Natural gas as a new fuel source has been widely used in various industries. In the coming years, there will be more natural gas pipeline construction and operations. The ball valve has outstanding advantages such as low-fluid resistance, good sealing performance, high reliability, and fast opening and closing speed. It has become the most widely used in the natural gas pipelines, but it is also the equipment with the most problems in pipelines operation. In this article SINOVAL discusses the selection of ball valve type for the natural gas pipeline, as well as the faults that occur during the operation and the countermeasures.
1 Ball Valve Structure
The ball valve is generally composed of valve body, ball, valve seat, valve stem and transmission device. The valve body structure can be divided into fully welded ball valve, top-mounted ball valve, 2 pieces ball valve, and 3 pieces valve body. The ball can be divided into the floating mounted ball valve and trunnion mounted type. According to API 6D standard, the valve seat structure is divided into Double Block and Bleed valve (DBB), Double Isolation and Bleed valve (DIB-1 and DIB-2). The DBB ball valve’s two valve seats are self pressure relief effect valve seat (SPE). When the pressure in the valve chamber is too high, the valve seat self-depresses. The two valve seats of the DIB-1 ball valve are double piston effect seats (DPE). After the valve seat is damaged on either side, the other valve seat can still ensure that the ball valve has a two-way sealing function. One end of the DIB-2 ball valve seat is SPE and the other end is DPE. The number of seat seals can be divided into single seals, double seals and multiple seals. Transmission can be divided into manual, electric, pneumatic, gas-liquid linkage.
2 Ball Valve Selection
Natural gas is generally collected from the natural gas wellhead to the natural gas gathering station and processing plant. After treatment, it is transported through the pipeline. During the transportation process, it’s pressed by the compressor station (compressor) to ensure the flow and flow rate.
(1) Natural gas pipelines are generally large in diameter and buried. The gas being transported comes from a natural gas processing plant. The gas is clean, has a low hydrocarbon and water dew point, a narrow range of temperature difference, high flow, and high pressure. Therefore, it is best to use large-diameter, buried, full welded, DIB-1 type ball valve. The fully welded ball valve meets the requirement of less leakage point and satisfies unattended requirements. DIB-1 type ball valve can realize two-way sealing. After one side seal leakage occurs, the first side seal can also play the role of cutting off, to meet the requirements of upstream and downstream cutting and maintaining the pipeline. Natural gas has low hydrocarbon and water dew point, stable temperature, and buried valve, so there is no problem that the pressure in the ball valve cavity is too high. Therefore, the buried globe valve on the gas pipeline may not be equipped with a valve body relief valve. That is, the leakage point can be reduced, as well as the workforce of the safety valve every year.
(2) Natural gas compressor stations have become an important part due to the obvious gas pressure drop during the transportation of natural gas. The valves in the compressor station are generally on the ground, and the valves have frequent switching characteristics. Therefore, it is best to use large-bore, above-ground, three pieces bolted ball valve, and DIB-1 (with valve body relief valve) type ball valves. The three pieces ball valve is easy to maintain. Under the condition that the sealing surface of the valve is damaged, offline maintenance can be performed and the valve seat seal can be replaced. Since the bolt connection of the valve body may increase the leakage point, it needs to strengthen the gas station inspection and keep routine maintenance. DIB-1 (with valve body safety valve) type ball valve can realize two-way sealing, and can meet the frequent venting operations upstream or downstream of the compressor station. At the same time, since the ball valve is installed on the ground, the temperature range is large, and the possibility of abnormal increase of gas in the valve chamber can not be excluded, the use of valve body safety valve can meet the requirements of safe operation.
3 Ball Valve Leakage Analysis
There are many reasons for valve body faults, including production problems, transportation problems, construction problems, and operational problems. Actually, there are few production problems and transportation problems. In today’s large-scale procurement and transportation, there are strict procurement processes, manufacturing monitoring processes, third-party out-factory inspection procedures, packaging and transportation requirements, plan of arrival inspection etc., so it will not be installed for valves that have problems. Even if they are accidentally installed, they will be replaced during airtight tests.
(1) Construction problems mainly refer to the inability to fully clean the pipe network after the completion of pipeline and ball valve construction. During the pipe network and ball valve hydrostatic test, impurities will enter the sealing surface when the pipeline is purged, and it causes damage to the ball valve sealing surface when switching the ball valve. Under the current strict construction procedures and supervisory supervision, this is rarely the case for other reasons including hoisting damage, overheated welding damage, and damage caused by inadequate protection.
