Pneumatic ball valve is a kind of pneumatic actuator widely used in modern automatic control system. The control signal drives the ball valve switch action through the pneumatic actuator to complete the switch control or adjustment control of the medium in the pipeline.
The first point: the choice of ball valve
Connection mode: flange connection, clamp connection, internal thread connection, external thread connection, quick assembly connection, welded connection (butt welding connection, socket welding connection)
Valve seat sealing: metal hard sealed ball valve, that is, the sealing surface of the valve seat and the sealing surface of the ball are metal to metal ball valve. Suitable for high temperature, containing solid particles, wear resistance. Soft seal ball valve, seat using polytetrafluoroethylene PTFE, para-polystyrene PPL elastic sealing material, sealing effect is good, can achieve zero leakage.
Valve material: WCB cast steel, low temperature steel, stainless steel 304, 304L, 316, 316L, duplex steel, titanium alloy, etc.
Operating temperature: normal temperature ball valve, -40℃ ~ 120℃. Medium temperature ball valve, 120 ~ 450℃. High temperature ball valve, ≥450℃. Low temperature ball valve -100 ~ -40℃. Ultra-low temperature ball valve ≤100℃.
Working pressure: low pressure ball valve, nominal pressure PN≤1.6MPa. Medium pressure ball valve, nominal pressure 2.0-6.4MPa. High pressure ball valve ≥10MPa. Vacuum ball valve, lower than one atmosphere pressure ball valve.
Structure: floating ball valve, fixed ball valve, V ball valve, eccentric half ball valve, rotary ball valve
Flow channel form: through ball valve, three-way ball valve (L-channel, T-channel), four-way ball valve
The second point: pneumatic actuator selection
The double acting piston type pneumatic actuator is mainly composed of cylinder, end cover and piston. Gear shaft. Limit block, adjusting screw, indicator and other parts. Use compressed air as power to push the piston movement. The piston is integrated into the rack to drive the gear shaft to rotate 90°, and then drive the ball valve switching action.
The single-acting piston type pneumatic actuator mainly adds a return spring between the piston and the end cap, which can rely on the driving force of the spring to reset the ball valve and keep the position open or closed when the air source pressure is faulty, so as to ensure the safety of the process system. Therefore, the selection of single-acting cylinders is to choose whether the ball valve is normally open or normally closed.
The main types of cylinders are GT cylinders, AT cylinders, AW cylinders and so on.
GT appeared earlier, AT is an improved GT, is now the mainstream product, can be installed with ball valve bracket free, faster than the bracket installation, convenient, but also more firm. The position of 0° and 90° can be adjusted to facilitate the installation of various solenoid valves, stroke switches, handwheel mechanism accessories. AW cylinder is mainly used for large diameter ball valve with large output force and adopts piston fork structure.
The third point: the selection of pneumatic accessories
Solenoid valve: The double-acting cylinder is generally equipped with two five-way solenoid valves or three five-way solenoid valves. The single acting cylinder can be equipped with two three-way solenoid valves. Voltage can choose DC24V, AC220V and so on. Explosion-proof requirements should be considered.
Stroke switch: The function is to convert the rotation of the actuator into a contact signal, output to the control instrument, and feedback the on-off status of the field ball valve. Commonly used mechanical, magnetic induction type. Explosion-proof requirements should also be considered.
Handwheel mechanism: installed between the ball valve and the cylinder, it can be changed to manual switch when the air source is faulty to ensure system safety and not delay production.
Air source processing components: there are two and three connectors, the function is filtration, pressure reduction, oil mist. It is recommended to install the cylinder to prevent the cylinder from getting stuck due to impurities.
Valve positioner: For the proportional adjustment pneumatic ball valve need to be installed, mostly used for pneumatic V-type ball valve. Enter 4-20
mA, to consider whether there is a feedback output signal. Whether explosion-proof is required. There are ordinary type, intelligent type.
Quick exhaust valve: speed up the pneumatic ball valve switching speed. Installed between the cylinder and the solenoid valve, so that the gas in the cylinder does not pass through the solenoid valve, quickly discharged.
Pneumatic amplifier: Installed in the air path to the cylinder to receive the positioner outlet pressure signal, provide a large flow to the actuator, used to improve the speed of the valve action. 1:1 (the ratio of signal to output). It is mainly used to transmit pneumatic signals to long distances (0-300 meters) to reduce the impact of transmission lag.
Pneumatic holding valve: It is mainly used for interlocking operation of the air source pressure, and when the air source pressure is lower than that, the valve supply gas pipeline is cut off, so that the valve maintains the position before the air source failure. When the air source pressure is restored, the air supply to the cylinder is resumed at the same time.
Pneumatic ball valve selection to consider the factors of ball valve, cylinder, accessories, each choice of error, will have an impact on the use of pneumatic ball valve, sometimes small. Sometimes the process requirements cannot be met. Therefore, the selection must be aware of the process parameters and requirements.
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