Reduce flare vent losses in natural gas processing units

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Reducing the flare vent loss in natural gas processing units – Zhongyuan Oilfield: Gu Ailian, Jiang Heping, Li Longhua, Cheng Yueling. I. Introduction The renovation and expansion project of the Zhongyuan-Third Oil and Gas Gas Processing Plant involves a medium-pressure cryogenic natural gas processing unit, whose production process is based on the design concepts provided by the German company LINDE. The main process units of the plant consist of raw material gas compression and drying, pressurized expansion refrigeration, propane-assisted precooling, NGL recovery, and NGL distillation. Since its commissioning on April 18, 2001, the product quality and yield of this device have shown an upward trend year by year. However, in the early stages of operation, the device shut down frequently, resulting in a rather passive production and operational situation; at the same time, torch venting caused significant losses. To this end, technical personnel actively carried out research on technical solutions and implemented process improvements. II. Analysis of the reasons for unplanned shutdown of the unit (1) Shutdown of the unit due to equipment failures: The adaptability issues of the compression unit and its auxiliary systems when operating conditions change can lead to damage to the unit’s seals, excessively high temperatures, and vibration, all of which can result in a shutdown and consequently in flare venting. (II) Shutdown of the device due to control system failures: Inadequate maintenance of the control system’s hardware and software, false signal detections, signal drift, changes in operating conditions, and failure to adjust certain parameter settings in a timely manner can all lead to the device shutting down as a result of interlocked failures. From April 2001 to December 2003, there were numerous shutdowns caused by control system failures, resulting in extensive flare venting. (III) Shutdown of the unit due to operational faults: Some newly recruited employees, owing to a lack of responsibility and low technical skills, fail to take appropriate actions in response to fluctuations in operating conditions when the unit shuts down, which leads to the unit stopping and subsequent venting through the flare. Alternatively, they fail to pay attention to cases where process parameters are reported either too high or too low, resulting in parameters being excessively high or low and causing a chain reaction that leads to the unit shutting down, thereby triggering flare venting. III. Losses due to unplanned shutdowns of the plant resulting in flare venting (I) Losses from natural gas flare venting during unplanned plant shutdowns Since the expansion project of the third gas processing plant involves a medium-pressure cryogenic natural gas processing unit, product separation is achieved through natural gas compression, refrigeration, and distillation; as a result, the pressure in the pressure-boosting and refrigeration units downstream of the plant is relatively high. In the event of an unplanned shutdown, if the operators on duty do not have time to reduce the load according to the procedures, the natural gas from those high-pressure units will flow into the lower-pressure units upstream (when the plant shuts down, the safety valves of the compressors open automatically). Due to the low pressure setting of the upstream safety valve, this inevitably causes the safety valve to activate, resulting in substantial venting through the flare. Process of the device’s pressure boosting unit. (II) Losses due to the venting of liquid hydrocarbons during unplanned plant shutdowns: To understand the losses associated with the emission of liquid hydrocarbons during unplanned plant shutdowns, a hydrocarbon emission flow diagram was prepared based on the original design. As can be seen from the diagram, when the unit shuts down unexpectedly, it causes the liquid levels in some separators to exceed the alarm limits, preventing the unit from starting up. As a result, the liquid hydrocarbons have to be discharged into the 8-V2 separator and then compressed and sent to the combustion chamber of the flare system for burning. IV. Feasible solutions to reduce torch venting losses in the plant. The number of unplanned shutdowns of the plant directly affects the volume of gas vented through the torch; therefore, finding ways to reduce the frequency of such shutdowns is not only practical but also an important factor in avoiding disruptions in production. Through measures such as in-depth research, technical problem-solving, enhanced inspection and maintenance of equipment and installations, maintenance of the control system’s software and hardware, and thorough overhauls, the frequency of unplanned shutdowns is gradually reduced, thereby achieving the goal of minimizing unplanned shutdowns and cutting torch venting losses. Each time the plant experiences an unplanned shutdown, the amount of gas released through its flare depends on the pressure relief in the plant’s pressurization and refrigeration units, as well as on the emission processes. Through technical analysis and evaluation, technical upgrades are implemented to prevent medium-pressure gas from flowing back upstream to the low-pressure areas for venting, thereby reducing the losses associated with flare venting. Recycle the liquid hydrocarbons emitted due to unplanned shutdowns of the equipment. The device releases liquid hydrocarbons each time it stops operating; calculations show that this amount is approximately 4.2 tons. Through technical efforts, light hydrocarbon recovery equipment was added to the original design, and the hydrocarbon liquid was incorporated into the reprocessing process, thereby reducing losses from flaring. V. Specific measures to reduce venting losses in the plant (1) Enhancing the sense of responsibility among operators To strengthen the sense of responsibility of operators, improve their technical skills, and reduce the number of plant shutdowns caused by human error, operators are given systematic training in four areas: process operations, equipment maintenance, electrical systems, and instrumentation. A testing system is also implemented after the training. Furthermore, by scientifically arranging the personnel and establishing teams where experienced workers mentor newcomers, the technical skills of the employees can be significantly improved. To enhance operators’ sense of responsibility for