(2) Operation problems mainly include internal leakage and external leakage of the ball valve. The external leakage position may be in the stem, grease injection nozzle, vent, drainage outlet, valve body safety valve, and three-stage bolt connection. The main reasons are as follows: ①The valve stem is rusted and detached, the limit bolts are inaccurate, and the ball valve switch is not in place, and the seal surface is worn for a long time. ②If the valve is not maintained or inactive for a long time, it will occur that the valve seat is locked to the ball body, the stem seal is locked to the valve stem, and the sealing surface is damaged when switching. ③Due to the lack of periodic maintenance of the ball valve or maintenance not in place, it causing the sealing grease to dry and harden, which keeps hindering the movement of the elastic valve seat or the movement of injection nozzle one-way valve, it will finally result in internal leakage and external leakage of the ball valve. ④Improper operation, the use of ball valve flow adjustment can result in damaging the sealing surface due to flushing.
4 Operational Countermeasures
The ball valve is an important equipment on the gas pipeline. Any problem will affect the operation. At the same time, the use of ball valves is large and the installation location is dispersed. Therefore, it is very important to monitor the operational status of the ball valve, improve the maintenance procedures, and strictly implement the operating procedures.
(1) During the construction period, before the hydrostatic pressure test, the ball valve shall be maintained with cleaning fluid and the ball valve shall be in half-opening and half-closing state. When the pipe network is hydrostatically tested, the ball can be evenly compressed to prevent damage due to ball deformation. . After the hydrostatic test, the cleaning fluid is injected again. During the ball valve switching process, the sealing surface can be kept moist, reducing the possibility of damaging the sealing surface with impurities. At the same time, the ball valve is in the fully open state, and the water in the pipe network is discharged and carried out. Purge work to reduce the flow of water and bring more impurities into the sealing surface. After the pipe network is purged and cleaned up, grease injection work is completed as soon as possible to reduce corrosion of the sealing surface by the cleaning liquid.
(2) During operation, building the ball valve status log to record each ball valve’s running status, maintenance date, and switch date in detail, it can prevent the occurrence of long-term non-action of the ball valve due to oversight, and improve the maintenance procedures of ball valves and strictly perform maintenance work on time, quality, and quantity. Among them, the grease injection of the valve stem is also indispensable to ensure that each ball valve is in the best operating condition.
(3) Ball valve internal leakage treatment. Ball valve internal leakage in the general on-site treatment, firstly, it needs to determine the cause of the ball valve leakage, targeting to processing. When the limit is inaccurate, use the ball valve 2% -5% spherical surface margin to make the ball valve limit adjustment through the opening of the valve cavity vent valve. After the limit adjustment, if the valve cavity is significantly reduced in the amount of venting, it can be further fine-tuned. When in the micro-leakage situation, it can lock limit, make the injection of cleaning fluid, and open and close the ball valve 2-4 times, the switching test after 24h, it generally can solve the problem of micro-leakage and followly carry out grease maintenance. After adjusting the limit, if there is still a lot of leakage, and you need to cut off, perform the sealing grease operation until it is sealed. It is recommended to inject a small amount of grease immediately after filling the sealing grease to prevent the location of the one-way valve of the grease injection nozzle from drying the sealing grease, which results in the leakage problems. For large leakage of the ball valve, the sealing surface is generally damaged, and it should be offline repaired at the right time.
(4) Ball valve external leakage treatment. Ball valve external leakage is generally from the microleakage to the larger leakage process. Except for special circumstances, when the valve stem leaks out, it’s only through the valve stem injection method of grease, if not solved, the gas at both ends of the valve needs to be released, and then the stem seal replacement work is performed. When the sewage outfall and the vent are leaked out, according to the actual situation, It is relatively simple to vent the valve chamber gas or close the root valve of the outfall and vent. When leakage occurs in the grease injection nozzle, this problem is more complicated. Firstly, based on the drawings, it need to be confirmed whether the grease injection nozzle is connected with the valve chamber or the external pipe, and then determined whether it can be replaced on-site according to the leakage amount and experience. This article suggests based on experience that the leakage of the grease injection nozzle should be replaced on-site as much as possible. When an external leakage occurs at the three-stage bolted joint, it is fixed well to tighten the screws evenly under normal circumstances. As the tightening of the screws will increase the valve switching torque, at the same time, in the actually situation, it is small by the distance between the bolts to be tightened and the bolts installed on the valve. It needs to be processed the professional tools according to bolt size and explosion-proof requirements，This article suggests that person with valve experience can work together with pipe workers to prevent over-tightening of the bolts to increased torque, which resulting in damage to the actuator during valve switching.
The ball valve can be determined in different situations through the function analysis of ball valve used in the natural gas pipelines. By discussing the problems of ball valves in construction and operation, it can put forward the related operational countermeasures to solve problems and also provide references for the construction and operation of natural gas pipelines in the next few years.