ALUMINUM (Al )
Aluminum pipe is manufactured in various wall thicknesses similar to copper and in sizes ranging from 1⁄8 to 12 in. Sizes above 3 in are not readily available. Aluminum is manufactured in various alloys; the most commonly used for facilities conforms to ASTM B-210. Joints are made using brazing or welding with special aluminum alloy filler metals.
Aluminum tubing is light in weight and generally used for specialty services, such as cryogenics (where ductility and strength are necessary) and for carrying compressed specialty gases (because of its corrosion resistance). It also resists many specialty chemicals and is resistant to atmospheric corrosion. It is not suitable for acids, mercury, and strong alkalies. It has a high rate of expansion.
BRASS (BR)
Brass is an alloy of copper and zinc. The proportion varies from 85% copper to
67% copper. Pipe with a high copper content is known as red brass, and that with
a lower content is known as yellow brass. When used for drainage systems, it is
obtained plain end. Joints for this pipe can be either threaded, flanged, brazed, or
soldered. Brass pipe for utility piping systems shall conform to ASTM B 43: Red Brass Tube, Seamless.
Brass is generally used for local branch drainage lines, where this alloy will resist specific corrosive drainage effluent and, in larger sizes, for potable and other water supply lines and to match existing work for alterations. Its advantages and disadvantages are the same as for copper.
Brass castings for pipe fittings and components of plumbing fixtures are not made with the same alloy as pipe and often contain lead. Pipe is lead free and brass fittings with lead are no longer permitted by code to be used for potable water.
CAST IRON SOIL PIPE (CI )
Technically known as gray cast iron, this pipe is a ferrous material alloyed with carbon in the form of free graphite flakes, silicon, and other impurities. It is avail- able in three classifications: service (standard) weight, extra heavy, and hub less. The pipe is commonly lined internally with cement or coal tar enamel, and coated externally with a variety of materials to reduce corrosion by soils.
Joints require two types of pipe ends: hub and spigot or hub less. The hub and spigot ends can be joined either by caulking or by the use of an elastodynamics compression gasket. Hub less pipe is joined by an external compression coupling.
Cast iron soil pipe should conform to the following standards:
1. ASTM A 74: Hub and Spigot Cast Iron Soil Pipe and Fittings
2. CISPI 301: Hubless Cast Iron Soil Pipe
3. CISPI 310: Hubless Cast Iron Fittings for Soil Pipe
Cast iron is well suited for sanitary effluent and can be used in any part of a gravity drainage and vent system. Advantages include an ability to withstand moderate external pressure (such as direct burial in soil), good fire resistance, good flow characteristics, and good corrosion resistance in most natural soils. Piping in use for over 100 years has been documented. Disadvantages are that the pipe is brittle and subject to breakage when roughly handled, it is subject to corrosive attack by aggressive soils and highly septic effluent, it is heavy, and it has a high initial material cost.
Cast iron pipe is manufactured with both inside and outside coated for corrosion resistance. A PE wrapping is often used to eliminate external corrosion of cast iron pipe buried underground on a site.
ACID-RESISTANT CAST IRON (AR)
Commonly referred to as high silicon iron pipe, acid-resistant CI is an alloy of gray cast iron containing between 14.25 and 15% silicon, and small amounts of manganese, sulfur, and carbon. It is available only in extra heavy pattern, with the same dimensions as CI piping.
Joints require two types of pipe ends: hub and spigot or hub less. The hub and spigot ends can be joined by caulking. Ru bless pipe is joined by a compression coupling.
Acid-resistant cast iron pipe shall conform to ASTM A 861 and ASTM A 518. This specialty piping material is used for drainage of various corrosive liquids, and since it is stronger than glass, is recommended for exposed or underground applications where there is a possibility of physical damage.
CARBON STEEL (ST )
Steel is a very broad category of piping because of the large number of alloys that have been produced. It is divided into two broad categories according to the method
of manufacture: mill pipe and fabricated pipe. Mill pipe is produced to meet finished pipe specifications. Fabricated pipe is manufactured from steel plate with spiral or straight welded seams.
