Registration of Plastic Fittings

By January 19, 2017Uncategorized

Registration of Plastic Fittings

PVE-5246, Last Updated: June 17/11, By: LRB

FEA of Plastic Valve Body

Plastic pipe that needs to be registered can be calculated to code rules found in B31.1 or B31.3. But a plastic pipe needs plastic fittings – how can they be registered? Plastic fittings are among the most difficult to get registered under the CRN system.

Burst Testing:

The good news is that TSSA has a standard to cover the burst test pressures used for unlisted materials (like plastics):

GUIDELINES FOR THE REGISTRATION OF NON-NUCLEAR FITTINGS IN THE PROVINCE OF ONTARIO (here):

Burst test factor of safety from the guideline (Appendix D):

  • 4 – for all materials not listed below
  • 6 – cast iron
  • 10 – glass
  • 10 – non-metallic, non-automated fabrication process

The bad news is that it is very hard to get a reviewer, even one from Ontario to accept the factors listed. Although moulded plastic parts are made with automated processes justifying a 4x factor, we invariably end up having to use a 10x factor. However we have seen rare cases where 4x has been allowed on moulded parts in Ontario. See more on CRN burst test problems here.

When more than one grade of plastic is used, each grade needs to be tested. That, combined with the need for conservative safety factors can make using burst testing unacceptable.

Calculations/FEA:

B31.3 provides allowed stress levels for calculating plastic fittings. B31.3 Table B-1 and B31.1 table III-3.2.1 provide HDS allowed stress levels for many common plastics. The Plastics Piping Institute has a larger list of HDS values here. Where the B31.1/31.3/PPI listings overlap, identical HDS values are provided. Most reviewers Canada wide allow the PPI values of HDS to be used with the occasional exception of some Alberta reviewers.

With an allowed HDS, the fitting can be calculated either by using standard code rules or as shown in the illustration above, by FEA. Simple or complex fittings can be registered at a higher operating pressure than would be allowed by burst test. When more than one grade of plastic is used, only the weakest has to be analyzed.