Pressure test for hot tap fittings and machine

Fillipo,

A hot tap needs to be treated as a repair or modification which cannot be hydrotested. Hence each weld needs to be treated as a "golden" weld, i.e. subject to more than one form of NDT and subject to supervision of the welding by suitable inspection personnel.

As there is basically a set of fillet welds ( MPI / dye pen) and one or two girth welds before you have a valve either bolted or welded onto the tee this is allowable. The girth welds should be able to be radiographed as well as MPI or dye pen or UT tested.

Doing a pressure test, perhaps in gas to avoid issues with trying to drain or remove water, is a good idea to prove the integrity of the hot tap operation, but should not be the only testing and inspection you do.

So,
What if such max external pressure is lower than the calcualted test pressure?
You need to reduce the test pressure on the tee. The potential to collapse and severely strain the existing pipe is real and creates a significant hazard as soon as you exceed the pressure inside the existing pipe.

is there any issue to reduce the test pressure to a lower value in order to avoid run pipe collapse?
Not IMHO. The "test" pressure of the tee is basically a leak test not a strength test as it can't reach the pressure required.

If i reduce the test pressure, then can i keep the current operating pressure after hot tap?
The current operating pressure / design pressure of the system remains unchanged providing the tee connection is designed for at least the same design pressure of the existing pipe. As noted above, this hot tap pressure test is simply a leak test and in many cases is not undertaken, but reliance is placed on the weld inspection of the fillet welds and the circumferential weld.

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Adding to LittleInch's thoughts, keep in mind that part of the pressure test is not so much for code of construction compliance but to verify that the hot tap rig itself is capable and has been properly assembled. You know, little things like verifying that the nuts on the studs between the block valve and hot tap machine were actually torqued, not just snugged (insert lunch break here) and not tightened. The last thing you need is to install the tee, punch a hole in the line, then discover that the block valve to tee flange bolts are loose, or that the hot tap machine has some issue, etc.