Pressure Withstanding Capacity
of Copper Piping System
By TK NG
When designing a fluid conveying piping system, one key consideration is the working pressure of the fluid which the system will have to handle. For a high-rise building, the fluid working pressure can change considerably from one location to another due to static pressure variation.
For a copper piping system, such as that for water supply, the following factors must be considered in the design and installation processes:
- Working pressure at different locations;
- Pressure withstanding capacities of different thicknesses and sizes of copper pipes;
- Annealing effect of silver brazing, if used for jointing the pipes, on the pressure withstanding capacity; and
- Pressure withstanding capacities of the types of pipe fittings used.
The first factor, working pressure, is the characteristic of a piping system that is under the control of the designer. The remaining three factors are all related to the material properties of the copper pipes/fittings adopted.
The pressure withstanding capacity of a copper pipe at temperatures up to 65oC can be estimated using formula P = 20ST/(D-T), where
P = pressure withstanding capacity (bar)
S = allowable stress (N/mm2)
T = wall thickness (mm)
D = outside diameter (mm)
According to the above formula, a larger pipe with outside diameter 10 times that of a smaller one would require a tenfold wall thickness to bear the same maximum working pressure. However, the wall thickness of copper pipe to BS EN 1057 only ranges from 0.6mm to 2.5mm (a factor of 4.17) for pipe sizes of 6mm to 159mm outside diameter. Hence, larger pipe sizes are generally expected to have lower pressure withstanding capacity. In the design process, designer must pay particular attention to the pressure rating of available copper pipes to ensure a sound design.
The allowable stress S is a property of the copper material. It depends on the pipe condition, i.e. whether it is annealed or not. Annealing softens the copper material and results in a lower allowable stress. The following table shows the allowable stress S at 65oC for different pipe conditions:
When silver brazing is adopted for jointing copper pipes instead of soldering, the higher temperature involved produces an annealing effect to the copper material. It reduces the pressure rating of the copper pipe. The annealed rating of copper pipe must therefore be used in the design calculation if silver brazing is specified.
For copper pipe fittings, the pressure withstanding capacity varies with the fitting type, jointing method, size and service temperature. In general, capillary fittings have higher pressure withstanding capacity than compression fittings given the same nominal size and service temperature. As there are also other types of fitting such as integral solder ring, press-fit, push-fit, etc., reference should always be made to the pressure information provided by the pipe and fitting manufacturers during design and installation.