I. Introduction
Backflushing of filter bags is a crucial measure for maintaining filter bag permeability and sustaining long-cycle production. The effectiveness of backflushing directly impacts the filter's function. In petrochemical production, backflushing in the polypropylene process is a prime example. A large amount of powdered polypropylene accumulates in the D802 tank, requiring nitrogen for transport. In D802, the polypropylene powder settles under gravity, while the nitrogen needs to be circulated from the filter bags. During nitrogen flow through the filter bags, the polypropylene powder adheres to them, clogging the bags and hindering nitrogen discharge. High-powered backflushing plays a vital role here, using pulsed backflushing to shake off the polypropylene powder and maintain filter bag permeability.
The extrusion granulation unit of Sinopec's polypropylene plant is the core unit of the polypropylene process, and the D802 backflushing system is a crucial guarantee for the stable operation of the granulation unit. The quality of backflushing directly affects the continuity of production. If backflushing malfunctions, filter bag blockage will cause excessive pressure inside D802, triggering PSV804 and resulting in a large amount of nitrogen gas carrying polypropylene powder to leak out, causing environmental pollution accidents and posing a risk of explosion. It will also cause the C802 blower inlet pressure to alarm low. If the D802 backflushing fails, it may cause production interruption at best, and in severe cases, trigger safety and environmental incidents. Under the current new normal of extremely stringent safety and environmental requirements, ensuring the reliable operation of D802 backflushing is of great significance to the stable production of the plant.
II. Mechanical Structure Design of the Backflush Device
This system is a novel filter backflushing system. Powder or dust-laden gas in a container passes through the filter, the clean gas is expelled, and dust particles fall and accumulate. After a certain filtration time, the filter needs to be backflushed to blow off the dust adsorbed on it; otherwise, the filter screen will become clogged, preventing gas from passing through and escaping. Currently, the mainstream filter backflushing method uses pulse solenoid valves, programmed to perform backflushing at set intervals based on the gain and loss of electrical current. This system uses a gas collection box, and through the cooperation of a solenoid valve and a pneumatic control valve, the gas in the collection box instantly and rapidly flows into the filter screen, implementing reverse pulse shaking backflushing. As shown in Figure 1, the backflushing structure cleverly utilizes the cooperation of low-power solenoid valves and pneumatic control valves to achieve high-power backflushing with minimal energy consumption, achieving remarkable results.
Figure 1 Backflush control system structure
As shown in Figure 2, under normal circumstances, the solenoid valve is not energized. The air path controlling the pneumatic control valve enters the air collection box through the pneumatic control valve, helping the air collection box to quickly fill with air. The sealing cover is slightly smaller in diameter than the guide sleeve. The gas in the air collection box enters the upper part of the sealing cover through the guide sleeve, making the pressure on the upper and lower parts of the sealing cover equal. Under the action of gravity, the sealing cover moves downward and falls onto the sealing port of the filter, sealing the air inlet of the filter. The higher the air pressure in the air box, the tighter the seal. There is an air source pressure setter in the air path. When the gas pressure in the air collection box reaches the set value, the air source and the pneumatic control valve stop supplying air to the air collection box, maintaining a high-pressure state in the air collection box. When the DCS sets the backflush interval to expire and backflush is required using the gas in the air collection box, the solenoid valve is energized, and the solenoid valve controls the switching of the pneumatic control valve. The gas on the upper part of the sealing cover is instantly discharged through the pneumatic control valve, causing the pressure on the upper part of the sealing cover to be lower than the pressure on the lower part. The sealing cover is pushed upward by the pressure on the lower part and moves away from the sealing port of the filter. The high-pressure gas in the air collection box instantly enters the filter to backflush the filter screen. The upper part of the sealing cover has a spring, which acts as a buffer to prevent impact damage to the upper part of the guide cylinder when the sealing cover moves upward. After all the gas in the gas collection box enters the filter screen, the pressure difference between the upper and lower parts of the sealing cover becomes equal again. Under the action of gravity, the sealing cover moves downward and covers the sealing opening, and the gas pressurizes the gas collection box. This cycle continues. This ingenious design enables high-power backflushing with minimal energy consumption.
Figure 2 Detailed diagram of the backflush structure
III. System Energy Saving Analysis
Pneumatic control valves and solenoid valves are the core control components of this system and also the main energy-saving components. As shown in Figure 3, the pneumatic control directional valve uses gas pressure to move the main valve core, thereby changing the direction of gas flow.
Figure 3 Internal structure of the pneumatic control valve
When the solenoid valve is not energized, the gas supply port P and port A are connected, and nitrogen can be supplied to the gas collection box through port A. When the solenoid valve is energized, the piston of the gas control valve moves to the right, compressing the spring, and the quick exhaust port A of the gas collection box and the exhaust port T of the gas control valve are connected, and the gas in the guide tube is quickly discharged.
In this control system, the pneumatic control valve plays a crucial role. Under the control of the solenoid valve, a suitable amount of gas is quickly discharged, so that the differential pressure at the top of the guide tube is lower than that at the bottom. Under the action of the pressure difference between the top and bottom of the sealing cover, the sealing cover moves upward quickly against gravity, and the gas in the gas collection box quickly enters the filter bag for backflushing.
Figure 4. Pneumatic circuit diagram of the control system
As shown in Figure 4, the pneumatic circuit diagram of the entire control system, the instantaneous exhaust volume and exhaust time of the quick-release valve are the core of the entire system. Excessive exhaust wastes nitrogen and causes the sealing cover to move upwards too quickly, resulting in excessive acceleration and potentially causing the sealing cover to impact the upper part of the gas collection box. Based on the weight of the backflushing steel sheet, the backflushing air pressure, and the air chamber pressure, the amount of gas instantaneously discharged by the solenoid valve or pneumatic control valve under the combined action of the air chamber pressure and the steel sheet's own weight is measured to ensure the sealing steel sheet jumps instantly at a speed that meets the backflushing requirements without causing excessive acceleration and impact to the upper cover. Precise calculations show that a solenoid valve or pneumatic control valve with a response time of 500ms and a flow rate KV value of 20L/M fully meets the requirements of the backflushing system.
Traditional dust collector solenoid valves are high-power solenoid valves, requiring a minimum power of 17 watts and a power supply voltage of 24VDC, consuming 596.7 kWh of electricity annually. In this system, the low-power solenoid valves consume only 1.7 watts and operate on a normal de-energization, energized-on-demand basis, eliminating the need for energy consumption by the pneumatic control valves. Using a control mode where only one solenoid valve is energized for 1 second per cycle, the annual power consumption is less than 3 kWh.
IV. Introduction to the Control System
DCS is the abbreviation for Distributed Control System, also known as a distributed control system in the domestic automation field.
DCS (Distributed Control System) is a control system based on microprocessors, combining computer technology, signal processing technology, measurement and control technology, communication networks, and human-machine interface technology to realize process control and factory management. Its essence is to use computer technology for centralized monitoring, operation, management, and decentralized control of production processes. The system inherits the advantages of conventional analog instrument control systems and centralized computer control systems, overcoming the disadvantages of highly concentrated hazards in single-microcomputer control systems and the limited functions and poor human-machine interaction of conventional instrument control systems. It can be conveniently used for production control and management of industrial plants, and its application in process automation fields such as chemical, power, and metallurgy is already widespread.
Traditional backflushing typically involves installing a differential pressure transmitter outside the filter, setting a differential pressure value, and indicating filter blockage when a certain differential pressure is reached, requiring backflushing for dust removal. However, in the polypropylene chemical industry, D802 requires nitrogen supplementation to maintain sufficient pressure for transporting polypropylene powder. Therefore, backflushing is usually time-based, using a timed backflushing method. This allows for periodic cleaning of the filter bags while simultaneously replenishing nitrogen supply. A program is written in the DCS to cyclically energize four solenoid valves. Each valve is energized for one second, then de-energized, and after a three-second wait, the next solenoid valve is energized, and so on. The three-second wait time is the time it takes for nitrogen to fill the gas collection box. When the nitrogen pressure is 0.6 MPa and the delivery pipeline is 1/2" full, the three-second time allows the pressure in the 0.5 m³ gas collection box to reach 0.2 MPa, concentrating the entire box's power to backflush the filter bags. The powerful airflow and agitation are sufficient to keep the filter bags clean at all times.
V. Conclusion
This backflush system was developed by the instrumentation technology research team of Sinopec Jinan Branch, breaking through foreign technological blockades and the dependence on imported spare parts for polypropylene backflush equipment. Since its implementation in 2013, it has performed exceptionally well, yielding significant economic benefits and adding an extra layer of safety to the plant. It offers valuable lessons for other plants in the industry.
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