Summary of Purging for the Separation Refrigeration Position

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Summary of Purging for the Refrigeration Unit Separation Process 1. Purpose of purging: After the process pipelines have been installed in the system and passed inspection, the pipelines of the process system are purged to remove dirt such as sediment, welding slag, and rust from within them. To prevent these debris from blocking pipes and equipment while driving, thereby avoiding damage to such equipment, valves, and instruments and ensuring the quality of the process materials, thus guaranteeing a smooth and safe operation of the system. 2. Purging principles and requirements: (1) During the purging process, it is necessary to ensure an adequate supply of air, so that the flow rate of the purging gas exceeds 20 m/s. (2) The purge air pressure is between 0.6 and 0.8 MPa. (3) All control valves are removed and replaced with bypasses; orifice plates and filters have their cores extracted; the connections between safety valves and pipelines are severed, and blind flanges are used for isolation. (4) Blow the main pipe first; the branch pipes can be blown only after the main pipe has passed the purging test. (5) All pipelines connected to vessels such as towers, tanks, and heat exchangers must be purged, and then reconnected once they pass the inspection. (6) Remove the short pipes at the inlet and outlet of the cold box, and connect them with temporary pipelines. The inlet and outlet pipelines of the compressor, expander, and pump are disconnected, and blind flanges are installed at the inlet and outlet of the casing for isolation. (7) Form purging teams in sub-regions, and provide training on the purging plan to team members prior to the purging process. (8) During purging, set up a warning line 15 meters away from the discharge outlet and place safety signs; personnel other than those involved in the purging operation are not allowed to enter. 3. Purging qualification standard: A painted wooden or aluminum plate is placed at the purging outlet; under the required purging pressure, the target plate is purged for 5 minutes. If there is no rust, dust, or pitting on the target plate, the purging is considered successful. 4. Purging system and gas source separation: The refrigeration purging system includes a hot zone, a methanation section, a cold zone system, a refrigeration compressor, and a flare system. Separation heat zone: In September 2012, when the pyrolysis gas compressor was not yet capable of providing purge air for each zone, in order to accelerate the purge process, the separation heat zone utilized the propylene double towers as a pressure storage source. A 3” temporary pipeline was installed via the utility plant’s air main to pressurize the propylene towers to 6–7 KG, thereby purging the pipelines connected to those towers. The butane stripping tower and the low-pressure propylene stripping tower utilize the plant air from the utility station; temporary pipelines are installed to pressurize these two towers to 6–7 KG. These towers are used as pressure storage sources to purge the auxiliary pipelines connected to them. Methanation system: In November 2012, after the purging of the fuel gas pipeline was completed at the cracking unit. Methanation utilizes 6–8 KG of compressed air provided by the burnt air compressor in the cracking unit; this air is delivered to the methanation system via fuel gas pipelines, where it is used to purge the methanation unit, the dryer, and the associated pipelines. Separation cold zone: In March 2013, after the pyrolysis gas compressor was put into operation, the cold zone utilized 9–10 KG of compressed air from the fifth stage outlet of the pyrolysis gas to purge the cold zone system (including the cryogenic tank, pre-demethanization tower, demethanization tower, off-gas distillation tower, deethanization tower, expander-recompressor, and other systems) through the process piping. Refrigeration compression system: The air supply for the propylene and ethylene compressors comes from the 4th stage outlet of the pyrolysis gas compressor. The outlet pressure at stage 4 of the pyrolysis gas compressor is 6–8 KG; purge air is introduced into the propylene and ethylene plant systems via 2 8” dry air lines and 2 6” dry air lines, respectively. Flare system: During the purging of the main system, emissions are made from each flare point, with the purging of the various branch lines of the system being completed first. Finally, pressurize each system, open the discharge lines to the flare tank simultaneously, and complete the purging of the flare main pipe. 5. Problems encountered during purging and solutions: The volume of the propylene twin towers in the heat separation area is nearly 4,000 m3; these tanks are filled with factory air to a pressure of 6–7 KG. Purging is carried out by controlling the air supply through the valves on the purging pipeline. An adequate amount of air is used, the pressure is maintained for a long time, resulting in an effective purging process. The propylene column reboilers are E-1530A/B/C/D, the intermediate reboilers are E-1540A/B/C/D, and the column top condensers are E-1535A/B/C/D. There are no valves to control either the gas phase or the liquid phase; all the pipelines have a diameter of over 30”. If purging is carried out with the system open, neither the gas flow rate nor the pressure can be maintained, which will inevitably affect the quality of the purging process. Therefore, these pipelines can only be purged using blasting. Kraft paper is used for blasting purging. The first issue that arises is determining how many sheets of kraft paper are needed to ensure the quality of the purging process, while at the same time preventing excessive pressure from occurring, so as to avoid the pipeline being displaced due to the large reaction forces