Sour Service - PWHT 1

[BigInch]

Can anyone tell me if the CSA Z662 code requires, or defines conditions under which PWHTs on certain cold, field bends, made on pipelines for sour service are, or are not required?

Independent events are seldomly independent.

 

[SNORGY]

BigInch:

My understanding of that Code is that PWHT does not become mandatory until the wall thickness is 38 mm or thereabouts. What is going to create your challenges are:

Z245.1 / Z245.11 limitations on ovality; read in conjunction with Z662 Clause 16 it might be the post-bend ovality that limits the bend radius.

At or above 12" (324 mm) OD, the Code constrains you to a change in direction of 1.5 degrees per pipe diameter, which works out to 60-D bends. I have always (hitherto) held to a rule of 2 to 3 percent outer fibre elongation; this seems to be consistent with industry practice where bends on the order of 20-D would generally be accepted as cold bends in the field.

Don't have the latest version of the Code in front of me but I can dive into it a bit more tomorrow if you like.

 

[SNORGY]

"...38 mm or thereabouts..."

Make that 31.8 mm.

 

[SNORGY]

The Code is silent with respect to when PWHT becomes mandatory after field bending. The best guideline I could find is to limit ovality to 5% or less (difference between Dmax and Dmin). As I mentioned, I try to limit field bends to less than 3% outer fibre elongation. I have also specified hardness testing on bend extrados to look for effects of strain hardening, whereas some shops that do "warm" bends sometimes use MPI to check for surface cracks after bending.

 

[BigInch]

Snorgy, I was hoping you would come to my rescue. I really depend on you for your Canadian expertise.

I said P"W"HT, but actually meant to consider only cold bending, which you have already answered with your strain limiting policy.

Thanks for your comment.

Independent events are seldomly independent.

 

[BigInch]

Yes I would appreciate you taking the time to have a quick look at the new version, if it is not too much trouble, as I have the impression that there may have been some revs in the 2011 corresponding to research conducted over the last few years or so. I'm in the wrong part of the world right now to be stocking up on Canadian codes. I'm just trying to learn what the current practices are around the Western world.

Independent events are seldomly independent.

 

[SNORGY]

BigInch:

These are some of the current salient excerpts from CSA Z662-11:

4.3.5.2 - Where steel pipe is intended to be heated, the effects of the time-temperature relationship on the
mechanical properties of the pipe shall be considered.
Note: Examples of post-manufacturing heating activities include fabrication, coating, and installation.

4.3.5.3 - The design pressure and design wall thickness for field bends made from steel pipe shall be in accordance
with the requirements for straight steel pipe.

6.2.3 - For steel piping, changes in direction may be made by the use of bends or elbows, or both, subject to the
following limitations:
(a) Bends shall be free from buckling, cracks, and other evidence of mechanical damage. Out-ofroundness
of the cross-section within the bend shall be controlled so that it is not detrimental to the
structural integrity and normal operation of the pipeline system. Where applicable, allowances shall
be made for the installation of liners and for the passage of internal inspection tools and pipeline
scrapers. Unless the effects of bending on the mechanical properties of the pipe are determined to be
within acceptable limits, the following limitations shall apply:
(i) Pipe bends shall have a difference between the maximum and minimum diameters not
exceeding 5% of the specified outside diameter of the pipe.
(ii) For field cold bends of pipe 323.9 mm OD or larger, the longitudinal axis of the pipe shall not be
deflected more than 1.5° in any length along the axis equal to the outside diameter of the pipe.
Note: For hot and induction bends, see Clause 4.3.5.2.
(b) For cold bends, any longitudinal pipe welds, where practicable, shall be on or near the neutral axis of
the bend.
(c) Circumferential welds that are subject to stress during bending shall be nondestructively inspected
after bending.
Note: Where practicable, circumferential welds should not be bent cold.
(d) Factory-made wrought steel welding elbows or transverse segments cut therefrom may be used for
changes in direction, provided that the remaining arc length measured along the crotch, for sizes
NPS 2 and larger, is at least 25 mm.
Note: Field-cut segments of elbows NPS 12 and larger should not be used.
(e) Pipe sections containing mechanical interference fit joints may be bent, provided that the mechanical
interference fit joints themselves remain straight and are not subjected to undue stresses during pipe
section bending or during installation.
(f) When bending coated pipe, care shall be taken to maintain the integrity of the coating. Where the
coating is damaged, it shall be repaired as specified in Clause 9.3.2.
(g) Mitred bends shall not be used.
Note: Deflections up to 3° caused by misalignment are not considered to be mitred bends.

In CSA Z245.11-05, the following is offered:

6.4.2 Bends
6.4.2.1
Hot bends shall be manufactured using any suitable thermal process. Hot bends of Grades 290 and higher
shall be post-bend heat treated, except that induction bends may be furnished without such heat
treatment, provided that the bending process is controlled within a temperature range of ±30 °C and
processing is such as to ensure proper cooling. Where post-bend heat treatment is required, bends shall be
heat treated in accordance with one or more of the procedures specified in Clause 8.
6.4.2.2
Bends formed below 620 °C shall be heat treated in accordance with one or more of the procedures
specified in Clause 8 if the wall thinning due to bending exceeds 10%.

My reasoning would be that if you can control:

* outer fibre elongation
* strain hardening / cracking at bend extrados
* ovality to less than 5%
* wall thinning to less than 10%

in a field bend, then heat treatment is not necessary.

 

[BigInch]

Snorgy, Thanks very much for those pointers. I can say they are exactly in line with our current practices here and in fact we limit ovals to around 2.5%. All of the other parameters are pretty much standard practice everywhere I've worked. We are bidding on a project where there is H2S content, but we have objected to the spec calling for heat treating every cold bend (when made within the above parameters), as we do not agree that the prevailing conditions warrant such extreme measures to be taken.

Owe you one.

Independent events are seldomly independent.

 

[SJones]

Assuming that ISO 15156-2 will apply, the limitations will be a maximum 5% outer fibre strain and achieving the maximum hardness of 22 HRC. Stress relief heat treatment is required thereafter.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.