Main Steam Lines Thermal Stress Analysis

[Boghi1990]

I have to determine the thermal stresses in the wall of pipe, during the start up/warm up of 18 inches and 12 inches with 8 inch and 7.5 inch insulation thickness steam lines. The length of the lines is about 190 feet and consists of a few bends, two Wye Tee fittings, pipe and insulation. These lines run from boiler stop valves to turbine main stop valves. The ambient temperature where this steam lines are installed is about 70F.

The heating rate of this start-up process is 200F/hr. The warming up process starts off at 100F and ends at 750F. During that period, the boiler stop valves are opened while the turbine main stop valves are fully closed. There are drain lines opened which allow any condensate to drain during the start up process. When there is not any condensate left in the steam lines, the drain lines closes off. This happens before the warm up process ends. After the start up process reaches 750F, the turbine main stop valves are opened.

A piping stress analysis (Caesar II) have been completed for these steam lines. The concern is now if the warming up proces will generate thermal stresses (circumferential & radial) in the pipe wall higher than the allowable stresses for the pipe material.

I need to investigate if the thermal stresses developed during this start up process are OK for the type of pipe of material, pipe wall thickness and the heating rate. The lines are are already installed, and the only parameter which can be changed is the heating rate of the start up process. Would the thermal stress be different in the straight run of pipe than in the bends or in the wye tee fitting?

I am wondering of what type of FEA could be the most appropiate for this situation. Do I need to model a transient(unsteady) coupled fluid thermal flow analysis, in order to get the temperature profile in the pipe wall thickness? Does this coupled fluid flow thermal analysis requires to model the whole length of these lines or could I model only a zsection of the steam piping? This model will lead to a large computational time.

I am also wondering if I could just model a transient heat transfer analysis(no CFD) to get the temperature profile in the pipe wall ? If I set up and analyse this transient heat transfer model, could I consider only a section of the steam lines ? Do I need to include the bend and the Wye Tee junction in this model? Attached is a print screen of the steam lines.

I would appreciate any suggestions or tips you might have.

Kind Regards,

Bogdan

 

[corus]

If the ends of the pipe are restrained then thermal expansion would cause axial stresses in the pipe together with bending at the joints though the stress limits would be higher as they'd be classed as secondary. Run a 2D thermal stress analysis to check if transient effects are significant, though I would doubt that. These would be significant if the temperature profile differed significantly from a linear temperature gradient. Use the temperature profile in a 3D model of the pipes to check the axial stresses from thermal expansion.

 

[Boghi1990]

Hi Corus,

Thank you for your response. The piping stress model (Caesar II) has got one end fully fixed (anchor), at the team sturbine end. The other end of the steam lines (boiler end) has 3 translations and one rotation(RZ)assigned as displacements. Now in the FEA model, I am considering to model only a section of the line and use the symmetry so I could model only a half or a quarter 3D FEA geometry. When I set up the structural analysis, how could I constrain the ends of the pipe? Should I constrain one end of the pipe like an anchor (all DOFs fixed) and the other end with imposed displacements or should I leave this end free to expand?

In my FEA model should I include the bend and the Wye Tee with the staight run of pipe or should I model only the staight run of pipe? I am not sure if the thermal stresses (radial & circumferantial) would be different in the straight run than in the bend or the Wye Tee fitting? Where I could find the stress limits (allowable stresses )?

I would appreciate any suggestions to the above questions.

Regards,

Bogdan

 

[corus]

There is symmetry at the half way point. You'd include all the bends up to that point. Restrain the pipe axially for symmetry. If this was in the Z axis, say, then the rotational restraints would be about X and Y axes.

In pressure vessels the allowable stress range for secondary thermal stresses plus primary stresses is twice yield but the pipe stress limit for thermal loads seems to be less. This link gives the allowable for thermal loads but you'd be better referring to the original design standard.

http://pipinganalysis.blogspot.co.uk/2011/07/allowable-stress-based-on-asme-b313.html