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The seven stages of chemical engineering technology from concept to industrialization (Issue 49/100) —— Technology finalization: Pipe material grades. Dear readers: Hello everyone! In the previous issue, we discussed the design of auxiliary pipelines. All pipelines on the PID—main process pipelines, start-up and shutdown pipelines, emergency discharge pipelines, purging and displacement pipelines, and pipelines for handling defective products—have now been drawn. Each pipeline now has information on its diameter and the medium it carries. In this episode, we’ll discuss the “handover” phase between the process engineering discipline and the pipe materials discipline—namely, pipe material grades. In simple terms, it involves determining, based on the characteristics of the medium flowing through the pipe, as well as the operating temperature and pressure, what material to use for each pipe, what wall thickness is required, what type of flanges and gaskets should be used, and whether insulation or trace heating is necessary. Only after this work is completed can the pipelines be said to have a complete “identity information”. Only then can the subsequent tasks of compiling the naming list and tallying materials for the piping discipline proceed. I. Pipeline data sheet: Requirements submitted by the process engineering team to downstream parties. Once the pipeline numbers are assigned, the process engineering team must prepare a pipeline data sheet for each pipeline and submit it to the pipeline materials engineering team. This data sheet serves as the input basis for the grading table prepared by materials specialists. It must contain at least the following information: pipe number, pipe diameter, operating temperature, operating pressure, maximum temperature, maximum pressure, design temperature, design pressure, medium name, and medium composition. There is a concept that is easily confused here: the relationship between operating conditions, maximum conditions, and design conditions. The operating temperature and operating pressure are the actual operating conditions inside the pipeline during normal operation; they can be directly read from the material balance table in the PFD. The maximum temperature and maximum pressure represent the highest values that can occur during the actual operation of the device. These values must be derived from a credible and well-founded \"worst-case scenario,\" rather than being determined by simply adding a certain safety factor arbitrarily. For example, for the overhead piping of a distillation column, the maximum operating pressure is usually taken as the overhead pressure when the cooling water supply to the condenser is interrupted; this pressure can be calculated through process simulation. The design temperature and design pressure are determined by adding a margin to the maximum operating conditions. Typically, the design pressure is set at approximately 1.1 times the maximum pressure, while the design temperature is obtained by adding a certain margin to the maximum temperature. The value of this margin must take into account the extent of operational fluctuations and the response accuracy of the control system. I’ve seen more than one project where operating conditions, maximum conditions, and design conditions were used interchangeably, or where the design conditions were set too conservatively. As a result, the pipe wall thickness became excessive and the flange rating was set too high, leading to increased material and installation costs. Conversely, if the design criteria are not properly met—such as by ignoring the special conditions during operation—the pipeline may experience overpressure or overheating under abnormal operating conditions. The selection of design parameters reflects engineering judgment; it must be based on evidence and analysis, rather than being decided arbitrarily. II. Index Table of Pipe Material Grades. In engineering firms or design institutes, pipe materials constitute a relatively independent and important specialty. Based on the pipe data sheets provided by the process engineering department, the materials department classifies the pipes and prepares an index table of pipe material grades. What is specified in the grading table? Each pipeline is assigned to a specific material grade based on the corrosivity of the medium, as well as the operating temperature and pressure. This grade specifies a series of selection criteria such as pipe specifications, wall thickness standards, flange types, gasket types, and bolt specifications. A grade is essentially a “material pack”; selecting a grade means choosing the specifications for the entire set of connectors. A complete set of material grade documents usually consists of three levels. The first layer is the index table – it lists all the material grades involved in this project along with their areas of application; it serves as a catalog and an overview of the detailed grade list. The second layer is a detailed grade table – each grade corresponds to a detailed table that lists the specific specifications and material requirements for all fittings, valves, flanges, gaskets, and bolts at that grade. The third layer is the connection table – it specifies how pipes of different diameters should be connected, and under what circumstances butt welding, socket welding, or threaded connections should be used. During the process package stage, it is actually sufficient to complete the material grade index table. The index table lists the material grades required