We are designing the high pressure steam piping for a power station. The main steam design pressure is 120 Bar(g), design temperature is 550 deg.C, material is ASTM A335 Grade P91 and pipe size is 500 mm (20 inch) nominal. Near the turbine end, the main steam piping branches out by using a fabricated 45 degree Y- Lateral, having the size 500 X 500 X 400 mm. After this branching, the 500 mm pipe is reduced to 400 mm and therefore we are running two 400 mm branches to the steam turbine.

I am thinking to model this fabricated Y-Lateral fitting by a simple node point in Caesar-II, having three branches (500mm, 500mm and 400 mm) connected to it and having a suitable SIF at this node point.

My question is, what is a good number for the SIF for this Y-lateral? For this size, I do not have a Latrolet (or equal). Also ASME B31.1, mandatory appendix D does not list any Y-Lateral fitting.

How do I proceed? I will appreciate any suggestion.

Regards,
PSTRN
21 April 2008

You are outside the scope of the piping Code. You really only have two choices here:

1) Ask the vendor you're going to purchase the fitting from, or
2) Perform your own FEA evaluation of the fitting and analytically determine the SIF.

As Richard says this is not for CAESAR II. Using FEA you may get it wrong also. Item is in creep range and you have to get it right. I would not risk this critical item. There are also manufacturing and welding issues. Find a supplier who has done this before and talk to them.

If this happens to be your first P91 job or first main steam line please talk to power station steam pipe designer. There is a long list of issues that have to be considered.

Richard,

If I replace the Y-lateral by a conventional reducing T (i.e 500X500X400 mm) or use an Olet of 400mm size, and change my 400 mm branch piping layout accordingly, can I use B31.1 Appendix-D or use the Olet manufacture' published SIF and then do the Caesar-II calculation?

Regards,

22 April 2008

The forging vendor can most likely provide you with the SIFs. But as in most cases with High Pressure heavy wall fittings, the SIFs will most likely be close to 1.0. Review the nominal stesses at the center point and see how much margin you have.

If you have stresses in the 80% plus range (SIF=1.0) then you probably have huge support loads that cannot be designed in the first place.