during blasting. Finally, by continuously adding kraft paper, it was determined that 25 to 30 layers of kraft paper were sufficient; at this level the system’s burst pressure was around 1 KG, which met the requirements for purging. These heat exchangers are all composed of 4 heat exchangers connected in parallel, and each of those parallel units has two feed streams; thus, whether it is the gas phase or liquid phase line of the reboiler or condenser, there are 8 connections in total. Based on the actual pipeline layout on site, if only one port is covered with kraft paper while the other 7 ports are fitted with blind flanges, that is, if blasting is carried out at a single port, dead zones will remain. Therefore, during the blasting process, we apply kraft paper to all 8 openings simultaneously and carry out blasting on them at the same time; this not only ensures that there are no dead corners in the pipeline but also maintains the flow rate of the gas or liquid main lines. However, during the blasting process, it was found that because the force exerted on each piece of kraft paper varied, one or two openings would burst first, and it was not possible to ensure that all 8 openings would explode simultaneously. To solve this problem, we attached a homemade sharp knife to the bottom of the kraft paper and tied it to a wire. Once one flange opening burst, we immediately manually caused the other 7 openings to burst as well. Using this method, we successfully resolved the most difficult area in the hot zone purging. The purging of the cold zone system in this system is carried out using compressed air from the fifth stage outlet of the pyrolysis gas, which features a large volume of gas and high pressure. During the purging process, the manhole of the cold flare tank is opened, and the outlet pressure of the fifth stage of the pyrolysis gas is adjusted via the V1365 overhead flare line. The main challenge in the cold zone system lies in the inlet and outlet pipelines of the cryogenic box. During purging, all short pipes at the inlet and outlet of the cold box are removed. The inlet and outlet pipelines are connected by short pipes, meaning the pipelines are in parallel. To prevent any dead corners during purging, all flanges at the outlet of the cold box are opened; this allows for an initial purge of the parallel pipelines, thereby removing the water and small debris inside. One discharge port is left open, while the remaining ones are covered with blind flanges. Each discharge port is then purged until it meets the required standards. Cold zone systems, particularly cold box systems, have complex processes, and frequent switching of purging procedures is required during the purging process. During the process transition, a target plate was installed at the fracture site; and to maintain a continuous air supply during target shooting, some temporary valves were added to the flanges of the process pipelines to control the flow. Since the cold zone purging uses gas from the fifth stage outlet of the pyrolysis gas compressor, which has high pressure and a large flow rate, significant reactive forces are generated at the purging discharge point, making it easy for the pipelines to vibrate and shift. To prevent such accidents, the two flanges at the discharge outlet are connected with wire, and the flanges are tightened to effectively reduce the reactive force from the discharge. The refrigeration compression system for the ethylene plant has two air supply sources: the fourth-stage outlet of the ethylene compressor, and two 6” dry air lines coming from the fourth-stage outlet of the pyrolysis gas compressor, located on the vapor line at the top of the first-stage suction tank. The entire ethylene plant system is connected through anti-surge valves. Since these valves are large and difficult to disassemble and adjust, the upstream ends of all anti-surge valves are sealed off; only after passing the testing is it that the gas supply is allowed to flow into the downstream section. During the purging of the pipelines attached to the two-stage suction tank V1670, it was found that the gas flow rate at the discharge outlet was low, insufficient to meet the requirements for purging. Upon investigation, it was found that the throttle of the anti-surge valve UV16051B in V1670 was severely restricted; ultimately, the anti-surge valve was removed and a temporary short pipe was used to make a connection, thereby ensuring the quality of purging for the auxiliary pipelines associated with tank V1670. The air source for the propylene plant system comes from two 8” dry air lines at the fourth-stage outlet of the pyrolysis gas compressor; the systems are also connected together using anti-surge valves. There are many users of the propylene system, and the heat exchangers of the refrigerant users have all been pressurized with water. During the purging of these heat exchangers, the liquid line flange was disconnected, and the gas supply was controlled using a valve on the liquid line to ensure that the purging of the liquid line met the required standards. The gas line is connected to the flange of the heat exchanger; purging is carried out there, and once the testing is successful the system is reset. The heat exchanger is then purged successfully through the gas line. The propylene inlet pipes are relatively large, with the largest being 42 inches; during the purging of these pipes, it was found that the volume of gas discharged was low. After discussion, a gas stream was introduced from the fifth-stage outlet of the pyrolysis gas compressor to V1630, ensuring an adequate gas supply for the propylene system. After the pyrolysis gas compressor was brought online to provide purge air for separation refrigeration, all the staff working on the separation unit worked tirelessly for over a month, and by early May they completed the purge work for that unit.