Click to go to the RathGibson home page

Bursting or Working Tubing or Pipe Pressures header image

Barlow's Formula

ASTM tubing and pipe specifications do not include any recommended service or burst pressure requirements. However, Barlow's formula is commonly used in the industry to approximate or predict the bursting pressures of ductile thin wall tubular [(Wall/ID) < 0.1] or cylindrical materials due to ID pressurization and generally yields conservative results or predictions. Other calculations are appropriate to heavy wall and brittle materials.

Working pressures or allowable pressures are arrived at using a safety factor (SF) to reduce the pressure from a level where bursting failure is very likely to where an acceptable level of risk is achieved. This is a complex matter where many issues must be considered such as: personnel hazard assessment, and general safety; corrosion; fatigue; fabrication changes (bends, flares); codes and insurance; seismic stability; and temperatures, to name a few. A number of these issues are addressed in a free publication (N° 9024) from the Nickel Development Institute (NiDI). Barlow's formula is merely a tool for burst estimation or prediction under ideal conditions at room temperature. Note that actual values may vary from these theoretical calculations for a number reasons.

Many engineers will use the ultimate tensile strength (UTS) to calculate a bursting pressure estimation and use the yield strength (YS) and a safety factor (SF) when arriving at a working pressure estimation. Using the yield strength and no safety factor (SF=1) will calculate an approximate theoretical pressure at which the tubing will begin to plastically deform.

Actual wall thickness or conservative wall estimates should be used. For example 16 gauge or 0.065" average wall tubing is likely to measure 0.060" thick, or near the low end of the allowable range. Actual pipe ODs should be used rather than Nominal Pipe Sizes (3 nps is nominally 3.5" OD). Consult the RathGibson's pipe page for help regarding pipe dimensions.
Barlow's Formula:

Barlow Calculation

Where:

      P = ID pressure (psi)
      T = Wall thickness (in)
      OD = Outside Diameter (in)
      SF = Safety factor (generally 1.5 – 10, 1 for collapsing pressure)
      S = Material strength (psi)

Ultimate Tensile strength or Yield strength can be used.

Ultimate should be used to determine the bursting pressure. Yield can be used for estimating pressures at which permanent deformation begins. A table is provided below which lists some minimum strength levels for guidance. Use of these values will yield conservative predictions relative to actual strength levels. For example S31803: the minimum yield strength is 65,000 psi, the actual yield strength may (does) average 100,000 psi for RathGibson products.

From Machinery's Handbook 25 p. 268, and ASTM-A370 A2.3.4.

OD (in) (in)
Wall (in) (in)
Strength Factor (psi)
Safety Factor (working)  
Safety Factor (bursting) 1  
   
Theoretical Working
Pressure
(selected SF)
(psi)
Theoretical Bursting or
Yeilding Pressure
(SF=1)
(psi)

Spec. Minimum Requirements

  Alloy Strength Yield (psi) Strength Ultimate (psi) ASTM Spec.
Stainless 304 30,000 75,000 A249
Stainless 304L 25,000 70,000 A249
Stainless 316 30,000 75,000 A249
Stainless 316L 25,000 70,000 A249
Stainless 309S 30,000 75,000 A249
Stainless 310S 30,000 75,000 A249
Stainless 321 30,000 71,000 A249
Stainless N08904 45,000 98,000 A249
6-Moly S31254 45,000 98,000 A249
6-Moly N08926 43,000 94,000 A249 & B674
6-Moly N08367 45,000 100,000 A249 & B676
Duplex S31803 65,000 90,000 A789
Duplex S32750 80,000 116,000 A789
Ferritic S44400 40,000 60,000 A268
Ni-Base N08810 25,000 65,000 B515
Ni-Base N08811 25,000 65,000 B515
Ni-Base N06600 35,000 80,000 B516
Ni-Base N10276 41,000 100,000 B626
Ni-Base N06022 45,000 100,000 B626
Ni-Base N06686 45,000 100,000 B626
Ni-Base N06059 45,000 100,000 B626
Ni-Base N06625 GR1 60,000 120,000 B704
Ni-Base N06625 GR2 40,000 100,000 B704
Ni-Base N08825 35,000 85,000 B704
Nickel N02200 15,000^ 55,000^ B730
Nickel N02201 12,000^ 50,000^ B730
Ni-Base N04400 28,000^ 70,000^ B730

^=Annealed

Back to top