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Urgent : Welding in Pressurized line with High Temperature 1

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ak1965

Mechanical
Jul 28, 2007
158
We are planning to weld a 4" weldolet of A 105 specifications to A213 P 11 High Pressure Steam Piping that is in service by taking a hot tapping.

1. We shall first weld the 4" weldolet to 6 " pipe that is carrying steam @ pressure 47 kg/cm^2 and 400 deg.C. while the 6" pipe is in service

2. Nex step would be to take a hot tapping from the 6" pipe thorugh this weldolet.

3. My query is that can i weld weldolet to the pipe in running condition. I suspect during welding, due to molten pool the actual sound wall thickness will reduce for the time being, making the pipe vulnerable for failure during the welding.

4.As per ASME Section II stress table, The maximum allowable stress would be around 1037 kg/cm^2 for F 11 material at 398.8 deg C.

5. Where as the actual hoop stress generated in 6" pipe (7.11 mm thk) considering the available sound wall around 3 mm (we presume 4 mm on outer side shall remain in molten state during welding).

6. Considering above I am skeptic that during welding due to execcsive hooop stress (1206 kg/cm^2),the pipe may fail and steam jet may come out in an uncontrolled manner.

We are planning to simulate the condition in our workshop.

Please opine for this unique problem.This is urgent please.

.
 
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I have seen hot tapping of steam lines in service.

3. My query is that can i weld weldolet to the pipe in running condition. I suspect during welding, due to molten pool the actual sound wall thickness will reduce for the time being, making the pipe vulnerable for failure during the welding.

The actual size of the weld fusion zone in a mushy state (L+S) relative the to the wall thickness will not result in a weakened condition where failure can ocurr from creep rupture. The pipe base material will draw heat from the weld region and the base material beneath will not be weakened, as well. When you look at the size of the weld region versus the circumference of the pipe, this is a small percentage that is affected at any given time.

 
Thanks....Metengr...

My worry that at 400 deg C the calculated strss value is 1037 kg/cm^2.

And actual hoop stress during welding due to reduced sound wall thickness shall be 1200 kg/cm^2.

Considering molten wall of around 4 mm out of total wall thickness of 7mm.

Will stiil the pipe hold the welding and will not pucnture under pressure?

 
ak1965
How are you calculating the 4mm wall affected material? The attachment weld will be a fillet weld by design on the OD surface of the main 6" pipe (0.28" wall thickness). The penetration of the fillet weld through thickness should be only a small percentage of the total wall thickness at the OD surface.
 
Metnengr

Please see the attchement sketch... it will make you more clear about my apprehension....!


I am not worried for the failure during the welding.... rather the safety of the welder during welding makes more sense to the apprehension...!
 
 http://files.engineering.com/getfile.aspx?folder=e72fab00-1d1a-4032-b88d-1d0d77d7f36b&file=On_Line_weld_Diagram_001.jpg
ak1965;
Thanks for the sketch, it helps me to visualize what you are thinking and concerns. The problem that I see with this approach is the hoop stress calc for the 4mm remaining wall under the weld region. You are assuming a 360 degree band of 4mm for this formula. This is really not a correct assumption. What you do need to consider is the following; what is the minimum wall thickness to sustain pressure and temperature using ASME B31.1 allowable stresses.

Assuming nominal wall thickness of the 6" pipe run (with no internally thinned material or creep damaged material) the actual wall thickness to sustain operating pressure and temperature will be below nominal wall thickness. Taking this into consideration, the remaining wall thickness (aka design margin) will be where the local melting from welding will occur and once again this will be a localized phenomenon, not 360 degrees.

In reality, during welding with the SMAW process and using stringer beads, the penetration depth into the pipe wall can be controlled by laying the beads on the surface of the pipe to achieve lower penetration. Second, welding is occurring at only one location at a time, similar to a line, and by the time the welder moves to a new location, the weld pool has solidified. You still have the code minimum wall thickness to rely on despite what is happening near the OD surface.

I understand your apprehension and I agree a mock-up should be used for your welders to measure penetration of the weld into the pipe wall. I see two requirements for this job - the welder needs to lay the weld beads on the OD surface of the pipe and not try to gouge into the pipe. Similar to buttering.

Have the welder work slowly with stringer bead sequences while welding on the pipe OD surface.
 
ak1965,

You could have more concern for the weldolet A105 material at 400 deg C service. Perhaps a weldolet of material to ASTM A182 Gr F11 would be more suitable and compatible to the A213 P11 piping.

The observations of metengr are good, and while many hot tap operations are performed every year the skills and the training are essential to the safe completion of a hot tap. Many hot taps done while the flow is cooling the weld heat zone have less concern of over-stress compared to zero flow high pressure hot taps.
 
Hire a contractor that does this all the time, you don't want to learn this by yourself.


 
metengr

thanks.. for useful and elaborate reply...!

We have concluded simulateion successfully.... heated a blind end pipe segment to 450 deg C with Nitrogen inside @ 48 kg/cm^2.

Welder could carry out TIG welding with ease with no deformation or puncture on the mock up....!


Using B31.1 stands good and logical....I shall do that as well... as the heat would be localized not a 360 degree. band.....!


@ ApC2Kp :

Thanks for your concern....! I know dissimlarity in the joint A 105 to P 11....! The factor is not unnoticed..! Infact we do not have F 11 weldolet readily available in our stores...!

This is a make shift arrnagment...!


@hacksaw

We have experience in hot tappings....! a really good one...!



Dear all.....after successful Mock up test...We have decided to go for welding tomorrow....!


Wish me all the best ........

 
Have you considered the welding control aspects?:
1) Have you got a qualified procedure for these conditions?
2) Most carbon steel welding would limit the interpass temperature to 250°C max. Here you are at 400°C
3) Most hot tapping / live welding is done with very detailed procedures, welder qualifications and controls.

On the plant I work on we would not tackle such a high temperature steam line.

We inlcude thickness checks of the pipe prior to welding to ensure we know the thickness (could have internal wear) and ensure there are no laminations.
 
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