The seven stages of chemical technology from concept to industrialization (Issue 65/100) -- 3D design

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Seven stages of chemical engineering technology from concept to industrialization (Issue 65/100) —— Engineering translation: Pipe stress analysis and 3D piping design. Dear readers: Hello! In the previous issue, we discussed the overall aspects of detailed design, mentioning that pipe stress analysis and 3D piping design are the two most technically demanding tasks in detailed design, and they have the greatest impact on the operational safety and usability of the installation. In this issue, we will discuss these two topics in detail. Pipe stress analysis addresses the issue of whether pipes will be damaged due to thermal expansion and contraction, while 3D piping design deals with determining the best way to arrange the pipes so that there are no conflicts and the system is easy to operate. One is responsible for safety, and the other for practicality; together they constitute the core work of the process piping discipline during the detailed design phase. I. Which pipes require stress analysis? Not all pipes need to undergo a detailed stress analysis. A medium-sized chemical plant may have thousands of pipes, and it is neither necessary nor possible to perform stress analysis on each one. What are the criteria? The following types of pipes must be included in the stress analysis. Pipelines operating at relatively high temperatures. High-temperature pipes have a large thermal expansion rate; if their ends are fixed rigidly, the resulting thermal stress can far exceed the allowable limits of the material. Generally, when the operating temperature exceeds the material’s creep temperature range, a detailed stress analysis is required. Carbon steel pipes with operating temperatures exceeding 300–400 degrees, and stainless steel pipes with temperatures exceeding 400–500 degrees, should all be included in the analysis. Pipes with a relatively large nominal diameter. Even at low temperatures, large-diameter pipes exert a significant load due to their own weight plus the weight of the fluid inside them; therefore, the spacing between supports and the type of supports required must be determined through stress analysis. Pipes connected to the outlets of sensitive equipment. Some equipment—especially high-speed compressors and turbines—has very strict limitations on the stress on pipe ends. Excessive stress on the pipe outlet can lead to deformation of the equipment housing, misalignment of the rotor, excessive vibration, and even damage to the equipment. Pipes connecting such devices must undergo stress analysis, even if the temperature is not high and the pipe diameter is small, to ensure that the forces and moments acting on the device’s connections remain within acceptable limits. Pipelines that transport extremely or highly hazardous media. Once a leak occurs in such pipelines, the consequences can be severe; therefore, their safety under various operating conditions must be thoroughly considered in stress analysis. Periodically operating pipelines that require fatigue analysis. If a pipeline is subjected to frequent cyclic changes in temperature and pressure during operation, even if the stress level at any given time remains within acceptable limits, long-term fatigue accumulation can still lead to crack initiation and propagation. In simple terms, pipes with high temperatures, large diameters, hazardous media, or those connecting critical equipment are all considered “high-risk pipes,” and should be included in stress analysis during the detailed design phase. II. How to perform stress analysis: Pipe stress analysis is usually carried out using specialized software such as CAESAR II. The basic steps of the analysis are as follows: establish a pipeline model – input the geometric layout of the pipeline, its diameter and wall thickness, material, operating temperature and pressure, as well as insulation details; apply constraint conditions – such as the location and type of supports and the position of fixing points; apply loads – including self-weight, temperature, pressure, wind load, and seismic load; perform operational calculations – to check whether stress ratios, displacements, forces on the supports, and loads at the pipeline ends exceed acceptable limits; if the results are not satisfactory, adjust the support configuration or the pipeline layout and recalculate. There are several key indicators of focus in stress analysis. The stress ratio is the ratio of the actual stress to the allowable stress; it must be less than 1, with a typical requirement of not exceeding 0.8 to 0.9 to allow for some margin. The displacement amount is the expansion displacement of the pipeline in its hot state, and it must remain within the range that can be safely absorbed by the pipeline itself and its supports. The stress on a support refers to the stress conditions that each support experiences in both hot and cold states, and it must not exceed the limits that the structure can withstand. The nozzle load is the force and torque exerted by the pipeline on the equipment’s nozzle, and it must remain within the limits specified by the equipment manufacturer. There are several common issues in stress analysis. One issue is improper setting of the fixing points – there are too many of them, preventing the pipes from expanding freely; as a result, thermal stress accumulates at certain weak points and leads to failure ； There are too few fixed points, resulting in excessive pipe displacement, which may lead to collisions with adjacent pipes or structures. Another issue is that the spring supports are selected to be either too small or too large – if they are too small, the spring gets compressed when the pipeline heats up and loses its supporting function; if they are too large, the spring pushes the pipeline upward in the cold state, resulting in a reverse load under cold conditions. Another issue is the neglect of accidental loads—wind load, seismic load, water hammer impact. Although these loads occur with low frequency, they can cause severe damage once they do occur; therefore, such extreme conditions need to be taken into account in stress analysis. After the stress analysis is completed, a stress analysis report is generated, which includes a conclusion regarding whether the pipeline stresses are within acceptable limits, information on the type and location of supports, the model and set load of spring supports, as well as results checking whether the loads on the equipment’s pipe connections exceed permissible values. This report serves as the basis for the subsequent preparation of pipeline construction drawings and the procurement of supports. III. The core value of 3D piping design: While 2D PID diagrams clearly show pipes, valves, and instruments, they are planar and based on logical structures. How pipes should navigate around equipment in space, pass through pipe racks, and avoid structural beams and columns – these cannot be shown on 2D drawings; they must be addressed in a 3D model. The core value of 3D piping design lies not in creating more realistic visuals, but in eliminating collisions, optimizing space utilization, and improving operability. In a medium-sized chemical plant, pipes, equipment, structures, electrical trays, and instrument panels are arranged intersectingly in three-dimensional space; it is extremely difficult to avoid collisions between pipes, between pipes and structures, and between pipes and equipment. If these collisions are not identified and resolved in the 3D model, temporary modifications will have to be made on-site: pipes must be