Steel pipe is manufactured by either the seamless (extruded) or welded method, and is available either plain (black) or galvanized (zinc plated inside, outside, or both). Wall thickness is expressed as ‘‘schedule,’’ and ranges from schedule 5 (light- est) to schedule 160. The wall thickness varies with the size of the pipe. The larger the schedule number, the thicker the pipe wall for a specific pipe size.
Steel pipe, depending upon type, can be obtained with threaded ends for screwed fittings, plain ends, and flanged and beveled ends for welding.
There is an extremely large number of steel pipe alloys available. The selection depends on the intended service. The steel pipe alloys most commonly used for service and utility systems conform to ASTM A 53: Steel Pipe, Welded or Seam- less, Black or Galvanized and ASTM A-106: Steel Pipe, Welded or Seamless, Black or Galvanized.
Steel pipe is generally used for pressure piping. Its advantages are long laying lengths, high internal and external strength, and the availability of varying pipe thickness to meet almost any design pressure. It has good flow characteristics and fire resistance and is low in initial cost. The most serious disadvantage is low corrosion resistance. This requires internal and external protection, with galvanizing the most commonly used method.
COPPER
Copper tubing is seamless, made from almost pure copper (99.9 percent), and is available in hard (annealed) and soft (drawn) form. It is manufactured in sizes ranging from 1⁄8 to 12 in, but sizes over 6 in are not generally available. All tubing is manufactured with plain ends only. Joints are made by soldering, and brazing, and with flared and flanged fittings.
The six types of copper tubing used most often are:
1. ASTM B-88 is the grade used most often for potable water, and also for compressed gases and vacuum systems where high purity is not a factor. It is seamless, available in nominal pipe sizes from 3⁄8 to 12 in, in hard and soft temper, and in three wall thickness grades—K (thickest wall), L, and M (thinnest wall). If patented flare fittings are used, the pipe must be obtained without outside diameter embossing, which would interfere with the sealing of the pipe wall against the side of the fitting.
2. ASTM B-819 is similar to B-88, except that it is available only in grades K and L and, in addition, the pipe is factory cleaned, capped, and specially marked. This pipe is required to be used for medical gas systems in health-care facilities. It should also be considered as the primary copper pipe for gases in laboratories as well.
3. ASTM B-75 is seamless, available as either hard or drawn, and in nominal sizes from 1⁄8 to 2 in O.D. The smaller sizes are often referred to as capillary tubing. This is the grade most often used for very small diameter pipe [1⁄4 in O.D. (6 mm) or less] connecting instruments to the distribution piping. This pipe is commonly joined by patented flare joints, which require temper 060 to seal correctly.
4. ASTM B-280, type ACR (air conditioning and refrigeration), is available cleaned and capped for field refrigeration piping and could also be considered for laboratory use. It is available only in hard temper, and its size is the actual O.D. of the pipe. This requires that all non-ACR fittings used in the system, which are manufactured in nominal pipe size, be dimensionally compatible with the ACR pipe, which is manufactured in actual pipe size.
5. ASTM B-306, copper drainage tube, is known as DWV (drainage, waste, and vent). This designation applies to copper tube used for non pressure drainage systems. This tube has the thinnest wall of any copper product. The preferred jointing method is soldering, which is adequate in strength and the least costly. Primary use is in residential construction as indirect waste lines and in larger projects for local branch lines where human waste is not discharged. Advantages are its light weight, ease of assembly, and smooth interior. Disadvantages include corrosive attack by ordinary sewage, poor fire resistance, and the need for dielectric connections to eliminate galvanic corrosion where this material is connected to any iron piping.
6. ASTM B-837 Type G is seamless tube, available in either hard (drawn) or soft (annealed) temper, and in nominal sizes from 3⁄8 to 11⁄8-in O.D. This tubing is identified by the O.D. rather than nominal size. This grade is manufactured specifically for natural gas and LP fuel gas systems. Fittings shall be similar to those used with type ACR copper. The pipe is joined by either brazing or flare type joints. Brazed joints are required for system pressures above 14 in WC where installed in inaccessible locations. Brazing alloys shall contain less than 0.05 percent phosphorus.
DUCTILE IRON PIPE (DI )
Ductile iron is a cast ferrous material alloyed with free nodular or spheroidal graphite in lieu of the flakes that are present in cast iron. This is achieved by the addition of a magnesium inoculation. It is used either as a gravity sewer or pressure pipe. Sizes available range from 3 to 54 in. There are eight pressure ratings—class 50 (125 psi) to class 56 (350 psi), and gravity sewer pipe. A cement or bituminous lining can be provided to resist internal corrosion. This pipe can be assembled with mechanical, gasket ed, or flanged joints. Ductile iron pipe shall conform to ASTM A 518 and ASTM A 861.
The advantages of ductile iron are the same as those for CI pipe, except that it is far stronger in terms of allowable pressure ratings and external load-bearing capacity. It is also not as brittle, allowing rougher handling. It has a higher initial cost than CI.
LEAD (LD)
Lead pipe is made from 99.7% pig lead with various alloys available for special applications. Joints for this pipe are either wiped, burned, or mechanically flanged. Lead pipe shall conform to WW-P-325a: Lead Pipe, Bends and Traps.
Uses of lead pipe include existing connections to floor-mounted water closets, radioactive wastes, and special laboratory corrosive wastes. It has very limited use in modern drainage systems.
STAINLESS STEEL (SS )
The term stainless steel encompasses a wide variety of alloys containing 11 to 30% chromium (Cr), 0 to 35% nickel (Ni), and 0 to 6% molybdenum (Mo) in various combinations as well as small amounts of other elements such as titanium, manganese, niobium, and nitrogen. It is widely used in the chemical, pharmaceutical, and food processing industries.
Pipe is available in sizes ranging from 1⁄8 to 48 in, and is manufactured in plain end, prepared end for welding, and flanged. Joints can be welded, threaded, or flanged. Wall thickness is expressed as a ‘‘schedule,’’ and ranges from schedule 5 (lightest) to schedule 160. The wall thickness varies with the size of the pipe. The larger the schedule number, the thicker the pipe wall for a specific pipe size. Stain- less steel pipe is also available as tubing with wall thickness designated as a decimal.
The composition of a stainless steel alloy determines its metallurgical structure or grade, and therefore its properties. There are five groups of stainless steel based on metallurgical structure: ferritic, austenitic, superaustenitic, martensitic, and duplex.
1. Ferritic stainless steels contain 12 to 30% (more typically 16 to 18%) Cr, 0 to 4% Ni, and 0 to 4% Mo with a low carbon content. This material is magnetic and has good ductility and cold form ability but is not hard enable by heat treatment. This class is generally less vulnerable to chloride-induced stress corrosion cracking. Its primary use is in transport of strong oxidizing fluids (such as nitric acid) in process environments, machinery, and kitchen equipment. This class is exemplified by type ASTM grade 430.
2. Austenitic stainless steels contain 17 to 27% Cr, 8 to 35% Ni, and 0 to 6% Mo. This material is typically nonmagnetic and readily weldable, and has good ductility (even at cryogenic temperatures) and cold form ability but is not hard enable by heat treatment. This class is more generally corrosion resistant, but is generally vulnerable to chloride-induced stress corrosion cracking. Regular carbon grades are susceptible to corrosion around welded joints due to migration of Cr away from the weld site. These problems are overcome by using a low carbon grade, indicated by an L suffix, that reduces carbon to below 0.035 percent. Grades within this class are the most commonly used stainless steels. This class is exemplified by type ASTM grades 304 and 316 (304L and 316L). A superaustenitic grade is also avail- able with superior resistance to chloride pitting.
3. Superaustenitic stainless steel alloys were created to better withstand corrosion in a more severe environment than conventional stainless steel. They are alloys of Ni, Cr, Mo, copper, and iron typified by UNS (Unified Numbering System) alloys N08020, N08024, and N08026.
4. Martensitic stainless steels contain 11 to 18% Cr, 0 to 6% Ni, 0 to 2% Mo, and 0.1 to 1% C. This class is magnetic, oxidation resistant, and hard enable by heat treatment. Little used in piping applications, its primary uses are in cutlery, turbine blades, and high-temperature parts. This class is exemplified by type ASTM grade 410.
5. Duplex stainless steel is characterized by a microstructure containing both ferritic and austenitic types with different grades, having a mixture of 40 to 60% of each for various alloys. Its advantages are good resistance to chloride-induced stress corrosion cracking and high mechanical strength properties along with good ductility and impact strength. Disadvantages include corrosion of pipe by reducing acids and weld site corrosion by oxidizing acids.
Stainless steel is available in various wall thicknesses. Pipe is commonly avail- able from schedule 5 to 80, and tubing from 0.028 to 0.188.
Stainless Steel Finishes
For stainless steel piping used in pharmaceutical, food-processing, chemical, and electronics applications, the interior and exterior of the piping are often required to be finished as required by FDA, EPA, USDA, or other applicable codes. Finishing the exterior makes it easier to keep clean. Finishing the interior will prevent the adherence of any solids, increase corrosion protection, and shorten pipe interior cleaning procedures. Finishing can be abrasive, electropolished, or both.
Abrasive finishes are mechanically produced by polishing and wearing away of the surface. This is often specified by a particular size or ‘‘grit’’ of the abrasive used, such as a 220 grit. This signifies the size of the abrasive passing through a specific size mesh. The larger the number, the finer the finish. Electropolishing is an electrochemical process using an electrical current to deposit metal from an anode to a cathode. Electropolishing, which is the opposite of electroplating, re- moves surface metal from microscopic high points faster than from low points. The metal to be polished is the sacrificial anode. Surface ions of iron are removed leaving a chromium-rich surface resulting in a smooth, corrosion-resistant pipe in- terior. Another method used to specify standard sheet and pipe exterior finishes uses numbers 1 to 8. An explanation of sheet finishes is given in Table 2.3. Table 2.4 gives the grit equivalent of microinch measurement.
Finishes are often indicated as Ra (arithmetic mean roughness average, or roughness average) and Rq, the equivalent of RMS (root mean square). Both of these are measured in micro inches and denote the smoothness of the surface. The smaller the number, the finer and finish. The Ra reading is approximately 87.5 percent of the Rq (RMS) reading. Other methods of expressing smoothness are center line average (CLA) and arithmetic average (AA).
Passivization
When the interior surface of stainless steel piping is required to have a very low rate of corrosion, it must be made passive. This is accomplished by the formation of a thin surface film that acts as a barrier to corrosion. The surface film (or passive film) is made thicker by exposing it to oxidizing and chelant solutions or electro polishing. Oxidizing solutions frequently used are nitric acid, ammonium persulfate, hydrogen peroxide, and citric acid. Chelants are nontoxic organic acids or their viable salts and nontoxic synergizing agents. ASTM A 380 suggests other passivation chemical combinations that have proven successful.
CORRUGATED STEEL PIPE
Corrugated steel pipe is available from 6 in (125 mm) to 96 in (2.66 m). It is fabricated from flat steel that has been rolled into various shapes and impressed with grooves around the circumference of the pipe, generally described as circular arcs connected by tangents. Corrugations are measured by pitch (the dimension from crest to crest at right angles to the corrugations) and depth.
Longitudinal pipe seams are riveted, welded, bolted or have helical lock seams. Joints are generally steel bands with a gasket under the sleeve and tightened by bolts inserted in an integral angle. Fittings are usually made from straight piping and shop fabricated into the desired shape.
Corrugated steel pipe shall conform to different AASHTO standards depending on the actual pipe material specification.
Aluminum pipe is manufactured in various wall thicknesses similar to copper and in sizes ranging from 1⁄8 to 12 in. Sizes above 3 in are not readily available. Aluminum is manufactured in various alloys; the most commonly used for facilities conforms to ASTM B-210. Joints are made using brazing or welding with special aluminum alloy filler metals.
Aluminum tubing is light in weight and generally used for specialty services, such as cryogenics (where ductility and strength are necessary) and for carrying compressed specialty gases (because of its corrosion resistance). It also resists many specialty chemicals and is resistant to atmospheric corrosion. It is not suitable for acids, mercury, and strong alkalies. It has a high rate of expansion.
BRASS (BR) Brass is an alloy of copper and zinc. The proportion varies from 85% copper to
67% copper. Pipe with a high copper content is known as red brass, and that with
a lower content is known as yellow brass. When used for drainage systems, it is
obtained plain end. Joints for this pipe can be either threaded, flanged, brazed, or
soldered. Brass pipe for utility piping systems shall conform to ASTM B 43: Red Brass Tube, Seamless.
Brass is generally used for local branch drainage lines, where this alloy will resist specific corrosive drainage effluent and, in larger sizes, for potable and other water supply lines and to match existing work for alterations. Its advantages and disadvantages are the same as for copper.
Brass castings for pipe fittings and components of plumbing fixtures are not made with the same alloy as pipe and often contain lead. Pipe is lead free and brass fittings with lead are no longer permitted by code to be used for potable water.
CAST IRON SOIL PIPE (CI ) Technically known as gray cast iron, this pipe is a ferrous material alloyed with carbon in the form of free graphite flakes, silicon, and other impurities. It is avail- able in three classifications: service (standard) weight, extra heavy, and hub less. The pipe is commonly lined internally with cement or coal tar enamel, and coated externally with a variety of materials to reduce corrosion by soils.
Joints require two types of pipe ends: hub and spigot or hub less. The hub and spigot ends can be joined either by caulking or by the use of an elastodynamics compression gasket. Hub less pipe is joined by an external compression coupling.
Cast iron soil pipe should conform to the following standards:
1. ASTM A 74: Hub and Spigot Cast Iron Soil Pipe and Fittings
2. CISPI 301: Hubless Cast Iron Soil Pipe
3. CISPI 310: Hubless Cast Iron Fittings for Soil Pipe
Cast iron is well suited for sanitary effluent and can be used in any part of a gravity drainage and vent system. Advantages include an ability to withstand moderate external pressure (such as direct burial in soil), good fire resistance, good flow characteristics, and good corrosion resistance in most natural soils. Piping in use for over 100 years has been documented. Disadvantages are that the pipe is brittle and subject to breakage when roughly handled, it is subject to corrosive attack by aggressive soils and highly septic effluent, it is heavy, and it has a high initial material cost.
Cast iron pipe is manufactured with both inside and outside coated for corrosion resistance. A PE wrapping is often used to eliminate external corrosion of cast iron pipe buried underground on a site.
ACID-RESISTANT CAST IRON (AR) Commonly referred to as high silicon iron pipe, acid-resistant CI is an alloy of gray cast iron containing between 14.25 and 15% silicon, and small amounts of manganese, sulfur, and carbon. It is available only in extra heavy pattern, with the same dimensions as CI piping.
Joints require two types of pipe ends: hub and spigot or hub less. The hub and spigot ends can be joined by caulking. Ru bless pipe is joined by a compression coupling.
Acid-resistant cast iron pipe shall conform to ASTM A 861 and ASTM A 518. This specialty piping material is used for drainage of various corrosive liquids, and since it is stronger than glass, is recommended for exposed or underground applications where there is a possibility of physical damage.
CARBON STEEL (ST ) Steel is a very broad category of piping because of the large number of alloys that have been produced. It is divided into two broad categories according to the method
of manufacture: mill pipe and fabricated pipe. Mill pipe is produced to meet finished pipe specifications. Fabricated pipe is manufactured from steel plate with spiral or straight welded seams.
Steel pipe is manufactured by either the seamless (extruded) or welded method, and is available either plain (black) or galvanized (zinc plated inside, outside, or both). Wall thickness is expressed as ‘‘schedule,’’ and ranges from schedule 5 (light- est) to schedule 160. The wall thickness varies with the size of the pipe. The larger the schedule number, the thicker the pipe wall for a specific pipe size.
Steel pipe, depending upon type, can be obtained with threaded ends for screwed fittings, plain ends, and flanged and beveled ends for welding.
There is an extremely large number of steel pipe alloys available. The selection depends on the intended service. The steel pipe alloys most commonly used for service and utility systems conform to ASTM A 53: Steel Pipe, Welded or Seam- less, Black or Galvanized and ASTM A-106: Steel Pipe, Welded or Seamless, Black or Galvanized.
Steel pipe is generally used for pressure piping. Its advantages are long laying lengths, high internal and external strength, and the availability of varying pipe thickness to meet almost any design pressure. It has good flow characteristics and fire resistance and is low in initial cost. The most serious disadvantage is low corrosion resistance. This requires internal and external protection, with galvanizing the most commonly used method.
COPPER Copper tubing is seamless, made from almost pure copper (99.9 percent), and is available in hard (annealed) and soft (drawn) form. It is manufactured in sizes ranging from 1⁄8 to 12 in, but sizes over 6 in are not generally available. All tubing is manufactured with plain ends only. Joints are made by soldering, and brazing, and with flared and flanged fittings.
The six types of copper tubing used most often are:
1. ASTM B-88 is the grade used most often for potable water, and also for compressed gases and vacuum systems where high purity is not a factor. It is seamless, available in nominal pipe sizes from 3⁄8 to 12 in, in hard and soft temper, and in three wall thickness grades—K (thickest wall), L, and M (thinnest wall). If patented flare fittings are used, the pipe must be obtained without outside diameter embossing, which would interfere with the sealing of the pipe wall against the side of the fitting.
2. ASTM B-819 is similar to B-88, except that it is available only in grades K and L and, in addition, the pipe is factory cleaned, capped, and specially marked. This pipe is required to be used for medical gas systems in health-care facilities. It should also be considered as the primary copper pipe for gases in laboratories as well.
3. ASTM B-75 is seamless, available as either hard or drawn, and in nominal sizes from 1⁄8 to 2 in O.D. The smaller sizes are often referred to as capillary tubing. This is the grade most often used for very small diameter pipe [1⁄4 in O.D. (6 mm) or less] connecting instruments to the distribution piping. This pipe is commonly joined by patented flare joints, which require temper 060 to seal correctly.
4. ASTM B-280, type ACR (air conditioning and refrigeration), is available cleaned and capped for field refrigeration piping and could also be considered for laboratory use. It is available only in hard temper, and its size is the actual O.D. of the pipe. This requires that all non-ACR fittings used in the system, which are manufactured in nominal pipe size, be dimensionally compatible with the ACR pipe, which is manufactured in actual pipe size.
5. ASTM B-306, copper drainage tube, is known as DWV (drainage, waste, and vent). This designation applies to copper tube used for non pressure drainage systems. This tube has the thinnest wall of any copper product. The preferred jointing method is soldering, which is adequate in strength and the least costly. Primary use is in residential construction as indirect waste lines and in larger projects for local branch lines where human waste is not discharged. Advantages are its light weight, ease of assembly, and smooth interior. Disadvantages include corrosive attack by ordinary sewage, poor fire resistance, and the need for dielectric connections to eliminate galvanic corrosion where this material is connected to any iron piping.
6. ASTM B-837 Type G is seamless tube, available in either hard (drawn) or soft (annealed) temper, and in nominal sizes from 3⁄8 to 11⁄8-in O.D. This tubing is identified by the O.D. rather than nominal size. This grade is manufactured specifically for natural gas and LP fuel gas systems. Fittings shall be similar to those used with type ACR copper. The pipe is joined by either brazing or flare type joints. Brazed joints are required for system pressures above 14 in WC where installed in inaccessible locations. Brazing alloys shall contain less than 0.05 percent phosphorus.
DUCTILE IRON PIPE (DI ) Ductile iron is a cast ferrous material alloyed with free nodular or spheroidal graphite in lieu of the flakes that are present in cast iron. This is achieved by the addition of a magnesium inoculation. It is used either as a gravity sewer or pressure pipe. Sizes available range from 3 to 54 in. There are eight pressure ratings—class 50 (125 psi) to class 56 (350 psi), and gravity sewer pipe. A cement or bituminous lining can be provided to resist internal corrosion. This pipe can be assembled with mechanical, gasket ed, or flanged joints. Ductile iron pipe shall conform to ASTM A 518 and ASTM A 861.
The advantages of ductile iron are the same as those for CI pipe, except that it is far stronger in terms of allowable pressure ratings and external load-bearing capacity. It is also not as brittle, allowing rougher handling. It has a higher initial cost than CI.
LEAD (LD) Lead pipe is made from 99.7% pig lead with various alloys available for special applications. Joints for this pipe are either wiped, burned, or mechanically flanged. Lead pipe shall conform to WW-P-325a: Lead Pipe, Bends and Traps.
Uses of lead pipe include existing connections to floor-mounted water closets, radioactive wastes, and special laboratory corrosive wastes. It has very limited use in modern drainage systems.
STAINLESS STEEL (SS ) The term stainless steel encompasses a wide variety of alloys containing 11 to 30% chromium (Cr), 0 to 35% nickel (Ni), and 0 to 6% molybdenum (Mo) in various combinations as well as small amounts of other elements such as titanium, manganese, niobium, and nitrogen. It is widely used in the chemical, pharmaceutical, and food processing industries.
Pipe is available in sizes ranging from 1⁄8 to 48 in, and is manufactured in plain end, prepared end for welding, and flanged. Joints can be welded, threaded, or flanged. Wall thickness is expressed as a ‘‘schedule,’’ and ranges from schedule 5 (lightest) to schedule 160. The wall thickness varies with the size of the pipe. The larger the schedule number, the thicker the pipe wall for a specific pipe size. Stain- less steel pipe is also available as tubing with wall thickness designated as a decimal.
The composition of a stainless steel alloy determines its metallurgical structure or grade, and therefore its properties. There are five groups of stainless steel based on metallurgical structure: ferritic, austenitic, superaustenitic, martensitic, and duplex.
1. Ferritic stainless steels contain 12 to 30% (more typically 16 to 18%) Cr, 0 to 4% Ni, and 0 to 4% Mo with a low carbon content. This material is magnetic and has good ductility and cold form ability but is not hard enable by heat treatment. This class is generally less vulnerable to chloride-induced stress corrosion cracking. Its primary use is in transport of strong oxidizing fluids (such as nitric acid) in process environments, machinery, and kitchen equipment. This class is exemplified by type ASTM grade 430.
2. Austenitic stainless steels contain 17 to 27% Cr, 8 to 35% Ni, and 0 to 6% Mo. This material is typically nonmagnetic and readily weldable, and has good ductility (even at cryogenic temperatures) and cold form ability but is not hard enable by heat treatment. This class is more generally corrosion resistant, but is generally vulnerable to chloride-induced stress corrosion cracking. Regular carbon grades are susceptible to corrosion around welded joints due to migration of Cr away from the weld site. These problems are overcome by using a low carbon grade, indicated by an L suffix, that reduces carbon to below 0.035 percent. Grades within this class are the most commonly used stainless steels. This class is exemplified by type ASTM grades 304 and 316 (304L and 316L). A superaustenitic grade is also avail- able with superior resistance to chloride pitting.
3. Superaustenitic stainless steel alloys were created to better withstand corrosion in a more severe environment than conventional stainless steel. They are alloys of Ni, Cr, Mo, copper, and iron typified by UNS (Unified Numbering System) alloys N08020, N08024, and N08026.
4. Martensitic stainless steels contain 11 to 18% Cr, 0 to 6% Ni, 0 to 2% Mo, and 0.1 to 1% C. This class is magnetic, oxidation resistant, and hard enable by heat treatment. Little used in piping applications, its primary uses are in cutlery, turbine blades, and high-temperature parts. This class is exemplified by type ASTM grade 410.
5. Duplex stainless steel is characterized by a microstructure containing both ferritic and austenitic types with different grades, having a mixture of 40 to 60% of each for various alloys. Its advantages are good resistance to chloride-induced stress corrosion cracking and high mechanical strength properties along with good ductility and impact strength. Disadvantages include corrosion of pipe by reducing acids and weld site corrosion by oxidizing acids.
Stainless steel is available in various wall thicknesses. Pipe is commonly avail- able from schedule 5 to 80, and tubing from 0.028 to 0.188.
Stainless Steel Finishes
For stainless steel piping used in pharmaceutical, food-processing, chemical, and electronics applications, the interior and exterior of the piping are often required to be finished as required by FDA, EPA, USDA, or other applicable codes. Finishing the exterior makes it easier to keep clean. Finishing the interior will prevent the adherence of any solids, increase corrosion protection, and shorten pipe interior cleaning procedures. Finishing can be abrasive, electropolished, or both.
Abrasive finishes are mechanically produced by polishing and wearing away of the surface. This is often specified by a particular size or ‘‘grit’’ of the abrasive used, such as a 220 grit. This signifies the size of the abrasive passing through a specific size mesh. The larger the number, the finer the finish. Electropolishing is an electrochemical process using an electrical current to deposit metal from an anode to a cathode. Electropolishing, which is the opposite of electroplating, re- moves surface metal from microscopic high points faster than from low points. The metal to be polished is the sacrificial anode. Surface ions of iron are removed leaving a chromium-rich surface resulting in a smooth, corrosion-resistant pipe in- terior. Another method used to specify standard sheet and pipe exterior finishes uses numbers 1 to 8. An explanation of sheet finishes is given in Table 2.3. Table 2.4 gives the grit equivalent of microinch measurement.
Finishes are often indicated as Ra (arithmetic mean roughness average, or roughness average) and Rq, the equivalent of RMS (root mean square). Both of these are measured in micro inches and denote the smoothness of the surface. The smaller the number, the finer and finish. The Ra reading is approximately 87.5 percent of the Rq (RMS) reading. Other methods of expressing smoothness are center line average (CLA) and arithmetic average (AA).
Passivization
When the interior surface of stainless steel piping is required to have a very low rate of corrosion, it must be made passive. This is accomplished by the formation of a thin surface film that acts as a barrier to corrosion. The surface film (or passive film) is made thicker by exposing it to oxidizing and chelant solutions or electro polishing. Oxidizing solutions frequently used are nitric acid, ammonium persulfate, hydrogen peroxide, and citric acid. Chelants are nontoxic organic acids or their viable salts and nontoxic synergizing agents. ASTM A 380 suggests other passivation chemical combinations that have proven successful.
CORRUGATED STEEL PIPE
Corrugated steel pipe is available from 6 in (125 mm) to 96 in (2.66 m). It is fabricated from flat steel that has been rolled into various shapes and impressed with grooves around the circumference of the pipe, generally described as circular arcs connected by tangents. Corrugations are measured by pitch (the dimension from crest to crest at right angles to the corrugations) and depth.
Longitudinal pipe seams are riveted, welded, bolted or have helical lock seams. Joints are generally steel bands with a gasket under the sleeve and tightened by bolts inserted in an integral angle. Fittings are usually made from straight piping and shop fabricated into the desired shape.
Corrugated steel pipe shall conform to different AASHTO standards depending on the actual pipe material specification.