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TECHNICAL TOOLS
Pressure Determinations
Barlow's Formula is commonly used to determine
1. Internal Pressure at Minimum Yield
2. Ultimate Bursting Pressure
3. Maximum Allowable Operating Pressure
4. Mill Hydrostatic Test Pressure
This formula is expressed as P =2St
D
where:
P = pressure, psig
t = nominal wall thickness, inch
D = outside diameter, inch
S = allowable stress, psi, which depends on the pressure being determined
To illustrate, assume a piping system 8 5/8" OD x 0.375" wall specified to ASTM A53 Grade B which has a specified yield strength (SMYS) of 35,000 psi and a specified minimum tensile strength (SMTS) of 60,000 psi.
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For Internal Pressure at Minimum Yield
S = SMYS (35,000 psi) and
| P = |
2St |
= |
(2) (35,000) (0.375) |
| D |
8.625 |
= 3043 or 3040 psig (rounded to nearest 1 0 psig)
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For Ultimate Bursting Pressure
S = SMYS (60,000 psi) and
| P = |
2St |
= |
(2) (60,000) (0.375) |
| D |
8.625 |
= 5217 or 5220 psig (rounded to nearest 10 psig)
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For Maximum Allowable Operating Pressure (MAOP)
= SMYS (35,000 psi) reduced by a design factor, for example 0.72 and | P= |
2St |
= |
(2) (35,000 x 0.72) (0.375) |
| D |
8.625 |
= 2191 or 2190 psig (rounded to nearest 10 psig)
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For Mill Hydrostatic Test Pressure
S = SMYS (35,000 psi) reduced by a factor depending on OD and grade (0.60 for 8 5/8" OD grade B) and
| P= |
2St |
= |
(2) (35,000 x 0.60) (0.375) |
| D |
8.625 |
= 1826 or 1830 psig (rounded to nearest 1 0 psig)
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