vent holes in reinforcing pads 4

[ALA445]

Dear All,
According to UG-37(g)of BPV Code VIII Div.1, vent holes in reinforcing plates and saddles of nozzles attached to outside of a vessel may be left open or may be plugged when the vessel is in service. My question is what about vent holes in re-pads of supports inside vessels? Any requirement to keep it open or to plug these after welding. What are the implications if left open for the vessels like 3 phase separators?

 

[0707]

Because of interstitial corrosion vent holes, in my opinion, in re-pads of supports inside vessels shell be plugged after welding.

luis

 

[MJCronin]

IMHO ....

First, you must understand that the original knowledge about repad vent holes has been long lost in the mists of time and brought to the grave by long-dead pressure vessel designers and engineers..... Some say that the practice grew out of the birch bark canoe building activities of North American Indians, Others claim that a young Irish barrel maker had a vision while bunging up whisky barrels in the John Jameson distillery in Dublin in the 1700s

Second, you must understand that, much like the Bible, the Torah, and the Quaran, the ASME code is an ancient lengthy, complex and often contradictory document that can mean many things to many people... The plugging of vent holes in repads has caused deep-seated bitter feelings in the past and small wars have erupted over this particular issue...

Third, there same to be only two possible arguments about plugging of the holes, and your beliefs about this must be made depending on which camp you deeply believe in.... FIRST REASON: Plugging is necessary for prevention of corrosion under the Pad.... SECOND REASON: Plugging is necessary to provide a weak point for indication of vessel cracking under the repad..... I believe the second is the best argument

Fourth, Understand that this hole plugging discussion has been made many many times on this particular forum ... I believe a book can be written about it.

For your the plugging of vent holes contained within the vessel, I do not believe it makes a difference either way

Whew !!!! .... I feel better !!!! ..... Now its time for a cup of coffee and my early morning medications

Thanks for reading !!!!

Good luck and best wishes !!!!

MJC

If anyone has any ancient books or knowledge about this topic, please share ...

MJCronin
Sr. Process Engineer

 

[r6155]

Vent hole IS NOT REQUIRED for inside pad.

Regards

 

[weldstan]

Having had to direct repairs to the vessel wall when an internal stiffener and pad did not have a weep hole installed prior to heating to 550F required for the internal coating, one may wish to consider weep holes especially when the vessel is subject to PWHT or other thermal treatment or operating at temperatures where the moisture inside of the pad can flash to steam. This was the cause of the vessel wall rupture and the rupture of the internal weld attaching the pad to the shell. Because of the internal coating the weep hole was drilled through the shell and tapped after thermal treatments. We had multiple very large vessels subject to this thermal coating treatment.

 

[r6155]

@ weldstan
It was a design mistake that you mention. Always avoid internal and external pads.

External reinforcing pad attachment welds and accessible surfaces of inside nozzle to
vessel wall welds shall be tested for the absence of leaks with a gauge
pressure of 15 psig (100 kPa) dry air or nitrogen and bubble forming solution.
This test shall be prior to the final hydrostatic or pneumatic test as applicable.

Regards

 

[0707]

Vent holes in reinforcing plates and saddles of nozzles attached to outside of a vessel, after being tested, due to the corrosion shall also be pluged.

luis

 

[blacksmith37]

For some processes and temperatures , hydrogen is diffusing through steel vessel walls. This hydrogen can recombine as a gas and build up pressure under a plugged external pad.

 

[SnTMan]

Previous discussion

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[DriveMeNuts]

Ahhh, the to plug or not to plug discussion does feel like groundhog day.
See you for the same discussion next month.

 

[0707]

blacksmith37

In situations where hidrogen difusion is a possibility are used reinforced forged pieces instead of reinforced pads.

luis

 

[r6155]

Normally vent hole is pluged with grease.
When there is thermal insulation, a pipe is placed that protrudes the thickness of the insulation.

Regards

 

[XL83NL]

First, you must understand that the original knowledge about repad vent holes has been long lost in the mists of time and brought to the grave by long-dead pressure vessel designers and engineers..... Some say that the practice grew out of the birch bark canoe building activities of North American Indians, Others claim that a young Irish barrel maker had a vision while bunging up whisky barrels in the John Jameson distillery in Dublin in the 1700s
...
If anyone has any ancient books or knowledge about this topic, please share ...

MJ, please pardon me for asking, but do you know the real historical reason for repads in PV's? I may have misunderstood you due to semantics or something else (English isn't my native so occasionally Im having trouble spotting sarcasm, etc.)

External reinforcing pad attachment welds and accessible surfaces of inside nozzle to
vessel wall welds shall be tested for the absence of leaks with a gauge
pressure of 15 psig (100 kPa) dry air or nitrogen and bubble forming solution.
This test shall be prior to the final hydrostatic or pneumatic test as applicable.

r, this sounds like its a citation of a Code section, but I dont think it isnt (looked it up and cant find it). Is this taken from something else, or is my VIII-1 copy hiding stuff for me?

Huub

 

[BJI]

Not from Division 1 but relevant from Division 2, 4.5.3.2 (d):

(d) Reinforcing plates and saddles attached to the outside of a vessel shall be provided with at least one vent hole
[maximum diameter 11 mm (7/16 in.)] that may be tapped with straight or tapered threads. These vent 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 plate and the vessel wall. Vent holes shall not be plugged during
heat treatment.

I have heard of the use of an external hole through the shell to vent an internal pad but I would prefer to avoid an internal pad under all circumstances.

 

[DekDee]

I am not an Engineer so would appreciate feedback on this subject (my apologies if it has been done to death in other posts)
A vent hole or weep hole in the external reinforcing pad is a practical requirement if you have a nozzle penetrating the vessel wall.
Any future leaks in the nozzle to shell weld will be exposed.
The same could be said for reinforcing pads for structural attachments that cover a vessel seam weld.
Any future leaks in the shell seam weld will be exposed.

Now we talk about internal re-pads - why would they need a vent / weep hole ?

Weldstan has given a very good alternative example but that is only for very specific vessels.
Having a vent/weep hole on an internal pad will lead to all sorts of corrosion issues.

Why is the vent / weep hole actually required ?

I have had numerous heated discussions with various Engineers over whether a vent/weep hole is required for repads on support legs for pipework (B31.3) - totally disagree with their responses but probably better to start a new post rather than hijack this one.
Cheers,
Shane

 

[r6155]

Like I said 26 Jul 20 Always avoid internal and external pads
See
1) ASME VIII Div 1 2015
11-1461 Revised UG-37(g) to delete the words “to test for tightness of welds that seal off the inside of
the vessel.” Revised UG-37(h) to clarify that all split re-pads require telltale holes, and deleted
“and shall be tested.”

2)PIP VESV1002 Vessel Fabrication Specification

3)Pressure Vessel Design Handbook 4th edi , by D. Moss

Responsible manufacturers do this test..

Regards

 

[ALA445]

Thank you all for the great replies. In my opinion, instead of vent hole in re-pads inside the vessel, we can leave one corner of re-pad un-welded which can be welded later once all the gases are out. Thanks again.