Pneumatic shut-off valve is an automation system, used as an automatic switch cut off a valve. Generally consists of valve + pneumatic actuator + solenoid valve. Pneumatic actuators are divided into three types: angular stroke cylinder type, multi-spring pneumatic thin film actuator type, straight stroke piston type. Cut-off valve through the reception of regulating instruments or central control room signal, control process pipeline fluid cut off or on. Widely used in beer, food and beverage, grain and oil, pharmaceutical, dairy, chemical and other industrial production sectors. Pneumatic shut-off valve air source requires filtered compressed air, the medium flowing through the valve body should be free of impurities and particles of liquid or gas.
Mainly includes: pneumatic globe valve, pneumatic ball valve, pneumatic membrane shut-off valve, pneumatic butterfly valve, pneumatic angle seat valve and so on. Should be based on the media pressure, temperature, flow and logistics characteristics to choose the right type of valve.
1.Mainly used for high-temperature steam or high-temperature thermal oil, and pneumatic control valves (proportional control valves) used in conjunction with some intermittent production equipment, when the temperature or pressure and other parameters beyond the set value, the requirements of the media cut off, do not allow over-temperature occasions. Connected with the temperature or pressure sensors and other signals, when the temperature and pressure of the detection point in the equipment exceeds the set limit value, automatically and quickly close the steam supply valve to stop the supply of steam.
2. It is used in the front end of the equipment which does not need to be adjusted and only needs to be cut off by automatic switching.
3. It is used in the position which is difficult to operate manually in the high air, and the position which is operated very frequently, and the pneumatic shut-off valve is used to replace the manual valve.
1. Selection of structural form
In the cut-off effect, the angle stroke valve is simpler than the straight stroke valve structure, sealing effect is more reliable and superior. The use of angle stroke cylinder valves are: pneumatic ball valves, pneumatic butterfly valves.
2. Selection of sealing surface
(1). face-to-face sealing
Common plunger valve spool, the sealing surface for the 60 °of the small cone, the width of this small cone is usually between 0.5mm ~ 2mm, to seal the good must ensure that the two conical surface good contact. But in fact, it is always subject to the influence of processing errors (such as concentricity, out of round, tilt, etc.), its sealing effect is not ideal.
(2). Spherical seal
The use of spherical surface of the spherical rotation and fixed valve fixed small cone tangent, if the hard seal, it is difficult to ensure the sealing performance, so most of the soft seal, such as pneumatic ball valves.
(3). Plane seal
Plane seal is the valve spool and valve seat surface are flat, the material and surface finish requirements are particularly high, general carbon steel or cast iron materials can not meet this requirement. Carbide or Stellite alloy must be used to ensure sealing.
3. Selection of sealing materials
(1). Soft seal
In addition to corrosion-resistant lined valves, the general soft seal valve refers to the valve core or seat one of the two non-metallic materials (mainly PTFE and nitrile rubber, etc.) of the sealing form of the valve. Soft seal valve sealing effect is the best, but in the pipeline installation, system cleaning may leave more or less unclean debris (such as welding slag, iron filings, etc.), When they flow through the regulating valve, it is easy to scratch the soft seal valve seat or spool, so that the leakage increases and the sealing reliability deteriorates. Therefore, the choice of soft seal structure must consider the cleanliness of the medium and put into operation before the strict flushing of the pipeline, and can only be used for low-temperature media.
(2). Hard seal
Hard seal and then surfacing wear-resistant alloy is the best choice for shut-off valve. This way in the consideration of sealing at the same time also consider the service life and reliability. Surfacing on the sealing surface of the Stellite alloy process, coupled with the valve inlet end of the installation of filters to avoid the entry of impurities, to ensure that the sealing performance of the steam and the long-lasting use (life is very long), and at the same time to solve the soft seal by the influence of the temperature and the generation of deformation of the situation. Enough to meet the requirements of strict cut-off, and it is durable, more cost-effective from an economic point of view.
4.Reliability of cut-off valve
(1). Soft seal, cut off the effect is good, but not suitable for particulate media or high temperature media, once scratched, the leakage will increase dramatically; high temperature leads to aging and deformation of the sealing surface material.
(2) The seal wear resistance considerations: for some very frequent switching, erosion conditions, must choose hard seal, and must be overlaid with wear-resistant alloy to improve the seal’s wear resistance, erosion resistance, scratch resistance, and improve the life of the valve.
(3) The viscous media containing particles, to consider the selection of hard seal pneumatic ball valves, butterfly valves or knife gate valves.
(4) For hard seal, there must be enough hardness difference, the same hardness material, it is easy to scratch and cause leakage.
IV. Summary
In summary, the reliability of the cut-off valve, you need to consider the characteristics of the medium, pressure, temperature, switching frequency and flow conditions and other factors to choose the type and brand of cut-off valve.
