Repair and Maintenance of High-pressure "Y" Globe Valves

Application and characteristics of "Y" globe valves
 
A y-type globe valve is widely used, belonging to the cut-off valve, which consists of a valve seat, valve disc, valve stem, valve cover, nut, handwheel, packing and gland. This kind of valve controls the opening and closing of the valve by matching the valve disc installed under the valve stem with the flange part of the valve body. The principle is to change the distance between the valve disc and the valve seat by rotating the handwheel to transmit the downward pressure of the valve lever so that the sealing surface of the valve flap and the sealing surface of the valve seat are closely attached to prevent the medium from circulating. Cut-off valves can be divided into straight-through type, straight-through type and right-angle type according to the fluid flow direction. The inlet and outlet channels of the straight cut-off valve are in a straight line, installed on a straight road, which is convenient to operate and widely used. However, when the fluid passes through the valve seat, it has to turn 90, and the resistance is large. The inlet and outlet channels of the DC cut-off valve are in a straight line, and the fluid flow is also in a straight line, so the fluid resistance is small; Right-angle globe valve, with its inlet and outlet channels at a right angle, is installed in vertically intersecting pipelines and is often used in high-pressure pipelines. Cut-off valves are simple in structure, convenient to manufacture and maintain, generally used for throttling, and only allow a one-way flow of media. Directionality should be paid attention to during installation. In actual production, it can also be used to adjust the flow rate, and the operation is determined by the valve opening. In practice, because of its small opening height and small up-and-down stroke, the opening and closing of the small-bore "Y" stop valve is basically not affected by the medium pressure in the pipeline, so it has a high utilization rate in plant production. However, the disadvantage of this kind of valve is that its structural length is large, especially in the case of a non-straight-through large-diameter structure, the medium resistance is large, the opening and closing is difficult, and the sealing surface is also vulnerable to erosion and corrosion. During long-term operation, the sealing is easy to fail, resulting in internal leakage of the valve. Therefore, during the 3-year overhaul period of the device, the stop valve must be inspected, especially in the operating environment with frequent opening and closing and unclean medium, and it should be regularly maintained.
 
Cause of incident

In March 2021, a pressure fluctuation occurred during the operation of the hydrogenation unit of a petrochemical company, which caused a pilot-operated safety valve with a working pressure of 15.6MPa to take off. When the standby unit was switched, it was found that the two "Y" cut-off valves in front of the valve had different degrees of leakage. As a result, the pilot valve can't be normally closed and cut off, which affects the normal operation of the cold high-pressure separation system of the device, and causes certain influences. The hydrogenation unit relies on high-pressure hydrogen to drive the raw oil to flow, which belongs to a high-temperature, high-pressure, flammable and explosive medium. Therefore, the requirements of valve equipment and facilities for sealing and cutting off are also high. The valve discussed in this paper is imported from VELAN and applied to the top of a cold high-pressure tank. The maximum operating pressure is 16.8MPa and the maximum operating temperature is 300℃. The medium is cold and hot high-pressure gas, which contains hydrogen sulfide and oil and gas. The system pressure fluctuation of the device and the dust particles in the medium lead to the overpressure and leakage of the pilot safety valve behind the Y-shaped cut-off valve. The frequent disassembly and commissioning of the safety valve lead to the frequent opening and closing of the cut-off valve, which damages the sealing surface for a long time.
 
On-site repair and maintenance
 
Repair the on-site stop valve. First, prepare materials and tools: grinding sandpaper, wrench, hand electric drill, valve wrench, pipe pliers, bolt loosening agent, grinding fixture and other tools. Then disassemble in sequence, first the handwheel and back cap, then the valve stem and disc are pulled out. The difficulty of the process lies in the second step. Because the spline at the bottom of the valve stem has not been lubricated for a long time, the accumulated impurities in the long-term operation cause it to be stuck and cannot be taken out. It is necessary to soak and lubricate it with kerosene with strong permeability for half a day, and then use the handwheel as a tool, and dismantle it by reverse force. After the valve stem and disc are taken out, put the parts in order, and check the damage to the valve seat and disc.
 
1. Inspection after the disintegration

After the valve is disassembled, it is observed that the sealing surfaces of the valve disc and the valve seat are slightly scratched and pitted, the bearing is rusted and stuck, and the packing of the valve seat is seriously aged. At the same time, it is also found that although there are some scratches on the sealing surface, it does not cause such a big leakage. As a result, the frequent opening and closing of the valve often causes slight damage to the sealing surface, but the failure to close the valve in place due to the lack of lubrication of the rotating parts is the main reason for the leakage of the Y-shaped cut-off valve.
 
2. Repair steps and operation contents

(1) After the valve is disassembled, it should be placed in order, and all parts should be polished, cleaned, de-rusted, decontaminated and degreased.
 