cut and rewelded, or elbows added to reroute them. This is not only costly and time-consuming, but the temporarily modified pipes often lack stress analysis and support calculations, posing safety risks. There are two types of collisions. A hard collision occurs when pipes interfere directly with steel structures, equipment, or other pipes in space – for example, when a pipe passes straight through the location of a steel structural beam or column; this must be eliminated. A soft collision occurs when, although the pipe does not come into direct contact with any objects, there is insufficient space for operating the valve handle, insufficient space for installing or removing flange bolts, too small a gap between the insulation layers, or when the instrument panels are blocked by the pipes and cannot be read. Soft collisions are more hidden than hard collisions; they can only be detected during the review of 3D models through specialized simulation of operation and maintenance paths. In a typical chemical plant, after detailed 3D piping design and collision checks, rework and changes caused by pipe collisions during on-site construction can be significantly reduced. 3D model review is one of the most effective ways to reduce on-site changes. IV. Virtual inspection: Walking through the path taken by the operator in the model. This is a very practical approach in 3D piping design, and it’s one of the tasks I highly recommend. In the 3D model, designers walk along the regular inspection routes from the first-person perspective of the operator. Go up the stairs, walk around the control platform, passing by every valve that needs to be operated, every instrument that needs to be read, and every sampling point from which samples need to be taken. Ask yourself a few questions before you leave. Is the height and orientation of the valve handwheel convenient for operation? Can one naturally grasp the handwheel while standing on the platform, or is it necessary to stand on tiptoes or bend over? Will the handwheel come into contact with the pipes or structures nearby when it rotates? Is the dial of the instrument that requires reading visible along the inspection route, or is it necessary to go behind the equipment and climb a ladder to take a reading? Is the height of the sampling port within the convenient operating range of 1.0 to 1.3 meters, and is there enough space below the sampling port to place the sampling bottle? Is the escape route on the operation platform unobstructed? Can operators evacuate quickly in an emergency? These problems are almost impossible to detect on 2D drawings. But when you walk the inspection route through the 3D model from a first-person perspective, these issues become apparent. Designers sit in their offices drawing diagrams, focusing on how to implement the functions, and often overlook the practical problems that operators face on a daily basis. Virtual inspections can compensate for the lack of this perspective. For the maintenance space, a similar method can also be used for simulation. Is there enough space to extract the core of the heat exchanger tube bundle – is the extraction length combined with the operating space sufficient? Is there enough lifting space when replacing the filter element? Is the opening direction of the manhole blocked by pipes or structures? It is much more cost-effective to simulate these issues using 3D models before the construction drawings are prepared, rather than having to make modifications after construction is complete when it turns out that there isn’t enough space for maintenance. V. Three stages of 3D model review The review of 3D models is usually carried out in three stages, with different focuses and different participants at each stage. 30% model review. At this stage, the general route of the pipeline has been determined, and the models of the main equipment as well as the structural framework have been established. The focus of the review is whether the equipment layout is reasonable, whether the route of the pipe trunks is economical, and whether sufficient space has been reserved for main operation pathways and maintenance areas. It is relatively easy to adjust the equipment layout issues identified in the 30% stage; simply moving the equipment model in the software is sufficient. If it is only during the 90% review that it is discovered that a certain reactor layout prevents operators from reaching the valves, this may require systematic rework such as adjusting the elevation of multiple pieces of equipment and recalculating the loads on the structural framework. 60% model review. The piping design has been refined to a considerable extent, with most of the pipelines, valves, and instruments already in place. The focus of the review has shifted from \"whether it can be fitted\" to \"how easy it is to operate and maintain\". Check the operating space for the handwheels of each manual valve one by one – ensure there is sufficient distance between the handwheel and the wall, platform, and adjacent equipment, and that adequate space is provided for valves with large diameters. Check each instrument on site to ensure that its dial is oriented for easy reading. Check one by one whether the location of the sampling port is convenient for operation. Check one by one whether the core-pulling space of the heat exchanger tube bundle is adequate, whether the manholes of the tower are oriented properly towards the operation platform, and whether there is sufficient lifting space at the top of the filter. For the review at this stage, it is recommended to involve personnel with driving and operation experience. By looking at the model, they can quickly identify which aspects of the design will cause operators to complain constantly. 90% model review. All pipes along with their supports and hangers, platforms, electrical cable trays, and instrument panels have been installed; this represents the final inspection stage before the issuance of the construction drawings. The focus of the review is to completely eliminate collisions—including hard collisions and soft collisions. Is there sufficient spacing between the insulation layers of the pipes? Is there enough safety distance between the electrical cable trays and the thermal pipes? Do the instrument panels and valve handles block each other? Are there any spatial interferences between the fire protection pipelines and the process pipelines? A final verification of constructability is also required – to check whether there is sufficient space for welding the pipes, whether there is enough room to install and remove the flange bolts, and whether the sequence in which the pipes are laid in the pipe tray matches the construction sequence. The construction engineer from the installation company should also participate in the 90% model review—they can raise many issues from the perspective of construction processes that designers might not think of. If the constructability review is carried out only after work begins on site, it will be too late. Preview for the next issue: Issue 66 – Special focus on safety facility design. The core techniques for pipeline stress analysis and 3D piping have been covered. Next, we move on to the stage of compiling the special section. The special section on safety facility design is currently the only one among the four special sections for chemical projects that still needs to be submitted to the relevant government authorities for review. The core question it must answer is: What protects the device in terms of safety? How is the independence of each protection layer verified? How are the conclusions of HAZOP and LOPA implemented in design? To be continued in the next issue.