API 650 telltales - to plug or not to plug
API 650 telltales - to plug or not to plug
(OP)
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!
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!





RE: API 650 telltales - to plug or not to plug
RE: API 650 telltales - to plug or not to plug
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.
RE: API 650 telltales - to plug or not to plug
RE: API 650 telltales - to plug or not to plug
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://ca
jt
RE: API 650 telltales - to plug or not to plug
RE: API 650 telltales - to plug or not to plug
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:
jt
RE: API 650 telltales - to plug or not to plug
RE: API 650 telltales - to plug or not to plug
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