I. Overview

The pneumatic control valve is composed of a pneumatic actuator and a control valve. The pneumatic actuator is powered by oil-free compressed air, receives a air signal of 20 to 100 kPa and is converted into displacement, and drives a control valve to regulate the flow of the fluid.

In order to improve the linearity of the valve position, overcome the influence of the friction of the valve stem and the pressure change of the medium to be adjusted as well as the speed of operation, the pneumatic valve positioner is matched with the control valve, so that the valve position can be correctly adjusted according to the adjustment signal. 

The quality of the air source should be free from obvious oil vapors, oils and other liquids. The number of solid particles contained in the air source of the control valve with positioner should be less than 0.1g/m3, and the particle execution should be less than 60μm, and the oil content should be less than 10g/m3.

The most common types are pneumatic diaphragm control valves and pneumatic piston control valves.

1. Pneumatic diaphragm control valve

The pneumatic source actuator has a maximum air supply pressure of 500 kPa. The actuator is divided into two types: positive action and negative action. The positive acting signal pressure is increased, the control valve is closed, and the air is closed. The reaction is the signal pressure is increased, and the control valve is also open, also known as air-opening.

2. Pneumatic piston control valve

The maximum air supply pressure of the pneumatic piston actuator is 700 kPa. Compared with pneumatic diaphragm actuators, it has a large output force under the same stroke conditions, so it is especially suitable for high static pressure and high differential pressure.

3. Valve positioner

There are two kinds of electrical signals and air signals.

Pneumatic valve positioner is used with pneumatic control valve. The air supply pressure of the positioner is related to the type of actuator and its pressure signal range (or spring pressure range). For example, when the ZPQ-01 positioner is matched with the ZM series pneumatic diaphragm actuator, if the actuator pressure signal range is 0.02~0.1Mpa, the air source pressure is 0.14Mpa; if the pressure signal range is 0.04~0.2Mpa, the air source pressure is 0.28Mpa; if the ZPQ-02 positioner is matched with the ZS-02 series piston actuator, the pressure signal range is 0.02~0.1Mpa, and the air source pressure is 0.5Mpa.

The electric signal valve positioner can also be called an electric-air valve positioner, which can convert a 0~10 mA or 4-20 mA DC electric signal into a standard air signal that drives a control valve.

II Commissioning of pneumatic control valve and related actuator

The main tasks of the commissioning of the pneumatic actuator are the cleaning of the air source pipe, the action direction of the valve, the adjustment of the valve positioner, and the linearity adjustment of the valve.

Steps for commissioning:

1. After the installation on site, check whether the parts are installed correctly and firmly, and check that the air source and air path and circuit are connected correctly.

2. From the main air supply tubes to the pneumatic actuator, purge the air supply line in sections. It is necessary to ensure that the pipeline is unblocked and the air source meets the air requirements. When purging, the air source filter of the pneumatic actuator should be opened and purged until the air source is qualified before reinstalling.

3. Check the pressure before the filter and pressure reducing valve to meet the requirements of the pressure reducing valve. Adjust the pressure after the pressure reducing valve to match the source pressure required by the nameplate or pneumatic actuator.

4. Switch the valve button to the “local control” file (some without this switch), and then manually operate the pneumatic actuator to confirm that the pneumatic actuator is moving in the correct direction, otherwise the corresponding air pipe should be exchanged. And the valve should not be stuck. For actuators with a air signal without positioner, the pneumatic command signal 0.02~0.1Mpa corresponds to full off and full open (the opposite of the air open actuator); for the pneumatic signal actuator with pneumatic positioner, the pneumatic command signal is first positioned After the amplifier is amplified, the actuator is driven; for the electric signal pneumatic actuator, the 4-20 mA signal corresponds to the 0.02-0.1 Mpa pneumatic signal, and then the pneumatic signal drives the actuator to operate.

5. Check and adjust the output of the valve positioner when the valve is fully open and fully closed. The output of the valve positioner should be the zero value of the air signal and electric signal when the valve is fully closed (for example: 0.02Mpa or 4mA). When the valve is fully open, it should correspond to its full value (for example: 0.1Mpa or 20mA).

6. Switch the actuator button into the "remote control" position (some without this switch), and carry out the remote electrical signal control test from a distant place (such as the centralized control room). The direction of action of the valve should be correct. The actual opening of the valve, as well as the position of the valve at the local and remote locations, should correspond exactly to the value of the action command issued to the valve.

7. Pneumatic actuators with linear adjustment function should also adjust the linearity of their working characteristics at a point commonly used in operation (generally near the intermediate valve position).

8. After the commissioning is completed, tighten all the screws and pipe joints and tighten all the screws.

III．Frequent failures of pneumatic actuator

The main reason for the valve jam is that the air source contains some water or the compressed air is unqualified, especially in the rainy season, the air is wet, and the water in the valve is easy to jam. The air source of the pneumatic actuator shall be clean, pressure-stabilized compressed air. Generally it is supplied from an oil-free air compressor to the air parent tube through the air storage tank, the filtration and decompression equipment. Each pneumatic actuator takes the air source from the parent tube. If the cleanliness or pressure of the air source of the parent pipe cannot meet the requirements of various types of pneumatic actuators, we can install air filters, air pressure reducing valves in front of the air pipes of each actuator.

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