The codes covering higher pressure natural gas systems in large-scale natural gas distribution systems are as follows: a) CFR-2011 Title 49 Volume 3 Part 192 Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards, and; b) ASME B31.8 Gas Transmission and Distribution Piping Systems.
Although these codes may not be specifically applicable to campus distribution systems, they offer many rules on pipe material selection and regulator selection that would be applicable to “best practice designs” for these smaller systems. Unfortunately, ANSI Z223.1/NFPA 54 National Fuel Gas Code and the International Fuel Gas Codes do not get into the selection of pipeline materials and pipe wall thicknesses.
The owner’s representative needs to work closely with the natural gas supplier and follow their 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 stated above.
ASME B31.8 covers the design, fabrication, installation, inspection, and testing of pipeline facilities used for the transportation of natural gas. This Code also covers safety aspects of the operation and maintenance of those facilities. Mandatory Appendix Q includes scope diagrams. Persons familiar with the ASME piping codes are aware that these codes provide methods for determining the strength and pressure sustaining capabilities of various piping materials and installation requirements. It is not the intent of this document to reduce these codes to two or three paragraphs.
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.
In the September 2019 Plumbing Engineer article on “Natural Gas Pipeline Systems on Site Distribution – Pipeline Materials and Installation Requirements,” the discussion centered on using carbon steel piping for above-ground installations and polyethylene (PE) piping for below-grade installations. From a strength standpoint for low pressure (less than 100 psig / 689 kPa) steel piping has significant excessive strength capacity. PE piping requires more careful examination of pipe strength considerations since PE is not nearly as rigid or strong as steel; plus, underground piping is exposed to the possibility of crushing under earth pressure where above-grade piping is not.
Engineering Controls – Pipeline Pressure
The Plastics Pipe Institute (PPI) Handbook of PE Pipe 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 above, the strength and rigidity of polyethylene 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.
The pressure rating for gas distribution and transmission pipe in U.S. federally regulated applications is determined by CFR Title 49 Part 192. CFR 192.121 requires that the maximum pressure rating (PR) of a PE pipe be determined based on its recommended hydrostatic design stress (HDS) that is equal to the material’s hydrostatic design basis (HDB) times a design factor (DF) of 0.32. (At the time of this writing, there have been discussions to increase the DF to 0.40. In Canada, gas distribution pipe is regulated per CSA Z662-07. CSA allows a design factor of 0.40 to be applied to the HDB to obtain the HDS for gas distribution pipe.
The HDB is determined through a series of studies and time related extrapolations. The development of these HDBs is outlined in Plastic Pipe Institute’s Handbook of PE Pipe. For plastic materials, their long-term working strength for a temperature is determined based on the result of a sustained-stress versus time-to-rupture (i.e. a stress-rupture) evaluation. The standard basis for determining a long-term hydrostatic strength (LTHS) value for PE piping materials is from results of pressure testing in water, or air, at the base temperature of 73°F (23°C). However, many commercial grades of PE materials also have an LTHS that has been determined at an elevated temperature, generally 140°F (60°C).
The LTHS of a PE pipe material is based on its value at 100,000 hours (11.4 years), however, this does not define its design life. The newer high-performance PE pipe materials – for example the PE4710 materials – exhibit no downturn prior to the 50-year intercept. Many evaluations have been conducted regarding the effect of a sustained temperature on a PE’s LTHS. While test results have shown that materials can be affected somewhat differently, they also show that over a range of about 30°F (17°C) above and below the base temperature of 73°F (23°C) the effect is sufficiently similar so that it can be represented by a common set of temperature compensating multipliers.
This is the equivalent of saying that for HDPE pipe meeting the requirements of ASTM D2513, the HDS is 500 psi (3,450 kPa) at 73°F (23°C) and, for MDPE pipe meeting ASTM D2513, the HDS is 400 psi (2,670 kPa) at 73°F (23°C). There are additional restrictions imposed by this Code, such as the maximum pressure at which a PE pipe may be operated (which at the time of this writing is 125 psig (860 kPa) for pipelines installed in location classes 3 and 4 and in distribution systems) and the acceptable range of operating temperatures. The Federal Regulations refer to PPI TR-4 for the HDS for various plastic piping materials at various temperatures. PPI TR-3 provides information for interpolating the information between tested temperature limits.
