The seven stages of chemical engineering technology from concept to industrialization (Issue 48/100) -- Auxiliary pipeline design

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The seven stages of chemical engineering technology from concept to industrialization (Issue 48/100) —— Technology finalization: Assistance in pipeline design. Dear friends: Hello everyone! In the previous issue, we discussed the milestone review for PID; all of the normal operating main process pipelines, control loops, and interlock logic were frozen. In this issue, we’ll discuss a component that is easily overlooked, but whose absence makes it impossible to drive, stop, or perform maintenance on a vehicle: auxiliary pipeline design. The main process pipelines are the arteries and veins of the plant, responsible for transporting materials during normal production. But the device cannot operate only under normal operating conditions. How to raise the temperature and build up the liquid level while operating the equipment; where to drain the materials when the equipment is shut down; how to perform emergency relief under accident conditions; how to isolate, purge, and displace the system prior to maintenance; and where to send defective products—none of these issues can be resolved by the main process piping. An independent auxiliary piping system is required to address them. In the projects I have worked on, the problems caused by inadequate design of auxiliary pipelines are far more numerous than those resulting from issues with the main process pipelines. The main process pipelines receive a lot of attention; they are drawn carefully and reviewed thoroughly, but the auxiliary pipelines are often the parts that are cut corners on during rush periods. As a result, the device was built, but when driving it was found that there was no nitrogen pipeline available for system purging; therefore, a rubber hose had to be used temporarily ； When parking, it was found that the materials could not be emptied completely; therefore, the flange had to be opened and the contents unloaded section by section ； Under accident conditions, it was found that the discharge path of the safety valve was unreasonable, and the high-temperature, high-pressure medium discharged posed a threat to the safety of the operators. If these problems can be resolved during the PID stage, it’s just a matter of drawing a few additional lines; otherwise, on-site operations will be plagued by complexity and inefficiencies every day. I. Pipeline system for driving: When the device is started from a cold, idle state, the primary challenges are: how to introduce the material, how to raise the temperature, and how to establish the liquid level. Many process engineers, when drawing PID diagrams, assume that the equipment is already in normal operating condition. But in practice, driving starts with an empty, cold, and stationary device. The reactor is empty; material needs to be added before heating it up. The distillation column is empty; it is necessary to establish a level in the reboiler first before starting the bottom pump. Only after establishing reflux can the column operate normally. The heat exchanger is cold; it is necessary to first circulate a heating medium to warm the pipes and equipment, in order to prevent thermal shock caused by introducing a high-temperature medium directly. If these auxiliary pipelines required during operation are not clearly shown on the PID, the operators will have to figure out ways to handle them on site when starting up the system. Common practices include connecting temporary hoses, reversing the flow direction, and various emergency procedures. Temporary pipelines pose high risks – the hose material may not be resistant to corrosion by the medium, temporary joints may leak, and the lack of fixed support can cause them to be forced apart by pressure. A normal driving process that relies on a large number of temporary pipelines cannot ensure safety or efficiency. The design of a pipeline for transporting liquids requires answering several questions. Before feeding, does the system need to be purged with nitrogen and undergo a gas-tightness test? Where does the nitrogen come from and where is it discharged? During the reactor heating phase, if normal operation relies on the reaction heat for self-heating, what should be done during the startup phase when there is no reaction heat available? It is necessary to activate heaters or steam preheating pipes; have these pipelines been connected yet? During the establishment of full reflux in the distillation column, is the refrigerant for the overhead condenser ready, and is the venting route for non-condensable gases unobstructed? By working backwards through each operational step, it is possible to identify which auxiliary pipelines are essential for starting up the system. The root valve of the startup pipeline is usually kept closed and is used only during the startup phase. This “normally closed” marking may seem unimportant, but it serves two purposes: one is to inform the operator that the valve should be closed under normal conditions ； Second, inform the inspection personnel that if they find this valve open while the device is operating normally, there is a problem. II. Parking pipelines: Parking is divided into normal parking and emergency parking. For normal shutdown, it is necessary to bring the device to a safe stop, empty all materials, and put the equipment in a condition suitable for maintenance. An emergency stop requires bringing the device to a safe state in the shortest possible time. The key to the design of shutdown piping is to address the issue of “where the materials go”. During normal shutdown, to which storage tank should the material in the reactor be transferred, to which container should the liquid at the bottom of the distillation tower be sent, and how should the accumulated liquid in the pipes be drained? For all of these, a clear discharge path must be indicated on the PID; it is not sufficient to simply mark it as “fully discharged”. For materials with high freezing points or high viscosity, the shutdown pipelines also require heat tracing for insulation. Otherwise, the material cools and solidifies inside the pipeline; when the system is restarted, the pipeline becomes blocked, and it’s necessary to disassemble it section by section for cleaning. Such lessons are common in the chemical industry; often, either there is a problem with the process itself, or the materials are not completely removed during shutdowns, and the pipes lack heat tracing protection. The relief piping in emergency shutdown situations is particularly critical. The temperature, pressure, and flow rate during emergency release can all reach their peaks in a short period of time. Whether the discharge destination is safe – whether it is discharged into a flare system or directly into the atmosphere, into which level of discharge manifold it is discharged, and whether there will be any conflicts with other discharge sources. All of these need to be considered one by one in the auxiliary pipeline design. III. Pipeline in accident conditions The pipelines in accident conditions mainly include safety valve discharge pipelines, emergency venting pipelines, and interlock discharge pipelines. These pipelines are silent during normal operation, but once activated, they carry out the core function of protecting equipment and personnel. There are several details in the design of the relief valve’s discharge pipeline that are easily overlooked. The pressure drop in the