API 650 telltales - to plug or not to plug 2

[alohapoint]

Hi all,
When installing a repad with a 1/4" threaded telltale per API 650, sections 3.7.5.1, and 3.7.6.1 state the tell tale hole "shall be open to atmosphere". In practice, threaded plugs are inserted with the logic that: a plug keeps moisture & rain out to avoid corrosion, prevents clogging with mud & moss, and they can be removed and checked periodically.

Why does API recommend "open to atmosphere"? What benefits make it better than using a plug?

Thanks!

 

[xema77]

The telltale holes should not be plugged as this would not allow them to do their job of indicating a leak under the repad. A good practice is to pack the holes with greases to protect from the elements.

 

[JStephen]

API doesn't explain their reasoning, so you generally get to guess, or ask someone in the know.

A second consideration is structural.

If you take a section of cylindrical plate and apply internal pressure, it will develop hoop stresses but will tend to round out. If you take a section of cylindrical plate, apply hoop forces to it, but no internal pressure, the hoop forces will tend to flatten it out.

If you have a tank shell with internal pressure, and then have one area that also has an equalizing external pressure, you get a certain tendence for that area to flatten out. If it has a repad welded on the outside, then the effect would be for the shell to make a chord straight across while the repad carried all of the hoop forces. So it would have the effect of transferring additional load from the shell to the repad.

I don't know how noticeable or important this effect is, but it could be another reason to require the tell-tale hole to be open.

 

[JohnGP]

The fact that they are called "telltale holes" suggests that it is all about indicating leakage, as xema77 pointed out, so should not be solidly plugged. ASME VIII states that they "may be used to provide some positive indication when the thickness has been reduced to a dangerous degree."

 

[jte]

JStephen is going down the right path: The reinforcing pad is not intended to handle an internal pressure loading. If the tell-tale hole is plugged and a leak allows the shell-pad space to become pressurized, then yes, the shell sees a local straightening. But the membrane stress is now carried through the repad. This load path has an offset where the load goes from the mid-thickness of the shell to the mid-thickness of the pad. This offset causes a moment. Larger than you might think. The throat of the repad weld must be designed to resist that moment. Run the numbers and you might be surprised at how stressed this weld can become.

In effect, this repad has now become a fillet welded lap patch and should be designed per Article 2.12 of ASME PCC-2. Cheap at about $0.50 per page:

http://catalog.asme.org/Codes/PrintBook/PCC2_2008_Repair_Pressure.cfm

jt

 

[JStephen]

If I remember correctly, the ASME reference about thickness being reduced is a different application altogether. There, the hole would be in the vessel wall, not in a repad. If you had a 2" thick wall, you drill a hole 1.75" into it, and then when corrosion hits 0.25", you have a visible indication in form of a leak. That wouldn't work on a repad.

 

[jte]

A bit more... API-650 states that tell-tale holes shall not be plugged:

Reinforcing plates for shell openings, or each segment of the plates if they are not made in one piece, shall be provided with a 6 mm (1/4 in.) diameter telltale hole. Such holes shall be located on the horizontal centerline and shall be open to the atmosphere.

5.7.5.1 has similar wording for manholes. Keep in mind that a repad in service which might be exposed to internal pressure is not in the same state of stress as it is when it is air tested in the fabrication process. A simple free body diagram will show that.

Section VIII Div. 1 is explicit about not plugging tell-tale holes with a plug which can withstand internal pressure:

These telltale holes may be left open or may be plugged when the vessel is in service. If the holes are plugged, the plugging material used shall not be capable of sustaining pressure between the reinforcing late and the vessel wall.

jt

 

[JohnGP]

Thanks JStephen, .....oops. jte has the correct reference.

 

[alohapoint]

Thank you. I was focusing too much on small weeps and not thinking about problems from the internal pressure.

Creating an internal pressure on a repad that wasn't designed for it is a risk to the repad, and therefore a risk to overall tank integrity. Allowing a uncontrolled leak through the telltale is better (lesser of two evils) than risking a catastrophic tank failure.

Thanks