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What's the purpose of the 1/4 NPT weep hole in a re-pad?

What's the purpose of the 1/4 NPT weep hole in a re-pad?

Would one of you fine fellers please explain to me what the purpose is of the weep hole in a nozzle repad.  As you know we usually call for those is a re-pad; usually it's a 1/4" hole with a 1/4NPT thread, and then it's filled with grease after fabrication.  What is the hole for?  I've heard several explanations, but I'd like to hear a definitive one.

Thanks ! ! !


RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

The primary purpose of the weep hole is for visual indication of leakage during hydrotest.

RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

...which is one of the reasons I've heard for the weep hole.  Not sure I understand the logic, there.  What will the hole tell you, that you lost integrity of the shell itself under the pad during the hydro?


RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?


When the pad is being welded, at the end the gases will be entrapped. This may bulge the pad sometimes. But the covering of the hole is to reduce corrosion afterwards.


Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

All of the answers are to a degree correct but I suggest thinking about the impact that a leak would have on the square inches of the annular space if the pressure were contained by not having a weep hole!  If your service ran at 100, 200, 300, etc. psi, would it be considered good engineering judgment to allow the annular space to see that kind of pressure if a leak occurred?  Bulging would be the least of the problem; “catastrophic failure” is a term that comes to mind.  I don’t think I’ve worked to a code that allowed more then 15 psi put on a repad for testing purposes.

Every time this repad theory comes up, I think back to a circumstance that I chuckle about today but wasn’t when it occurred and I’ll share it with you.  
Many years ago when I was a rookie doing QA on a SS field erected API tank, the foreman asked me what psi to test the manway repad.  I couldn’t remember but the engineer just happened to be walking by and I asked him.  He said “Oh, put 50-60 lbs on it”.  The foreman turned and shouted to his crew to come off the tank.  I asked him what was up and he said he was shutting the job down because he wouldn’t allow his company or crew to work on an unsafe project and he was calling his office immediately.  I innocently asked what was so unsafe and he replied that the idiot engineer was going to get somebody hurt and he and his crew wouldn’t be there to find out who.  Needless to say he explained what the problem was and we got the situation clamed down and finished the job but I never forgot his reaction.

"There's no such thing as a welding problem, there are only welding puzzles of assorted sizes!"

RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

Hello All,

I concur with Seldom.  All the other answers are nice "side issues" but the real reason for the hole is that you DO NOT want the re-pad to become a small pressure vessel of questionable MAWP.

The same issue comes up in piping system fabrication when welding on a dummy leg or a base elbow.  You need the "weep hole" there too.  It keeps the welded appurtenance from becoming a pressure vessel AND it allows the circumferential weld(s) to be finished without the weld metal being blown back into the welder's face by expanding gas/air.

Regards, John.

RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

I don´t know a requirement on this, This matter interested me since I´ve seen vessels with the thread and a screw that seal the repad. I saw once a repad with a nice SS screw with teflon tape (which looked very nice). I prefer a simple hole painted.

As little as I know I consider this a good practice rather than an important requirement. I agree with those who point out about avoiding the formation of a small pressure chamber. (I would add that is better to prevent trapped moisture to develope pressure if heated).

If somebody can provide a base for this it would be interesting.

RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

The saddle is a reinforcement, rather than a pressure retaining component. The branch line is welded to the main, and the saddle welded to both the branch and the main. There will be atmospheric pressure air trapped between the main & branch, and the saddle. My assumption has always been that if there's no hole to relieve the air pressure that increases as the final bit of weld is made on the saddle, the small amount of escaping air will just keep blowing molten metal out as the welder tries to finish.

RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

By coincidence I found a little reference on this.
API 650 ( (8th ed.)

"Manhole reinforcing plates, ...., ..., shall be provided with a 1/4" telltale hole (for the purpose of detecting leakage through the interior welds. Such holes shall be located substantially on the horizontal centerline and shall be open to the atmosphere."

RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

Leakage?!?!?!  Through the interior welds?  Hmmmm.  So that means they are checking for the presence of a burn-through of the fillet weld that attaches the repad to the vessel shell?


RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?


I believe the weld in concern is the nozzle to shell weld.
As an inspector, one should always include visual inspections of the weep hole when inspecting the rest of the vessel anyway. In my experiences, I have seen several occasions where the nozzle to shell weld has cracked or even been subject to severe corrosion and began leaking. This allowed product to completely fill the gap between the repad and the shell causing severe corrosion to the shell. Eventually, the crack(having been exposed to the now corrosive environment) propagated about 3" into the shell which finally allowed enough product to reach the weep hole. No one knows how long it took for these events to occur but needless to say did.

Recently, when inspecting large aboveground storage tanks, several clients are requesting Ultrasonic thickness testing of the repad looking for this type of corrosion.

As far as plugging the holes with grease goes, it is usually put there to keep water from being trapped behind the repad thus creating a corrosion problem. And, there will always be someone who sees the grease and assumes it to be a leak.

On painting the weep holes, that would only defeat the purpose of the hole being there in the first place! Now the hole is sealed up and product that might leak out, has no place to go.

Just my thoughts.....

Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico

RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

Hi Members & visitors !

I appreciated all the responses. As i can see it was a small question but with deep discussion.

I' ll add something me too :

Why this subject hole is treathed knowing that it's very seldom used ??

Can someone add a fruit to our dessert in the desert !!!

Thanks guys in advance !


RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

Hi again !

Sorry i mean "threaded" and not "treathed"



RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?


As Seldom stated on June 20, sometimes it is desirable to put a small pressure on the repad for testing purposes. Threading the connection allows the testing device to be connected.


RE: What's the purpose of the 1/4 NPT weep hole in a re-pad?

The 'weephole' serves several purposes:
1) By threading the hole (NPT), the fabricator can thread a test gauge for the 'air/soap' test (usually hold to 15 psig) to check for leakage.
2) The hole serves as an 'indicator' (weep) in the event of a subsequent minimal (pinhole) failure of the neck-to-vessel weld.  As noted by others, this also prevents the reinforcing pad from becoming a 'jacketed' vessel in the event of weld failure.
3) By packing with grease priror to shipment, it prevents corrosion from penetrating behind the pad, keeps bugs and 'critters' from plugging the hole, and provides minimal blockage in the event of a subsequent weld failure.

Hope that this helps...

Steve Halket

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