I have a 10" 150 psig steam line that runs between two boilers on a large university campus (transfer line).  One section of the pipe midway between the boilers is in a walkable tunnel. The pipe enters the west wall of the tunnel, turns north for 60' then then turns east and exits the east wall. We plan to cut an elbow (turning east) and install a "tee" to run a new 10" line down the tunnel to the north.  The university wants to take only a minimal (24 - 48 hr) shutdown to cut the elbow and install the tee, which is feasible with a pre fab tee, but my concern is the pressure boundary of the hydrostatic test.  

Relative to the tee, there are three boundaries for the pressure test:

1) NORTH: this will be a new north isolation valve on the north end of the tee.  New 10" piping will run north of the tee, and we will pressure test it after the tee is installed.
2) EAST: when we cut the elbow, the east side of the tee will be connected to existing pipe with an existing east isolation valve within 3 feet.
3) SOUTH: will cut the other end of the elbow, and install tee with new south isolation valve/ball vavles.  HOWEVER, the new weld on the other (south) side of the new south valve in the existing pipe will be outside the boundary established by the new south & north valves and the existing east valve.  To make things worse, there is no isolation valve in the 10" line from the tunnel back to the steam boiler (well over 1,000+ feet).

So the quesion is: if you have a new weld in a 150 psig steam line 5' away from an isolation valve and over 1,000' away from the other isolation valve, how would one pressure test the new weld?  Hydro test seems impossible due to length and short shutdown duration.  Pneumatic also seems impossible, and possibly dangerous.  Do pipefitters have an inflatable "pig" that can be placed in the pipe for the hydrotest, and then deflated and pulled out later?

Actually you cannot arbtitrarily substitute 100% RT or UT for a hydrostatic or pneumatic test in ASME B31.3.  This Code is very specific about when you can substitute NDE for the pressure test:

  1) There is a danger of cold brittle fracture

  2) There is a possibility of damage to internal refractory or linings.

Both of which would probably not apply in case described.  You must also realize the Codes are written for new construction and not for repairs/alterations to existing piping systems (but all repair/alteration work is to be done to the original Code of fabrication, as far as possible).

Legally then, you would have to test as much of this steam line as required between block-in points in order to test the new welded elbow installation.  This may not be practical.

In real life we have gotten around situations like this by making a presentation to the local Authorized Inspector.  This presentation must detail the reasons why a hydrostatic or pneumatic test is not feasible (reasons of cost or convenience are never accepted by the AI) and detail the amount and types of alternate tests to be performed.

Usually we perform 100% RT or 100% UT, 100% MT or 100% PT, we do as much visual inspections as possible of weld internal and external and we leave the welds exposed (uninsulated and unpainted) for a service test.

The main thing is, you must have the acceptance of the Authorized Inspector as this is a deviation from the strict, literal requirements of the Code. 

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