Max Span Limiting Criteria

[StevenHPerry]

B31.1 table 121.5 note C indicates that, in figuring the spans suggested, the max bending stress is 2300 psi and max sag is 0.1".

Is there any rhyme or reason behind the 2300 psi and 1/10 inch?

Can anyone guess why this suggested table would be included in B31.1 but not B31.3?

- Steve Perry

 

[MJCronin]

Steve...

My two cents:

The B31.1 span table is reasonable if perhaps a little too conservative IMHO. The basis for the spans, as you pointed out, is arbitrary.

After having worked for several major AE firms, I believe that this table is more of a "go-by"...and to be used when no other guidelines are available.

It is interesting to note that the table is really not applicable to piping > Schedule 80 or < Schedule 40.

Additionally, in systems with concentrated loads (valves, risers etc) additional rules and requirements must be developed.

In sloped steam piping systems, the 1/10 inch requirement can become irrelevant.

I have always been suspicious that this table was inserted in B31.1 to ensure a certain "minimum" amount of business for the pipe support companies.

My opinion only

-MJC

 

[BigInch]

Not familiar with B31.1, however these guidelines for bending stress usually come from an approximation of the longitudinal stress allowable - hoop stress allowable.

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[2ndIntegral]

My feeling is that B31.3 covers the distance between pipe supports, but only indirectly, as Big Inch suggested, through a longitudinal stress calculation. This is explained through Code Case 178 – which is based fundamentally on combined bending and torsion equations.

Another possible motive may lie in the far broader range of piping materials covered in the appendices of B31.3. Power piping is afforded the convenience of inserting a suggestion table as it principally concerns itself with a narrower (if not singular) range of piping materials.

On high temperature service I would prefer moving away from deflection based criteria as, very often, Young’s modulus and applied strains (for a broad suite of materials) are not as readily available for temperature ranges as values for yield stress.

MJC hit the mark; MSS 69 which indicates distances similar in conservatism to B31.1 does state that the indicated distances between pipe supports (and hence derived deflections) only apply where concentrated loads are not present. It also implicitly states that when span calculations are applied, the table should be ignored. The table is merely a starting point or a cheat-sheet when laying out the piping. It provides a means to generalize on support positions prior to sending through for formal stress analysis. Any engineer with the experience of having to renumber element / node lines on older pipe stress analysis programs can attest to the handiness of having (slightly) too many support points – albeit conservatively placed.

2I