Wye fitting forged block(seamless) or spherical(welded)

[paramarth]

EN 13480-3 piping code that Cylindrical Y-Pieces (Seamless) are not recommended in High temperature creep lines
One of the reason could be that forged and machined Y-Pieces have very high thickness in the crotch area (almost three to four times) compared to spherical welded Wye pieces. This can also affect start up of boiler-turbine in power plants
I find that many designers continue to use Forged and machined seamless Y-Pieces.
Can anybody throw light on which type of Y-Piece is preferred in Main steam and Reheat Piping based on their experience

 

[MJCronin]

I suggest that you include some pictures or sketches so we all understand exactly what you mean by the terms you use

MJCronin
Sr. Process Engineer

 

[weldstan]

We have seen weld fabricated wyes with almost no added material in the crotch with failure within predicted time of approximately 40,000hrs (after failure of course). These were made from P91 alloy and inspection indicated poor ID weld penetration. Replacement was done with forged wyes.
John Stanley P.E.

 

[paramarth]

I am attaching the pictures of the two wye pieces for your understanding (courtesy MEGA fitting)
The left side Y-Piece (larger one) is a spherical Y-Piece which is of welded construction widely used in Germany and Europe. Design is done mostly based on German TRD-301
The right side Y-Piece (smaller one) is seamless forged one. In this Y-Piece, the crotch thickness is very high compared to Spherical Y-Piece. Also in case of materials like F91, spools have to be welded on the three directions at manufacturing works before field erection as the area available for heat treatment will be very less and difficult at field.Design of forged Y-Pieces is done mainly based on KELLLOG's handbook and FEM.

 

[MJCronin]

In my opinion, forged wyes have been the de facto standard in US based high pressure steam systems.

All of the main steam, hot reheat and cold reheat steam systems that I have worked on for many years at Stone & Webster engineering were configured this way.

They have had a proven record and were always considered to be a very stiff element (like a valve) in any piping stress analysis.

WFI, of Houston Texas, has the most expertise in this area....... IMHO

http://www.wfi-intl.com/index.php?page=seamless-size-on-size-wye

MJCronin
Sr. Process Engineer

 

[XL83NL]

US and european markets, especially the German, are very different. US codes tend to have a higher safety margin and are therefore often considered to be thicker, so 'you know you're good', you could say.
The Germans have always engineered in a more precise manner, with lower margins. Therefore the designs and design rules can be different at some points. This may be such an example.
However I'm not that experienced to tell the historical difference, nor am I implying US codes are less correct for this matter (ASME codes don't have a world wide recognition for nothing).

 

[weldstan]

There may be considerably more reinforcement in the crotch than meets the eye. Only a cross sectional detail will define reinforcement.

 

[paramarth]

Thanks for your valuable comments

For a Supercritical unit with a Main steam pressure of 270 bar and temperature of 573 deg C, an F91 Y-Piece of Seamless Forged construction has a crotch thickness of nearly 210 mm, where as a Spherical Welded Y-Piece has a maximum thickness of only 125 mm.

What impact the high crotch thickness of a forged Y-Piece will have on the start up and also the fatigue is what disturbs me as a designer. Particularly once through supercritical units are designed for faster start ups and preferably have low wall thickness components. Should the ramping up of temperature the unit be decided based on the Y=Piece thickness ?

Or can the high crotch thickness be neglected as it is only in a small localised portion of the component ?

How this issue is seen in US based high pressure units. As I had mentioned earlier all German Supercritical and Ultra supercritical units are only using Spherical Welded Y-Pieces, where such an issue does not arise.

 

[MJCronin]

AAaahhh..... now we understand a bit more....

In the 1960s and 1970s, secondary stress affects of piping wall thickness transitions were not considered in forged wye fittings incorporated into powerplant systems. Quick start-ups were obviously not an issue in massive watertube coal fired boilers as they are in modern combined cycle power plants

If you are interested in the secondary stress effects in thickness transition, you may want to review the references in the Nuclear Power Section of ASME -III (I believe that it is section NB-3600). Additional information may be found in the section defining rules for Nuclear Vessels - NB-3200.

This may be of use:

https://www.ipen.br/biblioteca/2011/eventos/17030

If John Breen could share his knowledge about this issue it would be helpful....

MJCronin
Sr. Process Engineer