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Recently, I have been sharing with you the types, structures, and working principles of valves. 1. Control valves are mainly used to regulate the flow rate, pressure, etc. of a medium. Including control valves, throttle valves, pressure relief valves, etc. 2. Check valves are used to prevent the medium from flowing back. Including check valves of various structures. 3. Diversion valves are used to separate, distribute, or mix media. Including distribution valves and steam traps of various structures, etc. 4. Safety valves are used for safety protection when the pressure of the medium exceeds the specified level. Including various types of safety valves. 5. Shut-off valves are mainly used to cut off or connect the flow of a medium. These include gate valves, globe valves, diaphragm valves, ball valves, plug valves, butterfly valves, plunger valves, and metering needle valves. I. Control valves play a very important role in the automatic control of modern factories, as the production in such factories depends on the proper distribution and control of the flowing media. These controls, whether it is the exchange of energy, the reduction of pressure, or simply the feeding of materials into a container, all require certain final control elements to be implemented. Common control loops consist of three main parts; the first part is the sensing element, which is usually a transmitter. It is a device that can be used to measure the process parameters being monitored, such as pressure, liquid level, or temperature. The output of the transmitter is sent to the control instrument—the regulator, which determines and measures the deviation between the set value or desired value and the actual value of the process parameter, and sends correction signals one after another to the final control element—the control valve. The valve adjusts the flow rate of the fluid, enabling the process parameters to reach the desired values. Performance features: The control valve has a simple flow path and low resistance, and is generally suitable for use in the forward direction (installation). However, in applications with high pressure drops, the control valve is used in reverse direction to reduce unbalanced forces and minimize damage to the valve core; this also facilitates the flow of the medium and helps prevent coking and blockage of the control valve. Under normal circumstances, control valves are not recommended for use in reverse; they are only advised for such applications as those with high pressure differences, high viscosity, a tendency to coking, or media containing suspended particles. When used in reverse, operation at a low opening for extended periods should be avoided, especially during commissioning. II. Throttle Valve The external structure of a throttle valve (Choke valve) is no different from that of a globe valve; only the shape of their closing elements varies. The moving parts of throttle valves are mostly of conical streamlined design; by changing the cross-sectional area of the passage, flow rate and pressure can be regulated. Throttle valves are used to reduce the pressure of a fluid under conditions of very high pressure drops. Performance features: 1. It has a simple structure, making it easy to manufacture and maintain, with low costs. 2. The adjustment accuracy is not high, so it cannot be used for adjustment purposes. 3. The sealing surface is prone to erosion, and it cannot be used for cutting media. 4. Poor sealing performance. III. Pressure Reducer: A pressure reducer features a piston or diaphragm structure; the output pressure acts on this piston or diaphragm, overcoming the force of an adjustable spring to achieve equilibrium. Adjust the secondary pressure using the adjustment screw; set the spring load to open the main valve, allowing air flow from the inlet at initial pressure p1 to the outlet at secondary pressure p2. When the loop-connected outlet reaches the set pressure, the air inside acts on the diaphragm, generating a lifting force that opposes the spring force. If the flow rate decreases, p2 increases slightly, which in turn increases the force acting on the diaphragm relative to the spring force; the diaphragm and valve rise accordingly until they are once again in balance with the spring force. The air flow through the valve then decreases until its consumption is in balance with the output pressure. If the flow rate increases, p2 decreases slightly; this decrease reduces the force acting on the diaphragm relative to the spring force, causing the diaphragm and valve to descend until they are once again in balance with the spring force. The increased air flow passes through the valve until its consumption and output pressure are in balance. There is no air consumption; the valve is closed. Performance characteristics: Differences between pressure relief valves and relief valves (1) In the stationary state, the valve port of a pressure relief valve is always open, while that of a relief valve is always closed ； (2) The pressure relief valve controls the stability of the outlet pressure, while the relief valve controls the stability of the inlet pressure ； (3) The valve orifice of the pressure relief valve closes as the outlet pressure increases, while the valve orifice of the relief valve opens as the inlet pressure increases ； (4) Both the inlet and outlet of the pressure relief valve are part of the pressure oil circuit; the oil returning via the pilot valve must be directed back to the tank separately, whereas the oil from the relief valve flows back to the tank together with the oil at the outlet. IV. Check Valves A check valve is a valve whose valve disc opens and closes automatically due to the flow of the medium itself, and it is used to prevent the backflow of the medium. It is also known as a non-return valve, one-way valve, reverse flow valve, and backpressure valve. A check valve is a type of automatic valve whose main function is to prevent the backflow of fluid, stop the pump and driving motor from rotating in reverse, and prevent the leakage of fluid from containers. Performance features: A check valve is a valve used to prevent the backflow of fluid in a pipeline; it opens when the fluid flows in the correct direction and closes automatically when the fluid flows in the opposite direction. It is generally used in pipelines where the flow of the medium in the