Steam control valves are essential components in various industrial processes that involve the use of steam. These 阀门 enable operators to regulate the flow of steam with precision, allowing for optimal performance, energy efficiency, and safety.
Steam control valves play a crucial role in various industrial processes where precise control of steam flow is essential for optimal performance and safety. This comprehensive guide will delve into the functioning, types, selection criteria, and maintenance of steam control valves. Whether you’re new to the field or seeking to enhance your understanding, this article will provide valuable insights.
What is a steam control valve?
Steam control valves are valves that use compressed air as the power source, a cylinder as the actuator, and with the help of electrical valve positioners, converters, solenoid valves, holding valves, and other accessories to drive the valve to achieve switching or proportional adjustment, receiving control signals from industrial automation control systems to complete the adjustment of pipeline media flow, pressure, temperature, and other process parameters.
Steam control valves simultaneously reduce steam pressure and temperature to the level required for a given operating condition. These applications often involve high inlet temperatures and pressures, which must be greatly reduced. Therefore, these valve bodies are preferably manufactured by forging plus a combination of methods that can withstand steam loads at high pressures and temperatures. Forged materials allow higher design stresses, more optimized crystal structure, and inherent material consistency than cast bodies. Forged construction allows manufacturers to offer ratings up to Class 4500 and intermediate ratings more easily. However, cast bodies are still a good choice when pressures and temperatures are low or straight-through valves are required.
In order to cope with frequent extreme changes in steam characteristics due to decreasing temperature and pressure, the forged plus combination body type allows for the addition of an extended outlet port to control the outlet steam velocity at lower pressures. Similarly, in response to decreasing outlet pressures, the forged plus combination type of steam control valve allows the manufacturer to provide inlet and outlet connections with different pressure ratings to better match adjacent piping.
Other advantages of combining the de-tempering and depressurization functions in the same valve, as compared to two separate units, include:
- Improved mixing of the spray water due to optimization of the turbulent expansion area downstream of the pressure reducing element.
- It improved the adjustable ratio.
- Relatively simple installation and maintenance since it is one unit.
Working Tasks of Steam Regulating Valves
- Pressure-temperature regulation;
- Safety blocking function for subsequent systems.
How Steam Control Valves Work
Steam control valves operate by adjusting the steam passage through the valve body. This adjustment is achieved by moving a valve disc or plug, which controls the flow area. The valve can modulate the steam flow by changing the flow area to meet the desired operating conditions.
Areas of application for steam control valves
- Turbine high-pressure bypass valve during boiler start-up/commissioning;
- Receiving and transferring steam during turbine shutdown;
- Steam pre-treatment of steam lines.
Installation of Steam Control Valves
Steam conditioning valves are best installed with the working piston’s upper end in the horizontal piping’s lower part. The temperature sensor can be installed in any position, and the entire length must be immersed in the controlled medium.
Maintenance of Steam Regulating Valves
Steam control valves work accurately and normally to ensure the normal operation of process equipment and safe production is of great importance. Therefore, it is necessary to strengthen the maintenance of pneumatic control valves.
Maintenance of the key parts of the inspection
- Check the inner wall of the room: In the high-pressure differential and corrosive media occasions, the inner wall of the valve body, diaphragm valve’s diaphragm is often affected by the impact of the media and corrosion, must focus on checking the pressure and corrosion resistance;
- Check the valve seat: Due to the work of the media infiltration, the fixed valve seat with the inner surface of the thread is susceptible to corrosion and relaxation of the valve seat;
- Check the spool: The spool is one of the movable parts of the valve; by the media erosion is more serious, maintenance should carefully check whether the spool ministries are corroded, worn, especially in the case of high-pressure difference, spool wear due to cavitation caused by cavitation phenomenon is more serious. Serious damage to the spool should be replaced; check the sealing packing: check whether the packing asbestos rope is dry, such as the use of polytetrafluoroethylene filler; attention should be paid to checking whether the aging and whether the surface is damaged;
- Check whether the rubber film in the actuator is aging and whether there is cracking.
Routine Maintenance of Steam Regulating Valves
When the regulating valve uses graphite and asbestos as packing, lubricating oil should be added every three months to ensure that the regulating valve is flexible and usable. If the packing pressure cap is found to be very low, the packing should be supplemented, such as polytetrafluoroethylene dry packing hardening; it should be replaced promptly should pay attention to the operation of the control valve in the inspection tour to check the valve position indicator and the regulator output whether or not the match; positioner regulator valve should frequently check the gas source, found that the problem is handled promptly; should often be to keep the valve hygiene as well as the integrity of all parts of the valve for good use.
