Plumbing Tubes: Pressure Ratings

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:

Pressure Ratings

Where:

P = safe working pressure
f = safe working stress in tube wall MPa (from chart)
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:

Pressure Ratings

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



Pressure Ratings

P = allowable pressure
S = allowable stress
t = wall thickness
Dm = outside diameter (mm)







Allowable Stresses for Copper Tube as a Function of Temperature



Pressure Ratings
Conversion Factors
STRESS: Psi x 0.006895 = MPa
MPa x 145.04 = Psi

TEMPERATURE: °F = [(°F-32)/1.8] °C
°C = [(°C-1.8)/32] °F



Copper Underground: It’s Resistance to Soil Corrosion

In an attempt to categorise the nature and extent of the corrosion of copper pipe underground, studies by both the National Bureau of Standards in the States and the BNF Metal Research in the UK Classified the soils according to type.

An indication as to how various piping materials would behave when in these soils can therefore be obtained by matching the soil under question with the one described in the actual test. It should be borne in mind that various factors such as drainage, acidity and alkalinity as indicated by pH value, humus content and sulphate and chloride contents all influence the rate of corrosion. The worst case of corrosion occurred in soils with very poor drainage, whereas in well drained soil corrosion was always slight. It should be pointed out that in all cases other materials tested, lead, steel and cast iron, had higher pitting corrosion rates and in most cases higher averaged rates than copper.

Copper


Pressure Ratings

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