Copper and Copper Alloy - Tube and Pipe

( This document is an outgrowth of an earlier document which is reprinted, with permission from Standardization News, Vol. 15, No. 9, copyright American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103.)

• Introduction
• Tube and Pipe Manufacturing Processes
• Product Inspection
• Product Specifications
• Commodity Tube
• Plumbing Systems
• Fire Sprinkler Systems
• Copper Sovent® Drainage System
• Commercial Tube
• Pressure Ratings and Allowable Stresses
• Joining Tube
• Fittings
• Solders and Fluxes
• Brazing Alloys and Fluxes
• Corrosion Resistance
• Chlorination
• Offshore Platforms
• Enhanced Tube Surfaces
• Application Trends

Introduction

Modern technology has drawn on the unique combination of properties of copper and copper alloys in the form of tube and pipe products. Copper tube is used extensively to convey potable water in buildings and homes. Copper alloys are selected to convey many diverse fluids for the oil, chemical, process, and marine industries. Copper tube's second largest application is in air conditioning and refrigeration systems; its fastest growing use in is fire sprinkler systems in residential and office buildings. Copper is used for plumbing tube principally because of its corrosion resistance, machinability, and high level of heat transfer.

Chief tubular applications for copper in the transportation industry are for automotive and truck radiators, air conditioning systems, and hydraulic lines. In marine service, copper and copper alloy tube and pipe are used to carry potable water, seawater, and other fluids, but the chief application is alloy tube bundles for condensers and auxiliary heat exchangers. The food and beverage industries also use copper to carry process fluids for beer, spirits, cane sugar refining, and other food processing operations.

Tube and Pipe Manufacturing Processes

Until the mid 1960s, copper and copper alloy tube was produced exclusively by extrusion or piercing to produce tube shells for subsequent drawing. Extrusion involves heating a billet above the re-crystallization 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 and are inclined at an angle to the axes of the billet and are driven forward toward the piercing plug that 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 commercial. This was soon followed by forge and fusion welded heat exchanger tube in sizes up to 31/8 in. (7.94 cm).

Product Inspection

The original hydrostatic and pneumatic non-destructive tests have been large supplanted by the eddy current test because of its high production speed and insensitivity to operator fatigue. ASTM E 243, Practice for Electromagnetic (Eddy-Current) Testing of Seamless Copper and Copper Alloy Tubes, established the testing procedure for seamless copper and copper alloy tube for sizes up to 31/8 in. (7.94 cm). Machines can be calibrated to a wide range of sensitivities to reflect the acceptance level criteria of the product. It is being revised at present to accommodate welded tubular products.

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 36 tube and pipe specifications using US customary units, whereas there are only 11 companion hard metric specifications. The latter were developed to meet the projected needs of the ASME Boiler and Pressure Vessel Code. Four test methods, one plumbing tube classification document, and one standard practice also exist for tubular products.

Commodity Tube

The first of the three principal classes of copper tubular products is commonly referred to as commodity tube and includes the Types K (heaviest), L (standard), and M (lightest) wall thickness schedules of ASTM B 88, Specification for Seamless Copper Water Tube, and Type DWV; of B 306, Specification for Copper Drainage Tube (DWV). In each case, the actual outside diameter is 1/8 in (.32 cm) larger that the nominal or standard size.

Plumbing Systems

In the United States, copper tube for plumbing applications accounted for 8.8 percent of the total US market for copper and copper alloy mill products for 1986. All four wall thickness schedules are available in 20 ft (6.096 m) lengths in the drawn (hard) temper. Annealed (soft) temper straight length Types K and L are also available. Annealed (soft) temper coils in 60 (18.29 m) and 100 ft (30.48 m) lengths are frequently used as the underground service line between the water main in the street and the water meter in the home, avoiding the need for an intermediate joint. All copper tube produced to commodity or commercial tube classifications has a minimum chemical composition of 99.9 percent copper (including silver).

Fire Sprinkler Systems

Copper water tube and fittings in engineered soldered fire sprinkler systems are relatively new (introduced 1974) and rapidly expanding markets.

Historically, automatic fire sprinklers were installed in buildings to provide protection the structure and its combustible contents. The requirements governing system design, installation, and maintenance in building codes and standards are found in such standards as National Fire Protection Assn. (NFPA) 13. Copper provides multiple advantages over traditional systems. Dominating these advantages is copper's use of installation and smaller size for the same performance.

The increasing internal surface roughness common to traditional systems is not found in copper, as only slight changes in surface roughness normally occur throughout the service life of the tube. The narrowing of the inside diameter caused by corrosion build-up is also negligible. Copper tube is also resistant to galvanic corrosion when joined to steel portions of steel piping systems because of relative mass ratio of the two metals.

Additional advantages for copper relate to its low maintenance requirements and ductility, which accommodate building settling, distortion, or temperature extremes leading to simplified tube hanging and supporting. Its light weight affords reductions of up to 50 percent in shipment, fabrication, and installation in terms of hanger requirements and dead loads imposed on supporting structural elements and requiring small tube sizes. Copper installations provide considerable space savings particularly where concealment is desired.

Copper Sovent ^® Drainage System

Developed about two decades ago, the all-copper Sovent single stack drainage plumbing system substantially reduces the number of soil, waste, and vent stacks, the length of tube required, the number of fittings, and the size of the pipe chase. The system has four major design elements: a copper DWV stack, a copper Sovent aerator fitting at each floor level where soil fixtures are connected, copper DWV horizontal branches, and a copper Sovent de-aerator fitting at the base of the stack and at the upstream end of each horizontal offset. This arrangement of fittings, branches, and stack handles the same drainage fixture load as a conventional stack of the same diameter but without the need for the separate vent stack required in traditional systems.

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 and 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 with the tube cut to length on automatic machines and bent over mandrels to form the typical hairpins and return bends for air conditioning coils. Copper and copper alloy commercial tube accounted for 6.9 percent of the total US market for copper and copper alloy mill products.

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 B 31 Code for Pressure Piping.

Barlow formula for thin walled, hollow cylinders:
P = 2St _m
D-.08t _m

The value of S is an allowable design strength for continuous long-term service of the tube. It was adopted by the ASME Boiler and Pressure Vessel Code according to the rules of Appendix P of Section VIII of the code.

Allowable stresses for annealed and drawn tempers Types K, L, M, and DWV 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.

Joining Tube

The most common method of joining copper tubing systems is soldering with capillary fittings. Such joints are used 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 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.

Fittings

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

Solders and Fluxes

Soldered joints depend on capillary action drawing 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. Preferred fluxes are those that are only mildly corrosive. Solders for joining copper tube systems contained in ASTM B 32, Specification for Solder Metal, include 50-50 tin-lead, 95-5 tin-antimony, and the 96.5-3.5, 95-5, and 94-6 tin-silver alloys. Only lead-free solders are allowed for potable water systems.

Brazing Alloys and Fluxes

Strong, leak tight, brazed 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 percent silver and the second class are the copper alloys that contain phosphorus. 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.

Corrosion Resistance

The excellent corrosion resistance of copper alloys has long been recognized by the engineering community. This is due to the formation of a protective oxide film, which when combined with the inherent antifouling characteristics and high thermal conductivity results in their wide use. Typical applications include the large tonnage multistage flash evaporators for desalting plants, power utility surface condensers and petrochemical plant heat exchangers.

Clean, debris-free water, with a pH range of 7.2 to 8.5 and with adequate oxygen content, allows a protective film to form properly on the inside tube surface. Such film formation can be accelerated by ferrous sulfate additions, particularly in seawater, which greatly reduces the severe corrosion rate and effectively prevents both uniform and localized sulfide attack with concurrent improved resistance to impingement attack.

On-line tube cleaning effectively controls film formation while maintaining acceptable heat transfer efficiency. Water velocities must be sufficient to provide aerated water continuously and to prevent the settling of deposits, but not high enough to strip away the protective film. Generally, this design velocity is in the range of 4 to 8 ft/s, depending on the alloy. Maximum velocities in clean water service for the widely used condenser tube alloys range from 6 to 15 ft/s.

Excessive turbulence, industrial waste, sewage, or otherwise polluted waters are additional factors that influence the corrosion behavior of copper alloys and can influence maximum service velocities. For a seawater environment, minimum velocities in the order of 4 to 5 ft/s (1.22 to 1.52 m/s) are recommended.

Chlorination

Chlorine injection has been widely used for the control of algae and biorganisms in cooling water systems augmenting copper's inherent resistance to biofouling. It is normally accomplished by charging chlorine gas or sodium hypochlorite into the intake piping controlled to give a residual of 0.5 ppm chlorine.

Offshore Platforms

Based on the successful service history of Alloy C70600 as condenser and heat exchanger tubes in coastal power stations and chemical plants, the alloy is being increasingly specified for seawater piping on offshore platforms, especially those designed for long-term service in deep water locations. Design emphasis favors materials reliability and life cycle savings, which are provided by this alloy's properties.

Typical copper-nickel piping applications include fire protection systems, sanitary plumbing services, and process cooling lines, as well as seawater piping for flare booms, generator coolers, and distillation plants. The alloy is also successfully used for seawater feed lines to mud and cement pumps and for drill water supply.

As with copper fire sprinkler systems, the superior corrosion resistance of the copper-nickel alloy permits pipe downsizing according to the high Hazen-Williams friction factor, thus making it an economically favorable material choice.

Enhanced Tube Surfaces

Significantly improved efficiency of air conditioning heat exchangers has been achieved via internal or external (or both) ridging or fins on the condenser tube surface. This improvement is due to improved film coefficients on both sides, and thereby overall heat transfer. The increase in performance can lead to a reduction in size and weight of existing units and can aid the design of new units with constrained size and weight limitations.

Application Trends

Changes in the pattern of heat exchanger tube selection for condensers installed in new electric generating steam power plants have occurred in the recent past. These changes have been toward the use of fewer types of materials and thinner wall gages, as well as a general upgrading of materials, with a shift away from the older traditional alloys.

Alloy C70600 (90-10 Copper-Nickel) has become the most frequently specified material for general use in new construction, largely supplanting Alloys C443900-C44500 (Admiralty) and Alloy C68700 (Aluminum Brass).