for this project and the scope of application for each grade – for example, grade M1B is used for process fluid pipelines under normal temperature and pressure conditions, while grade M2A is used for steam pipelines under high temperature and pressure conditions. The detailed grading and take-over tables can be left for further development by the piping materials specialist during the basic design phase. The purpose of completing the index table at the process package stage is to enable the piping discipline to prepare the piping naming table based on the piping data tables and material grade index tables provided in the process package, thereby providing a basis for subsequent detailed design. Do not underestimate this step – if the material grade is not determined correctly, subsequent stress analysis, material statistics, and procurement specifications for the piping system will all be based on incorrect assumptions. III. Several Considerations for Material Selection There are three key factors to consider when selecting pipeline materials. The first is the corrosivity of the medium. The corrosive components present in the medium, such as acids, bases, salts, sulfides, and chlorides, determine the basic direction for selecting the material. Carbon steel is suitable for most organic media and utility process media at normal temperature and pressure ； Stainless steel is used in environments with acidic media or where a clean environment is required ； For highly corrosive media—wet chlorine, concentrated sulfuric acid, and hydrofluoric acid—it is necessary to consider special alloys, lined pipes, or non-metallic pipes. The corrosion data for each component in the material properties table serves as the basis for selecting the appropriate material. The second is operating temperature and pressure. The higher the temperature, the lower the allowable stress of the material, and the wall thickness needs to be increased accordingly. Temperature also affects the corrosion rate—as the same medium causes only mild corrosion of carbon steel at room temperature, its corrosion rate can increase significantly at high temperatures. Pressure determines the wall thickness of the pipe and the flange class – for the same diameter, if the design pressure increases from 1.0 MPa to 4.0 MPa, the flange class may change from PN16 to PN63, resulting in a significant increase in cost and weight. Third is the material compatibility and welding process. The materials of fittings, valves, flanges, gaskets, and bolts on the same pipeline must be compatible with each other. Stainless steel pipes are fitted with carbon steel flanges; welding dissimilar metals requires special welding techniques and welding materials. The material selection for gaskets and bolts also needs to be matched: high-temperature conditions require heat-resistant winding gaskets and high-temperature bolt materials, while low-temperature conditions demand attention to the material’s low-temperature impact toughness. There is a detail that is easily overlooked: the material of the pipe must match the material of the pipe ends of the connected equipment. If stainless steel is chosen for the pipeline material, while the pipe ends of the connected equipment are made of carbon steel, a connection between different types of steel is created. The requirements for transition fittings must be specified in the pipeline material grade table, to prevent mismatches in materials being discovered during installation. IV. Pipeline Insulation Design: While determining the material grades, the process engineering team must simultaneously complete the pipeline insulation design. Adiabatic design includes four types: insulation, heat retention, heat tracing, and scald protection. Insulation is used to reduce heat loss and is applicable to pipes whose operating temperature is higher than the ambient temperature. The selection of insulation material and thickness requires a comprehensive consideration of energy-saving benefits and insulation costs – the thicker the insulation, the greater the energy savings, but the higher the investment and the more space it occupies in the ductwork. There is a concept of economic thickness, at which the sum of the annual discounted cost of insulation investment and the heat loss cost is minimized. Insulation is used to reduce heat loss and prevent condensation outside the pipes, and it is applicable to pipes whose operating temperature is lower than the ambient temperature. The biggest concern in cold-insulation design is the penetration of moisture into the insulation layer. The sealing performance of the insulation layer must be much higher than that of the thermal insulation layer; once gaps appear, water vapor from the air can penetrate and condense, or even freeze, on the walls of the cold pipes, causing the thermal insulation layer to lose its effectiveness very quickly. The closed-cell content and water absorption rate of insulation materials are important criteria when selecting materials. Heat tracing is designed for pipelines carrying materials with high freezing points, high viscosity, or those that are sensitive to temperature. Steam tracing and electric tracing are the two main types of heating methods. Steam tracing has a low cost, but the temperature control is not very precise; it is suitable for applications where temperature requirements are not strict. Electric heat tracing offers precise temperature control and easy adjustment, making it suitable for systems that are sensitive to temperature. Special attention must be paid to the usage limitations of