Navigating ASME B31
While pipe wall thickness typically will be the limiting factor when designing for pressure, it is still important to verify that the fittings, components, and connections used are appropriate for the design conditions specified.
As a general rule, per Paragraphs 104.2, 104.7.1, 106, and 107, all valves, fittings, and other pressure-containing components manufactured in accordance with the standards listed in Table 126.1 shall be considered suitable for use under normal operating conditions at or below the specified pressure–temperature ratings specified in those standards. The user is cautioned that where certain standards or manufacturers may impose more restrictive allowances for variation from normal operation than those established by ASME B31.1, the more restrictive allowances shall apply.
At piping intersections, it is advisable to use a tee, lateral, cross, branch weld-on fitting, etc., manufactured in accordance with the standards listed in Table 126.1. In some cases a piping intersection may require a unique branch connection. Paragraph 104.3.1 provides additional requirements for branch connections to ensure there is sufficient piping material present to withstand the pressure.
To simplify design, the designer may choose to set design conditions conservatively high to meet the flange ratings of a certain pressure class (e.g. ASME Class 150, 300, etc.) defined by the pressure-temperature ratings for a particular material specified in ASME B16.5 Pipe Flanges and Flanged Fittings, or a similar standard listed in Table 126.1. This is acceptable provided it does not cause an unnecessary increase in wall thickness or other component design.
Piping structural integrity
An important part of piping design is to ensure that the piping system’s structural integrity is maintained once the effects of pressure, temperature, and external forces are applied. System structural integrity is often overlooked during design and can be one of the more costly parts of design if not done properly. Structural integrity is mainly discussed in two places, Paragraph 104.8: Analysis of Piping Components, and Paragraph 119: Expansion and Flexibility.
Paragraph 104.8 lays out the basic code equations for determining if a piping system has exceeded the code stress allowable. Those code equations are commonly referred to as the sustained loads, occasional loads, and displacement loads. The sustained loads are the effects of pressure and weight on a piping system. The occasional loads are the sustained loads plus the addition of wind, seismic, relief, and other short-term loads that might occur. It is assumed that each occasional load applied will not act simultaneously to the other occasional loads, thus each occasional load will be a separate load case when analyzed. The displacement loads are the effects of thermal growth, equipment displacements during operation, or any other displacement loading.
Paragraph 119 discusses how to handle piping expansion and flexibility in a piping system and how to determine reaction loads. A piping system’s flexibility is typically most important at equipment connections, because most equipment connections can withstand only a minimal amount of forces and moments applied at the point of connection. In most cases the thermal growth of the piping system has the highest impact on the reaction loads, so it is important to control thermal growth accordingly in the system.
To meet piping system flexibility and ensure that a system is properly supported, a good practice is to support steel piping in accordance with Table 121.5. If a designer strives to meet this table for standard support spacing, it accomplishes three things: minimizes dead weight deflection, decreases the sustain loads, and increases the available stress allowable for displacement loads. If a designer places the supports based on Table 121.5, it will typically yield dead weight displacement, or sag, less than 1/8-in. between pipe supports. Minimizing dead weight deflection helps reduce the chance of condensation pooling in piping for piping carrying vapor or gas. Following the spacing recommendations of Table 121.5 also allows the designer to reduce the sustained stress in the piping to approximately 50% of code sustained allowable. Per equation 1B, the stress allowable for displacement loads is negatively correlated with the sustained loads. So, by minimizing sustained loads it allows for the displacement stress allowables to be maximized. Suggested pipe support spacing is shown in Figure 3.
To help ensure that the piping system reaction loads are accounted for properly and code stresses are met, a common method is to perform computer-aided pipe stress analysis of the system. There are several different pipe stress analysis packages that can be used, such as Bentley AutoPIPE, Intergraph Caesar II, Piping Solutions Tri-Flex, or one of the other commercially available packages. The advantage to using computer-aided pipe stress analysis is it allows the designer to create a finite element model of the piping system that allows for easy verification and the ability to make necessary changes to a configuration. An example of a section of pipe modeled and analyzed is shown in Figure 4.
When designing a new system, typically the system designer will specify that all piping and components shall be fabricated, welded, assembled, etc., per the requirements of whichever code is being used. However, in certain retrofit or other applications, it may be beneficial for the specifying engineer to provide guidance on certain fabrication techniques, as specified in Chapter V.
