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2
- #1
Roger G
Mechanical
- Dec 18, 2023
- 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!!)
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!!)
