Preventing Coking of High-temperature Valves in the Coking Unit

Preventing Coking of High-temperature Valves in the Coking Unit

1. Overview
A 2.4Mt/a delayed coking unit adopts the process of two furnaces and four towers, and normally produces green coke for 24 hours. There are 16 high-temperature valves on the device. The medium is residual oil or oil and gas. The operating temperature is about 500°C. The medium is easy to coke. To prevent the medium from coking and affecting the opening and closing of the valve, each valve has a steam sealing line. Steam with pressure of 1.0MPa is continuously injected into parts such as valve cavities and packing to prevent coking. The total sealing steam consumption of these 16 high-temperature valves is about 6t/h at 1.0MPa. After entering the valve body, the steam will be converted into sulfur-containing sewage, which not only consumes high energy, but also increases the load of downstream sewage treatment plants.
 
2. High-temperature valves
The high-temperature valve is installed in the feed line and oil and gas line of the coke tower of the coking unit.
 
2.1. Feed-blocking valves
The feed blocking valve adopts a ball valve, which has 4 steam sealing steam injection points, namely valve cavities, packing box, inner and outer cavity of bellows. When the valve is fully opened or fully closed, the valve cavity, packing box and the outer cavity of the bellows are injected with sealing steam to form a closed high-pressure space between the ball, valve seat and packing. The steam is not consumed when the sphere and the valve seat are attached well. However, as the valve is opened and closed multiple times, there is slight wear between the ball and the valve seat, and a small amount of steam will leak. The packing is a non-metallic material, and the sealing is relatively good. The temperature of the steam seal pipeline is low and the leakage is less. The temperature of the steam seal pipeline of the valve cavity and the outer cavity of the bellows is high, and the leakage is much more. After the steam sealing steam is injected into the inner cavity of the bellows, a closed space cannot be formed when the valve is fully opened, but the steam continuously enters the high-temperature residual oil to prevent the high-temperature residual oil from entering the inner cavity of the bellows, thereby preventing the inner cavity of the bellows from coking and causing the valve not to be opened and closed well. When the valve is fully closed, after the steam sealing steam enters the inner cavity of the bellows, a closed high-pressure space can be formed in the pipeline between the feed-blocking valve and the four-way valve, and there will be a small number of leakages for steam.
 
In the opening and closing process of valves, the closed space between the valve cavity and the outer cavity of the bellows is destroyed, and a large amount of steam will be consumed at this time. To save steam consumption, an orifice plate is installed on each steam seal line. The steam consumption of the steam seal is mainly related to the steam pressure, steam temperature and the aperture of the orifice plate. The flow rate of the 1.0MPa steam superheated at 50°C through aperture orifice plates of 3/16 in and 1/4 in is 195kg/h and 340kg/h. The sealing steam consumption of each feed-blocking valve is a maximum of about 740kg/h, and the duration is about 30 seconds. Since the feed blocking valve is deactivated for 17.5 hours during the full closing period of the great steam blowing, coking and other stages in each production cycle within 48 hours, the average sealing steam consumption of each valve is about 174kg/h.
 
3. Conclusion
After the valve steam seal is changed to the rich gas seal, it will not affect the use of the valve, nor will it have a greater impact on the production process. After the actual use of the oil and gas isolation valve, the effect is good, and similar devices can also be used for reference.