the long-term operation of the plant, weekly assessments of production volume and yield are conducted for the operating shift, along with detailed reward and penalty systems. This encourages operators to continuously strive to avoid plant shutdowns, thereby reducing losses due to flare venting. (II) Reducing the failure rate of equipment and control systems: In order to minimize downtime caused by equipment failures, it is necessary for equipment operators to carry out regular inspections and maintenance of the equipment, conduct thorough overhauls, and continuously address any equipment failures. To reduce shutdowns caused by control system failures, the following measures have been taken: First, instrument maintenance personnel are organized to regularly inspect and maintain the contacts of the control system, thereby preventing shutdowns resulting from signal errors or drifts caused by aging, loosening, moisture damage, or short circuits, and eliminating signal interference. Second, the control system software is maintained on a regular basis, used in a proper manner, and any abnormalities are resolved promptly either by fixing them directly or by contacting the software developer, so as to avoid shutdowns due to software faults. Third, after the device has been operating for a while, some parameters may exceed their limits, and there may be serious control failures; these issues can only be addressed after the device is shut down for maintenance. Therefore, the periodic major maintenance of the device is utilized to resolve such faults, thereby reducing the number of shutdowns and ultimately minimizing losses associated with flare venting. (III) Technical renovation of the unit’s pressurized refrigeration unit: To address the issue of high flare gas discharge during unplanned shutdowns, technical improvements were carried out taking advantage of the major maintenance of the unit in March 2004. The details of these technical renovations are as follows: A check valve was added between the air cooler at the exit of the pressurizer and the gate valve; this constitutes the revised flow pattern for the unit’s pressurized refrigeration unit. Through the aforementioned technical modifications, when the expansion booster of the unit is shut down, it is prevented from happening that the medium-pressure gas within the process pipelines several hundred meters downstream of the new check valve, as well as in equipment such as the 1-V4 separator, 1-V5/AB dryers, 2-E1 and 2-E2 cryogenic tanks, and 2-V1 low-temperature separator, leaks into the lower-pressure areas upstream, resulting in significant gas venting. It is also prevented from occurring that the medium-pressure gas downstream causes a sudden increase in the pressure at the exit of the first stage of the feed gas compressor 1-K1, leading to shutdown of this compressor due to pressure buildup. Looking at the operation status from March 2004, after the equipment maintenance, up to the end of that year, it can be seen that since the amount of gas released through the flare during shutdowns was no longer as large as before the technical upgrades, the number of times the feed gas compressor had to stop operating also decreased significantly due to the shutdowns of the expansion boosters. (IV) Carry out process modifications to the unit’s hydrocarbon emission units in order to recover and reprocess the hydrocarbon liquids, thereby achieving zero loss in hydrocarbon emissions. To address the waste and environmental pollution caused by hydrocarbon emissions from the unit, technical improvements were implemented taking advantage of the major maintenance period of the unit in March 2004. The main aspects of this technical modification are: adding a small metering pump along with the corresponding piping between separators 8-V2 and 1-V7, in order to pump the hydrocarbon liquid from 8-V2 into 1-V7 for further processing. Process after technical modification of the device’s hydrocarbon emission unit. Through technical upgrades to the unit’s hydrocarbon emission system, the hydrocarbons released during unplanned shutdowns of the unit can be reprocessed. After nearly 9 months of operation, the results have been good. VI. Economic and social benefits of reducing venting losses from the plant’s torches. According to production operation statistics, from April to December 2004, there were 4 instances of unplanned shutdowns of the plant, a significant reduction compared to the number of such shutdowns during the same period in previous years. Based on calculations using the equation of state for non-ideal gases, natural gas savings from the technical upgrades until the end of the year amounted to 11.7×104 m3. At a cost of 1.0 yuan per m3, the total savings in RMB were: 11.7×1.0 = 11.7 (10,000 yuan). After deducting the investment of 12,000 yuan for the technical upgrades, the actual savings amounted to: 11.7–1.2 = 10.5 (10,000 yuan). Unplanned shutdowns of the equipment resulted in the emission of 2.4 tons of liquid hydrocarbons each time; after the technical upgrades, 26.8 tons less of liquid hydrocarbons were emitted compared to the same period in previous years. At a cost of 2,000 yuan per ton, this equates to 26.8×2000 = 5.36 (10,000 yuan) in savings. After deducting the investment of 18,000 yuan for the upgrades and the monthly operating costs of 200 yuan from the time of the upgrades until the end of the year, the net savings amounted to: 5.36–0.02×9–1.8 = 3.38 (10,000 yuan). In addition to these direct economic benefits, the technical upgrades also improved the system stability of the production equipment and reduced environmental pollution, resulting in significant social benefits as well. VII. Conclusion To continuously reduce the frequency of unplanned shutdowns of the equipment and minimize losses due to flare venting, the following recommendations are put forward: 1. Continue to organize various forms of technical training in order to improve the technical skills of operators and enhance their ability to analyze and identify abnormal conditions in the equipment. 2. Conduct proper evaluations of various aspects related to production operations within the teams; in addition to output and yield, the number of times the equipment stops operating can also be assessed, thereby encouraging team members to always strive to avoid such stops. 3. Strengthen the maintenance of equipment and automatic control systems, so as to continuously improve the ability of equipment and instrumentation maintenance personnel to troubleshoot faults. □