Weep-holes in reinforcing pads and 'similar'. 2

thread794-25516 and thread794-471960

These threads indicate 'incomplete knowledge' of this subject. I don't claim complete knowledge, and have not here covered everything I know on the topic, but hopefully I have filled in a few gaps. (Now retired, but with more than 41 years experience.)

Reasons for weep holes mentioned in those threads include:
- Prevent pressure build up during welding which may cause problems with blow-back when finishing the weld. I have never seen this: it could possibly happen under some circumstances, but as the welding slowly progresses the temperature, and hence pressure, of any trapped gas should stabilise.
- For high-temperature coating application such as metal spraying or galvanising, where trapped gas is said to blow the pad off with increased pressure, or trapped water would turn to steam. Bearing in mind that in this case the pad is already welded, it seems to me that this is pretty unlikely for dry gas: pressure is proportional to the absolute temperature, room temperature is about 300K, so at 900K (about 625C, ie the whole thing is red-hot, certainly way above temperature for most coatings or galvanising that I am aware of) the absolute pressure of any trapped gas is about 3 Atmospheres, 45psi, which is only 30 psi Gauge pressure - and you'd have to get the whole thing to this temperature to achieve that. OK, oversimplified, but that's where any argument has to start! Trapped water or liquid is different, and could cause very high pressure.
- Provide assurance, during the life of the vessel, that an internal weld has not failed, eg a nozzle weld cracked or corroded. At least one respondent on one of the other threads did not seem to think this can happen - it can! It has! It will again! This is definitely a reason for venting.
- Prevent pressure build-up from hydrogen. Yes, absolutely. More on this below.

It is not just reinforcing pads that this applies to. It also applies to slip-on flanges, any pad or support welded onto the shell (read pipe-wall etc too), external supports and clips, and any similar situation - and also internal pads, supports and clips.

Most of the above are fairly well covered through the other threads, but the issue of hydrogen is only briefly mentioned, so I'll elaborate on that.

The following basically assumes the material is carbon steel. Other materials may behave better or worse!

Hydrogen can be present either directly, from the service, or be a result of corrosion - specifically, acidic corrosion. This may not imply that an actual acid is present: it can be from sulphur compounds ('sour' service) or any low-pH service.

Hydrogen service is relatively complex, and I won't go into that, any inspector working on such equipment should be fully trained or otherwise supported by the owner. But note that hydrogen service is often also 'sour', with H2S.

Acid corrosion releases 'Nascent' hydrogen. 'Nascent' means 'new-born', and the thing about this is that it is atomic - hydrogen is released as individual atoms. Gaseous hydrogen is usually 'molecular': two atoms joined together. Hydrogen is the smallest atom there is, and atomic hydrogen, created by corrosion on the inside wall of a pressure vessel, can pass right through the wall by squeezing between the iron (etc) atoms that make up the steel. If the hydrogen emerges into the void behind a reinforcing pad, it will almost certainly encounter another hydrogen atom that has made the same journey, and they will combine to form a molecule. Molecular hydrogen is far less likely to diffuse through the steel, so does not escape. This can create enormous pressures, because the diffusion of atomic hydrogen is driven not by the pressure difference between the corroding surface and the void, but by the difference in partial pressure of atomic hydrogen. The partial pressure of a contained gas is the pressure that would exist if no other gas was present in that container. So, as most atomic hydrogen within the void is rapidly converted to molecular hydrogen, no matter what pressure of molecular hydrogen exists in the void, the partial pressure of atomic hydrogen is miniscule. The build up of pressure by molecular hydrogen is therefore, in principle, unlimited. Until, that is, it breaks out by over-stressing the walls of the void, or cracking a weld or etc. Drilling a weep-hole will prevent this.

