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When designing a pressure piping system, the designated engineer usually points out that the system piping should comply with one or more parts of the ASME B31 pressure piping code. How do engineers correctly follow the specification requirements when designing piping systems?
First, the engineer must determine which design code should be selected. For pressure piping systems, this is not necessarily limited to ASME B31. Other specifications issued by ASME, ANSI, NFPA or other governing organizations may depend on project location, application, etc. In ASME B31, there are currently seven active independent parts.
ASME B31.1 Power Piping: This section covers piping in power stations, industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems. This includes boiler exterior and non-boiler exterior piping used to install ASME Part I boilers. This section does not apply to equipment covered by the ASME Boiler and Pressure Vessel Code, certain low-pressure heating and cooling distribution piping, and various other systems shown in paragraph 100.1.3 of ASME B31.1. The origin of ASME B31.1 can be traced back to the 1920s, and the first official version was published in 1935. It should be noted that the first edition (including appendices) is less than 30 pages, while the current version is more than 300 pages long.
ASME B31.3 Process Piping: This section covers piping in petroleum refineries; chemical, pharmaceutical, textile, paper, semiconductor and cryogenic plants; and related processing plants and terminals. This section is very similar to ASME B31.1, especially when calculating the minimum wall thickness of straight pipes. This part was originally part of B31.1 and was first published separately in 1959.
ASME B31.4 Liquid and Mud Pipeline Transportation System: This section covers the pipeline system that transports mainly liquid products between the factory and the wharf, as well as in the wharf, pumping station, regulating station and metering station. This part was originally part of B31.1 and was first published separately in 1959.
ASME B31.5 Refrigeration piping and heat transfer components: This section covers refrigerant and secondary coolant piping. This part was originally part of B31.1 and was first published separately in 1962.
ASME B31.8 Gas Transmission and Distribution Piping System: This includes pipeline transportation products that are mainly gas between the source and the terminal, including compressors, conditioning and metering stations; and gas gathering pipelines. This part was originally part of B31.1 and was first published separately in 1955.
ASME B31.9 Building Service Piping: This section covers common pipes in industrial, institutional, commercial and public buildings; and multi-unit residential buildings, which do not require the size, pressure and temperature ranges covered in ASME B31.1. This section is similar to ASME B31.1 and B31.3, but is less conservative (especially when calculating minimum wall thickness) and contains less details. It is limited to the low-pressure, low-temperature applications specified in paragraph 900.1.2 of ASME B31.9. The book was first published in 1982.
ASME B31.12 Hydrogen pipelines and pipelines: This section covers pipelines in gaseous and liquid hydrogen services, as well as pipelines in gaseous hydrogen services. This section was first published in 2008.
The decision of which design code should be used ultimately rests with the owner. The introduction of ASME B31 states that “the owner is responsible for selecting the part of the specification that is closest to the recommended plumbing installation.” In some cases, “multiple code parts may apply to different parts of the installation.”
The 2012 edition of ASME B31.1 will be used as the main reference for subsequent discussions. The purpose of this article is to guide the designated engineer to complete some of the main steps of designing a pressure piping system that complies with ASME B31. Following the guidelines of ASME B31.1 can well represent the general system design. If ASME B31.3 or B31.9 is followed, a similar design method is used. The rest of ASME B31 is used for narrower applications, mainly for specific systems or applications, and will not be discussed further. Although the key steps in the design process will be highlighted here, this discussion is not exhaustive, and the complete code should always be referenced during system design. Unless otherwise stated, all references to the text refer to ASME B31.1.
After selecting the correct code, the system designer must also review any system-specific design requirements. Section 122 (Part 6) provides design requirements related to common systems in power piping applications, such as steam, water supply, venting and venting, instrumentation piping, and pressure reduction systems. ASME B31.3 contains a paragraph similar to ASME B31.1, but with fewer details. The precautions in section 122 include the specific pressure and temperature requirements of the system, as well as the definition of various jurisdictional restrictions drawn between the boiler itself, boiler external piping, and non-boiler external piping for connection to ASME Part I boilers Pipeline. Figure 2 shows these limitations of drum boilers.
