As for all materials, the allowable internal pressure for any copper tube in service is based on the formula used in the American Society of Mechanical Engineers Code for Pressure Piping (ASME B31):
P=allowable pressure, psi
S=maximum allowable stress in tension, psi
t min=wall thickness (min.), in.
D max=outside diameter (max.), in.
For copper tube, because of copper's superior corrosion resistance, the B31 code permits the factor C to be zero. Thus the formula becomes:
The value of S in the formula is the maximum allowable stress (ASME B31) for continuous long-term service of the tube material. It is only a small fraction of copper's ultimate tensile strength or of the burst strength of copper tube and has been confirmed to be safe by years of service experience and testing. The allowable stress value depends on the service temperature and on the temper of the tube, drawn or annealed.
In Tables 3a, b, c, and d, the rated internal working pressures are shown for both annealed (soft) and drawn (hard) Types K, L, M and DWV copper tube for service temperatures from 100°F to 400°F. The ratings for drawn tube can be used for soldered systems and systems using properly designed mechanical joints. Fittings manufacturers can provide information about the strength of their various types and sizes of fittings.
When welding or brazing is used to join tubes, the annealed ratings must be used, since the heating involved in these joining processes will anneal (soften) the hard tube. This is the reason that annealed ratings are shown in Table 3c for Type M and 3d for DWV tube, although they are not furnished in the annealed temper. Table 3e lists allowable internal working pressures for ACR tube.
In designing a system, joint ratings must also be considered, because the lower of the two ratings (tube or joint) will govern the installation. Most tubing systems are joined by soldering or brazing. Rated internal working pressures for such joints are shown in Table 4a. These ratings are for all types of tube with standard solder joint pressure fittings and DWV fittings. In soldered tubing systems, the rated strength of the joint often governs design.
When brazing, use the ratings for annealed tube found in Tables 3a-3e as brazing softens (anneals) the tube near the joints (the heat affected zone). Joint ratings at saturated steam temperatures are shown in Table 4a.
The pressures at which copper tube will actually burst are many times the rated working pressures. Compare the actual values in Table 5 with the rated working pressures found in Tables 3a, 3b and 3c. The very conservative working pressure ratings give added assurance that pressurized systems will operate successfully for long periods of time. The much higher burst pressures measured in tests indicate that tubes are well able to withstand unpredictable pressure surges that may occur during the long service life of the system. Similar conservative principles were applied in arriving at the working pressures for brazed and soldered joints. The allowable stresses for the soldered joints assure joint integrity under full rated load for extended periods of time. Short-term strength and burst pressures for soldered joints are many times higher. In addition, safety margins were factored into calculating the joint strengths.