In a reinforcing pad, this would normally crack a weld that holds the pad in place. In a slip-on flange, it usually cracks the inner fillet weld, at the end of the pipe. If the pipe is large diameter the wall can be severely distorted before the weld cracks. I have seen bulging in the neck of a manway (sour hydrogen service). We drilled the flange externally to release pressure, and it was over 30 minutes before we could no longer hear the gas escaping (drilling was stopped as soon as we detected gas escaping, so it was a pretty small hole, the drill tip just penetrating into the void.) Similar distortion can occur with slip-on flanges on manways in HF Acid service.

This process also leads to 'hydrogen blistering', where hydrogen builds up at laminar voids in steel (usually in poorer quality plate), deforming the plate, as in the photo attached.

With modern 'clean' steels hydrogen blistering should be far less of an issue, but the situation with reinforcement plates and other welded attachments has not changed. Fully-integrated reinforcement is the best way to address this. (But can lead to other problems if applied without due thought - but that's another story!)

Then there is the issue of plugging the hole. If hydrogen can build up from corrosion then, as explained, the pressure can be enormous. A threaded plug, or even a 'blob' of silicone sealant, will allow sufficient pressure to build up to crack a weld attaching the pad to the shell. Admittedly, as soon as it's cracked the pressure will release, but nevertheless the integrity of the reinforcement is compromised. Sealing the hole with a blob of heavy grease seems a far safer option.

Pipe trunnions (and similar) should also be vented. Until fairly recently it was common practice to install these with no end-caps, which frequently led to corrosion of the shell wall inside the trunnion - this was never painted, and the industrial, often coastal, atmosphere is corrosive enough to cause a major problem over a few years or decades! Project groups were generally uninterested in helping by welding caps over the ends of the trunnions, and this has lead to huge on-going costs for inspection, and, when needed, maintenance. Larger trunnions can provide irresistible nesting opportunity for birds, and the higher up a column the more likely they are to indulge, and the greater the cost of scaffold etc for access --. I assumed that capping the ends would resolve the issue, and, on hearing experiences of other who had failures inside trunnions that were capped I was pretty sceptical - surely the cap had only been installed long after construction? Then we had a failure on a pipe for which the trunnions had definitely been capped from new. To cut a long story short, the pipe (operating about 70C) had two trunnions opposite each other, one side had failed, the pipe wall being severely corroded over the entire surface inside the trunnion, the other side was pretty much as-new. The difference was in the drilled vent holes. One was situated right over the bracket that the trunnion was sitting on. When it rained, the trunnion cooled, causing a vacuum inside the trunnion, which sucked in water that was lying on the bracket. The other was drilled clear of the bracket. You can guess which side was corroded!! The second photo shows where the vent-hole was --. (It may only allow one photo??? Might have deleted the first - oh well!!) (It may only allow one photo??? Might have deleted the first - oh well!!)

Air at 15 psi maximum from the threaded hole, the inspector is inside the PV (if possible) and checks for any leaks (with a bubble forming solution) from the weld between the nozzle to the shell
Many inspectors ignore this test.
Plug the hole with heavy grease.

Regards

Vessels which process or store Cryogenic liquids (Liquid Nitrogen, etc) often have internal structures. The doubler pads for these structures require ventilation. They have a combination of a drilled hole at the top of the pad, and half an inch of incomplete weld at the base of the pad. During operation, the very low viscosity cryogen will accumulate behind the pad. When the vessel is shut down, the liquid cryogen will very quickly expand into gas, and needs to escape. Most of the liquid cryogen can just fall out under gravity. Any remaining cryogen can simply escape through the hole or gap.

Reasons I can think of for having a hole (or fillet weld gap) in a pad.

1. To prevent the gap under the pad from becoming a pressure vessel, of which there are many sub-reason for how this can happen. (Liquid cryogen build up, Nascent hydrogen build up, etc)
2. To prevent water or other corrosive substances building up under the pad, which go on to corrode the vessel, pad or weld. A hole or weld gap at the base of the pad can address this issue.
3. Where the pad is welded onto a vessel seam, it may be necessary to do a 15 psi soapsuds leak test of the vessel seam.
4. Prevent weld blow back.
5. It may be necessary to do a 15psi soap leak test of the pad fillet weld, and then completely pressure seal the hole with a threaded grommet. This scenario may be applicable for a vessel experiencing external pressure, such as ocean submersibles. It is not wise for corrosive seawater to make its way under the pad.

