This section will discuss various commercially available plastic piping materials and their properties.
Plastics, because of their unique chemical resistance and other characteristics, are widely used as raw material for both piping and elastodynamics sealing and gasket compounds. It is no longer justifiable to think of plastic pipe as merely a cheap substitute for other piping materials. Plastic has become the material of choice for piping systems used to convey various liquids, chemicals, pharmaceuticals, liquid fuels, and fuel gases, and those used for underground sewer water. Plastic pipe is available in a great variety of compositions. When used for plumbing systems, the restrictions imposed by the applicable code will be the single most important determining factor in the use and selection of any plastic pipe material.
Plastic pipe is manufactured in two types: thermoset (TS) and thermoplastic (TP). Thermoset piping is permanently rigid; examples are epoxy and phenolic. Thermoplastic material will soften when subject to any degree of heat and re harden upon removal of the heat. This will affect the strength of the pipe. Therefore, extreme care must be used when selecting the material type and support system for the material.
In general, the advantages of plastic pipe are excellent resistance to a very wide range of sanitary and chemical effluent and aggressive soils, long laying lengths, good flow characteristics, and economical initial system costs. Disadvantages are poor structural stability requiring close support, susceptibility of some materials to changes resulting from exposure to ultraviolet rays or sunlight, poor fire resistance, lowering of pressure ratings with elevated temperature, and production of toxic gases released by some materials when burning.
KEY PROPERTIES OF PLASTIC PIPE
The advantages of plastic pipe include:
1. Resistance to a very wide range of sanitary and chemical effluents
2. Resistance to aggressive soils
3. Availability in long lengths
4. Light weight
5. Low resistance to fluid flow
6. Generally low initial cost
Plastics, because of their unique chemical resistance and other characteristics, are widely used as raw material for both piping and elastodynamics sealing and gasket compounds. It is no longer justifiable to think of plastic pipe as merely a cheap substitute for other piping materials. Plastic has become the material of choice for piping systems used to convey various liquids, chemicals, pharmaceuticals, liquid fuels, and fuel gases, and those used for underground sewer water. Plastic pipe is available in a great variety of compositions. When used for plumbing systems, the restrictions imposed by the applicable code will be the single most important determining factor in the use and selection of any plastic pipe material.
Plastic pipe is manufactured in two types: thermoset (TS) and thermoplastic (TP). Thermoset piping is permanently rigid; examples are epoxy and phenolic. Thermoplastic material will soften when subject to any degree of heat and re harden upon removal of the heat. This will affect the strength of the pipe. Therefore, extreme care must be used when selecting the material type and support system for the material.
In general, the advantages of plastic pipe are excellent resistance to a very wide range of sanitary and chemical effluent and aggressive soils, long laying lengths, good flow characteristics, and economical initial system costs. Disadvantages are poor structural stability requiring close support, susceptibility of some materials to changes resulting from exposure to ultraviolet rays or sunlight, poor fire resistance, lowering of pressure ratings with elevated temperature, and production of toxic gases released by some materials when burning.
KEY PROPERTIES OF PLASTIC PIPE The advantages of plastic pipe include:
1. Resistance to a very wide range of sanitary and chemical effluents
2. Resistance to aggressive soils
3. Availability in long lengths
4. Light weight
5. Low resistance to fluid flow
6. Generally low initial cost
Disadvantages include:
1. Poor structural stability requiring additional support
2. Susceptibility of some types of plastics to physical changes resulting from exposure to sunlight
3. Generally low resistance to solvents
4. Poor fire resistance
5. Lowered pressure ratings at elevated temperature
6. Production of toxic smoke and gases, which are released upon combustion of some types of plastic pipe
DESCRIPTION AND CLASSIFICATION
Plastic pipe is as descriptive a phrase as metallic pipe. The properties of various plastic materials are obtained from the basic chemical composition of the polymer resin, additives, and the manufacturing process itself. In order to better understand the material called ‘‘plastic,’’ definitions of the basic terms and ingredients used by the plastic piping industry are necessary. Please understand that these are simplified definitions.
Plastic is a material whose essential ingredient is an organic substance of large molecular weight which at some stage in its manufacture can be shaped by flow and becomes solid in its finished state.
A polymer is a material consisting of molecules with a high molecular weight. A monomer is a chemical compound capable of reacting to form a polymer. Po- lymerization is a chemical reaction in which molecules of a monomer are linked together to form a polymer. When two or more monomers are used, the process is called copolymerization.
The following are common additives used in the manufacture of plastic piping: flame retardants, plasticizers to increase flexibility and workability, antioxidants to retard degradation from contact with air, stabilizers to retard degradation at higher temperatures, lubricants to aid in the extrusion process, pigment or dies to color the final product and protect against ultraviolet light, fillers to modify strength or lower cost, and modifiers to produce a special property response.
Another type of plastic is an elastomer. Used mostly for gaskets, an elastomer is a material that is capable of being repeatedly stretched to at least twice its original length at room temperature and which will return to its approximate original length upon release.
The following is a partial list of plastic pipe and elastomer materials available from all sources. The names in parentheses are trade names patented by various manufacturers. Only those piping materials that are commonly available will be discussed. Elastomers, indicated as (E), are listed only for reference since they are outside the scope of this chapter:
ABS acrylonitrile butadiene styrene; also (Buna-N) (E) BR butadiene (E)
CAB cellulose acetate butyrate (Celcon) CIIR chlorinated isobutene isoprene (E) CPE chlorinated polyethylene (E)
CPVC chlorinated polyvinyl chloride
CR chloroprene rubber (Neoprene) (E)
CSP chlorine sulphonyl polyethylene (Hypalon) (E) ECTFE ethylenechlorotrifluoroethylene
EP epoxide, epoxy
EPDM ethylene propylene-diene monomer (E)
EPM ethylene propylene terpolymer (E) FEP fluorinated ethylene propylene
FPM fluorine rubber (Viton) (E) HDPE high-density polyethylene
IIR isobutene isoprene (butyl) rubber (E) IR polyisoprene (E)
PA polyamide
PAEK polyaryl etherketone
PB polybutylene
PC polycarbonate
PCTFE polychlorotrifluoroethylene (Halar) PE polyethylene
PEX cross linked polyethylene
PF phenol-formaldehyde
PFA perflouralkoxy
PP polypropylene
PPS polyphenylene sulfide
PTFE polytetrafluoroethylene (Teflon) PEEK polyether etherketone
PFA perfluoroalkoxy
PS polysulfone
PVC polyvinyl chloride PVDC polyvinylidene chloride PVDF polyvinylidene fluoride
SBR styrene butadiene (E)
Plastic materials used for piping are divided into two basic groups, thermoplastic and thermosetting. Thermoplastics soften upon the application of heat and reharden upon cooling. This permits pipe to be extruded or molded into shapes. The most common piping materials are thermoplastic. Thermosetting plastics form permanent shapes only when cured by the application of heat or the use of a curing chemical. Once shaped, they cannot be reformed.
There are subclassifications of pipe based on the material used for the pipe itself. The two most common are polyolefins and fluoroplastics. Polyolefins, which are plastics formed by the polymerization of certain straight chain hydrocarbons, in- clude ethylene, propylene, and butylene. Piping includes PP, PE, and PB. Fluoro- plastics are polymers containing one or more atoms of fluorine. Piping includes PTFE, PVDF, CTFE, ETFE, PFA, and FEP.
PLASTIC PIPE STANDARDS AND NOMENCLATURE
A variety of standards and nomenclature is used to designate pressures and standard dimensions used for the procurement and identification of plastic pipe. Some are used to match existing metallic pipe specifications and others are unique to the plastic pipe industry. The following is an explanation of the terms used in various standards:
SDR. The standard dimensional ratio is the most commonly accepted measure for providing a pipe wall thickness category and constant mechanical properties for many plastic pipe materials. Used for solid homogeneous pipe, the SDR is found by dividing the average outside diameter of a pipe by the wall thickness. This designation has resulted in a series of preferred industry standard numbers that are constant for all sizes of pipe. It is possible for a pipe to have different SDRs depending on whether the I.D. or O.D. is the controlling factor.
DR. The dimensional ratio is often incorrectly used interchangeably with SDR. The DR is found in the same manner as above and means the same thing, but is used when the product does not have the preferred SDR number established by other prevailing standards. Pipe manufactured to pressure ratings for AWWA C-900 series standards uses this designation.
O.D. controlled. This designation is used when the outside diameter of the pipe is the controlling factor in the selection of the pipe.
I.D. controlled. This designation is used when the inside diameter of the pipe is the controlling factor in the selection of the pipe.
P.R. Pressure rated is used when the pressure rating is the controlling factor in the selection of the pipe rather than the dimensions of the pipe itself.
PS. Pipe stiffness is used only for sewer pipe. This designation is in PSI. The higher number has a thicker pipe wall.
Schedule. This designation is used to match the standard dimensions for me- tallic pipe sizes. The pressure rating of the pipe varies with pipe size. Some standards use iron pipe size (IPS) instead of schedule to keep the wall thickness consistent with iron pipe.
PSM. This is an arbitrary designation for products having certain dimensional characteristics unique to a very specific product.
The AWWA has several proprietary dimensional standards that are used to spec- ify plastic pipe used only for pressurized potable and fire water main distribution and transmission systems. The composition of the plastic piping material is refer- enced to ASTM standards. Since plastic pipe connects to or replaces cast iron and ductile iron pipe, these standards are O.D. controlled for use with O-ring gasketed joints only and are dimensionally compatible with these joints. These standards are:
AWWA C-900 4 to 12 in PVC AWWA C-901 1⁄2 to 3 in PE AWWA C-902 1⁄2 to 3 in PB AWWA C-903 Deleted
AWWA C-904 Fittings for C-900 pipe
AWWA C-905 14 to 36 in PVC
AWWA C-906 Larger diameter PE pipe
Trade laws allow import of Canadian pipe materials into the United States. The
Canadian Standards Association (CSA) has standards of their own, but many of them have not been completely coordinated with the United States standards for similar products. At this time, using CSA standards as reference for plastic products is not recommended.
There are three designations used in plastic drainage pipe standards: DWV, sewer, and drain. All standards are O.D. controlled and are non-pressure-rated. The only differences between identical materials with different designations are dimensions. Different materials with the same designations have the same dimensions.
For the most part, the codes are very specific about acceptable materials for use in plumbing systems, such as potable water and drainage, where approval of the authorities is required. Regarding uses such as draining chemicals from laboratories or industrial work, the codes are vague. When the chemical waste will be treated inside the project boundary, usually the materials (and design) used for the waste system do not fall under the jurisdiction of the plumbing code. In these cases, the engineer has the most latitude in the selection of materials used for drainage piping.
ELEMENTS FOR SELECTION
Due to the differences in manufacture, grade, and chemical composition of the pipe, test data must be obtained from the local supplier or manufacturer. Properties of similar materials from different manufacturers are often not the same. Very often, a range of values for properties such as tensile strength, maximum operating tem- perature, and hardness is given.
Elevated Temperature Considerations
Service temperatures in plastic piping systems depend on the type of plastic used. A maximum service temperature is generally fixed for thermoplastics, and identifies the upper limit to which the pipe may be heated without damage. When heated above this temperature, the pipe will soften and deform. Upon cooling, it will harden to the deformed shape and dimensions.
Long-Term Hydrostatic Strength
The design pressure for plastic pipe is based on long-term hydrostatic strength, which is determined by finding the estimated circumferential stress that, when ap- plied continuously, will produce failure of the pipe after 100,000 h at a specified temperature. In addition, a service factor is included in the design calculations. This factor takes into account certain variables together with a degree of safety appropriate to the installation. The service factor is usually selected by the design engineer, and referenced to a service design life of about 50 years. This design method does not include the fittings, joints, or cyclic effects such as water hammer.
Most pressure ratings for thermoplastic pipes are calculated assuming a water environment. As the temperature rises, the pipe becomes more ductile and loses strength, and therefore the rating of thermoplastic pipe must be decreased to allow for safe operation. These factors are different for each pipe material.
Fatigue Behavior
When surges and water hammer are likely to be encountered, additional allowance should be made or protective devices installed in the piping system to reduce the pressure.
Aging and Long-Term Degradation
Aging is the change in physical and chemical properties during storage or use, and is generally dependent on temperature. These changes can occur naturally through normal atmospheric or building temperature fluctuations, or can be developed artificially due to elevated temperatures of the fluid in the pipe. The origin of these changes within the pipe are in the molecular or crystallographic structure of the pipe. Plastics and elastomers experience chemical changes due to the influence of light, heat, oxygen, humidity, and radiation, all of which cause breaks in their molecular chains. One criterion for determining the onset of aging is the measure- ment of thermal stability (oxygen induction time) using differential scanning calorimetry.
