Pressure calculation

Pipe specifications do not include recommendations for maintenance or burst pressure requirements. However, Barlow's formula is commonly used in industry to approximate the burst pressures of thin-walled ductile iron pipe. Working pressures or allowable pressures are obtained by taking into account a safety factor (SF) to reduce the pressure from the level at which rupture is very likely to occur to an acceptable risk of rupture.

This is a complex matter with many factors to consider such as: what people have access to; general safety; corrosion; fatigue; manufacturing changes (bends, flares), codes and insurance; seismic stability and temperature to name a few.

Many engineers use the maximum tensile strength (UTS) to calculate a burst pressure and the yield strength (YS) plus a safety factor (SF) to calculate the working pressure. Using the yield strength without adding a safety factor (SF = 1) gives the approximate theoretical pressure at which the pipe will begin to plastically deform.

One should use the actual wall thickness or a conservatively assumed wall thickness for the calculations. For example, a wall thickness of 1.65 mm or 0.065" (aw) should probably be 0.060" or close to the minimum allowable wall thickness. One should also use the actual outside diameter of the pipe for calculation rather than the nominal pipe sizes (1" AD/25.40 mm is nominally 27.94 mm).

The formula according to Barlow for the pressure calculation is as follows:

Where:

P = max. working pressure (psig)

S = material strength (psi) (permissible stress)

T = wall thickness (in)

OD = outside diameter (in)

SF = Safety factor (generally 1.5 to 10)

The Barlow formula assumes optimum conditions at room temperature.

The material strength is determined by the tensile test, which measures the clamping force and the deformation on a test piece.

• the load that results in a permanent deformation of 0.2% is called the yield strength
• the load that results in fracture is called tensile strength

The strength of our bright annealed pipes (minimum requirements) is: