The selection of control valve

A, the importance of control valve selection The control valve is the actuator of the Auto-control system, and its application quality is directly reflected in the regulation quality of the system. As a terminal element in Process control, people have a newer understanding of its importance than in the past. The control valve application is good or bad, in addition to the product itself quality, the user is correct installation, use, maintenance, correct calculation, selection is very important.

As a result of the calculation of the selection of errors, resulting in the system to open and stop, and some even can not be used, so for users and system designers should recognize the importance of the valve in the field, the control valve must be selected to cause enough attention.

B, Control valve selection principle

1, according to the technological conditions, the selection of suitable structural forms and materials.

2, according to the characteristics of the Process object, select the flow characteristics of the control valve.

3, according to the process operating parameters, select the appropriate size of the control valve diameter.

4, according to the requirements of the process, select the necessary auxiliary devices. 5. Reasonable choice of executing agency. The response speed of the actuator should be able to meet the requirements of the process to control travel time: The selected control valve actuator should be able to face the requirements of the foot valve stroke and process to the leakage level. In some occasions, such as the selection of pressure control valve (including vent valve), should consider the actual possible pressure difference for appropriate amplification, that is, the Executive Body can provide greater force.

Otherwise, when the process of abnormal conditions, the control valve before and after the actual pressure difference is large, will not be closed or open the risk of failure.

Second, the control valve accessories In the production process, the control system to the valve put forward a variety of special requirements, therefore, the control valve must be equipped with a variety of ancillary devices (referred to as accessories) to meet the needs of the production process.

The accessories of the control valve include:

1. The valve positioner is used to improve the performance of the control valve regulation, and to achieve correct positioning.

2. The valve position switch shows the work location of the upper and lower limit stroke of the valve.

3, pneumatic protection valve when the control valve of the gas source failure, to maintain the valve in the gas source before the failure of the opening position.

4, solenoid valve to achieve the electromagnetic switch to ensure that the valve in the power failure valve in the desired safety open position.

5. When the controller of the control system fails, the handwheel mechanism can switch to the manual operation valve.

6. The pneumatic relay accelerates the action of the actuator and reduces the transmission time.

7. air filter Pressure reducer is used for purifying air source and regulating air pressure. 8, gas tank to ensure that when the gas source failure, so that no spring cylinder piston actuator can move the control valve action to the fault safe location.

Its size depends on the size of the cylinder, valve action time requirements and valve working conditions.

In short, the role of the attachment is to make the control valve function more perfect, more reasonable, more complete.

Third, the control valve maintenance The control valve has the characteristics of simple structure and reliable movement, but due to its direct contact with the process medium, its performance directly affects the system quality and environmental pollution, so the control valve must be regular maintenance and periodic overhaul, especially for the use of harsh conditions and important occasions, should pay attention to maintenance work.

Key Inspection Site

1, Valve body wall

For use in the high-pressure and corrosive media occasions control valve, valve body wall, diaphragm valve diaphragm is often affected by the impact of the medium and corrosion, must focus on the inspection of pressure, corrosion-resistant situation.

2, Valve seat Control valve work, due to the infiltration of the media, fixed seat thread internal surface is vulnerable to corrosion and make the seat loose, inspection should be noted.

The valve that works under the pressure difference, also should check the valve seat seal face whether flush bad.

3, Valve Core Valve core is to adjust the moving parts of the work, by the media scouring, corrosion is the most serious, overhaul should carefully check the valve core parts are corrosion, wear, especially in the case of high pressure poor wear more serious valve core (due to cavitation phenomenon), should be noted.

Valve core damage should be replaced, and also pay attention to whether the stem has a similar phenomenon, or with the spool connection loose.

4, diaphragm  "o" ring and other gaskets.

Should check the control valve in the diaphragm, "o" type of gasket is aging, crack damage.

5. Sealing Filler Attention should be paid to PTFE filler, sealed lubrication grease aging, the mating surface is damaged, should be replaced when necessary.