Copper/Alloy Tube and Pipe

Manufacturing Process

Until the mid 1960s, copper and copper alloy tube were produced exclusively by extrusion or piercing to produce tube shells for subsequent drawing. Extrusion involves heating a billet above the recrystallization temperature and then forcing the metal through an orifice in a die and over a mandrel held in position within the die orifice. The clearance between mandrel and die determines the wall thickness of the extruded tube shell. Extrusion pressure varies with alloy composition, lubricant and die design, with the copper-nickel alloys requiring very high pressures.

In rotary piercing, one end of a heated cylindrical billet is fed between rotating work rolls that lie in a horizontal plane. The rolls are inclined at an angle to the axes of the billet and are driven forward toward the piercing plug, which is held in position between the work rolls.

Extruded or pierced tube shells are then cold drawn to smaller sizes, mainly on bull blocks for copper, and draw benches for the copper alloys. With either type of machine, the metal is cold worked by pulling the tube though a die that reduces the diameter. Concurrently, wall thickness is reduced by drawing over a plug or mandrel that may be either fixed or floating. Tubes may be annealed at any intermediate stage, when drawing to smaller sizes is required because of the metal's work hardening. In the mid-1960s, welded copper tube produced from strip without the use of a filler metal became commercialized. This was soon followed by forge- and fusion-welded heat exchanger tube in sizes up to 3-1/8 in. (7.94 cm).

Product Specifications

The actual number of ASTM tube and pipe specifications under the jurisdiction of ASTM Committee B-5 on Copper and Copper Alloys requires analysis. There are numerous tube and pipe specifications using U.S. customary units and/or companion hard metric specifications. The latter were developed to meet the projected needs of the ASME Boiler and Pressure Vessel Code.

Commodity Tube

The first of the three principal classes of copper tubular products is commonly referred to as commodity tube; it includes the Types K (heaviest), L (standard), and M (lightest) wall thickness schedules as classified by ASTM B88, Specification for Seamless Copper Water Tube; Type DWV of ASTM B306, Specification for Copper Drainage Tube (DWV) and medical gas tube of ASTM B819, Specification for Seamless Copper Tube for Medical Gas Systems. In each case, the actual outside diameter is 1/8 in. (.32 cm) larger that the nominal or standard size. For a listing of these commodity tubes, please click here.

Commercial Tube

Commercial tube, largely directed to air conditioning and refrigeration field service applications (ACR Tube), is designated by its actual outside diameter. ACR tube is available in annealed (soft) coils or drawn (hard) straight lengths, as well as annealed straight lengths (on special order). It is intended for use in the field with special fittings for connections, repairs and alterations in air conditioning and refrigeration installations.

Level wound coils having continuous tube lengths are mounted on payout reels. The tube is cut to length on automatic machines and bent over mandrels to form the typical hairpins and return bends for air-conditioning coils.

Pressure Ratings and Allowable Stresses

The allowable internal pressure for any copper tube in service is based on the Barlow formula for thin-walled, hollow cylinders used in the ASME B31 Code for Pressure Piping.

Barlow formula for thin walled, hollow cylinders:

P =

where P = allowable pressure
S = allowable stress
t m= wall thickness
D = outside diameter

The value of S is the allowable design strength for continuous long-term service of the tube, as determined by the ASME Boiler and Pressure Vessel Code, Section 1-Materials.

Allowable stresses for annealed and drawn temper copper tube are shown in the table below. They are only a small fraction of copper's ultimate tensile or burst strength. In system design, joint ratings must also be considered, as the lower of the two ratings (tube or joint) will govern the installation. The rated joint strength in soldered tube systems often governs design. However, annealed ratings must be used in brazed systems since the brazing operation may anneal the tube near the joints.

Table 3. Allowable Stresses for Copper Tube as a Function of Temperature
Temperature (ºF)Allowable Stress (psi)
100 6000 10300
150 5100 10300
200 4900 10300
250 4800 10300
300 4700 10000
350 4000 9700
400 3000 9400
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Joining Tube

The most common method of joining copper tubing systems is soldering with capillary fittings. Such joints are commonly used in plumbing for water lines and sanitary drainage. Brazed joints with capillary fittings are used where greater strength is required or where service temperatures are as high as 350°F (176°C). Brazing is the required joining method for refrigeration and medical gas piping. Mechanical joints involving flared tube ends are frequently used for underground tubing, for joints where the use of heat is impractical and for joints that may have to be disconnected from time to time.

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Fittings for copper tube (water and drainage) are made to the American Society of Mechanical Engineers ASME/ANSI standards. Wrought and cast copper, and copper alloy pressure fittings are available in all standard tube sizes up to 10-inch, and in a wide variety of types to cover needs for plumbing, heating, air conditioning and fire sprinkler systems.

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Solders & Fluxes

Soldered joints depend on capillary action, which draws free-flowing molten solder into the gap between the fitting and the tube. Capillary action is most effective when the gap between surfaces to be joined is 0.002 to 0.005 in. To ensure that satisfactory joints are made, the tube's cut ends must be deburred followed by mechanical abrasion to remove any surface film or soil prior to application of the flux.

Fluxes used for soldering copper tube and fittings should meet the requirements of ASTM B813, Specification for Liquid and Paste Fluxes for Soldering Applications of Copper and Copper Alloy Tube. Solders for joining copper tube systems contained in ASTM B32, Specification for Solder Metal, include 50-50 tin-lead, 95-5 tin-antimony, and a variety of lead-free alloys of approximately 95% tin combined with copper, nickel, silver, zinc, antimony, bismuth or selenium. Only lead-free solders are allowed for potable water systems.

Brazing Alloys & Fluxes

Strong, leak-tight connections for copper tube may be made by brazing with filler metals that melt at temperatures in the range of 1100 to 1500°F (593.3 to 815.6°C). Brazing filler metals suitable for joining copper tube are of two classes: The first class contains 30 to 60% silver (the BAg series) and the second class are the copper alloys that contain phosphorus (the BCuP series). The two classes differ in their melting, fluxing and flow characteristics, but both provide the necessary strength with standard fittings. Flux may be omitted when joining copper tube to wrought copper fittings with alloys of the BCuP series as they are self-fluxing. Fluxes are required for joining to cast copper alloy fittings or when using BAg-series filler metals.

The fluxes used for brazing copper tube should meet AWS (American Welding Society) Standard A5.31 and be of classifications FB3-A or FB3-C.