drum itself... code/approach/feasibility/etc.





RE: steam drum repairs
1. What type of boiler is this (power or package boiler)?
2. How many years has the boiler been in service? Cycled?
Any weld repair to the steam drum will probably be under the National Board Inspection Code (NBIC), and will be required to be performed by an R Certificate holder.
Seal welding of previously rolled tubes is effective but extreme care must be used because corrosion products can be pulled into the seal weld.
Come back with more information, I might be able to help you based on our previous experience.
RE: steam drum repairs
You need what we old boilermakers call a "pocket". Cut the tube off past the outside(fireside) of the drum about 2-4 inches. You can do it from the outside if you can reach it or with a straight torch from inside of the tube. Leave the old tube and let it burn out.
Take a new piece of tube and orange peel one end and weld closed. Needs to be 2-3 inches past the outside of the drum.
Remove the old tube from the hole,prep and roll in with a "drum or Short roller". Make sense?
Make sure you get a qualified person/firm to do this. Seal welding is poor in this case since it is weak. A seal welded tube usually needs rerolled after welding. Bad practice for a watertube boiler. PWHT/Testing/Strength
RE: steam drum repairs
It is a package 1200# boiler that dates back to the late-60's. It was moved from one area of the site to another, years ago. It was partially retubed (d-tubes) over the past two years. I'm not sure of any retubing done before then.
RE: steam drum repairs
A two pass seal weld works nicely using SMAW or GTAW. I agree that the thermal stress from cycling resulted in the tube rolls breaking loose. However, seal welding is indeed effective and requires much less work.
RE: steam drum repairs
RE: steam drum repairs
One problem not mentioned is when rerolling tubes there is a tendency to over roll the hole and tube and the tube will spring back making it a little harder to accomplish a seal weld.
Normally when you get a true leaker, not a weeper that is an indication that something else is going on.
RE: steam drum repairs
RE: steam drum repairs
RE: steam drum repairs
Most seal welds are 3/8" or less in deposit thickness. By the way, on older B&W boilers they have rolled and seal welded their tubes in lower mud drums and steam drums from original installation.
RE: steam drum repairs
True, for 3/8 fillet welds on rolled tubes , no PWHT required, but does require removal of caustic deposits to prevent embrittlement of weld .
RE: steam drum repairs
In my opinion the use of "seal welding" a leaky water tube when it was not designed that way is a method of last resort.
Problems:
1.If the boiler was designed for roll joints and you seal weld for leaks,would this now be considered a strength joint? Would you need to justify it?
2.Cracks,good idea to check for same-PT or MT. Before and after.
3."Stuff" in the joint. 40 year old boiler with leaky tubes? No way to get that cleaned from the joint without tube removal. Forget GTAW,maybe you could use some P5.
So I think there are better ways. If your stuck and you must seal weld-better have your ducks in a row.
RE: steam drum repairs
Now, that is what I define as a headache.
That drum was weld repaired, re-drilled, and the boiler is still in service some 20 years later, so as they say, nothing is impossible.
The old boiler guru that designed and implemented the repair was a hero, and the project engineer, whose watch this happened on, got fired. And, as you guessed, so did the boilermaker.
Point being, there are always solutions available to your problem, Gibsi1.
rmw
RE: steam drum repairs
Seal welding boiler tubes is not such a big deal as long as, as deanc puts it, all the ducks are in a row. We do it fairly often but it is always considered a last resort.
If a tube roll is leaking and it has been rerolled and rerolled until finally the tube material is past it's modulus of elasticity, no amount of rolling is going to create a seal. The only options in this case are to replace the tube or seal weld.
If a tube has been leaking for a while and the tubesheet seating surface has been steam/water cut, no amount of rolling is going to create a seal. Either the tube has to be cut out, the tubesheet seating surface repaired and then a new tube installed or the tube is seal welded.
Even if a tube has been plugged it is still possible for the tube seal to leak. In this case the options again are to replace or seal weld.
For seal welding it is important to ensure the area to be welded is clean (the Fort McMurray case was unusual and unique. It is not normal to have such a high pH in the boiler water. There is more to this story than davefitz has written). The area should be examined with MT or PT prior to welding Preheat temperature control is essential, normally 200 F, maintained during welding and the area allowed to slow cool. The seal welds cannot be larger than 0.5" or else problems with PWHT requirements occur. The completed weld area is examined with MT or PT. The tube is then rerolled (unless there is a plug installed). All adjacent tubes are given a very light roll as well.