Ultraviolet Radiation (UVR)
UVR is a known source of degradation to plastic pipe. The effects can be reduced by adding pigments or covering the installed pipe with a jacket.
Flammability
During fire conditions, the degradation of plastics is greatly accelerated. In the early stages of a fire, most plastics melt and lose their structural shape and strength. As heat is added at a rising rate, plastics undergo a series of typical changes, which include chemical decomposition often releasing toxic chemicals. This decomposition occurs at a lower temperature than ignition. By the time ignition occurs or is possible, a relatively long period of chemical emission has elapsed.
When thermoplastic pipe burns, it releases smoke and toxic gases, provides heat that increases the intensity of a fire, and may provide a path for flame to spread along its length. In addition, open holes may develop at wall or ceiling penetrations which could provide a route for the passage of gases between rooms.
All organic materials are flammable, but this is particularly true of polyolefins. It is well proven that many polymers are as a result of their chemical composition difficult to ignite. Polymers can also be made much more difficult to ignite by the addition of flame retardants.
Acoustic Transmission
Because of its light weight, thermoplastic piping does little to reduce airborne sound. An appropriate thickness of insulation must be used to reduce noise.
Thermal Expansion
The amount of movement resulting from thermal effects is relatively high, thus requiring special attention to installation. As a general rule, runs in excess of 20 ft should be checked for the necessity of expansion offsets.
Corrosion Resistance
Corrosion occurs in two ways, as chemical and stress corrosion. There are two general types of chemical attack on plastic piping. The first is called solavation, which is the solubility, or absorption, of chemicals into the piping material from the fluids inside the pipe. This causes swelling and softening. The second type of attack occurs where the polymer or base resin molecules are somehow changed by a chemical agent, and the original properties of the plastic pipe cannot be restored upon removal of that chemical. Stress (or strain) corrosion weakens the pipe due to constant and repetitive movement and / or pressure surges.
The chemical resistance of the various types of plastics varies greatly not only among different types of plastic, but among different grades of the same type of plastic. Achieving full resistance is a function of the resistance of the compound used to make the pipe and the processing of the plastic.
The factors that determine the suitability and service life of any specific plastic pipe are
1. The specific chemicals and their concentrations
2. The jointing method
3. Dimensions of the pipe and fittings
4. Pressure inside the pipe
5. Ambient temperature and temperature of the fluid
6. Period of contact
7. Stress concentrations in the pipe and fittings
Abrasion
If a material such as sand, gravel, or slurry is transported in the piping system, or frequent cleaning with mechanical equipment is anticipated, resistance to abrasion should be investigated. Mechanical cleaning equipment manufacturers have data available from tests on various piping materials. Pipe manufacturers are also a source of information regarding various effluents. Additives can be used to increase the abrasion resistance of any pipe.
Biological Resistance
Very few types of plastic pipe can be degraded and / or deteriorated by the action of micro or macro organisms. For the most part, plastic pipe shows negligible or no susceptibility to bacterial attack. Refer to manufacturers for specific data.
Electrical Properties
Because plastic pipe is non conductive, electrostatic charging of a pipeline is possible if dry, electrically non conductive material is transported. All pipe materials with a specific resistance of 106 ohms per centimeter ( / cm) are considered non- conductive. Plastic pipe is generally not recommended to carry ignitable mixtures or electrically non conductive dry substances due to potential electrostatic charging and possible damage to the dry material inside the pipe.
Static electrical charges can be prevented by providing the pipe with a conductive coating of metallic powder or lagging the pipe with metallic foil that has been grounded.
Permeability
Permeation is a process where fluids pass either into or out of a piping system through the walls of a pipe. Permeation can occur through the walls of a susceptible plastic pipe, through gaskets or other jointing material, and through defects or inappropriately or incorrectly sealed pipe.
Organic matter that migrates from soil through the plastic pipe is called permeate, and the process is called permeation. Until additional scientific work is completed on permeation through plastic pipe, it is not recommended that plastic pipe be used to carry potable water in areas of contaminated soil.
Leaching
Leaching is a process where substances sometimes called extractable are released from the walls of the pipe material into the fluid, but not through the pipe walls. The most common extractable are inorganic chemicals and volatile organic com- pounds (VOCs).
Tests have shown that the rate of leaching from plastics in contact with high purity water usually decreases with time. The time it takes for any specific plastic to reach a steady state after being subject to immersion in the fluid (elution) in dynamic systems is a function of the water temperature and velocity. Experience has shown that leach-out in the first five days is considered a burn-out period. The release of VOCs from various plastic pipes in contact with high purity water at
74 F is shown in Fig. 2.1. Figure 2.2 shows calculated lengths of thermoplastics that will increase total organic compound (TQC) level of high purity water by 1 ppt when used as a transfer medium (assuming 4-in pipe and a water velocity of 6 ft / s).
1. Poor structural stability requiring additional support
2. Susceptibility of some types of plastics to physical changes resulting from exposure to sunlight
3. Generally low resistance to solvents
4. Poor fire resistance
5. Lowered pressure ratings at elevated temperature
6. Production of toxic smoke and gases, which are released upon combustion of some types of plastic pipe
DESCRIPTION AND CLASSIFICATION Plastic pipe is as descriptive a phrase as metallic pipe. The properties of various plastic materials are obtained from the basic chemical composition of the polymer resin, additives, and the manufacturing process itself. In order to better understand the material called ‘‘plastic,’’ definitions of the basic terms and ingredients used by the plastic piping industry are necessary. Please understand that these are simplified definitions.
Plastic is a material whose essential ingredient is an organic substance of large molecular weight which at some stage in its manufacture can be shaped by flow and becomes solid in its finished state.
A polymer is a material consisting of molecules with a high molecular weight. A monomer is a chemical compound capable of reacting to form a polymer. Po- lymerization is a chemical reaction in which molecules of a monomer are linked together to form a polymer. When two or more monomers are used, the process is called copolymerization.
The following are common additives used in the manufacture of plastic piping: flame retardants, plasticizers to increase flexibility and workability, antioxidants to retard degradation from contact with air, stabilizers to retard degradation at higher temperatures, lubricants to aid in the extrusion process, pigment or dies to color the final product and protect against ultraviolet light, fillers to modify strength or lower cost, and modifiers to produce a special property response.
Another type of plastic is an elastomer. Used mostly for gaskets, an elastomer is a material that is capable of being repeatedly stretched to at least twice its original length at room temperature and which will return to its approximate original length upon release.
The following is a partial list of plastic pipe and elastomer materials available from all sources. The names in parentheses are trade names patented by various manufacturers. Only those piping materials that are commonly available will be discussed. Elastomers, indicated as (E), are listed only for reference since they are outside the scope of this chapter:
ABS acrylonitrile butadiene styrene; also (Buna-N) (E) BR butadiene (E)
CAB cellulose acetate butyrate (Celcon) CIIR chlorinated isobutene isoprene (E) CPE chlorinated polyethylene (E)
CPVC chlorinated polyvinyl chloride
CR chloroprene rubber (Neoprene) (E)
CSP chlorine sulphonyl polyethylene (Hypalon) (E) ECTFE ethylenechlorotrifluoroethylene
EP epoxide, epoxy
EPDM ethylene propylene-diene monomer (E)
EPM ethylene propylene terpolymer (E) FEP fluorinated ethylene propylene
FPM fluorine rubber (Viton) (E) HDPE high-density polyethylene
IIR isobutene isoprene (butyl) rubber (E) IR polyisoprene (E)
PA polyamide
PAEK polyaryl etherketone
PB polybutylene
PC polycarbonate
PCTFE polychlorotrifluoroethylene (Halar) PE polyethylene
PEX cross linked polyethylene
PF phenol-formaldehyde
PFA perflouralkoxy
PP polypropylene
PPS polyphenylene sulfide
PTFE polytetrafluoroethylene (Teflon) PEEK polyether etherketone
PFA perfluoroalkoxy
PS polysulfone
PVC polyvinyl chloride PVDC polyvinylidene chloride PVDF polyvinylidene fluoride
SBR styrene butadiene (E)
Plastic materials used for piping are divided into two basic groups, thermoplastic and thermosetting. Thermoplastics soften upon the application of heat and reharden upon cooling. This permits pipe to be extruded or molded into shapes. The most common piping materials are thermoplastic. Thermosetting plastics form permanent shapes only when cured by the application of heat or the use of a curing chemical. Once shaped, they cannot be reformed.
There are subclassifications of pipe based on the material used for the pipe itself. The two most common are polyolefins and fluoroplastics. Polyolefins, which are plastics formed by the polymerization of certain straight chain hydrocarbons, in- clude ethylene, propylene, and butylene. Piping includes PP, PE, and PB. Fluoro- plastics are polymers containing one or more atoms of fluorine. Piping includes PTFE, PVDF, CTFE, ETFE, PFA, and FEP.
PLASTIC PIPE STANDARDS AND NOMENCLATURE A variety of standards and nomenclature is used to designate pressures and standard dimensions used for the procurement and identification of plastic pipe. Some are used to match existing metallic pipe specifications and others are unique to the plastic pipe industry. The following is an explanation of the terms used in various standards:
SDR. The standard dimensional ratio is the most commonly accepted measure for providing a pipe wall thickness category and constant mechanical properties for many plastic pipe materials. Used for solid homogeneous pipe, the SDR is found by dividing the average outside diameter of a pipe by the wall thickness. This designation has resulted in a series of preferred industry standard numbers that are constant for all sizes of pipe. It is possible for a pipe to have different SDRs depending on whether the I.D. or O.D. is the controlling factor.
DR. The dimensional ratio is often incorrectly used interchangeably with SDR. The DR is found in the same manner as above and means the same thing, but is used when the product does not have the preferred SDR number established by other prevailing standards. Pipe manufactured to pressure ratings for AWWA C-900 series standards uses this designation.
O.D. controlled. This designation is used when the outside diameter of the pipe is the controlling factor in the selection of the pipe.
I.D. controlled. This designation is used when the inside diameter of the pipe is the controlling factor in the selection of the pipe.
P.R. Pressure rated is used when the pressure rating is the controlling factor in the selection of the pipe rather than the dimensions of the pipe itself.
PS. Pipe stiffness is used only for sewer pipe. This designation is in PSI. The higher number has a thicker pipe wall.
Schedule. This designation is used to match the standard dimensions for me- tallic pipe sizes. The pressure rating of the pipe varies with pipe size. Some standards use iron pipe size (IPS) instead of schedule to keep the wall thickness consistent with iron pipe.
PSM. This is an arbitrary designation for products having certain dimensional characteristics unique to a very specific product.
The AWWA has several proprietary dimensional standards that are used to spec- ify plastic pipe used only for pressurized potable and fire water main distribution and transmission systems. The composition of the plastic piping material is refer- enced to ASTM standards. Since plastic pipe connects to or replaces cast iron and ductile iron pipe, these standards are O.D. controlled for use with O-ring gasketed joints only and are dimensionally compatible with these joints. These standards are:
AWWA C-900 4 to 12 in PVC AWWA C-901 1⁄2 to 3 in PE AWWA C-902 1⁄2 to 3 in PB AWWA C-903 Deleted
AWWA C-904 Fittings for C-900 pipe
AWWA C-905 14 to 36 in PVC
AWWA C-906 Larger diameter PE pipe
Trade laws allow import of Canadian pipe materials into the United States. The
Canadian Standards Association (CSA) has standards of their own, but many of them have not been completely coordinated with the United States standards for similar products. At this time, using CSA standards as reference for plastic products is not recommended.
There are three designations used in plastic drainage pipe standards: DWV, sewer, and drain. All standards are O.D. controlled and are non-pressure-rated. The only differences between identical materials with different designations are dimensions. Different materials with the same designations have the same dimensions.
For the most part, the codes are very specific about acceptable materials for use in plumbing systems, such as potable water and drainage, where approval of the authorities is required. Regarding uses such as draining chemicals from laboratories or industrial work, the codes are vague. When the chemical waste will be treated inside the project boundary, usually the materials (and design) used for the waste system do not fall under the jurisdiction of the plumbing code. In these cases, the engineer has the most latitude in the selection of materials used for drainage piping.
ELEMENTS FOR SELECTION Due to the differences in manufacture, grade, and chemical composition of the pipe, test data must be obtained from the local supplier or manufacturer. Properties of similar materials from different manufacturers are often not the same. Very often, a range of values for properties such as tensile strength, maximum operating tem- perature, and hardness is given.
Elevated Temperature Considerations
Service temperatures in plastic piping systems depend on the type of plastic used. A maximum service temperature is generally fixed for thermoplastics, and identifies the upper limit to which the pipe may be heated without damage. When heated above this temperature, the pipe will soften and deform. Upon cooling, it will harden to the deformed shape and dimensions.
Long-Term Hydrostatic Strength
The design pressure for plastic pipe is based on long-term hydrostatic strength, which is determined by finding the estimated circumferential stress that, when ap- plied continuously, will produce failure of the pipe after 100,000 h at a specified temperature. In addition, a service factor is included in the design calculations. This factor takes into account certain variables together with a degree of safety appropriate to the installation. The service factor is usually selected by the design engineer, and referenced to a service design life of about 50 years. This design method does not include the fittings, joints, or cyclic effects such as water hammer.
Most pressure ratings for thermoplastic pipes are calculated assuming a water environment. As the temperature rises, the pipe becomes more ductile and loses strength, and therefore the rating of thermoplastic pipe must be decreased to allow for safe operation. These factors are different for each pipe material.
Fatigue Behavior
When surges and water hammer are likely to be encountered, additional allowance should be made or protective devices installed in the piping system to reduce the pressure.
Aging and Long-Term Degradation
Aging is the change in physical and chemical properties during storage or use, and is generally dependent on temperature. These changes can occur naturally through normal atmospheric or building temperature fluctuations, or can be developed artificially due to elevated temperatures of the fluid in the pipe. The origin of these changes within the pipe are in the molecular or crystallographic structure of the pipe. Plastics and elastomers experience chemical changes due to the influence of light, heat, oxygen, humidity, and radiation, all of which cause breaks in their molecular chains. One criterion for determining the onset of aging is the measure- ment of thermal stability (oxygen induction time) using differential scanning calorimetry.
Ultraviolet Radiation (UVR)
UVR is a known source of degradation to plastic pipe. The effects can be reduced by adding pigments or covering the installed pipe with a jacket.
Flammability
During fire conditions, the degradation of plastics is greatly accelerated. In the early stages of a fire, most plastics melt and lose their structural shape and strength. As heat is added at a rising rate, plastics undergo a series of typical changes, which include chemical decomposition often releasing toxic chemicals. This decomposition occurs at a lower temperature than ignition. By the time ignition occurs or is possible, a relatively long period of chemical emission has elapsed.
When thermoplastic pipe burns, it releases smoke and toxic gases, provides heat that increases the intensity of a fire, and may provide a path for flame to spread along its length. In addition, open holes may develop at wall or ceiling penetrations which could provide a route for the passage of gases between rooms.
All organic materials are flammable, but this is particularly true of polyolefins. It is well proven that many polymers are as a result of their chemical composition difficult to ignite. Polymers can also be made much more difficult to ignite by the addition of flame retardants.
Acoustic Transmission
Because of its light weight, thermoplastic piping does little to reduce airborne sound. An appropriate thickness of insulation must be used to reduce noise.
Thermal Expansion
The amount of movement resulting from thermal effects is relatively high, thus requiring special attention to installation. As a general rule, runs in excess of 20 ft should be checked for the necessity of expansion offsets.
Corrosion Resistance
Corrosion occurs in two ways, as chemical and stress corrosion. There are two general types of chemical attack on plastic piping. The first is called solavation, which is the solubility, or absorption, of chemicals into the piping material from the fluids inside the pipe. This causes swelling and softening. The second type of attack occurs where the polymer or base resin molecules are somehow changed by a chemical agent, and the original properties of the plastic pipe cannot be restored upon removal of that chemical. Stress (or strain) corrosion weakens the pipe due to constant and repetitive movement and / or pressure surges.
The chemical resistance of the various types of plastics varies greatly not only among different types of plastic, but among different grades of the same type of plastic. Achieving full resistance is a function of the resistance of the compound used to make the pipe and the processing of the plastic.
The factors that determine the suitability and service life of any specific plastic pipe are
1. The specific chemicals and their concentrations
2. The jointing method
3. Dimensions of the pipe and fittings
4. Pressure inside the pipe
5. Ambient temperature and temperature of the fluid
6. Period of contact
7. Stress concentrations in the pipe and fittings
Abrasion
If a material such as sand, gravel, or slurry is transported in the piping system, or frequent cleaning with mechanical equipment is anticipated, resistance to abrasion should be investigated. Mechanical cleaning equipment manufacturers have data available from tests on various piping materials. Pipe manufacturers are also a source of information regarding various effluents. Additives can be used to increase the abrasion resistance of any pipe.
Biological Resistance
Very few types of plastic pipe can be degraded and / or deteriorated by the action of micro or macro organisms. For the most part, plastic pipe shows negligible or no susceptibility to bacterial attack. Refer to manufacturers for specific data.
Electrical Properties
Because plastic pipe is non conductive, electrostatic charging of a pipeline is possible if dry, electrically non conductive material is transported. All pipe materials with a specific resistance of 106 ohms per centimeter ( / cm) are considered non- conductive. Plastic pipe is generally not recommended to carry ignitable mixtures or electrically non conductive dry substances due to potential electrostatic charging and possible damage to the dry material inside the pipe.
Static electrical charges can be prevented by providing the pipe with a conductive coating of metallic powder or lagging the pipe with metallic foil that has been grounded.
Permeability
Permeation is a process where fluids pass either into or out of a piping system through the walls of a pipe. Permeation can occur through the walls of a susceptible plastic pipe, through gaskets or other jointing material, and through defects or inappropriately or incorrectly sealed pipe.
Organic matter that migrates from soil through the plastic pipe is called permeate, and the process is called permeation. Until additional scientific work is completed on permeation through plastic pipe, it is not recommended that plastic pipe be used to carry potable water in areas of contaminated soil.
Leaching
Leaching is a process where substances sometimes called extractable are released from the walls of the pipe material into the fluid, but not through the pipe walls. The most common extractable are inorganic chemicals and volatile organic com- pounds (VOCs).
Tests have shown that the rate of leaching from plastics in contact with high purity water usually decreases with time. The time it takes for any specific plastic to reach a steady state after being subject to immersion in the fluid (elution) in dynamic systems is a function of the water temperature and velocity. Experience has shown that leach-out in the first five days is considered a burn-out period. The release of VOCs from various plastic pipes in contact with high purity water at
74 F is shown in Fig. 2.1. Figure 2.2 shows calculated lengths of thermoplastics that will increase total organic compound (TQC) level of high purity water by 1 ppt when used as a transfer medium (assuming 4-in pipe and a water velocity of 6 ft / s).
Creep
When a load is continuously applied on a plastic material, it creates an instantaneous initial deformation that further increases at a decreasing rate. This further deformation is called creep. If the load is removed at any time, there is partial immediate recovery followed by a gradual creep recovery. If, however, the plastic is deformed (strained) to a given value that is maintained, the initial load (stress) created by the deformation slowly decreases at a decreasing rate. This is known as the stress relaxation response. The ratio of the actual values of stress to strain for a specific time under continuous stressing or straining is commonly referred to as the effective creep modulus, or the effective stress-relaxation modulus. This modulus is significantly influenced by time. Most pipe manufacturers are willing to provide values of effective moduli for specific materials and loading conditions. Experience has shown that all plastic pipe will creep.
The properties of plastic pipe are influenced by time of loading, temperature, and environment. Therefore, standard data sheet values for mechanical properties may not be satisfactory for design purposes. The stress-strain responses of plastic reflect its viscoelastic nature. The viscous, or fluid like, component tends to dampen or slow down the response between stress and strain.
When a load is continuously applied on a plastic material, it creates an instantaneous initial deformation that further increases at a decreasing rate. This further deformation is called creep. If the load is removed at any time, there is partial immediate recovery followed by a gradual creep recovery. If, however, the plastic is deformed (strained) to a given value that is maintained, the initial load (stress) created by the deformation slowly decreases at a decreasing rate. This is known as the stress relaxation response. The ratio of the actual values of stress to strain for a specific time under continuous stressing or straining is commonly referred to as the effective creep modulus, or the effective stress-relaxation modulus. This modulus is significantly influenced by time. Most pipe manufacturers are willing to provide values of effective moduli for specific materials and loading conditions. Experience has shown that all plastic pipe will creep.
The properties of plastic pipe are influenced by time of loading, temperature, and environment. Therefore, standard data sheet values for mechanical properties may not be satisfactory for design purposes. The stress-strain responses of plastic reflect its viscoelastic nature. The viscous, or fluid like, component tends to dampen or slow down the response between stress and strain.