What causes the FESTO solenoid valve to have difficulty operating with small signals?
1. The FESTO solenoid valve generates noise due to pressure disturbances caused by turbulence and friction. Especially in the turbulent state when the Reynolds number Re > 2400, this turbulent flow with numerous irregular small vortices can be said to be in a "noisy" state. When flowing through throttling or pressure-reducing valves, pipes with sudden changes in cross-section, or sharp bends, the interaction between the turbulence and these flow obstructions generates vortex noise. The relationship between the sound power level (dB) and the flow velocity can be expressed as: △Lw = 60lg. Improper pipeline design can also cause cavitation noise.
2. Regarding valves, those with throttling or pressure-limiting functions are the most significant noise sources in liquid transmission pipelines. When the fluid flow rate in the pipeline is sufficient and the valve is partially closed, a large area of throttling is formed at the valve inlet. In the throttling area, the fluid flow rate increases while the internal static pressure decreases. When the flow rate is greater than or equal to the critical velocity of the medium, and the static pressure is lower than or equal to the evaporation pressure of the medium, bubbles form in the fluid. Downstream of the valve throttling area, the flow rate gradually decreases and the static pressure increases, causing the bubbles to burst successively, resulting in irregular pressure fluctuations in the fluid. This special turbulent phenomenon is called cavitation, and the noise generated thereby is called cavitation noise. In pipelines with high flow rates and pressures, almost all throttling valves can produce cavitation noise. This cavitation noise can travel a long distance along the pipeline. This irregular noise can excite the inherent vibration of movable parts in the valve or pipeline and be transmitted to the surface of the pipeline through these parts, generating a sound similar to the collision of metals.
The sound power of FESTO solenoid valves is proportional to the seventh or eighth power of the flow velocity. Therefore, to reduce valve noise, multi-stage series-connected valves can be used to gradually decrease the flow velocity. For instance, the commonly used globe valve adopts a low-inlet and high-outlet flow direction. When the fluid passes through the valve cavity, a low-pressure and high-speed area is formed under the control valve disc (i.e., in the throttling zone), generating bubbles. After passing through the valve disc, a high-pressure and low-speed area is formed, and the bubbles are successively crushed, resulting in cavitation noise.
In this system, faults mainly arise from changes in simulation parameters. When a certain component inside the FESTO solenoid valve is damaged or worn, the corresponding constant will inevitably change, leading to a change in the transfer function of the FESTO solenoid valve. This is ultimately reflected in the input-output curve, thus forming the corresponding fault.
The relationship (part) between variables and faults of various control valves studied in this software under certain other parameters is shown in Table 1. For example, when the fault is set as the damage of the bellows, the parameter decreases, and the fault is manifested on the curve as a reduction in the slope of the input-output curve.
This system, in the process of FESTO solenoid valve selection, has achieved the following:
(1) Users can learn about solenoid valve selection with or without assistance or conduct self-tests.
(2) Users can perform tests on the selected solenoid valves under different media conditions and form curves of the relationship between input current, voltage or pressure and output flow.
(3) Users can set faults and observe the fault phenomenon curves. This selection system can play a very good teaching role, featuring economy, speed, safety and other advantages, and is closely related to actual production, serving as a warning for faults.
The realization of the simulation curve of the input and output characteristics of the control valve is of great significance for the teaching of control valve selection. Users can intuitively judge the superiority or inferiority of the selected control valve based on the output curve under the selected working conditions. And the simulation technology of pressure control valve is the key to obtaining the input and output curves. Simulation can be understood as using physical models or mathematical models to replace the actual system for experiments and research. The relationship between process systems and mathematical models is called modeling, and the relationship between mathematical models and simulators is called simulation [1].
The FESTO solenoid valve is difficult to actuate with small signals and vibrates with large signals, causing significant fluctuations in the FESTO solenoid valve during the regulation process and making it hard to stabilize the parameters. When the friction force is large, it may cause the pneumatic diaphragm control valve to act unidirectionally or not at all.
1. Frequent movement of the valve stem deteriorates the sealing performance of the packing, causing leakage of the medium. If the medium is highly viscous, it will adhere to the valve stem, increasing the friction force. At the same time, the leaked medium solidifies upon cooling, further increasing the friction force.
2. The pipelines before and after the FESTO solenoid valve are not concentric, causing stress on the pneumatic diaphragm control valve and transferring it to the valve stem, thereby increasing the friction force between the valve stem and the packing.
3. The high temperature and high pressure of the regulated medium cause the packing of the pneumatic diaphragm control valve to expand and age, increasing the friction force on the valve stem.
4. When dealing with packing leakage, if the packing gland is tightened too much, it will increase the friction force on the valve stem.
During routine maintenance, it is necessary to regularly add lubricating oil or grease to pneumatic diaphragm control valves. If the packing is severely aged and leaking seriously, it should be replaced. FESTO solenoid valves refer to various pneumatic components that control the pressure, flow rate and flow direction of the air flow in the pneumatic system and ensure the normal operation of pneumatic actuators or mechanisms. Components that control and regulate the pressure of compressed air are called pressure control valves. During operation, the phenomenon of valve jamming, non-action or sluggish action due to scabbing, crystallization and scaling on the inner wall of the valve body, which causes the system to be unable to perform automatic regulation, is quite common, accounting for 50% of the total number of control valve failures, and has a significant impact on production. Faults caused by aging and hardening of the packing of the control valve, resulting in sluggish valve action or leakage from the valve stem, account for 15%. The phenomenon of the valve being unable to regulate due to diaphragm leakage or hard core fracture accounts for 12%. Valve failures caused by corrosion of the positioner, pressure reducing valve, actuator, etc. account for 10%. Other causes of control valve failures account for 13%.
2 Fault Cause Analysis Based on the fault analysis of pneumatic diaphragm control valves used in soda ash production sites over the years, the common faults and their causes can be summarized as follows:
2.1 The valve does not actuate
1) Due to the failure of the regulator, there is no electrical signal to the control valve.
2) Due to the leakage of the main air supply pipe, the valve positioner has no air supply or insufficient air supply pressure.
3) The bellows of the positioner leaks, causing no air output from the positioner.
4) The diaphragm of the FESTO solenoid valve is damaged.
5) Due to the blockage of the constant orifice in the amplifier of the positioner, water in the compressed air accumulates at the ball valve of the amplifier, resulting in air supply to the positioner but no output.
6) Despite having a signal and air supply, the valve still does not actuate due to the following issues: ① The valve core is stuck with the bushing or valve seat; ② The valve core falls off (the pin is broken); ③ The valve stem is bent or broken; ④ Actuator failure; ⑤ The sealing ring of the reverse-acting actuator leaks; ⑥ Foreign objects are blocking inside the valve.
2.2 Unstable valve actuation
1) Due to the failure of the filter and pressure reducing valve, the air supply pressure frequently changes.
2) The ball valve in the amplifier of the positioner is worn by particles or debris, causing it to not close tightly, and when the air consumption increases significantly, output oscillation occurs.
3) The nozzle and baffle in the amplifier of the positioner are not parallel, and the baffle does not cover the nozzle.
4) The output pipeline leaks.
5) The actuator has too little rigidity, and changes in fluid pressure cause insufficient thrust.