CFR 192 Design of Natural Gas Piping Pressure Sustaining Capacity
In CFR 192, paragraph 192.121 covers the design of plastic pipe. Paragraph 192.123 covers the design limitations for plastic pipe.
Subject to the limitations of §192.123, the design pressure for plastic pipe is determined by either of the following formulas: CFR 192 para 192.121
P = 2 * S * t * DF / ( D – t )
P = 2 * S * DF / ( SDR – 1 )
P = Design pressure, gauge, psig (kPa).
S = For thermoplastic (PE) pipe, the HDB is determined in accordance with the listed specification at a temperature equal to 73 °F (23 °C), 100 °F (38 °C), 120 °F (49 °C), or 140 °F (60 °C). In the absence of an HDB established at the specified temperature, the HDB of a higher temperature may be used in determining a design pressure rating at the specified temperature by arithmetic interpolation using the procedure in Part D.2 of PPI TR–3/ 2008, HDB/PDB/SDB/MRS Policies (incorporated by reference, see §192.7).
t = Specified wall thickness, inches (mm).
D = Specified outside diameter, inches (mm).
SDR = Standard dimension ratio, the ratio of the average specified outside diameter to the minimum specified wall thickness, corresponding to a value from a common numbering system that was derived from the American National Standards Institute preferred number series 10.
DF = 0.32 (per discussion above)
CFR Design Limitations for Plastic Pipe
The design pressure may not exceed a gauge pressure of 125 psig (862 kPa) for plastic pipe used in: (1) (Low Pressure) Distribution systems; or (2) Classes 3 and 4 locations. When the pipe size is nominal pipe size NPS-12 (DN 300) or less; the material is a PE 2708 or a PE 4710 as specified within ASTM D2513; and the design pressure is determined in accordance with the design equation defined in §192.121.
Plastic pipe may not be used where (ground or) operating temperatures of the pipe will be: (1) below -20°F (-29°C), or -40 °F (-40°C) if all pipe and pipeline components whose operating temperature will be below -20 °F (-29 °C) have a temperature rating by the manufacturer consistent with that operating temperature; or (2) above the temperature at which the HDB used in the design formula under §192.121 is determined.
PE Material Designation
Standards for PE piping define acceptable materials in accordance with a standard designation code. This designation has been designed for the quick identification of the pipe material’s principal structural and design properties. As this section deals with this subject, it is appropriate to first describe the link between the code designations and these principal properties. For this purpose, and as an example, the significance of one designation, PE4710, is next explained.
The letters “PE” designate that it is a polyethylene piping material. The first digit, in this example the number “4,” identifies the PE resin’s density classification in accordance with ASTM D3350, Standard Specification for Polyethylene Plastic Pipe and Fittings Materials (range: 0 to 7). The second digit, in this example the number “7,” identifies the material’s standard classification for slow crack growth resistance – also, in accordance with ASTM D3350 (range: 0 to 8) – relating its capacity for resisting the initiation and propagation of slowly growing cracks when subjected to a sustained localized stress intensification. The third and fourth digits combined, the number “10” in this example, denote the material’s recommended hydrostatic design stress (HDS) for water at 73°F (23°C), in units of 100 psi. In this example the number “10” designates the HDS is 1,000 psi. Unfortunately, the HDS is a water designation and HDB and HDS are not specifically related for natural gas service.
Apparent Pressure Sustaining Capacity of PE Pipe
The following are examples of typical design stresses and service temperatures for PE materials. When considering the PE material to specify, the engineer should consult with prospective PE piping material manufacturers and PPI TR-4 to determine the material specification that will be used based on the system design pressure. Below is data presented by one of the PE pipe manufacturers.
