Design Inherently Safer Piping

Several regulations and standards should be considered when you are designing a piping system. This article highlights elements of these codes and standards that are commonly overlooked.

Over two decades working with and for many upstream and midstream engineering and operating companies, I have often seen inadequately designed onshore piping (for both gases and liquids). Although piping may be designed to have sufficient wall thickness for its maximum internal operating pressure, other factors, such as corrosion allowance, manufacturing tolerance, and thread depth, are frequently overlooked. This article discusses some of these factors that are important to onshore metal piping stress and overpressure protection.

The article reviews several regulations in the U.S. Code of Federal Regulations (CFR) and standards of the American Society of Mechanical Engineers (ASME) and American Petroleum Institute (API) that should be followed, and details how these requirements relate to each other. The article also presents some considerations beyond the regulations and standards that should be taken into account.

Designing natural gas piping

When designing natural gas piping, one of the first steps is to determine whether the pipeline will be used for production, gathering, transmission, or distribution. The piping is classified as production from the source to the limits of the production facility. API RP 80, “Guidance for the Definition of Onshore Gas Gathering Lines” (1), specifies how a pipeline will be classified after leaving the production facility: A pipeline is considered a gathering pipeline up to the first natural gas processing plant. After it leaves the plant, the pipeline is considered a transmission pipeline. Between the transmission line and individual users, it is considered a distribution pipeline.

The next step is to determine the class location according to 49 CFR 192, “Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards” (2). Class locations, which range from 1 for lowest risk to 4 for highest risk, help categorize risk based on the number of occupied buildings within 220 yards of each side of a one-mile-long pipe segment. A Class 1 location has 10 or fewer buildings intended for human occupancy per unit, while a Class 4 location has a prevalence of buildings with four or more stories above ground.

If the piping is a gathering pipeline, determine whether it is regulated by considering the stress tangent to the circumference of the pipe (known as hoop stress), the class location, and the required safety buffer (i.e., additional distance added to protect nearby populations), in accordance with Ref. 2. A table in Section 192.8(b) of Ref. 2 can help you determine whether the piping will be regulated. Follow Ref. 2 for transmission and regulated gathering piping.

Next, using the commonly available wall thicknesses, calculate the internal design pressure for steel piping (2):

where P is the internal design pressure, S is the yield strength, t is the nominal wall thickness of the pipe, D is the nominal outside diameter of the pipe, F is a design factor, E is the longitudinal joint factor, and T is a temperature derating factor. F, E, and T can all be found in Ref. 2.

Some facility locations and applications require more conservative design factors. For example, a design factor (F) of 0.60 or less must be used if the piping crosses a public road without a casing (i.e., external protection) or if the piping is supported by a vehicular or railroad bridge (2). Reference 2 has a wealth of information on these specific situations.

Reference 2 requires pipe to be designed with sufficient wall thickness, or installed with adequate protection, to withstand anticipated external pressures and loads that will be imposed on the pipe after installation. However, it does not provide any equations or methods to satisfy this requirement, only equations for internal pressure. External pressure is one factor that designers sometimes fail to take into account.

Other requirements that are commonly overlooked in the design of natural gas piping include:

• Threaded fittings on piping must meet a minimum metal thickness.
• Each pipeline must be designed with enough flexibility to prevent thermal expansion or contraction from causing excessive stresses in the pipe or components, excessive bending or unusual loads at joints, and undesirable forces or moments at points of connection to equipment or at anchorage or guide points.
• Each pipeline and its associated equipment must have enough supports or anchors to prevent undue strain on connected equipment, to resist longitudinal forces caused by a bend or offset in the pipe, and to prevent or damp out excessive vibration.
• Each exposed pipeline must have enough supports or anchors to protect the exposed pipe joints from the maximum end force caused by internal pressure and any additional forces caused by thermal expansion or contraction or by the weight of the pipe and its contents.
• Each support or anchor on an exposed pipeline must be made of durable, noncombustible material.

It is important to note that Ref. 2 specifies minimum federal standards, not best practices.

Designing liquid piping

One of the first steps in designing safer hazardous liquid piping is determining whether the piping is required to comply with 49 CFR Part 195, “Transportation of Hazardous Liquids by Pipeline” (3). For example, if the piping has a maximum operating pressure (MOP) greater than 20% of the specified minimum yield strength, or if the piping is carrying petroleum in a nonrural area, it must comply with this standard.

The internal design pressure for steel piping is calculated by (3):

where F_s is a design factor equal to 0.72 for steel and E_s is the seam joint factor.

Although this equation is very similar to Eq. 1, the joint factor (E or E_s) will vary depending on the specification or code requirements and the application. Table 1 highlights some of the differences between 49 CFR 192 (2), 49 CFR 195 (3), and ASME B31.8, “Gas Transmission and Distribution Piping Systems” (4). It is important to use the correct joint factor when calculating the internal design pressure for steel piping because it will be different depending on the specification and the pipe class.