Globe valve, plug valve, ball valve and check valve are the most commonly used valves in the ground engineering of gas treatment projects. Each type of valve may contain several different structural forms and is suitable for different production environments. The correct and reasonable selection of valves is the premise to ensure the design safety, stability and economy of the pipeline design. This paper introduces the structure and process characteristics of each type of valve, and gives the corresponding selection principles by comparing the advantages and disadvantages of each type of valve. In addition, this paper also gives the recommended valve selection method based on the selection of valves in the ground engineering of the actual gas treatment project.
The closing part of the globe valve is the valve disc, which moves along the centerline of the valve seat and is mainly used for cutting off.
Structural form of globe valve
The globe valve is divided into the following types according to different structural forms: (1) Straight-through globe valve: The medium inlet and outlet channels are 180° in the same direction, and the damage to the flow state is relatively small, so the pressure loss through the valve is relatively reduced. (2) DC globe valve: the flow channel of the valve body and the main channel form a diagonal line, so that the degree of damage to the flow state is smaller than that of the conventional globe valve, so the pressure loss through the valve is correspondingly smaller. (3) Angle globe valve: The fluid only needs to change direction once, so that the pressure drop through this valve is smaller than that of the conventional structure globe valve.
Process characteristics of globe valve
The characteristics of the globe valve: (1) The globe valve has a simple structure and is convenient to manufacture and maintain. (2) The sealing surface is not easy to be worn and scratched, and the sealing performance is good. There is no relative sliding between the valve disc and the sealing surface of the valve body when opening and closing, so the wear and abrasion are not serious, the sealing performance is good, and the service life is long. (3) When opening and closing, the valve disc travel is small, so the opening and closing torque is large and laborious, and it takes time. (4) Due to the tortuous medium channel in the valve body, the fluid resistance is large and the power consumption is large. (5) When the nominal pressure of the medium flow direction is PN≤16MPa, the forward flow is generally adopted, and the medium flows upward from the bottom of the valve disc; when the nominal pressure PN≥20MPa, the countercurrent flow is generally adopted, and the medium flows downward from the top of the valve disc to increase the sealing performance.
The stem axis of the globe valve is perpendicular to the sealing surface of the valve seat. The opening or closing stroke of the valve stem is relatively short, and it has a very reliable cut-off action, which makes this valve suitable for cutting or regulating and throttling the medium.
Selection principle of globe valve
According to the characteristics of different types of globe valves, the selection principles are as follows: (1) Globe valves should be selected for pipelines or devices with high temperature and high pressure medium; (2) It is suitable for places where pressure loss is not considered; (3) It is suitable for relatively small Valves, such as needle valves, instrument valves, sampling valves; (4) There are flow adjustment or pressure adjustment, but the adjustment accuracy is not high, and the diameter of the pipeline is relatively small, such as the nominal diameter ≤ 50mm pipeline; (5) It is not suitable for substances with high viscosity or containing easily precipitated particles.
The structure of the plug valve
The plug of the plug valve is divided into cylindrical plug and conical plug according to the shape. Cylindrical plug passages are generally rectangular, and conical plug passages are generally trapezoidal. These shapes are connected to the valve circular passage to generate pressure loss. There are several different forms of conical cocks, including tight-fitting, packing, oil-sealing, pressure-balanced, PTFE-lined, and multi-channel.
Process characteristics of plug valve
The advantages of the plug valve: (1) It can be opened and closed by turning 90°, the opening and closing is rapid, and the operation is convenient; (2) The flow direction of the medium is not limited by the installation direction; (3) The sealing performance is good, the parts are few, and the weight is light. , the flow resistance is small; (4) The plug valve is suitable for multi-channel structure, one valve can obtain two, three or even more valve flow channels, which can simplify the design of the pipeline system; (5) It is suitable for low temperature and viscosity Larger medium; (6) no vibration and low noise.
Disadvantages of plug valve: (1) Due to the large sealing area, large friction, more laborious opening and closing, and large operating torque; (2) Due to the limitation of the structure type, it is not suitable for high temperature and high pressure pipelines, and the general operating temperature is ≤ 300 ℃; (3) If a soft-sealed valve seat is used, a fire-resistant and anti-static structure should be designed.
Plug valve selection principle
According to the structural characteristics of the plug valve and the functions that can be achieved in the design, it can be selected according to the following principles: (1) It is used to distribute the medium and change the flow direction of the medium. The nominal diameter is not more than 300mm, it is recommended to use a multi-channel plug valve; (2) The plug valve is suitable for venting, sewage, bypassing and sending and receiving bulb pipelines, because the plug valve is resistant to erosion. (3) In the case of gas, natural gas, HVAC system pipelines and equipment, and the nominal diameter is not greater than 200nm, the packing type conical plug valve should be selected.
The structure of the ball valve
The structure of the ball valve is divided into floating ball type and fixed ball type according to the spherical structure. The channel of the ball valve is circular, and the non-reduced spherical flow channel is connected with the circular flow channel of the valve body in a straight pipe section, so the flow resistance is very small.
