Welding Valves 2

[Zeroman]

I was recently told from some field operators that they are having trouble opening and closing ball valves. As a matter of fact they are having such a hard time closing the valves that they are bending the handle. Are welders expected to remove the bonnet and the contents of the valve before welding? May we have insufficiently supported the valves and they are bending?!? Do you have any other ideas why these newly installed ball valves could be having troubles. Thanks for your help.

-Green Mech Eng

 

[atad]

I assume that when you say welding you are referring to socket weld flanged valves. If this is the case then it is normal practice to remove the centre section of the valve and weld the flanges to the pipe. When welding is complete and the pipe/flange assembly cooled, the valve centre section is reassembled.

A possible reason for difficulty in closing the valve is that the valve seats have become distorted/melted assuming they are PTFE or a polymer based seat material.

 

[Zeroman]

Thank you very much atad !!

Dave

 

[bcd]

Hello Zeroman,

Don't know the details of the type of valve, materials of construction, or installation technique. However, from the problem description, it could be caused by welding assembled valves in-line when they are closed. If the valves are soft-seated, the polymer can distort, mold itself around the obturator, and obstruct operation.

Everyone has an opinion on this subject, but it is also normal practice to weld fully assembled valves in-line without disassembly. Extra care must be taken, but it can and is done with regularity. If welding assembled valves, it is very important to open the valve before welding. It minimizes heat input because air can flow through, and helps prevent the seats from distorting. The benefit of welding assembled is it is faster, costs less, and eliminates the problem of a fully tested and functioning valve being disassembled and reassembled by the welder. There are many cases where the valves are damaged during disassembly/assembly. Pick your poison, damage from welding, or damage from the welders.

Some will argue that disassembly must be done before welding. However, the facts are that this is not always true.

bcd

 

[Celebur]

While welding of fully assembled valves is common, it is also common that this voids the manufacturer's warranty--but that's another issue.

Perhaps you should also look at the valve design. "Normal" turning torques depend on the design as well as the fluid pressure, so it could be that the design used is not good for the application. For example, is the ball trunnion mounted? If not, does the valve have a pressure-balance feature? And are the operators using it? Have you used this valve design before?

You need to look at what worked before and whether anything (design, supplier, etc.) has changed.

 

[Hush]

Here's the skinny from the valve manufacturer's perspective:

The warranty is void if our recommendations for welding aren't followed. This involves a high postheat requiring the valve be disassembled before welding.

The warranty is void if anyone but one of our service technicians dissasembles the valve.

In practical terms they're welded into lines all the time without disassembly. Make sure the valve is cracked open, watch your temperatures don't get too high, and change out the valve lube and sealants when you're done.

 

[PAN]

It seems that PTFE cannot withstand the heat of welding.

How about graphite rings? Should we weld without part removal? Please comment.

 

[scalleke]

Oh wonderfull PTFE. We all thought this was the answer to all our problems. hy should we not, PTFE can be heated to 450°F and there are even some modified versions out there that can accept temperatures higher than that.

HOWEVER.

The glass transition temperature of PTFE is about 295°F. Meaning that the molecular structure of Teflon will start changing (read deteriorating) at that temperature.

Another fact is that as is the case with all other materials, material properties of Teflon change as the temperature gets higher. Compression strength will definitely reduce.

The seats in the ball valve are installed with a certain compression ratio. The valve must seal eaven at low differential pressures right? This means that the seats are pre stressed and depending on the seat design that seat stress is more or less close to the maximum allowable compression stress of the PTFE. If the heat increase reduced the PTFE allowable compression stress to lower than the actual compression stress the seat will permanently deform and valve function will be compromized.

PAN, are you talking about graphite reinforced PTFE in which case the above explanations or are you talking about resin impregnated hardened carbon graphite. With the last one I do not see a problem as it is dimensionally stable and strong to very high temperatures.

Best Regards.

Scalleke

 

[PAN]

scalleke,
In my specific question, graphite can withstand upto 1000 F. The valve is not ball valve. It is sampling valve with graphite rings.

I realize that the heat of welding is higher than 1000 F. However, I'm not sure about the temperature close to the weld. Should we remove the part before welding? Please comment.

 

[scalleke]

Temperatures in the weld area are well above 1200°F. Ok but they are not even a short distance away from the weld. If the rings that you are talking about are compressed graphite rings and there is a reasonble distance between the rings and the weld area then you can leave them installed.

Depending on the fabrication of the graphite rings there will be certain additives in the rings and they are in fact the component that can get damaged. the graphite can hold the temperature but the additives will deteriorate. The rings might become brittle.

Then again . . . . Graphite is not a so solid material in all the type compressed rings that are manufactured and when you disassemble you risk damaging the rings.

I would proceed by doing a simple test weld on the type and dimension of the material that you are going to weld. Attach a couple of thermocouples to a test piece at different distances and weld according to the procedure that you will be using. Check the temperatures at the different distances. If the temperature is ok at the same distance as the rings will have to the welds, OK go ahead, weld and leave the seats installed. If the temperature is too high, disassemble and make sure to have spare seats ready.

In any case if you weld with all parts inline you will most likely violate your warranty. AND the safest course of action is to disassemble and weld. Which will cause troubles as the valve internals must be protected during welding and the welders will be careless.

Cover the basics, make a decission based on your situation.

scalleke