Cause of Leakage and Fire of Globe Valve Packing

When the newly built 6 000 kt/a atmospheric and decompression plant of a certain enterprise was started, the packing of the globe valve of the raw material line of the heating furnace leaked and caught fire, causing the plant to shut down. The disassembly analysis of the globe valve found that the gap between the valve stem shaft and the sealing seat hole was improper. Structural modification was carried out for the similar globe valve of the device, and the smooth operation of the device was realized.

The friction force of the sealing surface of the globe valve is smaller than that of the gate valve in the opening and closing process, which is wear-resistant, has good sealing performance, small friction force between the sealing surfaces, long life, small opening height, simple structure, convenient manufacturing and maintenance, small stroke, short opening and closing time and easy to work. It is often used in the auxiliary line valve of the control valve.
 
On October 6, 2012, during the operation of 6000 kt/a atmospheric and decompression of a certain oil refinery, the instrument verified the control valve FV-4008B of the raw material line of the atmospheric furnace, the medium was switched to the auxiliary line, and the front and rear valves of the control valve were closed. Open the sub-line globe valve. After the instrument verification control valve is completed, the medium is switched to the original process; the front and rear valves of the control valve are opened, and the auxiliary line valve is closed. In the process of closing the auxiliary line valve, the raw material oil leaked from the packing and caught fire. The fire is extinguished with an effort. Cut off the feeding; replace the accident valve, and disassemble and analyze the globe valve.
 
1. Leaked valves
1.1. The technical parameters of the valve are shown in Table 1
Table 1 Technical parameters of the valve

1.2. The structure of the valve is shown in Figure 1.
 

Figure 1 The structure of the valve
 
2. Cause of leakages
2.1. Inspection of disassembly of valves
After the fire, the valve was disassembled and inspected. After the valve was disassembled, it was found that most of the sealing seat on the valve had been unscrewed. The sealing seat is threaded; the packing lost the sealing pretightening force, and the packing was flushed by the medium. The valve stem lost its sealing protection; the medium leaked, and a fire occurred, forcing the device to shut down. After measurement, the matching gap between the sealing seat hole and the valve stem is 0.28 mm, and the standard gap should be 0.365 to 0.575 mm, as shown in Figure 2.


Figure 2 The structure of the valve after disassembly
 
2.2. Reasons for packing leakage
After inspecting the disassembled valve, it was considered that the gap between the valve stem and the upper sealing seat was too small, which is smaller than the standard gap. The standard gap is 0.365 to 0.575 mm, but the gap of the accident valve was only 0.28 mm. Under high temperature, the valve stem and the upper sealing seat are thermally expanded and stuck. Because the upper sealing seat has a positive thread, the thread rotates in an upward tightening direction in the valve opening process, and the thread becomes tighter and tighter; the sealing is strengthened, so the packing will not leak in the valve opening process. In the closing process of the valve, the screw thread rotates in the direction of loosening. Because the upper sealing seat and the valve bonnet are not welded, the upper sealing seat moves downward together with the valve stem, and the valve stem and the upper sealing surface separate from the valve seat; the gap becomes larger and larger. The pre-tightening force of the valve packing decreases, which cannot meet the requirements of the sealing medium, and the packing is flushed by the medium. The valve loses its sealing, and the medium leaks, causing a fire. The leaking part of the valve packing is shown in Figure 3.
 

Figure 3 Leakage locations of valve packing
 
3. Improvement measures
(1) After the accident, the valves of the same model were measured, and the gap between the valve stem and the upper sealing seat was 0.30 to 0.40 mm. The upper sealing seats of 8 stock valves were processed, and the diameter of the inner hole was enlarged; the gap was 0.65 to 0.70 mm, and the upper sealing seat and the valve bonnet were welded to prevent the upper sealing seat from being unscrewed when the valve is closed. It is installed on site and put into use. For the 7 replaced valves, the manufacturer replaced the upper sealing seat. After the replacement, the gap between the upper sealing seat and the valve stem was measured to be 0.58 to 0.70 mm, and the pressure test was qualified as a spare.
(2) When opening and closing the globe valve, use steady force without impact. When the valve is fully opened, turn the handwheel a little to prevent the threads from being loosened and damaged.
(3) Strengthen the post-maintenance of the packing. During the inspection, check whether there is any leakage in the stuffing box. The problems found in the inspection should be dealt with technically in time. The valve stem must be kept clean, and the packing needs to be filled with lubricating oil. The maintenance of the gland and bolts is also very critical, and some corrosive media will also damage them. For each overhaul of the control valve, the sealing packing must be replaced, and the gland and bolts depend on the situation.
(4) The filler cannot be used beyond the expiry date, and the use cycle should be controlled to be less than its working cycle, so that it can avoid unexpected situations.
 
4. Conclusion
The structural modification of 8 valves was done. The valves are in good condition after operating for 6 months, which proves that the valve modification is successful. The valve modification saved a lot of time for the smooth operation of the device. At the same time, it also improved the further understanding of the valve structure by the equipment technicians, and provided experience for the subsequent management of the equipment.