Valve is the control component in the pipeline fluid transportation system, which is used to change the channel section and medium flow direction. It has the functions of diversion, cut-off, throttling, check, diversion or overflow relief. Different valves have different effects, and they are all have their own advantage and disadvantages.
1. Gate Valve
Gate valve refers to the valve whose closing part (gate) moves along the vertical direction of the channel axis. It is mainly used to cut off the medium in the pipeline, that is, fully open or fully closed. In general, gate valve can not be used to regulate flow. It can be used for low temperature and high pressure as well as high temperature and high pressure, and can be made according to different materials of the valve. But gate valve is not generally used in pipelines conveying mud and other media.
- The fluid resistance is small;
- The torque required for opening and closing is small;
- It can be used in the ring network pipeline with medium flowing in two directions, that is, the flow direction of medium is not limited;
- When fully open, the erosion of sealing surface by working medium is smaller than that of stop valve;
- The structure of the body is simple and the manufacturing technology is better;
- The structure length is shorter.
- The external dimension and opening height are large, and the space for installation is also large;
- In the process of opening and closing, the sealing surface is relatively friction, and the friction is large, even it is easy to cause abrasion phenomenon in high temperature;
- Generally, gate valves have two sealing surfaces, which add some difficulties to processing, grinding and maintenance;
- The opening and closing time is long.
2. Butterfly Valve
Butterfly valve is used to rotate 90° with disc type opening and closing parts. A valve that opens, closes, and regulates the flow passage to the left and right.
- It has simple structure, small volume, light weight and low consumption, and is not used in large caliber valve;
- The opening and closing are quick, and the flow resistance is small;
- It can be used in medium with suspended solid particles, and can be used in powder and granular media according to the strength of sealing surface. It can be used for the two-way opening and closing of ventilation and dust removal pipeline, and widely used in gas pipelines and waterways of metallurgy, light industry, power and petrochemical systems.
- The flow regulation range is not large, when the opening reaches 30%, the flow will enter more than 95%;
- Due to the limitation of butterfly valve structure and sealing material, it is not suitable for high temperature and high pressure pipeline system. The general working temperature is below 300 ℃ and PN40;
- The sealing performance is poor compared with ball valve and stop valve, so it is not very high for sealing.
3. Ball Valve:
It is evolved from the plug valve. Its opening and closing parts are a sphere, which is rotated 90 ° around the axis of the stem. Achieve the purpose of opening and closing. Ball valve is mainly used in pipeline to cut off, distribute and change the flow direction of medium. The valve designed as V-shaped opening also has good flow regulation function.
- The flow resistance is the lowest (actually 0);
- It can be used in corrosive medium and low boiling point liquid reliably because it will not be stuck in operation (when there is no lubricant);
- In the range of pressure and temperature, the sealing can be realized completely;
- It can realize quick opening and closing, and the opening and closing time of some structures is only 0.05-0.1s, so as to ensure that it can be used in the automation system of the test bench. When the valve is opened and closed quickly, the operation is free of impact;
- The ball closing parts can be automatically located at the boundary position;
- The working medium is sealed reliably on both sides;
- When fully open and fully closed, the sealing surface of the ball and seat is isolated from the medium, so the medium passing through the valve at high speed will not cause the erosion of the sealing surface;
- It is considered as the most reasonable valve structure for low temperature medium system because of its compact structure and light weight;
- The valve body is symmetrical, especially the welded valve body structure, which can bear the stress from the pipe well;
- The closing parts can withstand the high pressure difference when closing. (11) ball valve with fully welded valve body can be buried directly underground, which makes the valve internals free from corrosion, and the maximum service life can reach 30 years, which is the most ideal valve for oil and natural gas pipelines.
- Because the main sealing ring material of valve seat is polytetrafluoroethylene, it is inert to almost all chemical substances, and has the characteristics of small friction coefficient, stable performance, not easy to aging, wide range of temperature application and excellent sealing performance. But the physical properties of polytetrafluoroethylene, including high expansion coefficient, sensitivity to cold flow and poor thermal conductivity, require the design of seat seal to be around these characteristics. Therefore, when the sealing material becomes hard, the reliability of the seal is destroyed. Moreover, the temperature resistance of polytetrafluoroethylene is low, and can only be used under the condition of less than 180 ℃. Above this temperature, the sealing material will age. But considering the long-term use, it will only be used at 120 ℃.
- Its regulating performance is worse than that of the stop valve, especially pneumatic valve (or electric valve).
4. Stop Valve
It refers to the valve that the closing member (disc) moves along the center line of the seat. According to this movement of disc, the change of valve seat opening is in positive proportion to disc stroke. Because the valve stem opening or closing stroke of this kind of valve is relatively short, and it has very reliable cut-off function, and because of the change of valve seat opening and disc stroke in a positive proportion, it is very suitable for the flow regulation. Therefore, this type of valve is very moving to cooperate for cutting or regulating and throttling.
- In the process of opening and closing, the friction between the disc and the sealing surface of the valve body is smaller than that of the gate valve, so it is wear-resistant.
- The opening height is only 1/4 of the valve seat channel, so it is much smaller than the gate valve;
- Generally, there is only one sealing surface on the valve body and disc, so the manufacturing process is better and easy to maintain;
- Because the filler is a mixture of asbestos and graphite, the temperature resistance is higher. Generally, stop valves are used for steam valves.
- Because the flow direction of medium through the valve changes, the minimum flow resistance of the stop valve is higher than that of most other types of valves;
- Because of the long stroke, the opening speed is slower than that of ball valve.
5. Plug Valve
It refers to the rotary valve with plunger shaped closure parts, passing through 90 °. A valve that makes the passage port on the valve plug connected or separated from the passage port on the valve body to realize opening or closing. The plug can be in a cylindrical or conical shape. The principle is similar to that of ball valve. Ball valve is developed on the basis of plug valve, which is mainly used for oil field exploitation and petrochemical industry.
6. Safety Valve
It refers to the overpressure protection device on pressure vessel, equipment or pipeline. When the pressure in equipment, vessel or pipeline rises more than the allowable value, the valve will open automatically, and then discharge in full quantity to prevent the equipment, vessel or pipeline and pressure from rising continuously; When the pressure is reduced to the specified value, the valve shall be closed automatically and timely to protect the safe operation of equipment, vessel or pipeline.
7. Steam Trap
In the medium of steam and compressed air, there will be some condensate formed. In order to ensure the efficiency and safe operation of the device, these useless and harmful media should be discharged in time to ensure the consumption and use of the device.
It has the following functions:
- it can quickly remove the condensate produced;
- Prevent steam leakage;
- Remove air and other non condensable gases.
8. Pressure Reducing Valve
It is a valve that reduces the inlet pressure to a certain required outlet pressure by adjusting, and relies on the energy of the medium itself to keep the outlet pressure stable automatically.
9. Check Valve
It’s also known as the counter-flow valve, check valve, back pressure valve and one-way valve. These valves are opened and closed by the power generated by the flow of the pipeline medium itself, and belong to an automatic valve. Check valve is used in pipeline system, its main function is to prevent medium backflow, prevent pump and drive motor from reversing, and discharge of container medium. Check valve can also be used to supply the auxiliary system with pressure above the system pressure. It can be divided into swing type (rotating according to center of gravity) and lifting type (moving along the axis).