The system designer must determine the operating pressure and temperature of the system, and the conditions that the system design should meet.
According to Article 101.2, the internal design pressure shall not be lower than the maximum continuous working pressure (MSOP) in the piping system, including the influence of the static pressure head. Piping subjected to external pressure should be designed for the maximum differential pressure expected under operating, closed, or test conditions. In addition, environmental impact needs to be considered. According to Article 101.4, if the cooling of the fluid may reduce the pressure in the pipeline below atmospheric pressure, the pipeline should be designed to withstand external pressure, or measures should be taken to break the vacuum. If the expansion of the fluid may increase the pressure, the piping system should be designed to withstand the increased pressure, or measures should be taken to relieve the overpressure.
According to paragraph 101.3.2, the pipeline should be designed to represent the metal temperature of the expected maximum sustained condition. For simplicity, it is usually assumed that the metal temperature is equal to the fluid temperature. If desired, the average metal temperature can be used, as long as the outer wall temperature is known. Special attention should also be paid to the fluid passing through the heat exchanger or from the combustion equipment to ensure that the most severe temperature conditions are taken into account.
Usually, the designer will add a safety margin to the maximum working pressure and/or temperature. The size of the margin depends on the application. When determining the design temperature, it is also important to consider material constraints. Specifying high design temperatures (greater than 750 F) may require the use of alloy materials instead of more standard carbon steel. The stress values in the mandatory appendix A are only provided for the allowable temperature of each material. For example, the stress value of carbon steel only provides up to 800 F. Long-term exposure of carbon steel to temperatures above 800 F may cause carbonization of the pipe, making it more brittle and prone to failure. If operating above 800 F, the accelerated creep damage associated with carbon steel should also be considered. For a complete discussion of material temperature limits, see paragraph 124.
Sometimes, specifying the test pressure for each system is also within the engineer's range. Paragraph 137 provides guidance on stress testing. Under normal circumstances, it is stipulated that the hydraulic test should be carried out at 1.5 times the design pressure; however, the hoop and longitudinal stresses in the pipeline should not exceed 90% of the material's yield strength during the pressure test in section 102.3.3 (B). For some non-boiler external piping systems, because it is difficult to isolate certain parts of the system, or simply because the system configuration allows for easy leak testing during initial service, in-service leak testing may be a more practical way to check for leaks. In the owner and With the agreement of the engineer, this is acceptable.
Once the design conditions are established, the pipeline can be specified. The first thing to determine is what material to use. As mentioned earlier, different materials have different temperature limits. Paragraph 105 provides additional restrictions on various piping materials. Material selection also depends on the system fluid, such as nickel alloys for corrosive chemical piping applications, stainless steel to transport clean instrument air, or carbon steel with high chromium content (greater than 0.1%) to prevent flow accelerated corrosion. Flow Accelerated Corrosion (FAC) is an erosion/corrosion phenomenon that has been shown to cause severe wall thinning and pipeline failures in some of the most critical piping systems. Failure to properly consider the possible thinning of pipeline components has already led to serious consequences. For example, in 2007, the desuperheating water pipeline of KCP&L's IATAN power station ruptured, causing the death of two workers and the third serious injury.
Equations 7 and 9 in paragraph 104.1.1 define the minimum wall thickness and maximum internal design pressure required in a straight pipe with internal pressure, respectively. The variables in these equations include the maximum allowable stress (from Mandatory Appendix A), the outer diameter of the pipe, the material coefficient (as given in Table 104.1.2 (A)), and any additional thickness allowance (described below). With so many variables involved, specifying the appropriate pipe material, nominal diameter, and wall thickness may be an iterative process. It may also include fluid velocity, pressure drop, and piping and pumping costs. Regardless of the application, the required minimum wall thickness must be verified.
