Problems and Measures in High Sulfur Natural Gas Field Valves (Part Two)
Dec 12, 2023
The overhaul valve maintenance work area of the high-sulfur gas field gas production center station includes three well stations and 10 valve chambers in the middle of the gas pipeline. It mainly covers the main process equipment including test separators, filter separators, gas-liquid separators, pigging areas and maintenance of all valves in the dehydration unit, supporting auxiliary production facilities, utility and fuel gas system areas. Valve maintenance includes gearbox maintenance, cleaning fluid and grease lubrication, and internal leakage inspection of valves. Gearbox rust removal, water removal, lubrication, functional testing and other operations are to ensure normal opening and closing of valves. The cleaning fluid can dissolve impurities between the valve ball and the valve seat, and the grease increases the lubrication of the valve sealing surface and reduces the probability of damage or scratches to the sealing surface. Internal leakage detection can determine whether the valve limit is correct, whether the sealing surface is intact, and whether it can function as an isolation barrier. In on-site production of high-sulfur gas fields, the replacement of filter elements of filter separators, process isolation, and temporary shutdown operations all require the valve to be opened or closed normally and quickly without internal leakages, which has a negative impact on the reliability and function of the valve. The health of the valves is worrying due to the harsh working conditions of high sulfur content and many impurities in high-sulfur gas fields, and the lack of maintenance since the start of production.
2. Problem occurring in the greasing nozzle
2.1 Leakages from the grease injection nozzle
The grease nozzle is an important part of the valve for grease maintenance. It is the only channel for cleaning fluid and grease to enter the valve. At the same time, when performing temporary leakage plugging, the sealing grease also enters the seal between the valve seat and the ball through the grease nozzle, achieving the effect of temporary closure. Inside the grease nozzle is a spring check valve, as shown in Figure 2. If it does not operate for a long time or is not lubricated for a long time, the one-way valve will suffer from corrosion and fracture of the spring; the spring will be stuck. The one-way valve cannot be reset, and sealing cannot be achieved, resulting in leakages. During the on-site grease injection maintenance of the valve, a backflow occurred after the grease injection nozzle was filled with cleaning fluid, and it was suspected that the one-way valve had failed.
Figure 2 Structural diagram of two types of grease nozzles commonly used on site
Solutions
Given the risk of leakage from the grease injection nozzle in the grease injection process, the following measures are formulated to reduce or eliminate the risk. After purging and replacing, perform grease injection. During the overhaul, the pipelines are purged and replaced before grease lubrication is performed. Use leak-locking connectors. For grease nozzles that may leak, use a leak-locking joint in advance, that is, add a one-way valve in front of the grease nozzle to prevent the pressure inside the valve cavity from dangerously releasing. During the overhaul in 2021, 3 old valve grease nozzle spare parts were replaced, and the remaining ones were not replaced. The external grease nozzle caps were temporarily used, and they will be replaced one by one in 2022 after the newly purchased parts are available. The grease injection valve purchase application form has been submitted.
2.2 Clogged drain nozzles
The drain nozzle is located at the bottom of the valve. Its main function is to discharge impurities inside the valve cavity and prevent impurities from scratching the sealing surface of the valve seat or valve ball. At the same time, through the leakage detection in the middle cavity of the drain nozzle, the current sealing status of the valve can be judged and whether there is internal leakage, which is very important for ball valves with shut-off. Given the high sulfur in high-sulfur gas fields, it is unacceptable for the drain nozzle to be exposed. The sewage nozzles on site are sealed with two O-rings, Figure 3. After the helium-nitrogen leakage test, the high-pressure nitrogen in the pipeline is used to conduct the valve internal leakage test. Some valves, such as the main shut-off valve on the upper part of the gas-liquid separator and the ball valve at the front of the test separator, had clogged drain nozzles, making it impossible to discharge sewage and detect leaks.
Figure 3 Structure of the sewage nozzle
Solutions
It is recommended to purchase a new customized sewage nozzle, disassemble and replace it for an overhaul next year, and clean the lower cavity of the valve body to ensure that the sewage nozzle is working properly. The main sizes of the sewage nozzle on site are 1/2 inch, 3/4 inch, and 1 inch. The purchase requisition form has been submitted. Periodic inspection and maintenance should be performed. During the annual overhaul, the valves are inspected for internal leakages and sewage discharge. At the same time, the condition of the sewage discharge nozzles is also thoroughly investigated. Problematic sewage discharge nozzles are recorded promptly and the causes of external leakage are analyzed.
3. Internal leakages of valves
During the overhaul in 2021, the internal leakage of the 373 ball valves in the high-sulfur gas field was tested. There are three different kinds of internal leakages, namely, valves with serious internal leakages, valves with slight internal leakages and valves without internal leakages. For valves with serious internal leakages, sealing grease should be injected. Slight internal leakages of valves can be temporarily blocked by injecting sealing grease. There are 14 valves with internal leakages in the well station, mainly testing the main valve and bypass valve at the front of the separator, plus the air inlet valve below the safety valve of the water jacket furnace. The specific internal leakage situation is shown in Figure 4. The ball valve with serious internal leakages is at the outlet of the water jacket furnace.
