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Pneumatic control valve selection and installation
Dec 14, 2018

Pneumatic control valve selection and installation


 

Pneumatic control valve is compressed gas as the source of power to cylinder as actuator, and with the aid of the valve positioner, converters, solenoid valve, valve, gas storage, gas filters, such as attachments to drive the valve, realize the switch quantity or proportion type adjustment, industrial automation control system receives the control signal to complete conditioning pipeline medium: flow, pressure, temperature, liquid level and other process parameters.Pneumatic control valve is characterized by simple control, rapid response, and the nature of safety, do not need to take additional explosion-proof measures.Today we will briefly introduce the pneumatic control valve selection and installation.

Pneumatic control valve selection and installation


First, the selection of the regulating valve


About the valve body

1. In the case of constant pressure difference, the top-guide type one-way regulating valve with unbalanced fluid pressure is a single-seat regulating valve with small size and simple structure, but it can adapt to harsh working conditions. Due to the use of an unbalanced spool, the combination is compacted with a small, multi-spring diaphragm actuator. This series of regulating valves is particularly suitable for a wide range of fluids due to the absence of a fluid pressure balancing structure.

2. Double seat control valve is used for high pressure and high pressure difference. However, if the leakage of the valve seat is relatively high, the sleeve single seat control valve should be used.

3. When the noise of the regulating system to the regulating valve is greater than 85 decibels, the noise reduction regulating valve should be selected.

4. For those with low pressure and large flow range, adjustable ball valves should be used.

5. For the valve with serious fluid flushing or working under the flash needle state, the valve is indented in principle, and the valve core and the valve seat are all surfacing with the Stellite alloy.

6. For the control valve working under strong acid and alkali medium, anti-corrosion measures should be taken. For example, corrosion-resistant stainless steel or the body is made of all lining PTFE to achieve anti-corrosion. All valves and actuators are sandblasted and all bolts and nuts are corrosion resistant for use in Xinjiang.

7. The material and pressure rating of the valve body, trim and seal of the regulating valve shall comply with the process conditions of the installation and the environmental requirements of the site. The regulating valve has a local mechanical indicating device that can indicate the opening of the valve in place.

8. Matching pipe fittings and supporting flanges, nuts (120%), nuts (120%), gaskets (200%). The mating flange material must be exactly the same as the process pipe material to facilitate welding during on-site construction.

9. Signs and nameplates Each valve has a stainless steel nameplate. The nameplate should be in an easy-to-see position. The nameplate should contain the following.

1) Name of the manufacturing plant

2) Valve's position number, manufacturing number and model number

3) Material, size and pressure rating of the valve body

The selection of the regulating valve should be calculated according to the parameters of the medium, flow, pressure, temperature, viscosity and process diameter after the valve is issued. It is calculated by the original manufacturer's original software and fully meets the design requirements.

About actuators

1. For the selection of actuators, it is necessary to select the common type of actuators for the southern part of China for the climate and conditions of the localities. For the cold regions in the north, it is necessary to use low-temperature actuators for working at high temperatures for a long time. The regulator actuator should use a high temperature actuator. The actuator used is of low temperature type, the connecting bracket is made of low temperature resistant cast steel, the rubber parts such as diaphragm and 0 type ring are made of low temperature resistant material, and the lubrication grease is also resistant to low temperature grease.

2. The actuator is currently adopted as a multi-spring structure, which is characterized by light weight, small size, high performance and large output force. The pneumatic actuator is matched with the type and size of the selected regulating valve; for the large-diameter regulating valve, a multi-spring film structure is selected.

Pneumatic control valve selection and installation

About attachments

1. The electric valve positioner should receive the analog signal of 4~20mADC, and the power supply voltage is 24VDC. The basic error is ≤±0.1%; the insensitive area fine adjustment range: 0.3~10%; the air consumption: <3NL/min (not related to the supply pressure).

2. The positioner is the soul and heart of the whole regulating valve, which directly affects the reliability of the regulating valve. In principle, the positioner should be imported as much as possible when selecting the adjusting valve. At the same time, for different regions, the ordinary type, high temperature type and low temperature type should be selected separately.

3. The electric valve positioner, pneumatic actuator and air filter pressure reducing valve of the regulating valve are used as an integral part of the regulating valve. The assembly kit is supplied to the owner and is assembled by the factory before the regulating valve is delivered. Qualified testing of all features.

4. The filter pressure reducing valve should be selected according to the size of the actuator to be equipped according to the regulating valve. According to the working environment, it is divided into normal temperature type, high temperature type and low temperature type.

5. All electrical parts of the positioner should meet the requirements of the on-site protection level.


Second, the installation of the regulating valve


1. In order to ensure the safety and reliability of the regulating valve, it should be installed horizontally as much as possible. The length of the straight pipe section of 10D should be ensured before and after the valve, and should be avoided in the case of high vibration intensity. If the pump outlet position should be extended, the pipeline should be used as much as possible to avoid damage to the regulating valve.

2. Make sure that the regulating valve is on the same line as the process line, and that the actuator is kept vertical.

3. The studs should be bolted. The gasket should use a metal-wound gasket that can be reused many times to ensure the sealing reliability of the regulating valve.

4. Ensure adequate maintenance space for the regulator valve in future use. The general regulating valve is equipped with a negative line. When the throttle valve is in an emergency, the negative line can be switched. However, when installing the negative line, it is necessary to leave a certain maintenance space for the regulating valve, generally not less than 50CM.

5. Replace the valve plug and valve seat. For the regulating valve with the hand wheel mechanism, the handle wheel mechanism is rotated to the limit position, and the special connecting wrench is used to loosen the cross connecting block of the adjusting valve push rod and the valve core, so that the connection between the actuator and the valve core is disengaged. Loosen the upper bonnet screw and remove the entire upper bonnet together with the actuator.

The valve seat can be replaced with a special tool and replaced with a new valve seat.

6, replace the actuator: the installation should pay attention to the vertical (vertical) or horizontal (bedroom) system of the gas tank. The cylinder must be free from the top and bottom, and there must be no jamming. In order to facilitate maintenance, the starter actuator should be installed close to the ground or floor. When the valve positioner or the first wheel mechanism is installed, the adjustment and operation convenience should be ensured. The role of the first round of the mechanism is: in the event of an accident, it can be used to directly operate the control valve without pneumatic pressure.

Pneumatic control valve selection and installation

The starting actuator should be installed at an ambient temperature not higher than +60 ° C and not lower than -40 ° C, and should be kept away from equipment with high vibration. In order to avoid heat aging of the diaphragm, the distance between the upper membrane cover of the control valve and the heat transfer pipe or equipment should be greater than 200mm. When the nominal diameter of the valve is different from the nominal diameter of the pipe, a different diameter pipe should be added between the two. . The pneumatic actuator should be mounted vertically on a horizontal pipe.

In special cases where horizontal or inclined installation is required, in addition to small diameter valves, support should generally be added. Even if it is installed upright, it should be supported when the valve has a large weight and vibration.

The direction of the fluid passing through the control valve is indicated on the valve body and cannot be reversed, just as the orifice plate cannot be reversed.

A shut-off valve is installed in front of and behind the control valve to remove the control valve during repair.

Before the control valve is installed, the pipeline should be cleaned to remove dirt and welding slag. The pipe and valve should be cleaned again after installation and the sealing performance of the valve to pipe connection should be checked. When the media is first introduced, the valve should be in the fully open position to prevent the magazine from getting stuck.