The common faults and causes of steam control valve
(A) regulating valve does not operate. Failure phenomena and causes are as follows:
- 1. No signal, no gas source. ① gas source is not open; ② due to the gas source containing water in the winter icing, resulting in air duct clogging or filter, pressure reducing valve clogging failure; ③ compressor failure; ④ gas source pipe leakage.
- 2. There is a gas source, no signal. ① regulator failure; ② signal pipe leakage; ③ positioner bellows leakage; ④ regulation network diaphragm damage.
- 3. There is no air source for the positioner. ① filter clogging; ② pressure reducing valve failure I; ③ pipe leakage or blockage.
- 4. The positioner has a gas source and no output. The throttle hole of the positioner is blocked.
- 5. Signal, no action. ① Valve spool off; ② Valve spool and society or with the valve seat stuck; ③ Valve stem bending or broken; ④ Valve seat spool freezing or coke block dirt; ⑤ Actuator spring rusted due to long-term unused.
(B) The action of the steam regulating valve is unstable. Fault phenomena and causes are as follows:
- 1. Unstable pressure of gas source. ① compressor capacity is too small; ② pressure reducing valve failure.
- 2 signal pressure instability. ① control system time constant (T = RC) is not appropriate; ② regulator output is not stable.
- 3. The gas source pressure is stable, and the signal pressure is also stable, but the action of the regulating valve is still unstable. ① the ball valve of the amplifier in the positioner is not closed tightly by the wear and tear of dirt, and the output will oscillate when the gas consumption increases; ② the nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle; ③ the output pipe and line leakage; ④ the rigidity of the actuator is too small; ⑤ the friction resistance in the movement of the valve stem is large, and the contact with the phase of the blocked part of the phenomenon.
Selection analysis of steam control valve
Selection of valve body type of control valve There are many types of valve bodies for control valves, and the commonly used types of valve bodies are straight-through single-seated, straight-through double-seated, angular, diaphragm, low-flow, three-way, eccentric rotary, butterfly, sleeve, sphere, and so on. In the specific selection, the following considerations can be made.
- (1) Spool shape structure is mainly based on the selected flow characteristics, unbalanced force, and other factors to consider.
- (2) Abrasion resistance: When the fluid medium is a suspension containing a high concentration of abrasive particles, the internal material of the valve should be hard.
- (3) Corrosion resistance due to the corrosive nature of the medium; try to choose a simple structure for the valve.
- (4) The temperature and pressure of the medium, when the temperature and pressure are high and vary greatly, should be selected to use the valve spool and seat materials subject to temperature and pressure changes of small valves.
- (5) Flash and cavitation are only produced in the liquid medium to prevent flash and cavitation. In the production process, flash and cavitation will form vibration and noise, shortening the valve’s service life, so the selection of valves should be prevented from producing flash and cavitation valves.
Selection of control valve actuator
To make the control valve work properly, the matching actuator must generate enough output force to ensure high sealing and valve opening.
There is generally no reset spring for double-acting pneumatic, hydraulic, and electric actuators. The size of the force is independent of its running direction. Therefore, the key to selecting the actuator is to find out the maximum output force and the rotational torque of the motor. For a single-acting pneumatic actuator, the output force is related to the opening of the valve, and the force on the regulating valve will also affect the motion characteristics. Therefore, it is required to establish a force balance in the entire opening range of the regulating valve.
Determination of the type of actuator
After determining the output force of the actuator, the corresponding actuator is selected according to the requirements of the process environment. When there are explosion-proof requirements on site, pneumatic actuators should be selected. Electric actuators should be selected as much as possible from the energy-saving perspective. If the adjustment precision is high, the hydraulic actuator can be selected. Such as the speed regulation of the transparent machine in the power plant, the reactor’s temperature regulation control in the refinery’s catalytic unit, etc.
Selection of the mode of action of the regulating valve
The mode of action of the regulating valve is only available when the pneumatic actuator is selected, and the combination of the positive and negative effects of the actuator and the positive and negative effects of the valve forms its mode of action. There are four kinds of combination forms, namely positive (gas-closed type), positive and negative (gas-open type), positive and negative (gas-open type), and negative and negative (gas-closed type). The action modes of the regulating valve formed by these four combinations are gas-open and gas-closed. For the choice of regulating valve mode of action, mainly from three aspects to consider: ① process production safety; ② the characteristics of the medium; ③ to ensure product quality, economic loss is minimal.