Pipe Span Factors - a note by Hassan Hajitabar.

In general Pipe span is limited by pipe material (allowable stress),
sectional modulus (nominal size and Sch. of pipe) and weight of its content
and insulation and design temperature of the pipe system which affects
allowable stress in calculation of pipe span. In general pipe span is
limited by allowable deflection and allowable bending and shear stress. To
simplify pipe support spacing calculation MSS- SP69 has provided recommended
practice for support spacing which has been accepted by ASME. These spans
are limited to max. combined stress(bending and shear) to 1500 PSI and max.
pipe sag of 0.1 inch we use allowable pipe span as a general and primary
solution for supporting but some points should be considered in supporting:
1- All span should be adjusted based on available structure for supporting.
2- In case of change of direction in horizontal pipes you should reduce pipe
span. as a good practice you can use 0.75 of span.
3- Span should be decreased based on concentrated weight and load in piping
system such as valve and flange. As a good practice you can use 0.75 of span
for one element and 0.6 of span for two elements in piping system.
4- finally you should consider maintenance requirement(for example for valve
maintenance and removal) you should consider supports as possible as near to
valves).
Also I should note hear that occasional loads such as wind and earthquake do
not concern span of weight support because using span is used for dead
loads. For this loads stress analyzer engineer should use proper guide and
other dynamic supports such as rigid strut and shock absorber with careful
attention to thermal expansion and load.
When you see various recommended span for a same size it may means using of
various safety factor, various fluid content, various design temp. Various
pipe materials and other design objects in calculation of max. allowable
span.

In general we have:

L < (10 * Z* F * S/W)^0.5
In which
L= Max. allowable span(mm)
Z= Pipe sectional modulus(mm3)
F= Safety factor
S=Allowable stress in design temp(N/mm2).
W=Weight per linear unit of pipe(N/mm)
I hope these all are useful for you.

Best Regards
Hassan Hajitabar

Piping Engineer
Engineering Department
Iranian Offshore Engineering & Construction Company (IOEC)
E-mail: Hajitabar@Ioec.com