1.1 Installation position
When installing the pneumatic control valve, its installation position requires a certain height from the ground. There is a specific space above. Below the valve to allow for the disassembly and repair of the valve. Finally, we must ensure easy operation, observation, and adjustment for control valves equipped with pneumatic valve positioners and handwheels.
The control valve needs to be installed on a horizontal pipeline perpendicular to the above and below pipeline and generally supported under the valve. For special occasions where we need to install the regulating valve horizontally on a vertical pipe, we should keep the control valve (except for small diameter control valves). We must avoid bringing additional stress to the control valve when installing it.
1.2 Temperature
The working temperature of the control valve should be at (-30 – +60) relative humidity, not more than 95%.
The front and rear position of the control valve should have a straight pipe section, the length of which is not less than ten times the diameter of the pipe (10D), to avoid the valve’s straight pipe being too short and affecting the flow characteristics.
1.4 The diameter of the control valve
When the diameter of the control valve is not the same as the process piping, we should use a reducer connection. For example, we can use a threaded link when installing a small diameter control valve. In addition, the fluid direction arrow on the valve body should be consistent with the fluid direction.
We have to set up a bypass pipe. Its purpose is to facilitate switching or manual operation, and we can regulate the valve for maintenance without stopping.
Before installing the control valve, we must thoroughly remove foreign objects from the pipeline, such as dirt, welding slag, etc.
2. Electric control valve installation principles.
The valve’s installation position, height, import, and export direction must meet the directional design requirements; the connection should be firm and tight.
Valves can be connected to the pipeline in various forms of the end. The most important connections are threaded, flanged, and welded connections. When using a flange connection, if the temperature exceeds 350 ℃, we should choose high temperature-resistant bolt material due to the bolts, flange, and gasket creep relaxation.
When installing the valve before carrying out an appearance check, the valve nameplate should comply with the current international standard GB12220 “General Valve Marking” provisions.
2.2 The strength testing of the control valves
When we test the strength, the experimental pressure for the nominal pressure of 1.5 times, the duration of not less than 5min, the valve shell, packing should be no leakage.
2.3 Testing the tightness
When we do the tightness test of the regulating valve, the experimental pressure is 0.3mpa. The testing pressure should be maintained during the duration of the experiment. The time should be by the provisions of Table 2. We are qualified with no leakage on the sealing surface of the valve flap. Its nominal diameter range is DN15-500.
3. Common faults of pneumatic and electric control valves
4.Common faults and causes of pneumatic control valves
4.1 Non-action of the regulating valve, failure phenomena, and reasons are as follows.
4.1.1 No signal, no gas source.
① The air source is not open.
② The air source contains water which freezes in winter, resulting in blockage of the air duct or blockage of the filter or pressure reducing valve which is out of order.
③ Compressor failure.
④ Leakage in the central air supply pipe.
4.1.2 With air source, no signal.
① Regulator failure.
② Positioner corrugated steel leaking air.
③ Damaged regulating mesh diaphragm.
4.1.3 Positioner without an air supply.
① Clogged filter.
② Faulty pressure reducing valve.
③ Leaking or blocked pipes.
4.1.4 Positioner with the air source, no output.
Blocked throttle hole in the positioner.
4.1.5 signal present, no action.
① Spool dislodged.
② Spool stuck with society or with the valve seat.
③ Valve stem bent or broken.
④ Valve seat spool frozen or coke block dirt.
⑤ Actuator spring repaired dead due to long-term non-use.
4.2 The control valve action is unstable. Fault phenomena and causes are as follows.
4.2.1 Unstable pressure of the gas source.
① Compressor capacity is too small.
② Faulty pressure reducing valve.
4.2.2 Unstable signal pressure.
① The time constant of the control system is not appropriate.
② Unstable regulator output.
4.2.3 Stable gas source and stable signal pressure, but unstable regulator valve action.
① The ball valve of the amplifier in the positioner is poorly worn by spoil, and output oscillation occurs when the air consumption is notably increased.
② The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle.
③ Air leakage from the output tube or line.
④ The rigidity of the actuator is too small.
⑤ Frictional resistance in the movement of the valve stem is high, and there is a blocking phenomenon with the phase contact part.
4.3 The failure phenomena and causes of vibration of the regulating valve are as follows.
4.3.1 The regulating valve vibrates at any opening degree.
① The support is unstable.
② There is a source of vibration nearby.
③ The Spool and bushing are badly worn.
4.3.2 The control valve vibrates when in the fully closed position.
① The control valve is selected large and is often used in small openings.
③ Single-seat valve media flow in the opposite direction to the closing direction.
4.4 The movement of the control valve is sluggish. The phenomenon of sluggishness and its causes are as follows.
4.4.1 Sluggish action of the valve stem in one direction only.
① Broken diaphragm leakage in the pneumatic film actuator.
② Leakage of the “O” seal in the actuator.
4.4.2 Stem is dull in reciprocating action.
① Clogging of the valve body with sticky material.
② Teflon is packing deterioration and hardening or graphite – asbestos packing lubricant dry.
③ Filler added too tight, friction resistance increased.
④ Due to the valve stem is not straight, resulting in high frictional resistance.
⑤ Pneumatic control valves without positioners can also lead to cause retardation.
4.5 Leakage of the control valve increases. The reasons for Leakage are as follows.
4.5.1 Large Leakage when the valve is fully closed.
① The valve spool is worn. The internal Leakage is serious
② The valve is not adjusted to close tightly
4.5.2 The valve does not reach the fully closed position
① The medium pressure difference is too significant. The actuator rigidity is slight. The valve can not close tightly
② There is a foreign body inside the valve
③ Bushing sintering
4.6 Flow adjustable range becomes smaller.
The main reason is that the Spool is corrupted to become smaller so that the adjustable minimum flow rate becomes more considerable.
Understand the failure phenomenon and causes of pneumatic control valve, you can take the suitable measures to solve.