opposite direction is not allowed, to prevent backflowing medium from damaging equipment and components. Prevents the rotary pump from reversing when it stops operating. In pipelines, check valves and shut-off valves are often used in series. This is due to the poor sealing performance of the check valve; when the pressure of the medium is low, a small amount of the medium leaks, and a shut-off valve is needed to ensure that the pipeline remains closed. The foot valve is also a type of check valve; it must be submerged in water and is specifically installed at the front end of the suction pipe of pumps that are unable to draw water by self-priming or that lack a vacuum pump for this purpose. V. Safety Valves A safety valve is a special type of valve in which the closing element remains in a closed position under external forces. When the pressure of the medium inside a device or pipeline rises above the specified value, this valve releases the medium outside the system, thereby preventing the pressure from exceeding that specified level. Safety valves belong to the category of automatic valves. They are primarily used in boilers, pressure vessels, and pipelines to ensure that pressure does not exceed specified limits, thereby playing a crucial role in protecting both personnel safety and equipment operation. Note: The safety valve must undergo a pressure test before it can be used. Performance features: 1. The weight-lever type safety valve has a simple structure; it is easy to adjust and provides relatively accurate performance. The load applied does not increase significantly as the valve disc rises. It is suitable for use in environments with high temperatures and was commonly used in the past, especially in boilers and pressure vessels operating at high temperatures. However, the weight-lever type safety valve has a relatively bulky structure; its loading mechanism is prone to vibration, and leakage often occurs as a result of this vibration ； It has a low seating pressure, and once opened, it is difficult to close and maintain a tight seal. 2. Spring-type safety valve: The spring-type safety valve is lightweight and compact, with high sensitivity. Its installation location is not restricted, and due to its low sensitivity to vibrations, it can be used in mobile pressure vessels. The disadvantage of this type of safety valve is that the applied load changes as the valve opens; that is, as the valve disc rises, the degree of compression of the spring increases, and the force acting on the valve disc also increases accordingly. This is detrimental to the rapid opening of the safety valve. Furthermore, the spring on the valve may lose its elasticity due to prolonged exposure to high temperatures. When used on containers with high temperatures, the insulation or heat dissipation of the spring often needs to be considered, which complicates the design. 3. Pulsating safety valves consist of a main valve and an auxiliary valve; the pulsing action of the auxiliary valve drives the main valve to operate. Due to their complex structure, they are typically used only in boilers and pressure vessels that require a large amount of safety discharge. VI. Directional control valves (switching valves, distribution valves) These valves are constructed by combining several types of direct-acting stop valves, and they are used to change the flow direction of the process medium as well as to control how multiple streams of medium converge or diverge. For example, fluid direction control in gear pump and polymer filter systems, as well as the multi-path distribution of process media in polymerization systems. Directional control valves are widely used in the chemical industry, oil industry, metallurgy, power sector, pharmaceutical industry, beer production, food processing, dairy beverages, cosmetics, as well as in pipeline systems for various engineering applications. Performance features: 1. Made of stainless steel ； 2. High polishing precision, meeting food-grade and pharmaceutical-grade requirements ； 3. The control system can be configured according to the actual needs of the user to achieve fully automated operation and improve work efficiency ； 4. The housing and key components are all manufactured by casting, resulting in a robust structure with high strength that resists deformation, as well as an attractive appearance ； 5. The conveying channel is smooth and polished, ensuring smooth material flow and preventing blockages ； 6. In applications that require pressure sealing, a special sealing structure can be employed to fully meet the usage requirements. VII. Thermostatic valve: This valve responds to changes in temperature within the chamber; when the bimetallic strip reaches a certain temperature, it deforms automatically, which in turn drives the valve core to move back into position and close the valve seat opening, thereby preventing steam from escaping. On the contrary, when there is condensed water in the valve chamber, the temperature within that chamber does not cause deformation of the bimetallic strip; as a result, the valve core and seat remain in an open position, allowing continuous drainage of the condensed water. Once all the condensed water has been drained, the temperature in the valve chamber changes immediately, causing the bimetallic strip to deform and thus close the valve seat hole. This repeated process of opening and closing achieves the effect of draining water while preventing air from entering. Suitable for the drainage and recovery of suspected water condensation in any thermal steam pipeline. Performance features: 1. Controls the fluid velocity at around 30 m/S to prevent cavitation damage; the fluid channel is of labyrinth design, which continuously changes the direction of the fluid ； A pressure difference of 25 MPa is allowed. 2. The throttling surface and the sealing surface are separated; different throttling elements are used depending on the hydrophobic flow rate. The surfaces of the components inside the valve are hardened, with a hardness level of up to HRC70, ensuring tight sealing and a long service life. 3. The valve body assembly features an internal pressure sealing structure; the greater the pressure difference, the better the sealing performance. 4. The valve body assembly and the actuator are connected in a floating manner, which eliminates jamming caused by misalignment between the valve core and the push rod.