5 replies to this topic

[ASH25320]

If welding is tobe done at pump suction which is running & carrying conductiing fluid, is there any chance of bearing damage or such thing due to this? what is the safe distance for carrying out welding on suction side? why welding is generally not allowed on a running turbine? Can somebody tell effects of static electricity & welding? Thanks in advance..

[Adriaan]

There are different welding methods, several of which uses electricity and THAT method no sane person would use on working equipment because of the charges involved.

Other forms of welding are thermal in nature (yes electric welding is too) and a running pump would cool the welding area so the weld would not be very good!

In any case welding weakens (as you are doing it) the area being worked on. Ergo; you do NOT weld anything in a system that is in operation.

[Art Montemayor]

With all due respect to Adriaan’s reply, I add the following comments not to correct or distract from this subject, but rather to lend some first-hand experience to it. I consider this a valuable subject and one that many young Chem Eng graduates out in industry will have to familiarize themselves with – sooner or later. And better sooner than later. Static electricity has nothing whatsoever to do with this thread, so the title is totally erroneous.

Contrary to what Adriaan states, there are many instances in the operation of a process plant or a pipeline where there is a definite and important need to weld on - and gain access to - an operating vessel or pipe that is under process pressure and temperature. Sometimes the pressure involved may be in the 1,000 psig range or more. I realize this may start to raise the hair on the back of some rookies out there, but it is best that they understand that this is not an extraordinary event if it has to be done. But bear in mind that the inherent hazard and the danger do not go away simply because there is a need. What I want to impress is the need to accept the fact that this may be a necessity sometime in your career and you should at least understand the principles and risks behind such a venture. I, personally, have made such connections into Natural Gas pipelines, steam mains, pressure vessels, and even Hydrogen lines. I’ve done it under a very strict and controlled procedure that called for careful planning and implementation. The basic technique that I will describe to you is called a “hot tap” and it is consider vital that all process and project engineers in industry know about it because they will be exposed to it sometime in their professional lives.

If you stop and consider that no production unit or operating plant can practically go through it’s economic life without some modification or revamp to its mechanical equipment you will quickly realize that sometimes that need has to be implemented without the plant or unit being shut down. For example, one typical hot tap is required when a new steam consumption point is introduced in a process plant or unit where it was not originally planned to have one. As a consequence, an available steam source exists only as an overhead steam main that passes near-by. What is the engineer to do? The answer is that you are forced to make a live tap into the outside steam main simply because you cannot afford to shut down the steam power plant (& thereby the entire plant complex) simply because of your local needs. Generally the common method used to make such a “tap” is carried out in the following manner:

1. Use is made of what is usually referred to as a “weldolet”, or a saddle-like fitting that has a nozzle. This fitting fits directly on top of a pipe or a vessel’s cylindrical shell and is fillet-welded employing an electric-arc welding method (rod &flux or TIG welding). A full-bore gate or ball valve is used to screw or weld to the nozzle.
2. Once the saddle assembly is welded to the pressurized pipe or vessel, a special bushing that is attached to the open side of the valve employed is employed with a soft packing used to allow an extended drill bit to be inserted into the valve and the attached nozzle. The packing material around the drill bit allows it to turn while sealing off any pressure inside the nozzle and valve.
3. An electric drill is used to turn the bit and drill through the vessel wall or pipe that is under pressure at the time. When the bit drills all the way through, the nozzle and the valve instantly come under pressure but the packing around the drill bit prevents any escape to the atmosphere.
4. Once the drilling is complete, the bit is extracted by pulling it out and once it is beyond the attached valve, the same valve is immediately closed.
5. The special bushing is removed from the assembly, as well as any nozzle extension and the required piping is now attached to the block valve that stands ready to furnish the fluid desired on the other side.

