The steam control valve uses compressed air as the power source, the cylinder as the actuator, and the help of an electrical valve positioner, converter, solenoid valve, position-keeping valve, and other accessories to drive the valve. The valve can be switched or proportionally adjusted to receive control signals from the industrial automation control system to adjust the pipeline medium’s flow, pressure, temperature, and other process parameters.
Basic Introduction of Steam Control Valve
Steam regulating valves are about flow, pressure, temperature, and other process parameters. Simple control and fast response characterize pneumatic control valves. It is also intrinsically safe and does not require additional explosion-proof measures.
Maintenance of steam control valves
Steam control valves work accurately and usually to ensure the normal operation of the process plant, and safe production has a fundamental significance.
The key inspection parts during the inspection of Control Valve
1. Check the inner wall of the interval.
In the high-pressure difference and corrosive media occasions, the valve body wall diaphragm valve diaphragm is often subject to the impact of media and corrosion. Therefore, we must focus on checking the pressure and corrosion resistance.
2. Check the valve seat.
Because of the medium infiltration work, the inner surface of the thread used to fix the valve seat is susceptible to corrosion and makes the valve seat slack.
3. Check the spool
The spool is among the movable parts of the control valve. Therefore, it is more serious erosion by the medium, and we must carefully check whether the spool ministries are rusted or worn. Especially in the case of high-pressure differences, the wear of the spool due to cavitation caused by the cavitation phenomenon is more serious. Therefore, you should replace the seriously damaged spool.
4. Check the sealing packing
We need to check whether the packing asbestos rope is dry. If PTFE packing is used, we should check whether it is aged and if its mating surface is damaged.
5. Check whether the rubber film in the actuator is aged and whether there is cracking.
Daily maintenance of steam regulating valves
When the regulating valve adopts graphite asbestos as packing, about three months should add lubricating oil to the packing once. Then, it can ensure that the valve is flexible and works well.
If the packing pressure cap is very low, we should replenish the packing.
If we find the Teflon packing hardened, we should replace it in time.
We should pay attention to the operation of the regulating valve during the inspection tour and check whether the valve position indicator and the regulator output match. We should always check the air source of the regulating valve with the positioner, and we need to find the problem and deal with it in time. We should often maintain the health of the regulating valve and the integrity of the components in good use.
Steam regulating valve common failures and the causes
The regulating valve does not act.
Failure phenomena and causes are as follows.
1. No signal, no gas source.
①Gas source is not open
② Due to the gas source containing water in winter icing, resulting in blockage of the air duct or filter, pressure-reducing valve blockage malfunction
③ Compressor failure
④Leakage of the main air supply pipe.
2. Air source, no signal
①Failure of regulator
②Leakage of signal pipe
③Leakage of positioner bellows
④Damaged diaphragm of regulating net.
3. No air source for the positioner
①Filter blockage
②Failure of a pressure-reducing valve
③Pipe leakage or blockage.
4. Positioner with the air source, no output.
The throttle hole of the positioner is blocked.
5. There is a signal but no action.
①Spool off
②The spool is jammed with the society or with the valve seat
③Valve stem bent or broken
④The valve seat spool is frozen or coke block dirt
⑤ Actuator spring rusted due to long-term non-use.
The action of the steam adjustment valve is unstable.
Fault phenomena and causes are as follows.
1. The pressure of the gas source is unstable.
① Compressor capacity is too small
②Failure of the pressure-reducing valve.
2. Unstable signal pressure
①The time constant of the control system (T=RC) is inappropriate.
②The regulator output is unstable.
3. Stable gas source and signal pressure, but unstable regulating valve action.
①The ball valve of the amplifier in the positioner is not closed tightly due to dirt wear, and the output oscillation occurs when the gas consumption is particularly increased
②The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle
③Air leakage from the output line
④The stiffness of the actuator is too low.
⑤ Frictional resistance in the movement of the valve stem is high, and there is blocking in the contact part with the phase.
Installation of steam control valves
Steam control valves are best installed with the working piston’s upper end in the horizontal line’s lower part. You can install the temperature sensor in any position, and it must immerse the entire length in the controlled medium。
Selection analysis
Types of valve bodies for control valves
There are many control valve bodies; we commonly use the single straight seat, straight double seat, angle, diaphragm, small flow, three-way, eccentric rotary, butterfly, sleeve, spherical, etc.
In the specific selection, we can make the following considerations.
(1) Spool shape structure.
We mainly consider factors such as flow characteristics and unbalanced forces based on the chosen.
(2) Wear resistance.
When the fluid medium is a suspended liquid containing a high concentration of abrasive particulates, the internal material of the valve should be hard.
(3) Corrosion resistance.
Due to the corrosive nature of the medium, we try to choose valves with a simple structure.
- Medium temperature and pressure
When the temperature and pressure of the medium are high and change a lot, we should choose the spool and seat material by the temperature and pressure change of the small valve.
(5) Prevention of flashing and cavitation
Flash steam and vapour corrosion only occur in liquid media. However, in the production process, flash and cavitation will form vibration and noise, shortening the valve’s service life. Therefore we should prevent flashing and cavitation when selecting valves.
The choice of a control valve actuator
To make the control valve work properly, the actuator should be able to produce enough output force to ensure a high degree of sealing and opening of the valve.
There is generally no reset spring for double-acting pneumatic, hydraulic, and electric actuators. The size of the acting force is not related to the direction of its operation. Therefore, the key to selecting an actuator is to find out the maximum output force and rotating torque of the motor. For single acting pneumatic actuators, the power output is related to the degree of opening of the valve.The force appearing on the regulating valve will also influence the movement characteristics. We, therefore, require a force balance to be established over the entire opening range of the regulating valve.
Determination of the actuator type
Once the output force of the actuator has been determined, we select the appropriate actuator according to the environmental requirements of the process used. For example, we should use pneumatic actuators for sites with explosion-proof requirements. In terms of energy saving, we should try to use electric actuators. If the adjustment precision is high, we can choose a liquid-acting actuator. For example, the speed adjustment of the transparent machine of the power plant, the temperature adjustment control of the reactor of the catalytic device of the oil refinery, etc.
Mode of action of the control valve
The mode of action of a control valve can only be used if a pneumatic actuator has been selected. The mode of action is formed by the combination of the positive and negative action of the actuator and of the valve.There are four combinations, i.e., positive-positive (air-off type), positive-versus (air-on type), anyway (air-on type), and negative-versus (air-off type). These four combinations form the regulating valve mode of action with two types gas-open and gas-closed. For the selection of the regulating valve mode of action, we mainly consider three aspects. First, they have processed production safety and the characteristics of the medium and ensured product quality so that the economic losses are minimized.
