Maximum Allowable Internal Pressure During Welding

[dnigos]

Hi -

I'm currently preparing a line for a hot tap and I have been religiously following the API 2201 in the process. Based on that document, I have a clear understand on metal minimum thickness and how to proceed & what precautions to take when welding onto a thin pipe. However, API seems to be silent on determining the maximum allowable internal pressure for welding the hot tap nozzle onto the pipe.

Our refinery uses the formula:

P = (2*t*SE)/(D-2Yt)

where t, measured thickness, is multiplied by 0.4 (how far HAZ is extended during welding), Y = 0.4, D is your pipe diameter, E = 1 and S is the allowable stress at Mean temperature which is significantly lower than my design allowable stress.

So, the result was a maximum internal pressure that is lower than our operating pressure. Seems like the only way to proceed is to lower the operating pressure below maximum.

Here's my pipe data
Design: 500 F @ 400#
Operat: 450 F @ 330#
Stress allowable at Design: 18900#
Pipe Material: A-106B
Size: 14"
THK: 0.375" Nom (assume minimal corrosion)

Any advice would be appreciated.

Thanks.

FYI: I've also looked at API 1104. No info there about Maximum Internal Pressure during welding.

DEX

 

[BigInch]

Lower the op pressure during welding.
The maximum temperature of the wall when welding should be considered. You may need to calculate a minimum flowrate past the welding area to keep the wall metal below the maximum temperature.


"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics

http://virtualpipeline.spaces.msn.com

 

[SJones]

Your calculations are along the right lines. You will derate based on allowable stress at temperature of the pipe wall and the reduction of the pipe wall thickness because of the molten weld pool. It's just how you have determined the temperature and the weld pool size that you need to feel confident about. Item 4 of Table 3 in the Annex A checklists intimates that pressure should be reduced to the lowest possible consistent with keeping the plant operating. In summary, you WILL be reducing the pressure!

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[MTPipeliner]

Here is a link to some software that can help you model in service welding on live lines.

http://www.ttoolboxes.com/products/PRCI_Thermal.cfm

It's not very cheap, but if you are going to be doing lots of hot tap work it may be worth looking in to.

The Edison Welding Institute (EWI) did quite a bit of work with this and is the basis for the program. I've read through some of the original reports and it's fairly complex and doesn't lend itself to simple formulas and calculations.

What is the fluid that is in the pipeline?

A couple observations from past work are

1. Wall thicknesses greater than .25" are difficult to burn through if you have a moving fluid inside the pipe.

2. A liquid fluid nearly always provides adequate cooling even at very low flow rates.

Something to keep in mind is hydrogen cracking that can be caused by the weld cooling too quickly. Low hydrogen welding rod should be used with enough heat to ensure that it doesn't cool too quickly.

 

[unclesyd]

Here is one companies capability. For someone well versed in the art you conditions should present no problems. The hottest line I've seen welded on was a 650 steam line at 750F.

http://www.tdwilliamson.com/PDF/MISCPDF/HighTemp_Tapping_Serv.pdf

 

[dnigos]

Reducing the operating pressure is an option, BUT this route is what we don't want to take ($$$$).

We are in the process of doing a UT to determine the line current thickness, but we should have sufficient wall thickness.

We have about 5 ft/s flow in the line.

The service is crude going to our heaters.

SJones: " Item 4 of Table 3 in the Annex A checklists " From what document??


Thanks!
Dex