Vapor Lock Problem

Date 04-11-2008 13:53 Edited 04-11-2008 15:12

A repair procedure that we often do in our shop involves boring out shaft journals of huge butterfly steam valves then press fitting in a large plug and reboring the the inner diameter to size. We are required to seal weld both sides of the plug (plug size can be anywhere from 9 to 11 inches in diameter). Welds MUST be pressure tight and are hydro'ed upon completion.

The problem: finishing off the last 1/2 inch of seal weld sometimes gets trickey becuase of air or vapor that is somehow trapped in this so-called press fit. Presumably the press is not perfect and so hot gas expansion blows out and prevents closing the weld. Sometimes we get it, sometimes we don't. Typically we'll use either a 9018 rod for the procedure, sometimes we'll use a 90S tig wire. We are not allowed to drill a vent hole anywhere else in the assy. The materials have a high carbon content so we always use a 400 degree preheat.

Has anyone experienced similar problems and how have you/would you solve this annoying problem?

Thanks for any ideas.

By GRoberts (***)

Date 04-11-2008 19:16

The same thing can happen when you don't properly vent a pipe closure weld. Can you make the groove deeper in the area where you last weld? Then you can do 2 passes. If the first one has the gas problem, it can be ground down some and then a 2nd pass put over it that shouldn't have the same problem. With a press fit, it is also possible that you could be generating your own gas from some contaminants that burn off ahead of the arc. So at your last spot of welding, it has no where to go except through the weld. Make sure that the parts are absolutely clean and see if this is a possibility as well.

By Tommyjoking (****)

Date 04-12-2008 01:48

Dang Tom....thats a tough nut to crack. Use a two pass procedure at the end of weldment as above......so you have to use the preheat per procedure or is this just to get the weld in there? When I have a gas/pressurization problem like yours...without a specified procedure I will simple let it cool down and then finish it off (2-3 inches max).....most of the time I can get a "seal" weld in place that way.

By DaveBoyer (*****)

Date 04-12-2008 02:02

GRoberts makes an interesting point about "contaminants" Do You lubricate the press fit surfaces or put them together dry?

By darren

Date 04-12-2008 05:49

and if you do use lube could you possibly replace the lube with a temperature differential fit i.e. super freeze the sleeve and heat the valve ? it will eliminate contaminants and provide a tighter fit leaving less gas within the fitment.

or perhaps heat to your maximum inter pass temp and weld on the cool down instead of the heat up?
darren

By tom cooper (**)

Date 04-14-2008 19:32

Well thanks for all the thought that you folks have put into this. I kind of mis-spoke when I said it was a press fit, it is actually a dry ice shrink fit (at -70 deg F) and we do a cleanliness inspection immediately prior, no lube is involved. That brings to mind the possibility that condensation could form and be trapped, waiting to come out as steam? We let the 400 deg preheat soak for about an hour before welding. That should be enough time to steam it all out.

Welding on the cool down might be something to try, thanks for that idea.

Why don't we have this kind of problem closing off the weld on a slip-on pipe flange? Difference in volume of the void between mated pieces?