the medium in the heat tracing design. For example, with certain materials that have a high freezing point, the heating temperature must not be too close to this freezing point; otherwise, even minor fluctuations in operation can cause blockages in the pipes. At the same time, for some media, not only the lowest temperature but also the highest tolerable temperature must be taken into account – excessively high heating temperatures can instead lead to the decomposition or polymerization of the material. The basis for the heat tracing design is the temperature operating range of the material, which should be determined during the phase of collecting material property data. For pipes with a heat-resistant insulation system operating at temperatures above 60 degrees, such insulation must be installed if they are in areas where people might come into contact with them. The purpose of heat protection and insulation is not energy savings, but the protection of people’s safety. The thickness of heat-resistant insulation is usually determined by the requirement that the temperature of the outer surface not be hot enough to cause burns, and there is no need to pursue a thickness that minimizes costs as is the case with ordinary insulation. Once the insulation design is completed, the insulation class for each pipeline is determined. The insulation class and material class are indicated together in the pipe code, forming the complete pipe code. Taking “P-10103-50-M1B-H” as an example – it is used for transporting process materials; the pipeline sequence number is 10103, the diameter is 50, the material grade is M1B, and the insulation grade is H. This complete pipeline number is indicated on each pipeline of the PID, and it serves as the unique identification for subsequent design, procurement, and installation work related to piping systems. V. Additional requirements for pipelines carrying special media: Some special media impose additional requirements on the grade of pipeline materials; they cannot be handled using conventional methods, and it is necessary to identify them at the process package stage. Pipelines for transporting oxygen require extremely high levels of cleanliness and must undergo degreasing treatment. Fittings and valves must not use oil-containing materials or lubricants; gaskets should not be made of materials with a high organic content. Fluoroplastic gaskets or wound gaskets without organic binders are typically used for flange gaskets. Oxygen pipelines also have flow rate limits; excessive flow rates can pose safety risks. When selecting the pipe diameter, it is necessary to ensure that the flow rate remains within safe limits. Pipelines for transporting hydrogen or hydrogen-containing media: under high temperature and pressure, hydrogen can cause hydrogen corrosion or hydrogen embrittlement in steel. The selection of material grade must take into account the requirements for resistance to hydrogen corrosion; where necessary, chromium-molybdenum alloy steel should be used instead of ordinary carbon steel. In pipelines carrying solid materials, if the flow rate is too low, solids tend to settle and cause blockages; whereas if the flow rate is too high, the pipe bends and valves are prone to erosion and wear. For elbows, large-radius elbows or wear-resistant elbows should be chosen; for valves, wear-resistant ball valves or plug valves are appropriate. The wall thickness of the pipes may need to have an additional corrosion allowance to withstand prolonged erosion. VI. Relationship between pipeline material grade and PID. The determination of pipeline material grades and the creation of PID should be an interactive process. The pipe number indicated on the PID includes information on the material grade. If the material grade changes – for example, if a pipe’s corrosivity assessment is upgraded due to a re-evaluation of the medium components, or if the material needs to be changed from stainless steel to a special alloy – the material grade indicated in the pipe designation must be updated, and the markings on the PID must be modified accordingly. Additionally, the applicable range of material grades is visually reflected on the PID. Pipes of the same material grade are usually grouped in the same process area on the PID. If a pipe uses a different material grade, it must be clearly indicated on the PID to facilitate identification during construction and installation. Only after the pipeline material grade index table is completed can the identity information of each pipeline on the PID be considered truly complete – with the diameter, material grade, and insulation grade combined into one set of data. This provides a unified data basis for all subsequent tasks related to pipeline management, such as the pipeline naming table, material statistics, stress analysis, and procurement specifications. Preview for the next issue: Issue 50 – Instrumentation and interlock design: The interlock logic diagrams are developed by the process engineering team. Once the pipe material grades are determined, each pipe in the PID system gets its own \"material identification tag\". Next, we move on to the aspects related to instruments and control within the sixteenth core task: instrument tag assignment, instrument data sheets, and interlock logic diagram design. Why must the interlock logic diagrams be led by the process engineering team, rather than being left to the instrumentation team? How to draw the cause-and-effect relationships, logic gates, and reset methods on an interlock logic diagram? To be continued in the next issue.