There are various methods for determining the recommended, allowable or rated internal pressure-temperature ratings for piping materials and systems. These include calculated ratings based on basic material properties, such as tensile and yield stress, piping dimensions and engineering correlations. Oftentimes this is preferred since it reduces the amount of testing required. However, pressure ratings based on actual material performance may also be developed and used. 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.
Rated Pressures Based on Calculation
As 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):
WHERE:
P=allowable pressure, psi
S=maximum allowable stress in tension, psi
t min=wall thickness (min.), in.
D max=outside diameter (max.), in.
C=a constant
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. 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. In addition, it does not account for the strain-hardening characteristics of copper tube that can increase the strength (true stress) over seven times.
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°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 14.3c for Type M and 14.3d for DWV tube, although they are not furnished in the annealed temper. Table 14.3e lists allowable internal working pressures for ACR tube.
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. Most tubing systems are joined by soldering or brazing. Rated internal working pressures for such joints are shown in Table 14.4a. 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 14.3a-e as brazing softens (anneals) the tube near the joints (the heat affected zone). Joint ratings at saturated steam temperatures are shown in Table 14.4a.
The pressures at which copper tube will actually burst are many times the rated working pressures. Compare the actual values in Table 14.5 with the rated working pressures found in Tables 14.3a-c. 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.
Rated Pressures Based on Performance Testing
Recognizing the limitations and overly conservative nature of establishing pressure ratings through calculation, it is possible to take advantage of the greater strength offered by thin-wall copper tube by establishing pressure ratings based on performance testing, such as burst and fatigue testing. This allows the system designer to specify copper tube with larger diameter to wall thickness ratios, thus reducing the amount of copper in the tube wall and optimizing both material use and cost.
Generally, performance testing is based on the operating regimes within which the piping system is expected to operate, with accelerated test methods and safe design factors applied to ensure that the tube is robust enough to withstand pressures well in excess of the test parameters.
An example of this performance rating is the testing required by the UL 207 Standard for Safety for Refrigerant-Containing Components and Accessories, Nonelectrical. Utilizing this standard, copper tube can be listed with a pressure rating higher than the calculated rated pressure shown in Tables 14.3a-e provided that the manufacturer can demonstrate for each tube size and wall thickness that the tube can withstand a pressure of three times the proposed rating, and withstand a pressure cyclic fatigue test for no less than 250,000 cycles without failure. Several manufacturers of copper tube and fittings have tested and received listings using this standard such that copper tube and fittings can be used in HVACR systems and equipment operating above the calculated rated pressures shown in Tables 14.3a-e.