Tell Tale hole in the vessel nozzle 1

[GasProcess]

Recently we hydrotested a high pressure vessel ( design pressure 125 Kg/cm2 ). The vesel is fabricated as per code ASME -VIII. Before hydrotesting the threaded bolts were removed from the Tell-tale holes of all the nozzles. At 140 Kg/cm2 water droops were observed from the Tell-tale holes of two of the nozzles.

1. Is the vessel ok for service at 100 kg/cm2.
2. what could be the source of the water drops.
3. Any further source for the Tell-tale holes litrature.

Thanks

 

[JoeTank]

The vessel did not pass the Code hydro, so it's not acceptable for use at 100kg/cm2 or any other pressure. The source of the leak is obviously faulty welding at the nozzle neck-to-shell wall joint.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[GasProcess]

Thanks Steve

Can you also point me to further reading for the Tell-tale hole and what is exact purpose on the nozzle.
Regards

 

[Shmulik]

GasProcess:
This question should be submitted in Boiler and Pressure Vessel engineering forum. Anyway: what you describe is exactly the tale that this hole should tell you, for this purpose it was made. The drops you see probably caused by an internal crack / leak at the internal shell-to-nozzle weld. What I would suggest you is performing of internal magnetic particles / die penetrate / ultrasonic NDT examination of welds (for all the nozzle connections, not only for those you can see a leakage).
Now this vessel is not permitted for use at any pressure, nor 100 kg/cm2 (!)
You have to get your AI approval for repairs (if any can be done).
(See Sec. VIII Div. 1 - UG-25 (e) ; UCL-25 ; UW-15 (d) for some further info).

 

[JoeTank]

The purpose of the tell-tale hole is to...

Facilitate pressure testing of the final as-welded assembly to assure leak tightness.

Provides a means of detecting a future leak due to cracking or corroding nozzle components... it remains open after testing.

I am aware of no technical papers that discuss the tell-tale hole.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[JStephen]

When you have two plates lapped, as in a repad, and pressurize the assembly, it is the pressure on the inner plate that keeps it round and against the outer plate. This in turn is what keeps the hoop tension in the two approximately equal.

If there is a leak between the inner and outer plates, there will be no net pressure acting on the inner plate. When the outer plate stretches in hoop tension, then the inner plate can just form a chord across the curve. Hoop stress in the inner plate is then much lower, which must raise hoop stress in the outer plate.

The tell-tale hole is one way of guaranteeing that you don't have such a leak. And it's not just the minor inconvenience of a leak that is involved, but getting a stress distribution that matches the analysis used for the occasion.

 

[unclesyd]

Don't let anyone tell you that it is ok to anyway plug or restrict the tell-tale hole.

As stated above you will have to get to inside and do an NDT test on the affected nozzles. If you can do the testing immediately thee is an old trick to locate the defect. I assume that you have someone versed in the art of penetrant testing so have them wipe the area dry and just spray the developer on the suspect area without any prep. With a good light you could see wet spot appear in the developer at the defect. If you see nothing do the standard PT test after well drying the area, preferably whit a little heat to drive the water out. You can do this with a grinder where you could see a water bubble form.

You mentioned plug in the tell-tale holes.
Was this vessel stored outside or wetted down sometime prior to the hydrotest without the plug?
I have seen small amounts of liquid come from tell-tales where moisture came from the outside through condensate or in one case rain.

Make sure you follow the proper vessel entry procedures.

 

[GasProcess]

Thanks guys this is a lot of information for Tell-tale hole.

Yes we have done the DP test and found that there is a crack on the internal cladding of the vessel ( There is a 3 mm internal cladding of Incoalloy 825 over the base metal of 216 Gr-60 )So we can not do MPI. The plugs were already in place on the holes and were removed only after the hydrotest pressure was applied. so there is no chance of out side water gettig in the holes.

Now we are finalizing a plan for ascertaining the corrosion / errosion of the internal cladding and the base metal. If there are any more suggessions please share.

Thanks all for the insight on the Tell-tale hole