![]() ![]() 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. The very conservative working pressure ratings give added assurance that pressurized systems will operate successfully for long periods of time. Compare the actual values in Table 14.5 with the rated working pressures found in Tables 14.3a-c. The pressures at which copper tube will actually burst are many times the rated working pressures. Joint ratings at saturated steam temperatures are shown in Table 14.4a. When brazing, use the ratings for annealed tube found in Tables 14.3a-e as brazing softens (anneals) the tube near the joints (the heat affected zone). In soldered tubing systems, the rated strength of the joint often governs design. These ratings are for all types of tube with standard solder joint pressure fittings and DWV fittings. Rated internal working pressures for such joints are shown in Table 14.4a. Most tubing systems are joined by soldering or brazing. In designing a system, tube, fitting and joint ratings must be considered collectively, because the lower of the ratings (tube, fitting or joint) will govern the maximum installation design pressure. Table 14.3e lists allowable internal working pressures for ACR tube. This is the reason that annealed ratings are shown in Table 14.3c for Type M and 14.3d for DWV tube, although they are not furnished in the annealed temper. 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. Fittings manufacturers can provide information about the strength of their various types and sizes of fittings. The ratings for drawn tube can be used for soldered systems and systems using properly designed mechanical joints. In Tables 14.3a-d, the calculated rated internal working pressures based on the ASME (Boardman) equation are shown for both annealed (soft) and drawn (hard) Types K, L, M and DWV copper tube for service temperatures from 100☏ to 400☏. In addition, it does not account for the strain-hardening characteristics of copper tube that can increase the strength (true stress) over seven times. The downside of utilizing this calculated pressure rating is that it underestimates the actual safe performance of the tube since it is overly conservative when applied to thin wall tubing (where the diameter to wall thickness ratio is greater than 10) like the commercially available copper tubes covered in this handbook. The allowable stress value depends on the service temperature and on the temper of the tube, drawn or annealed. ![]() 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 value of S in the formula is the maximum allowable stress (ASME B31) for continuous long-term service of the tube material. C=a constantįor copper tube, because of copper's superior corrosion resistance, the B31 code permits the factor C to be zero. P=allowable pressure, psi S=maximum allowable stress in tension, psi t min=wall thickness (min.), in. Rated Pressures Based on CalculationĪs for many piping materials, the calculated allowable internal pressure for copper tube in service is commonly based on the formula used in the American Society of Mechanical Engineers Code for Pressure Piping (ASME B31): These generally require more extensive testing across the product size range and anticipated stress/strain regimes than the calculated methods, but can provide more accurate and robust ratings. However, pressure ratings based on actual material performance may also be developed and used. Oftentimes this is preferred since it reduces the amount of testing required. These include calculated ratings based on basic material properties, such as tensile and yield stress, piping dimensions and engineering correlations. There are various methods for determining the recommended, allowable or rated internal pressure-temperature ratings for piping materials and systems. Design and Installation Data: Pressure Ratings and Burst Strength
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