Additional thickness allowance may be added to compensate for a variety of reasons, including FAC. Due to the threads, grooves, etc. required to manufacture mechanical joints, it may be necessary to consider the allowance for material removal. According to Article 102.4.2, the minimum tolerance shall be equal to the thread depth plus the machining tolerance. It may also be necessary to allow margin to provide additional strength to prevent damage, collapse, excessive sag or buckling of the pipeline due to superimposed loads or other reasons discussed in paragraph 102.4.4. Margins can also be added to account for welded joints (paragraph 102.4.3) and elbows (paragraph 102.4.5). Finally, the margin can be increased to compensate for corrosion and/or erosion. According to Article 102.4.1, the thickness of this margin is judged by the designer and should be consistent with the expected life of the pipeline.
Non-mandatory Appendix IV provides guidelines for controlling corrosion. Protective coatings, cathodic protection, and electrical insulation (for example, insulating flanges) are all methods to prevent external corrosion of buried pipes or underwater pipelines. Corrosion inhibitors or linings can be used to prevent internal corrosion. Attention should also be paid to the use of water pressure test water of appropriate purity and, if necessary, to completely empty the pipeline after the water pressure test.
The minimum pipe wall thickness or plan required by previous calculations may not be constant within the pipe diameter range, and different plans may need to be specified for different diameters. The corresponding schedule and wall thickness values are defined in ASME B36.10 welded and seamless forged steel pipes.
When specifying the pipe material and performing the calculations discussed earlier, it is important to ensure that the maximum allowable stress value used in the calculation matches the specified material. For example, if A312 304L stainless steel pipe is incorrectly specified instead of A312 304 stainless steel pipe, the provided wall thickness may be insufficient due to the significant difference in the maximum allowable stress value between the two materials. Similarly, the manufacturing method of the pipeline should also be appropriately specified. For example, if the maximum allowable stress value of seamless pipe is used for calculation, seamless pipe should be specified. If not, the manufacturer/installer may provide seam welded pipe, which may result in insufficient wall thickness due to the lower maximum allowable stress value.
For example, suppose the pipe is sized for water with a design temperature of 300 F and a design pressure of 1,200 psig. 2 inches and 3 inches. Carbon steel (A53 B grade seamless) wire will be used. Determine the appropriate piping schedule to meet the requirements of ASME B31.1 Formula 9. First, specify the design conditions:
Next, determine the maximum allowable stress value of Class A53 B at the above specified design temperature from Table A-1. Please note that the value of seamless tube is used because seamless tube will be specified:
A thickness allowance must also be added. For this application, 1/16 inch. Assumed corrosion allowance. A separate milling tolerance will be added later.
The value of y is determined by Table 104.1.2(A):
3 inches. The pipeline will be specified first. Assuming a No. 40 pipe and a 12.5% milling tolerance, calculate the maximum pressure:
The wall thickness of 40 pipe is suitable for 3 inches. The pipeline under the above design conditions. Next, check the 2-in. The pipeline uses the same assumption:
2 inches. Under the design conditions specified above, the pipe will require a thicker wall thickness than Schedule 40. Try 2 inches. Schedule 80 tube:
Schedule 80 pipe is suitable for 2 inches. The pipeline under the above design conditions.
Although pipe wall thickness is usually the limiting factor for pressure design, it is still important to verify that the fittings, components, and connections used are suitable for the specified design conditions.
As a general rule, in accordance with paragraphs 104.2, 104.7.1, 106 and 107, all valves, fittings and other pressure-bearing parts manufactured according to the standards listed in Table 126.1 shall be considered suitable for use under normal operating conditions or below these The specified pressure-temperature rating specified in the standard. Users should be aware that if certain standards or manufacturers may impose stricter limits on deviations from normal operation than those specified in ASME B31.1, the stricter limits should apply.
At the intersection of pipelines, it is recommended to use tee, horizontal, cross, and branch welding fittings manufactured in accordance with the standards listed in Table 126.1. In some cases, pipeline intersections may require unique branch connections. Section 104.3.1 provides additional requirements for branch connections to ensure that there is sufficient piping material to withstand the pressure.