The ball of the floating ball valve is controlled by the valve stem to freely float in the two valve seat sealing rings, rotate and seal, and the ball is pressed against the valve seat sealing ring at the outlet end by the pre-tightening force and medium pressure applied to the two valve seat sealing rings. , which is a single-sided forced seal. In addition to the dead weight of the ball, it also bears the full load of the working medium, and the sealing specific pressure of the ball and the sealing surface of the valve seat is related to the medium pressure.
The ball of the fixed ball valve is connected with the upper and lower valve stems to be fixed and will not move, and the valve stem can be equipped with bearings to reduce friction. In addition, the supporting force is transmitted from the valve seat sealing ring to the ball valve stem, the operating torque is small, and the movable valve seat presses the sealing ring on the ball under the action of the medium pressure, which plays a sealing role.
Process characteristics of ball valve
The advantages of the ball valve include: (1) Small fluid resistance (the resistance coefficient of the full-port flow channel valve body is equal to the pipe section of the same length); (2) The structure is simple, the valve body is symmetrical, the volume is small, it can flow in both directions, and it can be sealed in both directions; ( 3) It is tight and reliable, and the sealing surface material of the ball valve is widely used in plastic, which has good sealing performance; (4) It is easy to operate, open and close quickly, and it only needs to rotate 90° from fully open to fully closed, which is convenient for long-distance control; (5) Convenient Maintenance, the ball valve has a simple structure, the sealing ring is generally movable, and it is more convenient to disassemble and replace; (6) The sealing pair of the valve is completely isolated from the medium in the fully open or fully closed position, and the medium passing through at high speed will not erode the sealing pair; ( 7) When the ball is opened and closed, it is a process of rotating and wiping, so most ball valves can be used for media with suspended solid particles, and suitable wear-resistant valve seat sealing materials can also be used for powder media; (8) Straight-through type The ball valve is widely used for cutting off. The three-way, four-way and five-way ball valve is used for distributing the medium and changing the flow direction of the medium.
Disadvantages of ball valves: (1) If the valve seat sealing ring and other parts are made of non-metallic materials, the pressure-temperature rating is lower than the pressure-rating value of the ball valve shell (metal), so that the actual use pressure and temperature of the ball valve are limited by the medium. ; (2) When non-metallic materials such as valve seat sealing ring are used, the fire resistance and anti-static structure performance of the ball valve should be considered; (3) It is difficult to process and grind the sphere, and the processing of the metal-sealed auxiliary ball valve (hard-sealed ball net) is more troublesome; (4) It is not suitable for media with coking and high viscosity.
Selection principle of ball valve
According to the characteristics of different types of ball valves, the selection principles are as follows: (1) It is suitable for medium and low temperature (≤150℃), high pressure and high viscosity medium; (2) It cannot be used for flow regulation; (3) It is used for fast start-up (4) High-pressure and large-diameter ball valves should use fixed ball valves.
The structure of the check valve
Check valve can be divided into swing type and lift type according to the structure. The closing member (disc) of the swing check valve rotates around a fixed axis in the valve body. The closing member (disc) of the lift check valve moves along the centerline of the valve seat.
Process characteristics of check valve
Advantages of check valve: (1) The working load changes greatly; (2) The number of opening and closing of the valve is small, the valve is in the closed or open state for a long period of time, and the moving parts (such as the valve disc) do not move for a long time; Off (or from off to on) switching action is flexible and fast.
Disadvantages of check valve: (1) When the valve is closed to open, the valve flap gradually opens, the valve flap hinders the flow of the medium, and the fluid resistance is large; (2) The valve flap assembly relies on its own weight and spring force (such as when an auxiliary spring is added). ) and so on play a sealing role, once the medium pressure is slightly greater than its sealing force, the valve will leak, and the sealing performance will be poor; (3) When the valve is closed, the medium quickly flows backwards and impacts the valve disc, which may cause water hammer (also known as water sleep) phenomenon; (4) When the medium flows, it will generate impact through the valve disc assembly, which may generate movement and noise.
Selection principle of check valve
(1) It is suitable for clean medium, not suitable for medium containing solid particles and high viscosity; (2) For high and medium pressure check valve < DN50mm, vertical lift type and straight-through lift should be selected. Check valve; (3) For low pressure check valve < DN50mm, butterfly type, vertical lift type and diaphragm type check valve should be selected; (4) For high and medium pressure check valve with 50m≤DN≤600mm, it should be selected Swing check valve.
Globe valve, plug valve, ball valve and check valve are the most commonly used valves in oil and gas field ground engineering. According to different classification standards, each type of valve can be divided into various types, which are used according to the type of valve, material of parts and transmission mode, etc. The characteristics can determine the use range of the valve, and the installation, repair and maintenance methods of the valve can be determined according to the structural characteristics such as the size and connection method of the valve. Fully consider the advantages and disadvantages of each valve, combine the characteristics of the actual oil and gas field surface engineering projects, and select the corresponding valve according to the valve selection principles and methods recommended in this paper to ensure that the valve can fully play a role in the pipeline system, improve the design quality, and make the pipeline system safe. operation, effectively reducing the design cost.