Converting these Hydrostatic Design Bases with the SDR’s, the following design pressure ratings become apparent using equations from CFR 192 para 192.121
Buried PE Pipe Design
PPI Design of PE Piping Systems, Chapter 6 Section 3 describes how to calculate the soil pressure acting on PE pipe due to soil weight and surface loads, how to determine the resulting deflection based on pipe and soil properties, and how to calculate the allowable (safe) soil pressure for wall compression (crushing) and ring buckling for PE pipe. The calculations used basically follow the Iowa Formula for plastic pipe installation. The concern here being that the pipe can be accidently crushed due to soil pressure or imposed surface loads; especially when there is no pressure in the pipe. Also, 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.
Detailed calculations are not always necessary to determine the suitability of a PE pipe for an application. Pressure pipes, that fall within the Design Window given in AWWA M-55 “PE Pipe – Design and Installation” regarding pipe SDR, installation, and burial depth, meet specified deflection limits for PE pipe, have a safety factor of at least 2.0 against buckling, and do not exceed the allowable material compressive stress for PE. Thus, the designer need not perform extensive calculations for pipes that are sized and installed in accordance with the M-55 Design Window.
AWWA M-55 Design Window specifications are:
a) Pipe made from stress-rated PE material.
b) Essentially no dead surface load imposed over the pipe, no ground water above the surface, and provisions for preventing flotation of shallow cover pipe have been provided.
c) The embedment materials are coarse-grained, compacted to at least 85% Standard Proctor Density, and have an E’ of at least 1,000 psi (6.9 MPa). The native soil must be stable; in other words, the native soil must have an E’ of at least 1,000 psi (6.9 MPa).
d) The unit weight of the native soil does not exceed 120 pcf (18.87 kN/m3).
e) The pipe is installed in accordance with manufacturer’s recommendations for controlling shear and bending loads and minimum bending radius and installed in accordance with ASTM D2774 for pressure pipes.
AWWA M-55 Design Window Maximum and Minimum Depth of Cover Requiring No Calculations
The minimum burial depth for Natural Gas piping per CFR 192 is 36" (915 mm) to the top of the pipe, unless it is buried in rock, then 24" (610 mm) is acceptable. CFR 192 further requires that in areas where deep tilling or other activities could threaten the pipeline, the top of the pipeline must be installed at least one foot below the deepest expected penetration of the soil.
Minimum burial depth is 12" to 18" (305 to 457 mm) in IFGC and NFPA 54. ASME B31.8 requires service lines to be installed at a depth that will protect them from excessive external loading and local activities, such as gardening. It is required that a minimum of 12” (300 mm) of cover be provided in private property and a minimum of 18” (460 mm) of cover be provided in streets and roads. AGA recommends a minimum pipe depth for mains to be 24” (610 mm).
If the burial installation falls outside of the AWWA M-55 Design Window, the PPI Design of PE Piping Systems, Chapter 6, Section 3 provides detailed calculations for verifying the capacity of the piping system to resist crushing. Keep in mind, that most of the constraints outlined in the AWWA M-55 Design Window are recommended installation techniques for plastic as well as PE piping.
Installation of 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 that 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 of an appropriate soil support.
Once the piping is installed, if the piping burial depth is less than 36” (914 mm) 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 pipe is generally well restrained by soil loads and will experience very little lateral movement. However, restrained longitudinal pipe end movements and resulting loads that result should be addressed.
Transitions to other pipe materials that use mechanical couplings will need to have reactions calculated due to temperature changes. Generally, fused connections will easily withstand these forces; however, the end connection (the anodeless riser) will need to be restrained.
The size of the anchoring block will vary depending on soil conditions and the thrust load as calculated via the following equations:
σ = E * α * ∆T
F = σ * π / 4 * (Do^2 – Di^2)
F = Force created by expansion/contraction, pounds (Newtons)
σ = Stress in pipe created by the expansion/contraction of the material, psi (N/mm2)
α = thermal expansion coefficient for pipe material, in/in/°F, (mm/mm/°C)
E = Apparent modulus of elasticity for the pipe material, psi (N/mm2)
∆T = Pipe material temperature change, °F (°C)
Do= Outside diameter of the pipe material, inches (mm)
Di = Inside diameter of the pipe material, inches (mm)
Once the forces are determined based on the soil temperatures are expected, then anchor blocks will need to be designed.