Technical

Pressure calculation

Safe tube pressure

Pressure formula for stainless steel & nickel tubes

Tubing 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 tubes. Working pressures or allowable pressures are calculated by using a safety factor (SF) to reduce the pressure from a level where bursting failure is very likely to an acceptable level of risk. This is a complex matter where many issues must be considered such as: personnel assessment and general safety, corrosion, fatigue, fabrication changes (bends, flares), codes and insurance, seismic stability and temperatures, to name a few.

Many engineers will use the ultimate tensile strength (UTS) to calculate a bursting pressure estimation and use the yield strength (YS) together with 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 1.65 mm wall or 0.065" average wall tubing is likely to measure 0.060" thick, or near the low end of the allowable range. Actual tube ODs should be used rather than Nominal tube Sizes (1” OD, 25.40 mm is nominally 27.94 mm).


Barlow's formula can be used to estimate burst pressure of tubes.

Barlow´s Formel

Where:

P = max. working pressure (psi)

S = material strength (psi)

T = wall thickness (in)

OD = outside diameter (in)

SF = safety factor (in general 1.5 to 10)

The Barlow estimate is based on ideal conditions at room temperature.


Material Strength

The strength of a material is determined by the tension test which measures the tension force and the deformation of the test specimen.

  • the stress which gives a permanent deformation of 0.2% is called the yield strength
  • the stress which gives rupture is called the ultimate strength

Strength of our bright annealed tube materials with minimum requirements:

Alloy              ASTM             Strength Yield (PSI) Strength Ultimate (PSI)
304 A269 30.000 75.000
304L A269 25.000 70.000
316 A269 30.000 75.000
316L A269 25.000 70.000
316Ti A269 30.000 71.000
321 A269 30.000 71.000
310S A269 30.000 75.000
317L A213 35.000 75.000
904L A269 45.000 98.000
6Mo A269 45.000 98.000
31803 A789 65.000 90.000
32750 A789 80.000 116.000
400 B165 28.000 70.000
600 B167 35.000 80.000
625 B444 60.000 120.000
825 B423 35.000 85.000
C276 B622 41.000 100.000
C22 B622 45.000 100.000
TiGr2 B338 40.000 50.000