Problem description
The problems exhibited by the high-temperature butterfly valve of the fourth unit of the heavy oil catalytic cracking unit after operation include:
1.The original valve manufacturer experienced the same malfunction after replacing the valve three times and running for less than 5 months;
2.The instrument control system of the high-temperature butterfly valve feedback valve body action is not smooth, and the actual feedback only reaches 1.5% when executing 50% opening;
3.The feedback from the instrument control system of the high-temperature butterfly valve caused a malfunction in the execution of the action, resulting in a high-temperature alarm in the entire system, which lasted for more than 2 months;
4.After the instrument control system of the high-temperature butterfly valve gives a closing signal, the valve cannot be closed smoothly; 5. Due to design flaws and limitations in the transmission mechanism of the high-temperature butterfly valve, it cannot be manually operated on site and cannot achieve the required closed state, resulting in a high-temperature alarm in the entire system and equipment paralysis and inability to operate normally.
Fault analysis
The reasons for the above problems are as follows:
1.When the high-temperature butterfly valve is in the open state, the sealing surface of the valve plate is deformed due to poor rigidity, causing the valve plate to jam when opened. The main factor causing deformation is due to the difference in stiffness of the valve plate, which mainly comes from the deformation of the valve plate, valve stem, transmission device, and accessories under gravity after the high-temperature butterfly valve is subjected to high temperature. The valve plate, valve stem, transmission device, and accessories deform into an elliptical shape, causing them to jam during opening and closing;
2. The given valve position adjustment system of the instrument control system cannot control the valve plate to execute the opening or closing program, and the corresponding action received by the transmission device is invalid, unable to achieve the instructions issued by the program, thus issuing a fault alarm. When the fault alarm cannot be eliminated, it will cause the temperature of the heavy oil catalytic cracking unit to be too high, and the high-temperature flue gas containing a large amount of catalyst dust and water vapor will accumulate, which may lead to dangerous safety accidents in the equipment.
Solution
The problem with the instrument control system of the high-temperature butterfly valve has been resolved and restored to normal use after replacing the valve and transmission device. The main problem is that after disassembling the valve body for maintenance, it was found that the valve stem and valve plate had completely deformed, causing irreparable damage. Please refer to the attached figure for key performance. In addition, deformation and visible gaps were found at the connection between the transmission device and the valve stem. Through research, a new structure has been designed to address the safety hazards mentioned above, and after strength verification, the new structure can meet the mechanical strength design requirements and process requirements. The main function of the valve stem is to complete the corresponding motion action according to the received instructions under the control of process requirements and instrument signals. The energy is transmitted to the valve plate through the valve stem to achieve the opening requirements of the valve, maintain the relative position of the valve plate to the valve body, and the valve stem must withstand the force of the medium and the transmission force of the driving device to the valve plate, ensuring the function of regulating, controlling or cutting off.