outlet pipeline must not be too high – if the back pressure is too high after the safety valve activates, the valve core cannot return to its proper position, which may lead to continuous leakage or damage of the safety valve. If the discharge medium contains liquids or condensable gases, the discharge pipeline must be equipped with a liquid separation tank or a condensate collection system to prevent liquids from accumulating at the lowest points of the pipeline and blocking gas flow. The height and direction of the discharge outlet of the relief pipeline must be such that the discharged substances do not pose a hazard to operators or surrounding equipment. The emergency vent line is used to quickly release the pressure in the system to a safe level under accident conditions. The vent valve should be placed at the highest point of the system, where gas accumulates, ensuring the best venting effect. The diameter of the vent pipeline must meet the requirements for the maximum vent flow rate; a too small diameter will result in delayed pressure relief and an escalation of the incident. The interlock relief pipeline, in conjunction with the interlock shutdown mechanism, facilitates system depressurization, unloading, and cooling. The interlock logic diagrams and the relief pipelines on the PID must correspond one-to-one; there should be no situation where the interlock diagram requires a relief pipeline, but no corresponding pipeline can be found on the PID. IV. Inspection and purging of pipelines: After the plant is shut down and before maintenance work begins, it is necessary to isolate, purge, and replace the equipment and pipelines to ensure the safety of maintenance personnel when entering confined spaces or opening up the equipment. Isolation means separating the equipment to be repaired from the operating system. The most common method is to use a blind flange or a double-valve isolation on the connecting pipeline. The dual-valve isolation method involves two valves connected in series, with a drain valve placed in between – this way, even if one of the valves leaks, the fluid will be discharged through the drain valve rather than entering the maintenance area. The PID must clearly indicate the locations of the isolation valves and drain valves; it is not sufficient to simply state “install blind flanges during maintenance”. Displacement involves using an inert gas to completely replace the flammable or toxic gases inside the equipment. Nitrogen is the most commonly used displacement medium. Where nitrogen is introduced, where waste gas is discharged, and what the process of displacement entails – all of these require clear displacement pipelines to be illustrated on the PID. Many people do not forget the nitrogen inlet pipeline, but often forget the exhaust gas outlet pipeline. Where are the exhaust gases discharged to? – If they are sent to a flare, does the flare system have the capacity to handle them? ； If it is discharged directly into the atmosphere, has a toxicity assessment and dispersion analysis been conducted? The setup of the discharge pipeline directly affects the safety of the displacement operation. Purging is carried out in several ways, including chemical cleaning, water flushing, and steam purging. Purging lines typically use the plant’s utility pipelines as a source for the purging medium, which is connected temporarily to the system to be purged. The PID must indicate the locations of the purging inlet and outlet at least, to provide clear guidance for on-site operations. V. Non-conforming product pipeline: During the initial startup of the equipment and in cases of abnormal operation, a large amount of material that does not meet the product specifications is produced. If defective products have no proper disposal method, they either end up in the storage tanks of qualified products, contaminating the entire batch, or operators have to transport them in barrels on a temporary basis – such temporary handling in barrels is not only inconvenient but also poses safety and environmental risks. The design of a defective product pipeline requires answering several questions. From which sampling point or discharge point are the defective products separated, and to which storage tank for defective products are they sent for temporary storage? Is the capacity of this storage tank sufficient to hold the amount of defective products generated during the startup phase? What to do with the materials in the tank containing defective products going forward – send them back to the upstream process for reprocessing, sell them as by-products, or dispose of them as waste liquid. These have different destinations, and the accompanying pipelines and conveying equipment also vary. The defective products generated during driving tend to be unstable in both quantity and composition. If the pipeline design for defective products is proper, operators can smoothly transfer the defective materials to the temporary storage system, and only after the equipment is operating stably and the products meet the quality standards can they be transferred back to the storage tanks for qualified products. The entire process does not require shutting down the equipment or temporary takeover; it is easy to operate and safe to control. VI. Connection of auxiliary pipelines to equipment ports Once the design of the auxiliary pipelines is completed, there is one task that must not be overlooked: adding the ports of these auxiliary pipelines to the equipment data sheet. Many equipment data sheets take into account only the process connections required for normal operation when they are prepared, without providing for auxiliary connections. It was not until the piping for operation, purging, and discharge was drawn on the PID that it was discovered that no suitable pipe connections could be found to connect these pipelines to the equipment. At this point, it is necessary to check the requirements for auxiliary ports for each device, and add the required number and specifications of such ports. I generally recommend that towers and storage tanks be equipped with at least two spare pipe connections, with diameters ranging from DN25 to DN80 being suitable. These spare ports are useful during the design phase of auxiliary pipelines, or to avoid having to make holes in new equipment during future plant modifications. The spare pipe outlet is normally sealed with a blind flange; it can be put into use by simply connecting the pipeline when needed. Only after all the auxiliary pipe connections are completed does the PID truly become the complete version that includes all the pipelines. This includes normal operation pipelines, startup and shutdown pipelines, emergency discharge pipelines, purging and replacement pipelines, and pipelines for handling defective products – any pipeline that may be needed under any operating condition of the plant can be found on the PID. Preview for the next issue: Issue 49 – Pipe Material Grades: Material Selection and Insulation Design. All auxiliary pipelines have been designed, and all pipes listed in the PID now have their diameter and fluid type information. The next step is to determine the material, wall thickness, flange class, gasket type, and insulation scheme for each pipeline, based on the properties of the medium and the operating conditions. This is the role of the pipeline material grading system – the process engineering department provides the necessary conditions, while the pipeline material specialty develops the grading tables. To be continued in the next issue.