In order to simplify the design, the designer can choose to set the design conditions conservatively to meet the flange level of a specific pressure level (such as ASME level 150, 300, etc.), which is determined by the pressure-temperature level of the specific material specified in ASME B16 Definition. 5 Pipe flanges and flange fittings, or similar standards listed in Table 126.1. This is acceptable, provided it does not cause unnecessary increases in wall thickness or other component design.
An important part of piping design is to ensure that the structural integrity of the piping system is maintained under the influence of pressure, temperature and external forces. The structural integrity of the system is often overlooked in the design process. If it is not done well, it may become one of the more costly parts of the design. Structural integrity is mainly discussed in two places, paragraph 104.8: pipe component analysis, and paragraph 119: expansion and flexibility.
Paragraph 104.8 lists the basic code equations used to determine whether the piping system exceeds the code allowable stress. These code equations are commonly referred to as continuous loads, incident loads, and displacement loads. Continuous load is the effect of pressure and weight on the piping system. Incidental loads are continuous loads plus wind loads, seismic loads, terrain loads and other possible short-term loads. It is assumed that each incidental load applied will not act on other incidental loads at the same time, so each incidental load will be a separate load case in the analysis. Displacement load is the effect of thermal growth, equipment displacement during operation, or any other displacement load.
Paragraph 119 discusses how to deal with pipeline expansion and flexibility in piping systems, and how to determine reaction loads. The flexibility of the piping system is usually the most important at the equipment connection, because most equipment connections can only withstand the least amount of force and moment applied at the connection point. In most cases, the thermal growth of the piping system has the greatest impact on the reaction load, so it is very important to control the thermal growth in the system accordingly.
In order to meet the flexibility of the piping system and ensure that the system is properly supported, it is good practice to support the steel pipe in accordance with Table 121.5. If the designer strives to meet the standard support spacing of this table, it will accomplish three things: minimize the deflection under its own weight, reduce the support load, and increase the allowable available stress for the displacement load. If the designer places a support according to Table 121.5, it will usually produce a displacement or sag of less than 1/8 inch of its own weight. Between the tube supports. Minimizing the self-weight deflection helps to reduce the chance of condensation pools in pipes carrying steam or gas. Following the spacing recommendations in Table 121.5 also allows the designer to reduce the continuous stress in the pipeline to approximately 50% of the continuous allowable specification. According to Equation 1B, the allowable stress of the displacement load is negatively related to the continuous load. Therefore, by minimizing the continuous load, the maximum displacement stress is allowed. The recommended spacing between pipe supports is shown in Figure 3.
To help ensure that the reaction loads of the piping system are properly considered and the specified stresses are met, a common method is to perform computer-aided piping stress analysis on the system. Many different pipe stress analysis software packages can be used, such as Bentley AutoPIPE, Intergraph Caesar II, Piping Solutions Tri-Flex or one of the other commercially available software packages. The advantage of using computer-aided pipeline stress analysis is that it allows the designer to create a finite element model of the pipeline system in order to facilitate verification and the ability to make necessary changes to the configuration. Figure 4 shows an example of modeling and analysis of a section of pipeline.
When designing a new system, the system designer usually specifies that all piping and components should be manufactured, welded, assembled, etc. in accordance with the requirements of any specifications used. However, in certain transformations or other applications, it may be beneficial to appoint an engineer to provide guidance on certain manufacturing techniques, as described in Chapter 5.
A common problem encountered in retrofit applications is weld preheating (paragraph 131) and post-weld heat treatment (paragraph 132). Among other benefits, these heat treatments can also be used to relieve stress, prevent cracking, and improve weld strength. The items that affect the pre- and post-weld heat treatment requirements include but are not limited to the following: P number grouping, material chemical composition, and material thickness at the joint to be welded. Each material listed in the mandatory Appendix A has an assigned P number. For preheating, Article 131 specifies the minimum temperature to which the base metal must be heated before welding. For post-weld heat treatment, Table 132 provides the holding temperature range and length of time for holding the welded area. Heating and cooling rates, temperature measurement methods, heating techniques, and other procedures should strictly follow the guidelines stipulated by the specification. Failure to perform heat treatment correctly may have unexpected adverse effects on the welding area.
