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On-site emergency handling for the detachment of the valve disc in process pipeline gate valves In current chemical industry operations, safe and continuous production is fundamental to ensuring the efficiency of a plant or even an entire enterprise; therefore, ensuring the continuous operation of such plants holds a very important role in daily equipment management. In the daily operation of the plant, there are many factors that can affect its continuous operation; the most common ones include leaks in containers and pipelines, failures in moving equipment, damage to stationary equipment, and blockages in pipelines. Pipeline blockage accounts for a significant proportion of the faults that disrupt the continuous operation of devices. Since pipeline blockages not only lead to a reduction or cessation in the normal, continuous flow of the medium, but can also cause equipment damage due to overpressure, dealing with such issues under pressure is extremely difficult due to the enclosed environment. Therefore, finding a way to resolve pipeline blockages without disrupting production and ensuring smooth flow of the medium remains a challenging problem that needs to be addressed on a regular basis. Generally, when the device is initially designed, measures are taken to prevent medium coking and salt buildup from blocking the pipelines, and appropriate treatment methods are prepared in advance. Under normal conditions, such pipeline blockages do not occur; and even if they do arise, the existing treatment measures ensure that production is not affected ； What is truly difficult to handle or unpredictable is the pipeline blockage caused by the detachment of the valve discs in pipelines; in such cases, if it is not possible to remove or replace the valves in the process pipelines, it will inevitably pose a direct threat to normal production, with severe consequences. In chemical production, valves are frequently used for stopping and controlling the flow of media in pipelines. There are many types and quantities of such valves, including gate valves, globe valves, check valves, plug valves, and ball valves ； Among them, gate valves are widely used in production due to their simple structure and ease of use. Through continuous efforts in exploration, research, and testing, the problem of the valve disc falling off in straight-rod wedge-type rigid single-disc gate valves has been resolved. These valves are now being used in production facilities, with excellent results achieved. II. Structure of Gate Valves A gate valve is a type of valve in which the opening and closing action (of the valve disc) is carried out by a valve stem, which moves up and down along the sealing surface of the valve seat, thereby allowing the flow of fluid to be either enabled or blocked. Gate valves have low flow resistance and require little effort to open and close, making them widely used for controlling the flow in pipelines carrying various types of media. When the gate valve is partially open, vortices are generated on the back side of the gate, which can lead to erosion and vibration of the gate as well as damage to the sealing surface of the valve seat; repair is difficult. Therefore, gate valves are generally not used for throttling. The structure of the gate valve is shown in Figure 1, and it mainly consists of components such as the valve body, valve cover, gate plate, valve stem, and handwheel. Based on the structure of the valve stem, gate valves are divided into open stem and hidden stem types ； Based on the structure of the valve disc, they can be classified into wedge-type rigid single-disc gate valves, wedge-type elastic single-disc gate valves, wedge-type double-disc gate valves, and parallel-disc gate valves. The one shown in Figure 1 is a stem-mounted wedge-type rigid single-disc gate valve. 1. Valve body 2. Gasket 3. Nut 4. Double-headed bolt 5. Valve cover 6. Packing 7. Packing gland 8. Stem nut 9. Handwheel 10. Stem 11. Gate valve Figure 1 Structure of gate valve (1) Rod-type wedge-shaped rigid single-gate valve The structure of its gate is shown in Figure 2. The gate is a wedge-shaped unit, with its sealing surface forming an angle with the vertical center line of the gate. It is characterized by a simple structure, small size, and reliable performance. However, the machining precision of the wedge angle at the sealing surfaces of the gate and valve seat is very high, making both machining and maintenance difficult. Moreover, during the opening and closing process, the sealing surface is prone to scratches, and the gate is likely to get stuck when temperatures change. In such cases, if there is severe corrosion at the junction between the valve stem and the gate, or if there are quality issues, trying to open the valve forcefully can cause the junction between them to separate, resulting in the gate falling off. This type of gate is suitable for gate valves under various pressures in normal and medium temperature conditions. Figure 2: Wedge-type rigid gate (2) Wedge-type elastic single-gate valve. The structure of its gate is shown in Figure 3; an annular groove is provided in the middle of the gate, or it is formed by welding two gate plates together, with a hollow space in between. The wedge angle is designed in the same way as that of the rigid gate. It features a simple structure, reliable sealing surfaces, and the ability to self-compensate for deformation of the valve body caused by abnormal loads, thereby preventing the valve disc from getting stuck. However, the closing torque should not be too high to avoid exceeding the elastic range of the valve