Seal welds from inside a drum are not considered strength welds. The welds are not holding the tubes in place and the pressure is actually working to try and force the weld into the leak.
Seal welded plugs from the outside of a drum (this happens on small drums, headers, etc.) are considered strength welds and the calculations must be performed to show the welds are sufficient to hold pressure and to hold the plugs into place.
All the above must be accepted by your friendly, neighborhood Authorized Inspector prior to the repair.
RE: steam drum repairs
GTAW or GMAW at low to med heat is needed, 3/16th to 1/4" toes are suffient,
if you go bigger, the HAZ is greater than the tube can hold,
light re-roll after welding,
'hope this help.ER
RE: steam drum repairs
RE: steam drum repairs
If the drum is OK, and the tube ID surfaces look ok (no axial corrosion fatigue cracks that can sometimes ocurr in tube stubs), and your water leaks are at the tube roll joints, then I would seal weld all of the tubes in the steam drum. Partial seal welding (not all tubes) will not work. It is all or nothing from our experiences.
RE: steam drum repairs
best regards,
athomas236
RE: steam drum repairs
In a 30 plus year old boiler, you have to be suspicious of how many times it might have already been retubed, and/or re-rolled, with the potential for the tube holes in the drum having suffered from previous over rolling. Time to get out the mic's.
rmw
RE: steam drum repairs
One idea to reduce residual stress at the heat affected zone at the drum side is to run a 2 or 3 pass weld with the 1st or 2nd bead favoriing the drum side. Then weld another pass overlapping 1/2 of the prior bead. This may slightly temper the underlying bead.
Ligament craks are sometimes repaired using techniques similar to those in the NBIC. These are half bead temper techniques that I have used on cracked ligaments extend through the entire ligament Elevated preheat, tight preheat/interpass controls, bead sequence, and grinding are all used to reduce/modify the stresses from welding.
I welded a CJP PQR for this technique on 2-1/4" PLate with no strongbacks holding it down. The angular distortion was < 5 Degrees.
You received some great advice above. I hope it works well for you.
Gerald Austin
Iuka, Mississippi
http://www.weldinginspectionsvcs.com
RE: steam drum repairs
RE: steam drum repairs
Thanks!
Ike
RE: steam drum repairs
ER
RE: steam drum repairs
Not being negative-just been down this road before.
RE: steam drum repairs
Talk to your boiler inspector or the people who issue your operation cert.
RE: steam drum repairs
About rerolling the remaining tubes. Perhaps there was a low water condition not serious enough to damage the drum and tubes but enough to have loosened them when they were flared into the drum during installation.
RE: steam drum repairs
1. This is a repair not an alteration (refer to the 2001 NBIC, 2003 Addendum, RD 2040 a) and Figure RD 2040 d) for seal welding tubes.
2. Starting in Section I, tubes can be expanded into a component as long as they are under 6" diameter. Reference is then made to PWT- 11.1. When you read PWT-11.1, there are 4 permissible methods to install rolled tubes;
a. expanded and flared
b. expanded and beaded
c.expanded, flared, seal welded, re-expanded
d. expanded, seal welded, re-expanded
The issue for tube stick out is before flaring not after. Seal weld deposits are limited to 3/8".
Seal welding is a last resort. However, I have to respectfully disagree with deanc. You MUST complete seal welding - otherwise you are wasting time and $$$$ by trying to re-roll tubes. Re-rolling is not effective once you commit to seal welding the tubes.
RE: steam drum repairs
Yes,metengr gibsi1 may be past the point of no return and it would be wise to "seal weld" all the tubes.
Here is the problem-old boiler,problems with obtaining a good weld because of the flare. Been there-the tubes may be very close,the flare may be large,the tubes may be thin,and the joint is probably full of unweldable "stuff". The welder is unable to obtain a good joint,leak proof,with the flare. Therefore you remove some of it. Does it still fall within UW-11,maybe-maybe not. If you need to remove enough of the flare for a good weld it may become an alteration(always need to ask the jurisdiction,yes?)
As stated,once you are already here,may as weld complete the job in a satisfactory manner. Think I would of rerolled after the first seal weld-no matter now.
What to do when a job goes south? Well just my opinion.