Figure 4 Leak detection at well stations
Solutions
The anti-sulfur ball valves currently used in high-sulfur gas fields are mainly used for testing main pipeline valves such as test separators, filter separators, gas-liquid separators and pigging areas. They all use the surfacing process of INCONEL625 plus ENP. When scratches appear on the sphere, the ENP needs to be deplated and then surfaced twice, resulting in a longer maintenance period and higher maintenance costs. At the same time, there is also the risk of incomplete deplating and the new coating falling off. Therefore, it is recommended that high-sulfur gas fields should not use this solution for ball valves in the future, and instead purchase new valves with A350 plus full surfacing as the base material or 625 as the base material.
2. Problem occurring in the greasing nozzle
2.1 Leakages from the grease injection nozzle
The grease nozzle is an important part of the valve for grease maintenance. It is the only channel for cleaning fluid and grease to enter the valve. At the same time, when performing temporary leakage plugging, the sealing grease also enters the seal between the valve seat and the ball through the grease nozzle, achieving the effect of temporary closure. Inside the grease nozzle is a spring check valve, as shown in Figure 2. If it does not operate for a long time or is not lubricated for a long time, the one-way valve will suffer from corrosion and fracture of the spring; the spring will be stuck. The one-way valve cannot be reset, and sealing cannot be achieved, resulting in leakages. During the on-site grease injection maintenance of the valve, a backflow occurred after the grease injection nozzle was filled with cleaning fluid, and it was suspected that the one-way valve had failed.
Figure 2 Structural diagram of two types of grease nozzles commonly used on site
Solutions
Given the risk of leakage from the grease injection nozzle in the grease injection process, the following measures are formulated to reduce or eliminate the risk. After purging and replacing, perform grease injection. During the overhaul, the pipelines are purged and replaced before grease lubrication is performed. Use leak-locking connectors. For grease nozzles that may leak, use a leak-locking joint in advance, that is, add a one-way valve in front of the grease nozzle to prevent the pressure inside the valve cavity from dangerously releasing. During the overhaul in 2021, 3 old valve grease nozzle spare parts were replaced, and the remaining ones were not replaced. The external grease nozzle caps were temporarily used, and they will be replaced one by one in 2022 after the newly purchased parts are available. The grease injection valve purchase application form has been submitted.
2.2 Clogged drain nozzles
The drain nozzle is located at the bottom of the valve. Its main function is to discharge impurities inside the valve cavity and prevent impurities from scratching the sealing surface of the valve seat or valve ball. At the same time, through the leakage detection in the middle cavity of the drain nozzle, the current sealing status of the valve can be judged and whether there is internal leakage, which is very important for ball valves with shut-off. Given the high sulfur in high-sulfur gas fields, it is unacceptable for the drain nozzle to be exposed. The sewage nozzles on site are sealed with two O-rings, Figure 3. After the helium-nitrogen leakage test, the high-pressure nitrogen in the pipeline is used to conduct the valve internal leakage test. Some valves, such as the main shut-off valve on the upper part of the gas-liquid separator and the ball valve at the front of the test separator, had clogged drain nozzles, making it impossible to discharge sewage and detect leaks.
Figure 3 Structure of the sewage nozzle
Solutions
It is recommended to purchase a new customized sewage nozzle, disassemble and replace it for an overhaul next year, and clean the lower cavity of the valve body to ensure that the sewage nozzle is working properly. The main sizes of the sewage nozzle on site are 1/2 inch, 3/4 inch, and 1 inch. The purchase requisition form has been submitted. Periodic inspection and maintenance should be performed. During the annual overhaul, the valves are inspected for internal leakages and sewage discharge. At the same time, the condition of the sewage discharge nozzles is also thoroughly investigated. Problematic sewage discharge nozzles are recorded promptly and the causes of external leakage are analyzed.
3. Internal leakages of valves
During the overhaul in 2021, the internal leakage of the 373 ball valves in the high-sulfur gas field was tested. There are three different kinds of internal leakages, namely, valves with serious internal leakages, valves with slight internal leakages and valves without internal leakages. For valves with serious internal leakages, sealing grease should be injected. Slight internal leakages of valves can be temporarily blocked by injecting sealing grease. There are 14 valves with internal leakages in the well station, mainly testing the main valve and bypass valve at the front of the separator, plus the air inlet valve below the safety valve of the water jacket furnace. The specific internal leakage situation is shown in Figure 4. The ball valve with serious internal leakages is at the outlet of the water jacket furnace.
Figure 4 Leak detection at well stations
Solutions
The anti-sulfur ball valves currently used in high-sulfur gas fields are mainly used for testing main pipeline valves such as test separators, filter separators, gas-liquid separators and pigging areas. They all use the surfacing process of INCONEL625 plus ENP. When scratches appear on the sphere, the ENP needs to be deplated and then surfaced twice, resulting in a longer maintenance period and higher maintenance costs. At the same time, there is also the risk of incomplete deplating and the new coating falling off. Therefore, it is recommended that high-sulfur gas fields should not use this solution for ball valves in the future, and instead purchase new valves with A350 plus full surfacing as the base material or 625 as the base material.
