Oct 26 2015, By: Matt Hiskett

Why Cold Stretching?

Cold stretching has been in use for more than 30 years, but not under ASME code rules until 2008. Cold stretching (or cold working) increases the yield limit of a stainless steel material by work hardening it after all forming and welded steps are complete. A vessel can be cold stretched by over pressurizing it 1.5 to 1.6 times the maximum allowable working pressure (MAWP) under careful control of the deformation vs pressure as outlined in VIII-1 App 44. The resulting permanent plastic deformation increases the diameter up to 7% and the volume by 14%.

Stainless_Steel_Stress_Strain_Curve_With_Cold_Working

Stainless steel stress strain curve showing an increase in yield strength after cold working.

The ASME allowed stress in a vessel made of SA-240 304 stainless is 20,000 psi for cryogenic applications. After cold stretching, the allowable stress is increased to 39,300 psi (Table 44-1-1 in ASME VIII-1). The higher allowable is a combination of both an increase in the yield stress and a decrease in the factor of safety.

Cold_Stretched_Cyrogenic_Storage_Vessel

A cold stretched cryogenic storage vessel. The cold stretched vessel is not visible, contained inside a non-stretched vacuum vessel that can be seen in this photograph.

To date the only cold stretched vessels we have seen are used for cryogenic storage and we do not think this process would be practical for a vessel without an outer containment vessel.

Cold Stretching and The CRN process

Cold stretched vessels are easy to register if you have all the required documentation, however much of the required information is not in writing on the jurisdictions websites – refer to this chart:

Requirement ASME App. 44 BC AB SK MN ON QC ACI*
Drawings (with additional notes). Drawings are a standard requirement of CRN registration. yes yes yes yes yes yes yes
Calculations. Calculations are a standard requirement of CRN registration. yes yes yes yes yes yes yes
Manufacturers Design Report (MDR) including U-DR-1 yes yes
Certificate of compliance (for MDR and UDS) yes
User design report unique to the specific vessel being registered yes
Signed letter from the owner of the vessel. yes yes yes yes yes yes yes yes
Cold stretch procedure (44-6-3) yes yes yes yes yes yes yes yes
ABSA IB13-016 yes yes
Limited Ownership yes yes yes yes yes yes yes yes
*ACI covers the registrations in MB, NS, PEI, NFLD, YK, NV and NT

The biggest problem with registering cold stretched vessels is App. 44-2: “The use of this mandatory appendix is contingent upon the approval from the user of the user’s designated agent.” Anyone building cold stretched vessels must have a letter from the customer stating that they understand that they are purchasing a cold stretched vessel built to appendix 44. Cold stretched vessels cannot be modified or repaired without re-stretching.

What’s included in the CRN submission?

As with all submission packages, the standard vessel drawings and calculations must be included in the submission. Drawings for a cold stretched vessel must have a few additional notes added:

  • The minimum and maximum cold stretching pressure (1.5xMAWP and 1.6xMAWP)
  • A note referencing the Cold Stretching Procedure (CSP)
  • Reference to Appendix 44
  • Warning: Cold-Stretched Vessel: Do not weld, grind, or in any way modify the pressure boundary

What’s in a Manufactures Design Report (MDR)?

An MDR is a vessel design specific document that includes the design conditions for each design. It should include the following:

  • A completed U-DR-1 (User Design Requirements) form, found in Appendix K-K of ASME VIII-1
  • Alberta Specific Information to include:
    • Reference to the design drawing
    • Number of units to be built
    • Serial numbers for the units
    • Reference to the CSP
    • A Certification of Compliance, as found in ASME VIII-2, Table 2-B.1

What about a User Design Specification (UDS)?

The UDS is also vessel specific; however it covers the entire process, including fabrication. In addition to the information found in the MDR, it also needs to include a document, similar to your standard CSP, but less generic. For example the edited CSP should make reference to the vessel diameter, nozzle sizes, and Pressure/Temperature ratings. As with the MDR, the UDS requires a certificate of compliance as found in ASME VIII-2, Table 2-A.1.

Is my current CRN valid for a different customer?

Based on our experience and discussions with the various registration agencies, we have concluded that you can submit multiple user acceptance letters with a CRN submission, which will allow that CRN to be used for multiple customers.

It is our opinion that after a registration is completed in a province a revision to the CRN can be obtained to include additional customer by submitting the following documents:

  • A New CRN application form or a revision to existing CRN.
  • One user acceptance letter for each new customer to be added.
  • A cover letter stating that a new customer is being added and that the registered design remains the same.