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Can Copper Tube Be Used in Steam and Steam Condensate Piping Systems?
Everyone knows that copper tube has a proven history in use in a variety of applications, however a common question is "can I use copper tube in a steam system?" Just like in water systems, copper tube has long been used in both steam and steam condensate piping systems. The key to its successful use in either high- or low-pressure steam systems is in choosing the appropriate joining method for the service.
Let's consider the most commonly used joining techniques, soldering and brazing. And let's relate everything back to Table 4. Pressure-Temperature Ratings of Soldered and Brazed Joints as found in the CDA Copper Tube Handbook. If we look at the ratings for soldered joints we see that there are four alloy designations provided in the first column of Table 4. They are Sn50, Sb5, Alloy E (a proprietary solder containing no lead), and Alloy HB (a proprietary solder containing no lead). In each case the maximum pressure rating from 1/8" through 12" (nominal) or standard tube sizes is 15 psig for saturated steam temperatures.
If we look at the same pressure ratings for saturated steam for joints made using an alloy having a melting temperature above 1100°F, brazing alloys as defined by the American Welding Society, we see that the pressure rating increases to 120 psig for all sizes from 1/8" through 12".
Why the difference? What are these maximum allowable pressures based on? Unlike the pressure ratings shown for services other than steam, these pressure ratings are controlled by service temperature, not pressure.
Temperature and pressure are directly proportional for steam. As the pressure in the system is increased, the temperature increases accordingly. Saturated steam, a condition where steam contains as much water as it can and still be a vapor, at 15 psig has an absolute pressure at sea level of 29.7 psia (pounds per square inch absolute). At this pressure it would have a corresponding temperature of approximately 250°F which is the maximum recommended temperature for soldered joints as shown in Table 4 of the Copper Tube Handbook. Therefore, rather than the allowable pressure of the soldered joints controlling the rating, the allowable temperature is the controlling factor, leading to the rating of 15 psig regardless of the solder alloy used.
Brazing alloys, with a minimum melting temperature of 1100°F, can withstand higher temperature service without a loss of integrity and therefore have a higher maximum temperature limit. In fact, the allowable temperature for brazed joints exceeds that of the copper alloys that it is used to join, therefore the temperature rating of the copper tube and fittings are the controlling factor. Table 3a and Table 3b of the Copper Tube Handbook shows that the ASME Boiler and Pressure Vessel Code limits the use of copper tube and fittings to a maximum service temperatures of 400°F or less. Using a similar analysis as above, the absolute pressure at which saturated steam has a temperature of approximately 400°F is at 120 psig, the maximum pressure allowable for brazed joints for saturated steam service shown in Table 4.
As with any piping system, the pressure rating of the system is controlled by the lowest allowable pressure of the tube, fitting, joint or joining material. For steam systems constructed using copper tube of Types K or L, the maximum allowable pressure at which the system could be designed would be 120 psig. As shown in Tables 3a and 3b of the Copper Tube Handbook, the lowest maximum operating pressure for Type L copper tube is 127 psig (corresponds to 12-inch nominal Type L tube in annealed form). Since this is more than the allowable pressure for the brazed joint, the 120 psig allowable for the joint is the controlling factor, regardless of the fact that smaller diameter tubes have higher allowable pressures. However, to use copper tube and fittings in a steam system at this pressure the joints must be brazed.
As long as these temperature and pressure limits are met, copper tube and fittings can be used in both high- and low-pressure steam systems. The system must still be designed and installed to meet the requirements of all applicable local, state and federal construction and safety codes for steam applications. Back to Top
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