Pressure RatingsWorking Pressures (Temp. not exceeding 65°C)
Working Pressures are based on a working stress in the copper tube wall of 55.55 MPa which provides for a factor of safety on the ultimate strength of between 5.24 & 7.1.Working Pressures (Temp. exceeding 65°C)
For working temperatures exceeding 65°C the safe working stress can be determined from the chart.
Having established the safe working stress at a given temperature the safe working pressure for a tube of specific dimensions can be calculated from the following formula:
P = safe working pressure (bar)
f = safe working stress in tube wall MPa (from chart: N/mm²)
t = tube wall thickness (mm)
D = tube outside diameter (mm)Example:
The safe working stress for a fully annealed tube at 150°C is found from the chart to be 34 MPa. For a tube of this quality, 15.88mm o.d. by 0.81mm thick, the safe working pressure is thus: NOTE:
If brazing tube and fittings, the temperature required to melt the filler material (650°C - 750°C) is sufficient to fully anneal the tube. Pressure ratings should therefore be calculated using the formula as shown above. Pressure Ratings and Allowable Stresses
The allowable internal pressure for any copper tube in service is based on the Barlow formula for thin-walled, hollow cylinders used in the ASME B31 Code, for Pressure Piping.
The value of ‘S’ is the allowable design strength for continuous long-term service of the tube, as determined by the ASME Boiler and Pressure Vessel Code, Section 1 - Material.
Allowable stresses for annealed and drawn temper copper tube are shown in the table below. They are only a small fraction of copper’s ultimate tensile or burst strength. In system design, joint ratings must also be considered, as the lower of the two ratings (tube or joint) will govern the installation. The rated joint strength in soldered tube systems often governs design. However, annealed ratings must be used in brazed systems since the brazing operation may anneal the tube near the joints. Barlow formula for thin walled, hollow cylinders
P = allowable pressure
S = allowable stress
t = wall thickness
Dm = outside diameter (mm)Allowable Stresses for Copper Tube as a Function of Temperature
|Temperature (°F)||Allowable Stress (psi)|
Psi x 0.006895 = MPa
MPa x 145.04 = PsiTEMPERATURE:
°F = [(°F-32)/1.8] °C
°C = [(°C-1.8)/32] °F