Page 235 - Boiler_Operators_Handbook,_Second_Edition

220 Boiler Operator’s Handbook

should also remove any scale from the end of the tube, company or state boiler inspector to be certain you can
making certain it is clean, round, and smooth so there’s re-weld them under the local law. Most states require
a good metal to metal fit between the plug and tube. all welded repairs be performed by an authorized con-
Gently tap the plug into the tube, the water pressure will tractor that is approved by the State or holds a National
hold it and hammering excessively can distort the drum Board Certificate of Authorization to repair boilers,
or header. what we call an “R” stamp. It really is a stamp, the au-
Plugging of fire tubes requires not only a plug but a thorized company actually has a steel stamp that is used
means of holding them in because the water will leak to to mark the boiler when the welded repair is done.
the fireside of the tube and apply pressure to the plugs. The first step in replacing a tube is removing the
A piece of cold rolled steel rod longer than the tube and old one. Whenever it’s possible the tube should be cut
threaded at the ends is required along with nuts and off and removed, leaving the ends in the drum, header
plugs that are bored to accept the steel rod. At least with or tube sheet. Replacement of some water tubes in bent
the rod you are certain you’ve got the right tube ends. tube watertube boilers requires removal of other tubes
The rod has to be large enough to overcome the force of to gain access to the tube that’s to be removed. It’s pos-
the water pressure against the plug and produce enough sible that you will have to remove several good tubes to
force to seal the plug and the end of the tube. The tube remove a defective one.
end must be cleaned as described for watertube boilers. Removing a tube from a firetube boiler is pretty
The plugs also have to have a means of sealing the space much restricted to pulling it out of the hole it’s in-
between the rod and the plug. An advantage of plug- stalled in. If the tube is heavily scaled it may be neces-
ging a firetube boiler is you can tighten the plug while sary to remove it from the inside and that could require
the boiler is under hydrostatic test to try to seal a leak. removal of several other tubes. A single tube replace-
You can also plug a boiler while it’s under water pres- ment in a firetube boiler is seldom located where the
sure but, for most operations, the plugging of a firetube tube can be removed via a handhole or manhole. The
boiler is so involved that it’s much easier to just replace holes in the tubesheet of a firetube boiler are made a
the tube. bit larger than the tube so slight accumulations of scale
A common repair for many water tube boilers in- will slip through the hole. In some cases the scale is
volves replacing a section of the boiler tube. Frequently stripped from the tube as it is removed. In extreme situ-
it’s only a portion of the half of the tube that faces the ations it’s necessary to split and collapse the entire tube
furnace. When bulges or blisters form due to scale to get it out.
buildup, and sometimes rupture, the rest of the tube is Removing the tube requires crushing or cutting
still intact and the original thickness. The repair requires away the tube end where it is expanded into the drum,
a skilled boilermaker welder. The tube is cut out around header or tube sheet. I’ve seen several boilers seriously
the failure, normally in an elliptical form, and a piece damaged by inexperienced or careless contractors cut-
cut from another tube is inserted in its place with the ting the tubes with a torch and one case where a repeat
edges of the original tube and patch butt welded. Since repair was necessary in very few months because the
the tube walls are so thin (less than 1/8 inch) the weld is contractor cut the tube sheet with a torch and put new
normally made by GTAW (gas shielded tungsten tip arc tubes in without repairing the cuts. If your personnel or
welding also called TIG) welding. a contractor uses a torch to cut the tubes inspect every
Entire sections of boiler tubes can be removed and opening to ensure the tube holes are smooth and clean
replaced in a similar manner. The elliptical patch is used so a new tube will seat properly in the hole when it’s
at either end so the welder can reach through the open- expanded.
ing provided for it to reach the butt joint at the back of The best way to remove a tube end is to chisel it
the tube. The welder has to work on the inside of the out, making certain you never touch the tube sheet,
tube at the back because there’s no room to get to it from drum or header with the chisel. It eliminates the risk
the back. Once the back is welded the patch is set and of cutting the inside of the tube hole but it takes longer
welded to complete the repair. That method is referred and, quite frankly, takes more skill. By cutting a shal-
to as using a “window weld.” low (about half the tube thickness) groove through the
Replacing a boiler tube is best done by a boiler- tube where it’s expanded you produce the same effect
maker who has the skill and experience necessary to do as flame cutting. After the tube is cut by driving it to
the job right but you can do it if you have the tools. If the the center you can collapse the tube into the middle,
tube is welded you should check with your insurance away from the tube hole, so the end or whole tube, can