There are variations to the technique, but the mechanical basics are very much as I’ve described them. I have seen some hazardous and dangerous situations averted or totally mitigated with this basic technique. One outstanding example occurred in 2000 at Conroe, TX when a runaway locomotive ran right through our plant’s gate on the spur that received full, pressurized tankcars of Ethylene Oxide (EO). The night that this near disaster occurred, we had a series of full, EO tank cars sitting on the spur line. The locomotive slammed into the first tanker and literally caved in the 2:1 ellipsoidal head of the tanker, but miraculously the tanker held its charge without bursting. The tanker however, wound up 30 yards away, on its side. The next morning, after thanking our God for sparing us, we were faced with the task of having to “safe” the situation by evacuating the tanker. The normal lines were totally useless and we had no option but to make a hot tap into the pressurized vessel full of EO. A specialist was called in immediately who does this type of hazardous hot taps. Needless to say, he showed up in a very expensive automobile and proceeded to organize every tool and procedure he needed for the very dangerous procedure. He carried it out flawlessly and the situation was saved. When we later found out how much he charged for the service, we understood why he drove one of the most expensive cars in the USA. I mention this incident just to show to what extremes the technique is used sometimes – albeit reluctantly. But you may not have any option. That’s why I consider the subject important to know about and to be aware that you may someday be called upon to contemplate such a procedure.

Electric arc welding is not static electricity. The two are totally different in nature and in application. Welding can also be done via the autogenous method – by using a flame from an oxy-acetylene torch and literally fusing the parent metal to the new metal you want to introduce. This method is with a live flame whereas the electric arc uses an electric current to induce the fusion. There are different methods of achieving the weld between two metals, so you should be specific in describing what you mean. What has been described in the original post is not coherent in that certain things that do not occur during “welding” are assumed.

I certainly do not recommend any one to undertake welding on a pressurized pipe or vessel – especially doing it with the autogenous method. The electric arc can be used, but once again caution and experience are the basic pre-requisites and a total familiarity with the metals involved and the stresses that can be built up are a strict necessity. If you are not a welder nor have you welded before, it is difficult to explain the techniques and the procedures that must be undertaken as a minimum for the sake of safety and success. There is no substitute for experience in this matter and certainly, a young engineer who has little or no familiarity with the technique has no business applying it or trying to supervise it.

[Nirav]

hello,

Art always gives very detailed explanation which is very helpful to understand the scenario.

I have heard about 'hot tapping' but never saw it practically yet; since it is rare and I do not have a long experience. The following links give very good animation about 'hot tapping' and it would be really helpful to those like me to visualise the process.

But first, read the brief procedure explained by Art.

http://www.teamindus...aps/hottap.html
http://www.teamindus...s/HotTapWeb.swf

Warm regards,

[Art Montemayor]

Nirav:

Thank you very much for your contribution. I like your response better than I do mine. Your basic Web research surfaced the very company that did the expert tapping of the EO tank car incident that I described in my post. I owe you a token of gratitude because it corrected what my memory failed to tell me: the incident happened in January of 1998 - not in 2000. My, how time does fly.

Also, the tap that succeeded in evacuating all the EO was a COLD tap, not a hot tap. The fine details and a complete report of the incident are reported on the website of the company involved: Team Industrial Services of Alvin, Texas -- right down the road from where I was born and raised. In my haste to write my response during lunch hour, I didn't do a careful writeup and used what my memory recalled or remembered about the incident. I was there; but I have been at so many others similar to that type that sometimes the details of all of them get hazy with onset of age and other priorities.

I highly recommend any interested young engineer to visit this website and read and study the many incident reports and case histories that they have archived in their website. The reading is not only interesting, but it also is a lot of mental fun finding out how human engineering ingenuity and resourcefulness resolves what would normally be catastrophic situations. Note the techniques used and the procedures carried out. The whole excercise is one that all engineers should be aware of or have knowledge of where this type of technical problem solving can be obtained. I think you will agree that the subject is an interesting one.

[Adriaan]

Dear Art,

of course sometimes it is unavoidable to do welding on a system in operation; this is a specialist job however (the company I work for contracts such things out to a specialist company). My post was mainly intended to convey one message: DON'T DO IT. At least don't do it yourself get a specialist to do it for you.