Repairing holes in valves

[kclim]

We have a sliding gate valve, with a hole in the valve body. The valve is a feedwater valve, constructed from carbon steel (casting) and operating at 200C and 18 MPa. The hole is on a cylindrical portion of the valve body where the wall is about 75 mm thick.

The suggested method of repair was to 'rebore' the hole by drilling a tapped hole, insert the appropriate threaded plug and then seal weld the plug from both sides.

Intuitively, this sounds reasonable. I was wondering if there are any codes/standards covering this type of repair?

ASME B16.34 seemed to have some info, but I couldn't get anything definitive out of it.

 

[metengr]

If the valve is ASME B16.34, the requirements of ASME Section VIII, Div 1 must be followed for welding. What size is the hole, and what caused it? Can you plug weld the hole from the OD and back grind the ID to assure a full penetration weld?

 

[kclim]

The reason there is a hole in the valve is because site wanted to attempt to align the disc in the valve without opening it. They figured they could play gynecologist and manipulate the disc from the hole using steel rods, as opposed to opening the valve (which I'm told would have cost a fortune).

Size of the hole would be no more than an inch, maybe even half. I tried looking at ASME VIII on plug welds (UW-17), although it appears more applicable when joining two overlapping sections (where minimum plug diameter makes sense), rather than filling holes (where min plug diameter seems inappropriate).

Or are you suggesting to use a backing plate, plug weld to UW 17 followed by removing/grinding the backing plate?

 

[metengr]

kclim;
Understanding your dilemma now puts me in a better position to offer advice. Since your objective is to avoid opening the valve you have two options;

1. You drill an access hole to manipulate the disc and it is unsuccessful. Now you must open the valve with a hole in it. In this situation, I would use a backing bar and plug weld the hole, remove the bar to assure full penetration and your done.

2. You drill the hole and you are successful in manipulating the disc and now you avoid opening the valve. So, in this situation, I would consider a fully threaded plug and use a partial penetration weld, not a seal weld because it might leak over time.


The problem with option 2 has nothing to do with the repair I mentioned, it has more to do with understanding what happened inside the valve. Are you planning to insert a borescope to see the condition of the disc and valve bore? You might have to bear the cost of opening this valve to really understand what is going on.

 

[dugal]

A complete valve change out might be considered if the valves functions are in question.