plate. Elastic valve plates are suitable for medium and small diameter gate valves operating under various pressures and temperatures, as well as in applications where frequent opening and closing is required. However, it requires the medium to have few solid impurities, and it is not suitable for media that tend to coking. Figure 3 Wedge-shaped elastic gate valve: In daily production, the problem of the detachment of wedge-shaped rigid single-gate valve gates is quite common. Practice has shown that this type of valve gate detachment can be resolved ； Other types of valve plate detachment issues are less common, remaining only at the theoretical level and not verified through practice; therefore, they are not discussed in detail here. III. On-site emergency handling when the valve disc falls off 1. Principle of handling When the valve disc of a gate valve falls off, it is in the fully closed position. Due to the high precision of the wedge angle between the valve disc and the valve seat sealing surface, a sealed cavity is formed at the top of the valve disc and within the valve, as shown in Figure 4. When the valve plate comes off, a small amount of fluid remains in the cavity. Thanks to the excellent sealing performance between the gate and the valve seat surfaces, when a hole is drilled at the position corresponding to the center of the valve stem, no large amount of fluid leaks out, thereby preventing safety accidents and creating the conditions necessary for threading to be carried out next. By utilizing this principle, a special device (or nut) is first welded to the valve body at a position opposite the center of the valve stem. When a hole is drilled through the valve body at that center point, the threads of the special welding device (or nut) are used to enlarge the diameter of the drilled hole. A special push rod can then be used to push the valve disc away from the sealing surface of the valve seat; by properly controlling the movement of the push rod and the valve stem, the valve can continue to function normally. Figure 4: Schematic diagram of the cavity 2. Treatment measures 1) For ordinary low-pressure valves with a diameter of DN80 or less, and where the fluid flowing through them is non-toxic and non-flammable, an M14 nut can be welded at the position corresponding to the center of the valve stem. A φ12mm hole is drilled at the bottom of the valve, also at the position corresponding to the center of the valve stem, through the central hole of the nut. The threads of the welded nut are used to thread the hole, after which a special push rod is used to push against the valve disc in order to open the valve and operate its switch. 2) For valves with a diameter of DN80 or larger, or those that operate under high pressure and have flammable or toxic fluids as the flow medium, it is necessary to weld a special device at the position corresponding to the center of the valve stem. A through-hole is drilled at the bottom of the valve, also at the position corresponding to the center of the valve stem, using this central hole in the special welding device. The threads in this central hole are then used to thread the drilled hole, after which a special push rod is used to push the valve disc, thereby opening the valve and enabling further control over its opening and closing. The resulting structure after installation is shown in Figure 5. 1. Valve 2. Valve disc 3. Special nut 4. Screw rod 5. Packing Figure 5: Schematic diagram of the device after installation. Before using the push rod, it is necessary to determine an appropriate sealing method between the push rod and the valve body, based on the pressure and temperature of the medium conveyed by the valve. Generally, corresponding nuts and washers are pre-installed on the push rod. After the valve is opened and the flow rate is adjusted, sealing material such as asbestos rope or PTFE tape is wrapped around the push rod and the base of the valve body; finally, the nuts are tightened to achieve sealing. For valves under high pressure, specially processed gaskets can also be used for sealing. 3. Precautions during processing 1) Since the structural shapes and dimensions of valves vary among different manufacturers, it is advisable to use the dimensions recommended in the table below for drilling, in order to prevent excessive drilling from damaging the sealing surfaces of the gate and valve seat. Recommended Drill Size Table 2) For valves whose operating medium is flammable gas or liquid, butter should be applied to the drill bit before drilling, and a water hose should be used to keep flushing continuously to prevent the medium from catching fire. 3) Since the bottom of the valve is subjected to significant stress when the push rod is used to push the valve open, this method is not suitable for cast-iron valves, in order to prevent the valve body from cracking due to its low strength. 4) The accurate positioning of the valve plate at the center of the bottom of the valve body is a key factor determining the quality of drilling; therefore, it is necessary to carry out careful measurements and determine its position with precision. This method was employed to resolve the problem of valve plate detachment in the diesel valves and the feed water valves for the coking cycle water in the plant’s first atmospheric and vacuum distillation unit, achieving good economic benefits. IV. Conclusion The on-site emergency handling of valve disc detachment has completely resolved the problem of valve disc detachment in pressurized pipelines, which has long plagued production, and has played a very positive role in ensuring the continuous operation of the equipment.