【Ethylene Cracking】Process flow of cracking gas main valve and coke cleaning valve
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During normal ethylene production, the large valve for the pyrolysis gas is kept fully open while the coking removal valve is closed, to ensure that the pyrolysis products are incorporated into the main pipeline network； During coke cleaning operations or plant shutdown, the main valve for pyrolysis gas is closed, thereby isolating each individual pyrolysis furnace from the system. This prevents the pyrolysis gas in the pipelines from coming into contact with burnt gas, ensuring safe operation. The large valve for cracking gas operates on a periodic basis, usually once every 3 months or so; the coke cleaning valve operates approximately once a month. The entire cycle takes around 4 months. Due to differences in production processes among various factories, these cycles vary as well.
In the 1 million-ton ethylene project of Sino-Saudi (Tianjin) Petrochemical Co., Ltd., the large valves for the pyrolysis gas and the valve used for coking removal in the pyrolyzer unit are both double-plate full-bore valves manufactured by the German company “Zimmermann&Jansen”. These valves are used together, and they play a crucial role in the process; they represent key control points in the pyrolyzer control system. The large valve for pyrolysis gas is located before the pyrolyzer output is connected to the main pipeline, while the valve for coking removal is situated at the starting point of the line through which the coked gas is discharged (from the coking tank and the furnace). Therefore, the operating condition of the cracking gas main valve and the coking cleaning valve has a direct impact on the safe production of ethylene. In fact, this valve would regularly experience problems such as lagging or sticking, which posed a serious threat to the safe production of ethylene. Therefore, to ensure safe operation of the process, the valve is regularly maintained according to the situation, with some wear-prone parts being replaced.
Performance parameters and structural features of the pyrolysis gas main valve: 1. It features a double-disc gate valve with bimetallic sealing surfaces, which ensures good sealing performance. Sealing is achieved by the compression of a wedge element between the two discs when the valve is closed; when the valve is opened, the wedge element relaxes, resulting in less friction between the valve discs and the sealing elements, thus facilitating opening of the valve. Considering the operating conditions, the large valves for pyrolysis gas and the coking removal valves must be able to withstand high temperatures as well as wear; therefore, only the use of high-temperature resistant metal seals can extend the service life of these seals. Therefore, this type of valve is generally preferred for ethylene production.
2. Generally, they have a large diameter; as a result, double-disc gate valves with bimetallic sealing surfaces are very large in size, weighing around 28 tons and reaching heights of up to 8 meters； The coke cleaning valve is relatively small, reaching up to 5 meters in height, with a weight of around 2 tons. 3. Since the pyrolysis gas main valve and the coking removal valve serve to isolate either the pipeline network or the external atmosphere, the process requires that they be closed tightly with zero leakage in order to ensure safety. At the same time, the pyrolysis gas main valve and the coking removal valve are connected by mechanical interlocking: the coking removal valve closes when the pyrolysis gas main valve opens, and the pyrolysis gas main valve closes when the coking removal valve opens. 4. The actuator is an electric angular travel actuator, featuring high output torque, long travel distance, and good stability.
For the isolation and regulation of the inlet pipelines of the gas turbine in the catalytic cracking units of various refineries, given the large diameter of these process pipelines and the fact that the process medium is high-temperature flue gas (up to 750°C), triple-eccentric butterfly valves are primarily used. The largest valve sizes used in domestic installations reach DN1,800 mm, and those equipped with hydraulic actuators can achieve a closing time of less than 0.5 seconds. Triple-eccentric butterfly valves are also widely used for the isolation and regulation of other pipelines such as those at the inlet and outlet of the main fans, vent valves, the inlet and outlet pipelines of compressors, flare vent valves, the inlet and outlet of slurry pumps, as well as various high, medium, and low-pressure steam pipelines. Liquefied petroleum gas is highly prone to leakage; therefore, to ensure high flow capacity while preventing leaks, it is more reliable to use triple-eccentric butterfly valves in the main inlet and outlet pipelines of its storage tanks. Furthermore, in sulfur recovery units, liquid sulfur that needs to be transported in a molten state currently requires the use of three-eccentric butterfly valves equipped with steam jackets.