A common issue encountered in retrofit applications is preheating of welds (Paragraph 131) and postweld heat treatment (Paragraph 132). Among other benefits, these heat treatments are applied to relieve stress, prevent cracking, and improve strength at the weld. Items affecting pre- and postweld heat treatment requirements include, but are not limited to, the following: P-number grouping, material chemistry, and thickness of the materials at the joint to be welded. Each material listed in Mandatory Appendix A has an assigned P-number. For preheating, Paragraph 131 provides a minimum temperature to which the base metal must be heated before welding can occur. For postweld heat treatment, Table 132 provides a holding temperature range and a length of time at which to hold the weld area. Heating and cooling rates, methods of temperature measurement, heating techniques, and other procedures should closely follow the guidelines specified by code. Unintended adverse effects to the weld area may result due to failure to properly perform heat treatment.
Another potential area of concern in a pressure piping system is pipe bends. Pipe bending can cause wall thinning that can result in insufficient wall thickness. Per Paragraph 102.4.5, pipe bends are allowed by code as long as the minimum wall thickness satisfies the same equations used to calculate minimum wall thickness for straight pipe. Typically, an allowance is added to account for wall thinning. Table 102.4.5 provides recommended bend thinning allowances for different bend radii. Pre- and/or post-bending heat treatment may be required for pipe bends as well. Paragraph 129 provides guidance for the fabrication of pipe bends.
For many pressure piping systems it is necessary to incorporate a safety, or relief, valve to prevent overpressurization in the system. For these applications, Nonmandatory Appendix II: Rules for the Design of Safety Valve Installations is a very valuable and sometimes lesser known resource.
Per Paragraph II-1.2, a safety valve is characterized by full opening pop action and is used for gas or vapor service, while a relief valve opens relative to static upstream pressure and is used primarily for liquid service.
Safety valve installations are characterized by whether they are an open or a closed discharge system. In an open discharge installation, the elbow at the outlet of the safety valve will normally discharge into a vent stack open to atmosphere. Typically, this will result in less back pressure. If enough back pressure is generated in the vent pipe, a portion of the exhaust may discharge, or blowback, from the inlet end of the vent pipe. The vent pipe should be sized large enough to prevent blowback. In a closed discharge application, pressure can build up at the safety valve outlet due to air compressing in the discharge pipe, possibly causing pressure waves to propagate. In Paragraph II-2.2.2 it is recommended that design pressure of the closed discharge pipe be greater than the steady state operating pressure by a factor of at least two. Figures 5 and 6 illustrate an open and a closed safety valve installation, respectively.
A safety valve installation can be subjected to a variety of forces that are summarized in Paragraph II-2. These forces include the effects of thermal expansion, interaction from the simultaneous discharge of multiple safety valves, seismic and/or vibration effects, and the effects of pressure during a relieving event. While the design pressure up to the safety valve outlet should match that of the run pipe, the design pressure in the discharge system is dependent on the configuration of the discharge system and the properties of the safety valve. Equations are provided in Paragraph II-2.2 to determine the pressure and velocity at the discharge elbow, vent pipe inlet, and vent pipe outlet for both an open and a closed discharge system. Using this information, the reaction forces at the various points in the discharge system can be calculated and accounted for.
A sample problem is provided in Paragraph II-7 for an open discharge application. Other methods exist for the calculation of flow properties in a safety valve discharge system, and the reader is cautioned to verify that the method used is sufficiently conservative. One such method is described in “Analysis of Power Plant Safety and Relief Valve Vent Stacks” by G. S. Liao, published by ASME in the Journal of Engineering for Power in October 1975.
Safety valves should be located a minimum distance of straight pipe away from any bends. This minimum distance is dependent on the service and geometry of the system as defined in Paragraph II-5.2.1. For installations with multiple safety valves, recommended spacing of the valve branch connections is dependent on the radii of the branch and run piping as indicated in Note (10)(c) of Table D-1. Per Paragraph II-5.7.1, it may be necessary for pipe supports located on the safety valve discharge to be connected to the run pipe instead of the adjacent structure to minimize the effects of thermal expansion and seismic interaction. A summary of these and other design considerations in the design of safety valve installations can be found in Paragraph II-5.
Obviously, it is impossible to cover all the design requirements of ASME B31 within the confines of this article. But any specifying engineer involved in the design of pressure piping systems should, at a minimum, be familiar with this design code. Hopefully, armed with the information above, the reader will find ASME B31 a more valuable and approachable resource.
Monte K. Engelkemier is a project principal at Stanley Consultants. Engelkemier is a member of the Iowa Engineering Society, NSPE, and ASME, where he serves on the B31.1 Power Piping Code committee and subcommittees. He has more than 12 years of hands-on experience in pipe system layout, design, support assessment, and stress analysis. Matt Wilkey is a mechanical engineer at Stanley Consultants. He has more than 6 years of professional experience in piping system design for various utility, municipal, institutional, and industrial clients and has been a member of ASME and the Iowa Engineering Society.