Weld strength calculation

[virjil]

Can somebody help me in calculating the weld strength of a 20" SCH 80 pipe connected perpendicular to a 42" diameter, 9 mm thick (not seamless) pipe, SS304 material. It has a reinforcing pad of CS material of 9 mm thickness.The 42" diameter run is installed vertically and is heated to 600 deg C. On my actual measurement, maximum deflection of the 20" diameter pipe wrt x-axis from the support, due to linear thermal expansion of the 42" diameter pipe, is 60 mm.
We would like to increase the operating temperature of the assembly to about 750 deg C. I am worried that the welds on the tie point will not anymore carry the stress induced by thermal expansion.

 

[arto]

Differential thermal expansion on steel-on-stainless may cause extremely bad stresses. Once a client had the ends fall on a "brand new" vessel 3 months old due to stress corrosion cracking.

Also the c'stl falls off the chart in B31.3 in those tempperature ranges, & there's not much left of the 304 stainless - you might want to look into high temp materials for this application.

 

[EdDanzer]

If the weld filler material is the same as the base material, and is welded 100%+ with no flaws. The weld should have the structural properties as the parent material. If the pieces have a different chemistry than the weld then it will be difficult to determine functionality. It might pay to have a welding engineer look at the project.

 

[Rich2001]

One has to determine the alloying composition of the weld. This is a function of the filler materials, base metals and welding process (type, heat input, etc.). Basically you need to run a set of dilution calculations to determine what the alloy is, ferrite number … then estimate its' mechanical properties.

In general Stainless (if austenitic SS) if expand 30% more than the C.STL. yielding a large thermal stress at the joint. One has to model not only the joint but the complete piping system.

[bold]A complete Pipe stress analysis is required for the whole piping network. Include a transient temperature - stress analysis as the system is heating up. [/bold]

The creep strength of 316 at 1300F (704C) is 7.5 ksi (1% elongation 10000 hours) while at 1100F (593C) the creep strength is 17.4 ksi (almost a 60% loss in creep strength).

A good piping stress Finite element program with a competent piping engineer and a welding engineer is required. the cost will be less than the cost of an accident or emergency repair.

Carbon Steel - the temperature of 750C is higher than AC₁ the transformation temperature.

Be very, very careful.

 

[virjil]

Actually, each of the two- 20" branch pipings have 2 sets of single volute expansion joints. I believe they can carry the thermal expansion loads. But all of you were suggesting that the problem can be on the metallurgical transformations that happens to steels subjected to elevated temperatures. Thanks guys.