Another area of potential concern in pressure piping systems is elbows. The bending of the pipe will cause the wall thickness to become thinner, resulting in insufficient wall thickness. According to paragraph 102.4.5, as long as the minimum wall thickness satisfies the same formula used to calculate the minimum wall thickness of straight pipes, the code allows pipe bends. Usually, a margin is added to account for the thinning of the wall thickness. Table 102.4.5 provides the recommended bending refinement allowance for different bending radii. Bending pipes may also require heat treatment before and/or after bending. Paragraph 129 provides guidelines for the manufacture of elbows.
For many pressure piping systems, a safety valve or pressure relief valve must be included to prevent overpressure in the system. For these applications, non-mandatory Appendix II: Safety Valve Installation Design Rules is a very valuable but sometimes little-known resource.
According to paragraph II-1.2, the safety valve is characterized by a fully open pop-up action for gas or steam service, while the safety valve is opened relative to the upstream static pressure and is mainly used for liquid service.
The characteristic of safety valve devices is whether they are an open discharge system or a closed discharge system. In an open discharge device, the elbow at the outlet of the safety valve is usually discharged into the exhaust pipe to the atmosphere. Generally, this will result in less back pressure. If sufficient back pressure is generated in the exhaust pipe, part of the exhaust gas may be discharged or blown back from the inlet end of the exhaust pipe. The size of the exhaust pipe should be large enough to prevent blowback. In closed discharge applications, the pressure at the outlet of the safety valve will increase due to the compression of the air in the discharge pipe, which may cause pressure waves to propagate. In Article II-2.2.2, it is recommended that the design pressure of the closed discharge pipe be at least twice as large as the steady-state working pressure. Figure 5 and Figure 6 respectively show the opening and closing of the safety valve installation.
Safety valve installation may be subject to various forces summarized in paragraph II-2. These forces include the effects of thermal expansion, the interaction of simultaneous discharge of multiple safety valves, the effects of earthquakes and/or vibrations, and the effects of pressure during pressure relief events. Although the design pressure to the outlet of the safety valve should match the operating pipe, the design pressure in the exhaust system depends on the configuration of the exhaust system and the performance of the safety valve. Paragraph II-2.2 provides equations for determining the pressure and velocity at the discharge elbow, discharge pipe inlet, and discharge pipe outlet of open and closed discharge systems. Using this information, the reaction forces at various points in the exhaust system can be calculated and accounted for.
An example question is provided in paragraph II-7 for open emission applications. There are other methods for calculating the flow characteristics in the discharge system of the safety valve. Readers should pay attention to verifying whether the method used is conservative enough. In October 1975, ASME published GS Liao's "Analysis of Power Plant Safety and Pressure Relief Valve Exhaust Pipe" in the "Power Engineering Journal" describing one such method.
The safety valve should be located at the minimum distance away from any straight pipe elbows. The minimum distance depends on the service and geometry of the system defined in paragraph II-5.2.1. For installations with multiple safety valves, the recommended valve branch pipe connection distance depends on the radius of the branch pipe and pipeline, as shown in Note (10)(c) of Table D-1. According to Article II-5.7.1, it may be necessary to connect the pipe support at the discharge port of the safety valve to the running 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 devices can be found in paragraph II-5.
Obviously, it is impossible to cover all the design requirements of ASME B31 within the scope of this article. However, any designated engineer involved in the design of pressure piping systems should at least be familiar with this design specification. I hope that through the above information, readers will find that ASME B31 is a more valuable and easier to obtain resource.
Monte K. Engelkemier is the project leader of Stanley Consultants. Engelkemier is a member of the Iowa Engineering Society, NSPE and ASME, and he serves on the B31.1 Power Pipeline Code Committee and Subcommittee. He has more than 12 years of practical experience in piping system layout, design, support evaluation and stress analysis. Matt Wilkey is a mechanical engineer at Stanley Consultants. He has more than 6 years of professional experience in designing piping systems for various utility, municipal, institutional and industrial customers, and is a member of ASME and the Iowa Institute of Engineering.
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