Safe working stresses

Estimate the safe working pressure for a 4 in. (100 mm) dia., schedule 40 pipe, carbon steel, butt welded, working temperature 100°C. The safe working stress for butt welded steel pipe up to 120°C is 6000 lb/in2 (41.4 N/mm2). 

For temperatures up to 250°F, the recommended safe working stress is 9000 psi for lap-welded steel pipe and 6500 psi for butt-welded steel pipe. If the schedule number is known, the safe working pressure can be estimated directly from Eq. (14). 

S. = maximum cross-sectional area of plummet, ft2 Ss = safe working stress, lbf/in.2 ST = tensile strength, lbf/in.2 t = minimum wall thickness, in., see Table 4 T = temperature, °R u = internal energy, ft lbf/lbm v = specific volume of fluid, ft3/lbm... 

At upper bound longitudinal limit at 0 = 0, values for tensile strength over estimate the safe working stress load necessary to suit end-use applications. 

Table 25.1 gives the safe working stress for polypropylene pipes under various conditions. The figures incorporate a true safety factor of 1.3, which ensures safe working at the specified time. 

Table 25.1 Safe Working Stress (a) of Pipe Made From Propathene...

A more sophisticated procedure is described in ASTM D 1780 Standard practice for conducting creep tests of metal-to-metal adhesives. This standard covers the determination of the amount of creep of metal-to-metal adhesive bonds due to the combined effects of temperature, tensile shear stress, and time. The primary intention of this test procedure using single lap shear specimens is to provide basic data for the choice of safe working stresses for applications in which the allowable deformation within the service life of the structure is the criterion of failure. The standard is intended to be used with standard testing machines in combination with extensom-eters allowing to measure shear strain close to the bond line of the employed lap shear specimen. 

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