Ascending Efficiency: Rising Stem Valves in Critical Process Control
Nov 21,2023Read More
Control valve consists of two main assemblies: valve body assembly and actuator assembly (or actuator system). The control valve can be divided into four types: single seat series control valve, double seat series control valve, sleeve series control valve and self operated series control valve. Variants of the four types of control valves can bring many different applicable structures, each with its special applications, characteristics, advantages and disadvantages.
1. The control valve shall be installed in accordance with the provisions of the design drawings and design documents. The installation position shall have sufficient operation space for installation, operation and maintenance; Its installation position shall avoid those with vibration, humidity, vulnerable to mechanical damage, strong electromagnetic interference, high temperature, severe temperature change and corrosive gas; The installation shall be firm and flat; The actuator of the control valve shall be on the upper part of the regulating mechanism.
2. The installation of control valves shall comply with the provisions of relevant national standards, such as quality inspection and evaluation standards for construction and installation engineering, industrial automation instrument construction and acceptance specifications, electrical equipment installation engineering construction and acceptance specifications, etc.
3. The equipment, auxiliary equipment and main materials required for the installation of the control valve shall comply with the relevant provisions of the current national or ministerial standards.
4. The flow direction of the fluid in the control valve shall be consistent with the arrow marked on the valve body of the control valve, except in some special circumstances.
5. The connection of the control valve includes threaded connection, flange connection, welding connection, etc. The threaded connection is used for small diameter control valves. When installing threaded control valves, removable movable connectors must be installed at the same time. Flange connection includes flange connection and clamping connection. The nominal diameter of the connecting flange shall be consistent with the connecting pipe diameter of the control valve. Flanges also have different pressure levels, and the pressure level of flanges shall be consistent with that of control valves. The inner diameter of the flange shall be equal to the inner diameter of the pipe. When the control valve is connected, new protrusions shall not be introduced into the connecting pipeline. For example, sealing gaskets, welding joints, etc. shall not protrude in the pipe.
6. The upstream and downstream shut-off valves and bypass valves shall be installed at the same time as the control valve. The pipeline resistance and the influence on the fluid flow state should be taken into consideration when it comes to the pipe section length between the upstream and downstream shut-off valves and the control valve.
The importance of control valve selection：
The control valve acts as an actuator in the automatic control system, and its application quality is directly reflected in the adjustment quality of the system. As a terminal element in the process control, people have a newer understanding of its importance than in the past. In addition to the quality of the product itself, whether the user has correctly installed, used, and maintained, the correct calculation and selection of the control valve also play an important part in the control valve application. Due to the error of calculation and selection, the system breaks down, and some even cannot be put into use. Therefore, users and system designers should recognize the importance of the selection of the control valve.
Principles of control valve selection：
1. According to the process conditions, select the appropriate structural form and material.
2. According to the characteristics of the technological object, select the flow characteristics of the control valve.
3. According to the process operating parameters, select the appropriate control valve size.
4. According to the requirements of the process, select the required auxiliary devices.
5. Reasonable selection of actuator. The reacting speed of the actuator should be able to meet the process.
The control valve is featured with its simple structure and reliable action. However, because it is in direct contact with the process medium, the performance of control valve directly affects the system quality and environmental pollution. Therefore, the control valve must be maintained and overhauled regularly, especially in bad and important occasions. Followings are some key inspection parts:
1. Inner wall of valve body
For the control valve used in the occasion of high pressure difference and corrosive medium, the inner wall of the valve body and the diaphragm of the diaphragm valve are often impacted and corroded by the medium, so the pressure resistance and corrosion resistance must be checked.
2. Valve seat
During the operation of the control valve, due to the infiltration of medium, the inner surface of the thread used to fix the valve seat is easy to be corroded, which makes the valve seat loose. Pay attention to it during inspection. For valves operating under high pressure difference, check whether the sealing surface of the valve seat is damaged.
3. Valve core
The valve core is a movable part during regulation, which is most seriously scoured and corroded by the medium. During maintenance, carefully check whether all parts of the valve core are corroded and worn, especially under the condition of high pressure difference, the wear of the valve core is more serious (due to cavitation), which should be paid attention to. Replace the valve core when it is seriously damaged. In addition, pay attention to whether the valve stem has similar phenomena, or the connection with the valve core is loose.
"O" rings and other gaskets. Check whether the diaphragm and "O" gasket in the control valve are aging and cracked.
5. Sealing packing
Pay attention to whether the PTFE packing and sealing lubricating grease are aging and whether the mating surface is damaged, and replace it if necessary.
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