I have a question regarding the welding of firebox sheets. Is it the general practice in our industry today to make these full penetration welds WITHOUT a backing strip? If the answer is yes, I am curious to know what reasons one might have for not using a backing strip.

aw90h@cs.com

There are a number of reasons. A backing strip adds no real strength to the joint. The extra metal thickness will unnecessarily interfere with heat transfer. In many locations it is very difficult to place one properly. and the unwelded overlap between trhe parent metal and the backing strip can allow moisture to be trapped against the sheets.

In a properly set up joint, with the correct proceedure and rods, a backing strip is unnecessary. If you are really worried about the root pass, consider using GTAW (TiG) for it.

The guys at Colorado Boiler did a lot of patchwork without backing strips just fine, but when we replaced a big chunk of tube sheet they wanted to use them. Seems to me that any ASME recommended practice that the qualified welder is comfortable with will work just fine.

Maybe your insurer has an opinion?

Dave

lathro19@idt.net

> We have gone both ways at The Valley. We have left it largely up to the welder to make the call, depending on the postion he must weld in. The question is, are you sure that you have a full penetration weld without a backing strip? If you can visually inspect the backside of the weld there is no problem, but in some cases, especially with long welds in inaccessable locations, you can't do a visual. You could do an X-ray, but perhaps this is overkill for a stayed surface.
The backing strip insures that you have full penetration. Allen is quite correct that the reminants of the strips could hold moisture, but if a boiler is properly drained and blown out while still warm, this ought not be a problem. Over time the strips rust away to nothingness, they are very thin (we have used 8 gauge material).
I would note that in the steam era (that is before you and I started working on them) boilermakers/welders didn't use backing strips, and they didn't always fit up things too well either. I have seen gaps between sheets of over 1/2 inch that was patiently filled in with weld. Of course with that much of a gap you can't help but have full penetration.
TIG will produce a very nice weld, but it is slow going and again, perhaps overkill for the application.


jdconrad@snet.net

Mark,

I just graduated in welding at SLCC in Salt Lake. As part of our schooling and in order to pass the course we had to weld many plates using ASME code and full penetration welds on square butt joints and v-groove joints. All of the plates had to be x-rayed after welding. A passing weld according to ASME was a passing weld in the class.

We found that a backing strip is actually harder to do and pass in a full penetration weld in a v-groove or butt joint weld. The reason is, you run the risk of slag inclusions in the weld when using a backing strip. Allen said that the backing strip must be placed well, this is so true. A backing strip that is not on correctly will only increase the risk of slag inclusions in the joint.

We were taught by a boiler maker to use a backing strip to set up the joint, then tack the joint then before welding, cut the backing strip off the joint. This will give you a good set-up and allow for a good deep weld with little or less risk of inclusions.

ASME does allow a certain amount of inclusions in a weld per inch of weld. My opinion, why run the risk when you know the risk is there. I weld without the backing strip unless the procedure clearly calls for a backing strip. I agree with the other comment made, check with the insurer. He/she may want a backing strip, in such case, that is how it is!

John E. Rimmasch
HVRR

The backing strip insures that you have full
> penetration. Allen is quite correct that the
> reminants of the strips could hold moisture,
> but if a boiler is properly drained and
> blown out while still warm, this ought not
> be a problem. Over time the strips rust away
> to nothingness, they are very thin (we have
> used 8 gauge material).
> I would note that in the steam era (that is
> before you and I started working on them)
> boilermakers/welders didn't use backing
> strips, and they didn't always fit up things
> too well either. I have seen gaps between
> sheets of over 1/2 inch that was patiently
> filled in with weld. Of course with that
> much of a gap you can't help but have full
> penetration.
> TIG will produce a very nice weld, but it is
> slow going and again, perhaps overkill for
> the application.


jrimmasch@yahoo.com