Causes of regulating valve failure
Regulating valve is also known as control valve. In the field of industrial automation process control, it accepts the control signal output by the adjustment control unit and uses power operation to change the final control element of process parameters such as medium flow, pressure, temperature, and liquid level. Generally composed of actuators and valves. According to the characteristics of the stroke, the control valve can be divided into straight stroke and angular stroke; according to the power used by the actuator, it can be divided into three types: pneumatic control valve, electric control valve, and hydraulic control valve; according to their functions and characteristics There are three types of linear characteristics, equal percentage characteristics and parabolic characteristics. The regulating valve is suitable for air, water, steam, various corrosive media, mud, oil and other media. Commonly used classification of control valves: pneumatic control valves, electric control valves, hydraulic control valves, self-operated control valves.
The regulating valve is the terminal execution device in the automatic regulating system, which regulates the production process by receiving the control signal.
Jamming is a frequent problem in regulating valves, which often occurs in newly put into operation systems and re-commissioned periods after overhaul. This is due to the accumulation of welding slag, iron filings and other impurities inside the pipeline at the orifice and guide part to cause blockage. After the jam occurs, the medium will not flow smoothly or the packing of the regulating valve will be too tight during maintenance, which will increase the friction, resulting in the phenomenon of small signal inaction and large signal action.
Regulating valve leakage generally includes internal leakage, packing leakage, and leakage caused by deformation of the valve core and valve seat.
- (1) Valve leakage is due to the improper length of the valve stem. The length of the valve stem of the air-opening valve is too long, so that the upward or downward distance of the valve stem is too short, resulting in a gap between the valve core and the valve seat. Contact, resulting in valve leakage.
- (2) Packing leaks. Packing the packing into the stuffing box will exert axial pressure on the regulating valve. Due to the plastic deformation of the packing, radial pressure will be generated, and it will be in close contact with the valve stem, but the contact is uneven. During the operation of the regulating valve, the stem and packing will move axially. In a high-temperature and high-pressure environment, it is prone to filler leakage due to the influence of high-permeability fluid media.
- (3) The valve core and valve seat are deformed, leaked, eroded and impacted, causing them to be deformed. As time goes by, the valve core and valve seat will not match, and there is a gap between them, resulting in lax closing. A leak has occurred.
The reason for the oscillation of the control valve is that the spring stiffness of the control valve is insufficient, the output signal of the control valve is unstable and produces sharp changes, the frequency of the control valve is the same as the frequency of the system, and the pipeline and the base vibrate violently.
There is also improper selection of the regulating valve. When the regulating valve is working at a small opening, drastic changes in flow resistance, flow rate and pressure occur. When this change exceeds the stiffness of the regulating valve, the stability of the regulating valve is reduced, which is serious. Under the circumstances, the regulating valve oscillates.
The failure phenomena of the control valve not working include:
- (1) No signal, no gas source. The cause is that the air source is not turned on or the air source is faulty, the air pipe is blocked or the pressure reducing valve fails due to the blockage, and the compressor has a fault light;
- (2) There is air source but no signal. The cause is that the positioner fails due to air leakage due to the rupture of the bellows, or the converter fails;
- (3) The positioner has no air source. The cause is that the filter and the pressure reducer are blocked and fail;
- (4) The positioner has air supply but no output. The cause is that there is a blockage in the orifice of the positioner and the failure occurs;
- (5) There is signal but no action. The cause is that the valve core falls off; the valve stem is bent or broken.
Unstable control valve action The reasons for the unstable control valve:
The compressor capacity is too small, causing the pressure reducing valve to fail, resulting in unstable air source pressure;
The regulator is unstable, and the time constant of the control system is inappropriate, resulting in unstable signal pressure;
The frictional resistance of the valve stem in motion is very large, and the blocking phenomenon occurs during the contact process;
The ball valve of the amplifier in the positioner is not tightly closed due to dirt and wear, and the output will oscillate when the air consumption increases.
Slow control valve action
The pneumatic membrane is damaged and leaked during operation, which makes the valve stem sluggish in unilateral action; the positioner has a weak reaction performance, which will cause the control valve to move sluggishly; there are obstructions such as rust in the control valve, which will cause blockage to make the control valve slow action in axial movement; poor packing quality or too tight packing, which increases frictional resistance, resulting in slow motion of the regulating valve.
Source: China Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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