SAFE WORKING PRESSURE
USING A.S.M.E. B31.1
Relying on the ASME's expertise in high pressure applications will eliminate the danger of
pipe or tube rupture. The traditional method of selecting pipe for hydraulic systems that
use ultimate burst pressure divided by an arbitrary safety factor fails to take into account
the actual structural capabilities of the pipe. In understanding the B31 1 formula below,
you will have the opportunity to maximize safety and enhance system performance.
The formula for calculating safe working pressure is :
Do - 2y(tm-A)
The left column below is an explanation of the meaning of each value. The right hand column
is the value for 2" SCH 80 A106 B and A106 C.
|P = Pressure
||(B) 2574 or (C) 3004
||SE = 15000 for A106 B or 17500 for A106 C
||(B) 15000 or (C) 17500
|tm = Wall thickness (specified)
|A = Allowances (see additional allowances)
||.02725 (12.5% wall deviation)
|Do = Outside diameter
|y = .4
ADDITIONAL ALLOWANCES BEYOND 12.5% WALL DEVIATION FOR:
BENDING will reduce safe working pressure by the following amounts: 6 D= 6%, 5 D= 8%,
4D= 14%, 3 D = 25%. More information in ASME B 31.1 paragraph 102.4.5.
THREADING you need to make allowance for the depth of the thread. More information
in ASME B 31.1 paragraph 102.4.2.
CORROSION factors need to be addressed if the pipe is exposed to a corrosive environment.
The additional wall thickness needs to be calculated such that over the desired life of the
system that there remains sufficient wall thickness throughout the pipe's life. More information
in ASME B31.1 paragraph 102.4.1. Future articles will deal with organic coatings that will
mitigate this issue at a cost significantly lower than stainless steel. If you have current
needs for organic coatings--call.
ACTUAL WALL THICKNESS can positively influence pressure ratings. Theses are specified
in ASME B31.1 paragraph 102.4.4. Section D o. This paragraph allows for actual wall thickness
and outside diameter to be used in calculating working pressure without relying on 12.5% wall
deviations. You will realize extra value when you use actual wall thickness to calculate safe
working pressure. Our measurement capabilities plus our stock of pipe will allow us to assist
you in getting the best pipe for your systems.
The actual formula for the two different grades of pipe work as follows:
2" SCH 80 A106 B
2.375 - .8(.218 - .02725)
2" SCH 80 A106 C
35000(.218 - .02725)
2.375 - .8(.218 - .02725)
The 14% extra working pressure that Grade C offers in this example would be sufficient to
safely allow downgrades for bending. Additionally, this will reduce installed cost by not
having to use 2 1/2" SCH 160 A106 B for a system operating at 2500 psi, as well as reducing
heat if 2" SCH 160 A106 B were used.
The ASME B31.1 formula (Table 102.3.2 (C)) predicts a life expectancy of 7000 complete full
temperature cycles ~ 19 years of service life for the pipe. Now you can have the confidence
of knowing the pipe will be both safe and provide the best value for years to come.
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