Pressing of pipes

[PrageethV]

After joining 2-3 pipes with butt joint, straighness of the centre line is deviated due to the thermal stresses and fitup face out.
To bring back the component to a straight pipe, mechanical pressing is done using hydralic press, supporting the component as a simply supported beam and the load from centre. Plastic deformation is obtained with max deforming force of 500 tons. Is the same process acceptable to creep strength materials like Grade 91/92 ? What is the max thickness of the pipe that can be straightess corrected with this available pressing force

 

[metengr]

For creep strength enhanced ferritic steel, there is a sensitivity to reduced creep strength with the introduction of remnant cold work. So, to be safe, you would need to perform a subcritical thermal treatment after pressing.

 

[weldstan]

With the heavy wall thickness for most P91 applications and its high yield strength, cold straightening will prove difficult with your equipment, excepting the small bore, thinner wall spools. As metengr indicated reduction in creep life must be expected, even with subcritical tempering performed after bending. Only renormalizing and tempering fully restores creep properties after cold bending. Because of a number of poor results with cold straightening in the field, we have simply prohibited it to be used.

 

[PrageethV]

I am incharge of header Manufacturing. From Fitup stage the straightness is lost. While welding the nipple/tubes to the body pipe , again bow(what we call for the bend in the body pipe) is induced. In PWHT , in furnace , while cooling the nipple/tubes act as heat transfer fins and again bow is induced.
I came to know that many world class manufacturers does it without bow correction(what we call for straightening the header ) How can it be done. The tolerance for straightness is +-9 from the mean theoritical central line.

 

[metengr]

It can be done by proper staging. We have ordered several safe-ended Grade 91 headers from a large and reputable boiler manufacturer with no issues. I reviewed most of the fabrication.

 

[PrageethV]

please provide me some stage checks to ensure staightness. There are many practical difficulties in maintaining the straightness .
The reasons are
1. There is some amount of face out
2. Root welding is given priority while fitup. A uniform root gap of 2-2.5 is maintained. So the small amount of Faceout reflects as a Bow
3. Furnace is optimised with accumulating a number of headers. In furnace itself some bow is induced.