Re: [PipingDesign] B31.3 allowable excursions over design pressure - non ferrous materials

On Jul 2, 2008, at 5:52 AM, Steve McKenzie wrote:

> Could someone versed in the above code in its entirety, please refer
> me to where or how I can find exactly what B31.3 permits in the way
> of excursions over design pressure.

Here's the Code language--
302.2.4 Allowances for Pressure and Temperature Variations. Occasional variations of pressure and/or temperature may occur in a piping system. Such variations shall be considered in selecting design pressure (para. 301.2) and design temperature (para. 301.3). The most severe coincident pressure and temperature shall determine the design conditions unless all of the following criteria are met.

(a) The piping system shall have no pressure containing components
of cast iron or other nonductile metal.

(b) Nominal pressure stresses shall not exceed the yield strength at
temperature (see para. 302.3 of this Code and Sy data in BPV Code, Section II, Part D, Table Y-1).

(c) Combined longitudinal stresses shall not exceed the limits
established in para. 302.3.6.

(d) The total number of pressure-temperature variations above the
design conditions shall not exceed 1000 during the life of the piping system.

(e) In no case shall the increased pressure exceed the test pressure
used under para. 345 for the piping system.

(f) Occasional variations above design conditions shall remain
within one of the following limits for pressure design.

                 (1) Subject to the owner's approval, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than:

                 (a) 33% for no more than 10 hr at any one time and no more than 100 hr/yr; or

                 (b) 20% for no more than 50 hr at any one time and no more than 500 hr/yr.

                 The effects of such variations shall be determined by the designer to be safe over the service life of the piping system by methods acceptable to the owner. (See Appendix V.)

                 (2) When the variation is self-limiting (e.g., due to a pressure relieving event), and lasts no more than 50 hr at any one time and not more than 500 hr/year, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than 20%.

(g) The combined effects of the sustained and cyclic variations on
the serviceability of all components in the system shall have been evaluated.

(h) Temperature variations below the minimum temperature shown in
Appendix A are not permitted unless the requirements of para. 323.2.2 are met for the lowest temperature during the variation.

(i) The application of pressures exceeding pressure temperature
ratings of valves may under certain conditions cause loss of seat tightness or difficulty of operation. The differential pressure on the valve closure element should not exceed the maximum differential pressure rating established by the valve manufacturer. Such applications are the owner's responsibility.

All the clauses and conditions make this a fairly tough read, and you'll need to sort out what applies to your system. Nothing about Titanium is specifically referenced, but if titanium is considered non-ductile, variations are out because there's no way to absorb any local yielding. Frankly, I think this whole thing is nearly as nutty as the clause in Section IX that allows lower P-numbers to be used for WPQ's written for higher P numbers. It gives careless or ignorant operators a license to abuse their systems and claim that the Code permits it when they get haled into court after someone gets killed. But there you have it. The whole damn Code is predicated on preventing cut corners by people who don't care who gets hurt, and this clause just panders to such people. And that's my little homily.

Christopher Wright P.E. |"They couldn't hit an elephant at chrisw@skypoint.com | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania