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For the purposes of this article, discussion will be limited to pipelines installed in Location Classes 3 and 4 (suburban and urban environments) and in low pressure distribution systems and service lines (downstream of transmission systems) and for pipelines NPS 12 (DN 300) and smaller.
The codes covering higher pressure natural gas systems in large-scale natural gas distribution systems are: CFR-2019 Title 49 Volume 3 Part 192 Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards; ASME B31.8 Gas Transmission and Distribution Piping Systems; and ANSI Z223.1/NFPA 54 National Fuel Gas Code and the International Fuel Gas Codes.
These codes do not make recommendations on natural gas piping system materials; but provide rules and regulations for each type of piping system if it is used. The 49 CFR part 192 provides requirements for gas distribution piping for materials that have been incorporated by reference into 192.7. The owner’s representative needs to work closely with the natural gas supplier and follow his rules, regulations, and recommendations in any installation since the supplier will be most familiar with the local rules applicable to the project. These will have precedence over the codes listed earlier.
Above-Ground Pipeline Materials
The material of choice for most above ground installations of natural gas piping is ASTM A53/A53M, black steel, Schedule 40, Type E or S, Grade B. carbon steel pipe. Steel piping is used for systems operating in excess of 1,000 psig (6,900 kPa) and delivering natural gas to locations in excess of 300 miles from the last compressor station. It is also used in many low-pressure aboveground distribution systems.
Normally, when operating at pressures over 2.0 psig (14 kPa), welded pipe joints are recommended. Fittings would be ASTM A234/A234M wrought-steel welding fittings suitable for butt welding and socket welding. Larger pipe, generally NPS-2-1/2 (DN 65) and larger, would have flanged fittings and valves. Pipes NPS 2 (DN 50) and smaller would still be threaded; generally threading and unions would be limited to installation of valves and regulators that would require periodic maintenance and replacement. It is recommended that all piping exposed to outdoor weather be painted.
Other piping materials include aluminum tubing, copper tubing, and corrugated stainless steel (CSST).
Below-Grade Pipeline Materials
According to 2008 seminar materials prepared by the Plastics Pipe Institute (PPI), 95 percent of the (new) piping materials installed for distributing natural gas is polyethylene (PE) pipe. In 1992, a survey revealed that about two-thirds of gas piping purchased was medium density polyethylene (MDPE) and 1/3 was high density polyethylene (HDPE). By 1998, the proportion of MDPE was closer to 85 percent. The distribution of gas mains vs gas service pipe was about 50-50.
Other pipeline materials used below-grade are fiberglass reinforced plastic (FRP), PVC, PE coated carbon steel, and cast iron. Metal pipes normally require PE covers and cathodic protection. As of 2017, nearly 4 billion feet or 750,000 miles (1.2 billion meters) of fused PE pipe has been installed serving 49.4 million PE gas service lines. PE pipe was first used in 1957 natural gas gathering lines. PE pipe was first used in 1961 for natural gas distribution lines.
According to the Plastics Pipe Institute, PE pipe represents the material of choice for most gas distribution applications in North America and throughout the world. For US applications, its use is governed by CFR Title 49 Part 192 which prescribes the design, installation, maintenance, repair, and integrity management of plastic piping. In Canada, the operation is regulated through CSA Z662 clause 12. PE pipe for gas distribution applications range from NPS 1/2 (DN 15) diameter through NPS 24 (DN 600) diameter, but current regulations only allow PE up to NPS 12 (DN 300) nominal diameter. Typical pressure capabilities are up through 80 psig (550 kPa) for MDPE pipes and up through 125 psig (860 kPa) for HDPE pipes.
Advantages of Using Polyethylene Piping
over Metal Piping Systems
The history of PE in gas distribution has been well documented since its’ initial use in the 1950s. Polyethylene (PE) piping provides superior performance to steel, aluminum, ductile iron, and cast iron piping systems in many applications and provides the following advantages: a) Corrosion resistant both inside or outside; b) Earthquake resistant; c) 100-year service life; d) Polyethylene pipe has a smoother interior surface; e) Joints are as strong as main line piping material; f) Allows for drilling and pulling of piping under roadways; g) Due to polyethylene’s flexibility, smaller sizes can be coiled and delivered in spools with lengths up to 500 feet, resulting in less field joints being required; h) Also, due to PE’s flexibility, PE pipe can be field bent to a radius of about 30 times the nominal pipe diameter or less depending on wall thickness. NPS 12 (DN-300) PE pipe, for example, can be cold formed in the field to a 32-foot (9.75 meters) radius. This eliminates many of the fittings otherwise required for directional changes in piping systems and it also facilitates installation; i) Temperature Resistance: PE pipe’s typical operating temperature range is from 0°F to 140°F ( 18°C to 40°C) for pressure service; j) Sustainable: Requires less energy to produce and to join this type of piping than its metal counterparts, and; k) Light Weight: Easier to handle for installers
Limitations of Polyethylene Piping
As with all materials there are limitations on the proper use of PE for gas distribution including: PE is primarily limited to underground service due to potential affects from UV exposure; and the strength and rigidity of PE is not nearly as robust as its metal counterparts. As a result, it is limited to lower pressure applications (< 125 psig). Pipe wall thickness requirements are greater than for metal piping, resulting in smaller inside diameters for a given size pipe.
PE has a higher coefficient of thermal expansion than metal. PE has the potential to be damaged or crushed by backfill material. PE is more susceptible to damage by construction activities that occur after the piping system is installed. PE piping is not detectible by itself in underground service. Although these limitations are apparent, proper engineering design is used to offset these potential disadvantages.
Engineering Controls – Pipeline Pressure
The maximum allowable operating pressure for gas distribution and transmission pipe in US federally regulated applications is determined by CFR Title 49 Part 192. Theory and material strengths are discussed in ASME B31.8 and further explained in the PPI Handbook of PE Pipe. The handbook provides an extensive look at the long term strength of PE piping materials along with the required attributes that should be used in the design of PE piping systems for natural gas systems.
As mentioned, the strength and rigidity of PE piping is not nearly as robust as its metal counterparts. The obvious engineering solution is to include sufficient material/wall thickness on the piping system to provide the pressure sustaining capacity required by the application.
Also, the ground temperature influences the strength of the piping material. These effects will be discussed in a future Plumbing Engineer magazine article.
for Underground PE Piping
Underground, PE natural-gas piping should be installed according to ASTM D 2774 Standard Practice for Underground Installation of Thermoplastic Pressure Piping. ASTM D 2774 covers trenching, bedding, protecting, and backfill aspects for installing pressurized plastic underground utility piping systems. ASTM D 2774 is supplemented by ASTM F1688, Standard Guide for Construction Procedures for Buried Plastic Pipe; this guide contains general construction information applicable for plastic pipe and supplements the installation standards for the various types of pipe including PE pipe. Flexible pipe, such as thermoplastic and ﬁberglass, are typically designed to rely on the stiffness of the soil surrounding the pipe for support. The contract documents should describe the requirements for an appropriate soil support backfill material.
The PPI in its design of Design of PE Piping Systems, Chapter 6 Section 3, discusses the methods of determining the minimum pipe SDR requirements with respect to soil type, pipe burial depth, and expected surface loads. Basically, if the native soil is of poor quality, the stability of the soil can be improved in the area of the pipe trench with improved backfill materials. The calculations used basically follow the Iowa Formula for plastic pipe installation. Once the piping is installed, if the piping burial depth is less than 36 inches it is important to protect the pipelines by covering the trenches with protective barriers if major truck loads are expected to cross the pipeline path.
Thermal Expansion and Contraction
A buried PE pipe is generally well restrained by soil loads and will experience very little lateral movement. However, restrained longitudinal pipe end movements and resulting loads need to be addressed.
Transitions to other pipe materials that use the bell and spigot assembly technique need to be calculated using the thrust loads delivered by the pressure plus the potential of the load due to temperature changes. Merely fixing the end of the PE to the mating material may result in upstream joints pulling apart unless those connections are properly restrained. The number of joints that need to be restrained to prevent bell and spigot pull out may be calculated using techniques recommended by the manufacturer of the alternate piping material.
Polyethylene Piping Installation Requirements
U.S. government specifications reference publication AGA XR0603 (2006; 8th Ed) AGA Plastic Pipe Manual for Gas Service for many of the installation requirements and techniques for installing underground plastic pipe.
Underground PE piping has the following code required burial requirements. In many localities, the pipe burial depth is required to be at or below the frost line. All plastic pipe must be installed below grade. Due to concerns about PE piping with respect to UV radiation tolerance, anodeless risers are normally recommended where piping extends above grade. The pipe should be buried a minimum of 3 feet (0.91 m) belowground, and a 14-AWG corrosion-resistant tracer wire should be placed in the pipe trench 6 inches (152.4 mm) over the pipe. Another detection method is to put a warning tape containing metallic material with the words “natural gas” on it. This will allow location by a metal detector and warn of the gas line immediately below the tape if digging takes place without the pipe being located beforehand. NFPA 54 and CFR Part 192 permit shallower pipe burials, but shallower burials are subject to crushing by H20 truck loads.
For mains, CFR 192.327 states a main may be installed with less than 24 inches (610 millimeters) of cover if the law of the state or municipality: establishes a minimum cover of less than 24 inches (610 millimeters); requires that mains be installed in a common trench with other utility lines; and provides adequately for prevention of damage to the pipe by external forces.
For Service Lines: CFR 192.361; each buried service line must be installed with at least 12 inches of cover on private property and at least 18 inches of cover in streets and roads. However, where an underground structure prevents installation at those depths, the service line must be able to withstand any anticipated external load. This conforms to NFPA 54 and IFGC requirements.
In accordance with CFR Part 192 Subpart H, the utility operator must install an excess flow valve (EFV) on any new or replaced service line serving a single-family residence after February 12, 2010, unless one or more of the following conditions is present: The service line does not operate at a pressure of 10 psig (69 kPa) or greater throughout the year; The operator has prior experience with contaminants in the gas stream that could interfere with the EFV’s operation or cause loss of service to a residence; An EFV could interfere with necessary operation or maintenance activities, such as blowing liquids from the line; or An EFV meeting performance standards in CFR paragraph 192.381 is not commercially available to the operator.
The utility operator must mark or otherwise identify the presence of an EFV in the service line. They are required to locate an EFV as close as practical to the fitting connecting the service line to its source of gas supply. ASTM F2138 and ASTM F1802 cover the specification and testing of EFV’s.
Polyethylene Pipe Jointing Procedures
PE piping must be joined using procedures conforming to CFR 192that have been qualified by test in accordance with CFR 192.283 and proven to make satisfactory joints. Personnel making the joints in plastic pipe must be qualified in accordance with CFR 192.285, under the submitted and approved procedure by making a satisfactory specimen joint that passes the required inspection and test. Joints in plastic pipe must be inspected by a person qualified by CFR 192.287 under the applicable procedure. The 2019 CFR now requires heat fusion joints be made in accordance with the procedure in ASTM F2620 incorporated by reference into 192.7.
PE piping design should avoid making indiscriminate heat fusion connections between plastic pipe or fittings made from different PE resins by classification or by manufacturer if other alternative joining procedures are available. If heat fusion joining of dissimilar polyethylene is required, special procedures are required and the joining methods need to be tested and qualified. AGA XR0603 discusses and illustrates the following types of joints: heat fusion, butt fusion, saddle fusion, socket fusion, heat-fusion using resins with differing cell classifications, electrofusion, adhesive (FRP piping), solvent cement (PVC piping), mechanical fittings, flanges, flange adaptors, transition fittings, and other special purpose fittings.
Connections Between Metallic and Plastic Piping
Normally manufactured steel gas transition fittings conforming to AGA XR0603 requirements are used for jointing steel and polyethylene pipe. Mechanical fittings, flanges, flange adaptors, and other proprietary designs may also be utilized as mechanical connectors or transition fittings for both similar and dissimilar pipe. They must satisfy the requirements of CFR 192.273, 192.281, and 192.283, and state codes.
Key Standards and Documents
In addition to discussions of installation and joining techniques, AGA XR0603 discusses: normal coil handling, tube cutting, and cold weather coil handling. From an installation standpoint, Service Connections, Main Connections, Service Head Adaptors, Insert Service Risers (Anodeless Risers), and Excessive Flow Valves/Flow Limiters are discussed.
Standards covering manufacture and installation of PE piping systems include: ASTM D2513 (2012 ae) Polyethylene (PE) Gas Pressure Pipe, Tubing, and Fittings; ASTM D2683 (2014) Standard Specification for Socket-Type Polyethylene Fittings for Outside Diameter-Controlled Polyethylene Pipe and Tubing; ASTM D2774 (2012) Underground Installation of Thermoplastic Pressure Piping; ASTM D3261 (2016) Standard Specification for Butt Heat Fusion Polyethylene (PE) Plastic Fittings for Polyethylene (PE) Plastic Pipe and Tubing; ASTM F1802 (2015) Standard Test Method for Performance Testing of Excess Flow Valves; ASTM F2138 (2012; R 2017) Standard Specification for Excess Flow Valves for Natural Gas Service; ASTM F2786 (2010) Standard Practice for Field Leak Testing of Polyethylene (PE) Pressure Piping Systems Using Gaseous Media Under Pressure (Pneumatic Leak Testing), and ANSI MSS SP-142 (2012) Excess Flow Valves for Fuel Gas Service, NPS 1.5 through 12.
Service connections are the pipes that extend from the local distribution main to the building or facility where the meter and regulator are located. CFR 192.361 and 192.363 state the general material requirements for service lines. CFR 192.273, 192.281, 192.367, 192.369, and 192.375 cover specific requirements of plastic services or their connections to mains of various materials. AGA XR0603 includes procedures, recommendations, and remarks that are intended to illustrate these requirements as they apply to plastic.
Plastic service lines may be connected to metal mains using fittings with compression ends or other transitions connections. Plastic fittings are commonly used to connect plastic services to plastic mains.
Plastic service fittings may be mechanical type, adhesive bonded, solvent cemented, or fusion joined to either or both the main and the service line. Each joint in the service line to main connection must be designed and installed to, “sustain the longitudinal pullout or thrust forces caused by contraction or expansion of the piping or by anticipating external or internal loading” to comply with CFR 192.273. In addition, mechanical type connections to the main must comply with CFR 292.367(b).
When mechanical fittings are used, they should be of the full encirclement type and of a design, which will not damage the pipe. Where used with thermoplastic pipe, the type and grade material, and pipe wall thickness must be considered in determining the suitability of the mechanical service connection. It is important that the joint is designed to prevent rotation on the main.
The connection of the plastic pipe of the service tee has been shown by experience to be a critical junction. Compaction of backfill under the service tee and pipes is of prime importance to provide adequate support and to avoid the potentially high tensile, shear or bending stress on the plastic at this point. Installation of a protective sleeve over this connection can be used in addition to proper backfill to minimize these stresses. Historically, some incidents have been attributed to failures at the connection as detailed in report number SIR-98/01 published by the National Transportation Safety Board. Manufacturer’s instructions should be carefully followed.
In closing, PE piping has had a successful history of use of more than 60 years and has included advancements in standards, regulations, and materials.