Calculations of Maximum Allowable Working Pressure

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UFC 3-430-07
24 July 2003
including Change 1, Jan 2004 and Change 2, March 2005
When inspection indicates that the thickness of the plate or the strength of any joint is
less than that on which the current maximum allowable working pressure is based, or
when it is impracticable to apply the required hydrostatic test, calculate a new maximum
allowable working pressure in accordance with the rules of the Code. The following
factors of safety must be used and increased by the inspector if the condition and safety
of the boiler demand it:
The lowest factor of safety permissible on existing installations is be 4.5
except for horizontal return tubular boilers having continuous longitudinal
lap seams more than 3.7 m (12 feet) in length where the factor of safety is
8, and when this latter type of boiler is removed from its existing setting, it
must not be reinstalled for gage pressures in excess of 103.42 kPa (15
Reinstalled or secondhand boilers have a minimum factor of safety of 6
when the longitudinal seams are of lap riveted construction, and a
minimum factor of safety of 5 when the longitudinal seams are of butt and
double strap construction.
The maximum allowable working pressure for a vessel in operation must
be computed with the appropriate formulas in the Code, using dimensions
actually determined by the inspection for the thickness and twice the
estimated corrosion allowance before the next inspection, and making
suitable allowance for the other loadings enumerated in the Code to be
considered in the design of a vessel. The maximum allowable working
pressure of vessels designed and built with one or more openings, for
which the closures are auxiliary equipment not part of the pressure
vessels, must be determined only after due consideration of the auxiliary
equipment to be used as closures. The minimum factor of safety must not
be less than 4 and must be increased by the inspector if the condition and
safety of the vessel demand it. The condition of the vessel and the
particular service to which it is subject are the determining factors.
FACTOR OF SAFETY. The factor of safety is "built in" to the ASME Code
formulas of Section I. For reference, the original formula is:
P = (TS x t x E) / (R x FS)
P = pressure in psi
TS = tensile strength, psi
t = thickness, inches
E = efficiency of the longitudinal seam
R = inside radius, inches
FS = factor of safety


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