When performing line cleaning, the heat exchanger first cleans the main line and then the auxiliary line – how can one distinguish between the main line and the auxiliary line?

This is a quite practical issue; it’s easy to get confused in chemical processing environments. The main line usually refers to the pipeline through which the medium flows continuously during normal operation, such as the feed or discharge pipes in the main process ； Side lines are often bypass, backup, or discharge pipelines, such as the bypass for control valves or the line used for draining waste. You can distinguish it from several aspects: by looking at the drawings and pipeline numbers. On the design PID diagram, different line styles are used for identification – main lines are usually represented by thick solid lines, while secondary lines use thin solid lines. The letters or numbers at the end of the pipeline numbers also differ, such as “-P-” and “-B-”. Check the valve positions: Valves on the main line (such as the inlet and outlet valves of heat exchangers) are kept open during normal operation, while valves on the bypass lines are usually closed and only opened when switching or draining is required. Observe the operating habits: If, when scanning the lines in the past, one particular line was always used first, it is likely to be the main line; however, it is still best to verify by referring to the operation sheet or process procedures, as different devices may use different naming conventions. Additionally, the scanning order is to scan the main line first and then the secondary line, primarily to prevent residues from the main line from getting into the secondary line. However, it is advisable to review this process with the on-duty technician or engineer before carrying out the operation, as the markings on older devices can sometimes wear out. Safety first, right~

Why is the control valve connected to the secondary line first and then the main line?