(2) Check the quality of all parts of the valve. The "y" globe valve for this repair is DN50, Class1690LB. In a high-temperature environment, the quality of valve parts should be strictly controlled. In addition to checking the valve disc and valve seat, the valve body, packing, spline, bolt and nut should also be strictly checked, cleaned and greased, so as to have zero tolerance for defective parts and components and ensure the safety of use.
 
(3) The inspection found that the sealing surfaces of the valve seat and disc are slightly corroded and damaged, and need to be ground. First, grind the valve disc: it is easy to grind the valve disc. After the valve stem and the valve disc are pulled out, rough grinding is carried out with sandpaper, deep scratches and pit erosion of the valve disc and the valve seat are ground off, and then grinding paste prepared by kerosene and grinding powder is used for fine grinding. When grinding, check the disc surface frequently, and adjust the angle and strength until the sealing surface is smooth and metallic luster appears. This valve is a welded globe valve with a long valve body structure. The grinding of the valve seat is not conducive to the entry operation with the grinding fixture, so the opposite grinding is selected. Before grinding, the jig should be used for rough grinding. The specification of the stop valve is DN50, and the structure of the valve body leads to poor visual effects, except that the hand cannot enter and it is inconvenient to operate. It takes experience and hands touch to judge the grinding degree. The rough grinding of the valve seat is similar to the grinding of the valve disc, mainly to remove oil stains, deep scratches and pit corrosion on the surface. Then, finish grinding with grinding: use the qualified valve disc, add the ground paste made of kerosene and grinding powder from coarse to fine, and grind clockwise to avoid new scratches on the valve seat caused by excessive grinding. From coarse to fine, then to fine grinding, and finally grinding to the degree of metallic luster to mirror surface.
 
(4) If the rolling bearing is damaged, replace it with a new one.
 
(5) All sealing packings shall be replaced, and the stuffing box shall be cleaned and processed.
 
(6) All processed and repaired parts shall be inspected before being reassembled.
 
Check and inspect the newly processed parts and repair valve clacks and valve seat sealing surfaces. The red lead powder diluted with kerosene is used for the inspection of the sealing surface, and it is evenly coated on the sealing line of the valve disc with the appropriate width. Then, it is combined with the valve seat through guide control and then pulled out after proper force rotation. Use a flashlight to observe the sealing line formed by the valve flap and valve seat, and check whether it is closed. If it is a closed line, the repair condition is qualified; otherwise, it needs to be re-researched.
 
(7) All parts of the valve should be cleaned and assembled. After assembly, the valve switch is flexible, the lubrication is in place, and the repair operation is finished.
 
3. Daily operation and maintenance

During the production and operation of the high-pressure cut-off valves, attention should be paid to daily maintenance. The valve stem thread often rubs against the valve stem nut, so it needs to be lubricated regularly. For valves that don't open and close frequently, turn the handwheel regularly to prevent them from being stuck. Every time the valve is opened and closed, it should be turned back half a circle, which has prevented false closing; When closing the valve laboriously, use a special wrench, and try to avoid using pipe tongs. Do not use too much force or tools to close the valve excessively. For outdoor valves, the valve stem should be sheathed to prevent dust and other impurities from being involved in nuts when opening and closing the valve. When LDAR is used to detect the leakage of packing, it should not be tightened by excessive force, and its compression space should be reduced as much as possible for the protection of packing. In case of leakage, it is best to directly cut off the valve and then replace the packing. Do not operate under pressure when replacing, especially the medium with high temperature, high pressure and easy corrosion, so as to prevent personal injury. The valve parts should be checked regularly for defects, corrosion, etc., without sand holes, air holes or cracks in appearance. The packing gland should be flattened and the switch should be flexible. The pressure and temperature grade of the valve used shall be consistent with the working pressure of the pipeline. Always keep the valve clean. You can't rely on the valve to support other heavy objects, and you can't stand beside the valve. The valve body and handwheel of the valve should be painted and anticorrosive according to the management requirements of process equipment. The valves on the system pipeline should be numbered according to the process requirements, and the valves should be listed with the number when opening and closing to prevent misoperation.
 
Conclusion
 
The sealing surface of the "Y" cut-off valve is easily eroded by impurities, resulting in sealing failure. Therefore, regular inspection, operation and maintenance should be carried out during normal production and operation. Summary of field application: As the safety umbrella of the device, this high-temperature and high-pressure "Y" globe valve play a very important role in the production device, so the daily opening and closing should be moderate. Due to the dust particles mixed in the medium, the sealing surface may cause sealing failure under the condition of repeated opening and closing. Therefore, when the safety valve is put into service, the first cut-off valve should be opened first, and the second cut-off valve should be gradually opened, and observation should be stopped to ensure the safety of production and use of the Y-shaped cut-off valve. Due to the influence of factors such as medium characteristics, operating environment, frequency of operation, product quality and improper use and maintenance, valves will have various failures. If they are not discharged in time, it will not only affect normal production, but also sometimes bring disaster. Therefore, maintenance should be strengthened to discharge the failures in time.