Safety concern about welding on vacuum tanks in a VOD system

[KevinMH]

We have two tanks which are used as part of a VOD system coupled with mechanical vacuum pumps. The tanks are both probably converted pressure vessels (I know for certain that one of the 2 tanks is – based on the name plate, it was designed for a working pressure of 100 psig.) The name plate on the other tank is now too close to a wall to view and ascertain if it has the same working pressure. The tanks are probably made of “mild steel” and operate at “room temperature”, approximately 50 degrees F in the winter and up to 120 degrees F in the summer with a starting vacuum of 1 torr, or 1000 microns. In operation, the tanks are pumped down, a ladle is placed in the VOD chamber and the combination of pumps and pre-evacuated tanks is used to reach a vacuum adequate to remove nitrogen and oxygen to customer ordered levels.
We recently moved the tanks, and our engineering department decided to cut part of the support off the side of one tank and to weld material to support a catwalk on the other tank. Welding is being done with an electric arc with no pre or post heat. Cut-off was done with an oxy-acetylene torch. I’m not certain of the rod grade being used for the welding. I’ve mostly been peripherally involved in work on pressure vessels, but those situations were better defined than this and the organization had planned methods and certified/qualified welders.

Are there safety issues with the practice described above, or am I just being unnecessarily cautious based on varied experience with pressure vessels?

 

[metengr]

KevinMH;
Considering your information and service duty of the tanks, I would have an organization that has a current National Board R-Certificate to weld attachments to these tanks. An organization having a valid R-Certificate has qualified welding procedures to perform this work, and would perform necessary nondestructive testing (NDT) to ensure a quality weld.

 

[KevinMH]

Meteng - thank you for the reply, it basically agrees with my gut feeling/knowledge. I know we haven't done either step, having discovered the tanks being welded on this past Friday and being the metallurgist in the group with the most experience in wrought materials. Most of our final products are iron powder related and the metallurgists involved with them are more narrowly focused than my experience has been.