Deciding to not drill a hole in a pad is perhaps the more difficult decision.
I cannot think of a lot of definitive reasons for not drilling a hole.
1. for internal pads where the process fluid remains liquid and is very corrosive, unhygienic etc. This may apply to food processing.
2. For some applications where the vessel experiences external pressure.

I agree with the two comments above (r6155 and DriveMeNuts).

r6155: Another reason for plugging an internal vent is to avoid the possibility of entrapped material escaping and compromising safety for intrusive inspection. Probably only a real risk if the material is lethal or poisonous (nickel carbonyl, HF or similar). We had a few vessels in such service (questionable, at least) that had been installed second-hand, and in my view should not have been used. But it goes to show that compliance with a design standard is not always sufficient to ensure PV safety in practice! The failure of the exchanger at Longford is a case in point, there are other examples.
Welded internal linings - usually for corrosion resistance - are another issue. These are in two general types: clad linings and shingle linings. The difference is that, with a clad lining, relatively small plates (perhaps 150 x 300mm?) are welded all-round direct to the shell, and to each other. They may be only 3mm or 1/8" thick. Each is drilled and tapped so an air test, as described by r6155, can be applied, and then the hole is usually welded over and checked with dye penetrant (or etc). In a shingle lining, if it extends 360 degrees around the vessel, only the top is usually welded to the shell, with the plates overlapping vertically and the laps seal welded. If a second row of plates is required then it is only welded to the top row, not to the shell. The bottom is left unsealed. Even though there is usually a considerable galvanic difference between plate and shell (eg stainless steel or monel plate over carbon steel shell) I have never seen an issue with corrosion under the lining, probably partly because the lining extends below the level where corrosion occurs, and partly because there is no flow, so the supply of 'active corrodent' is limited. (Have sometimes seen minor corrosion on the shell immediately adjacent, both top and bottom.)

DriveMeNuts: I had little experience with cryogenics (some with pumps and compressors) so your comment is particularly interesting! I would doubt that corrosion is a common issue at cryogenic temperature, and I am not aware of cryogenic processes that involve much in the way of corrosive fluids anyway - not to say there are none, but nitrogen, ethylene, propylene, methane etc are not corrosive. (In the wrong situation oxygen is, but not when dry or flow rate is low - here, potentially, starteth another complex discussion which I will avoid! Suffice to say that storage tanks are usually lined with something that won't corrode under storage conditions. Stainless steel or even aluminium (I think?) are common.)
I agree with your conclusion: not drilling a hole would need to be carefully considered, but I am still of the view that some of the reasons are basically spurious - blow-back during welding should not occur under normal circumstances, for example. Providing reinforcement with a butt-welded thicker shell insert is the safest way in most cases, but at a cost.

Drilling through the shell from the outside can work under many circumstances. I briefly discussed hydrogen blistering in my original post: I have on occasion drilled through the outer part of a blister to prevent further growth, even, with suitable precautions, in anhydrous HF service. As it didn't go with the first post (despite the talk of lethal materials etc that is not an intended pun!), here is the photo of blisters that I tried to send before. The photo is a bit skewed, the blisters follow the line of a laminar inclusion in the plate, and are not right along the bottom of the vessel, which is aligned with the boot and nozzle.

a)Try to avoid the pad, it makes in-service inspection difficult. .
The structural details of supporting lugs, rings, saddles, straps, and other types of supports shall be given special design consideration to minimize local stresses in attachment areas.

b) Nozzles shall not be located in Category A or B joints. When adjacent to Category A or B joints, the nearest edge of the nozzle‐to‐shell weld shall be at least five times the nominal thickness of the shell from the nearest edge of the Category A or B joint

Regards