What happens when the control valve fails?
（Ⅰ）The method that increases life expectancy
1. Wide open working life extension method
Allow the regulator to start at maximum opening as much as possible, such as 90%. In this way, cavitation, erosion and other damage occurred on the spool head.
With the failure of the spool, the flow rate increases, the corresponding valve closed a point, so that the continuous destruction, gradually closed, make full use of the whole spool, until the core and sealing surface destruction, can not be used.
At the same time, the large opening work throttle gap, erosion weakened, this than the beginning let the valve in the middle of the opening and small opening on the work to improve life 1 ~ 5 times. If some chemical plant USES this method, the service life of valve raised 2 times.
2. Decreases, increase work opening and improve service life
Decrease S, that is, increase the loss of the system in addition to the regulating valve, so that the pressure drop allocated to the valve is reduced, in order to ensure the flow through the regulating valve, it is necessary to increase the opening of the regulating valve, at the same time, the pressure drop on the valve is reduced, so that cavitation and erosion are also weakened.
Specific measures include:
Orifice throttling consumption pressure drop is set behind the valve.
Close the manual valve in series on the line until the regulator has a better working opening.
At the beginning of the valve selection in the small open degree of work, the use of this method is very simple, convenient and effective.
3. The method of reducing caliber increasing work opening and improving life span
By reducing the diameter of the valve to increase the operating degree.
Specific measures include:
Change a small one gear diameter valve, such as DN32 to DN25;
Body unchanged, replace spool seat with small seat diameter.
For example, when a chemical plant is under overhaul, the throttle part dgl0 is replaced with dg8, and the service life is doubled.
4. Transfer damage location to improve life
Move the damage to a secondary position to protect the sealing face and throttling face of the spool seat.
5. Increasing throttling channel and increasing life span
The simplest way to increase the throttling passage is to thicken the seat so that the seat hole grows to form a longer throttling passage.
On the one hand, the sudden expansion of flow closure throttling can be delayed and the damage position can be transferred away from the sealing surface. On the other hand, increased throttling resistance, reduced the pressure recovery degree, so that cavitation weakened.
Some seat hole design into the bench, wave type, is to increase resistance, weaken cavitation. This method is often used in the introduction of high-pressure valves and in the improvement of old valves and is very effective.
6. Change the flow to improve life expectancy
Open type flow towards the open direction, cavitation, erosion mainly in the sealing surface, so that the root of the valve core and valve seat sealing surface will be damaged soon; Flow closure type flow towards the closed direction, cavitation, erosion in the throttling, seat sealing surface below, protect the sealing surface and the root of the valve core, extend the life.
When the problem of prolonging service life is more prominent, the valve used for artificial open flow can prolonging service life by 1 ~ 2 times only by changing the flow direction.
7.Use special materials to improve the life of the method
In order to resist cavitation (damage shape such as honeycomb dot) and scour (streamline channel), special material can be used to make throttle parts. This special material is 6yc-1, A4 steel, stele, hard alloy and so on.
In order to resist corrosion, can be changed to more resistant to corrosion, and a certain mechanical properties, physical properties of the material. This material can be divided into two categories: non-metallic materials (such as rubber, tetrafluoron, ceramics, etc.) and metallic materials (such as monel, haltzite, etc.).
8. Changing valve structure to improve service life
To change the structure of the valve or choose a longer life of the valve to achieve the purpose of improving life, such as the choice of multistage valve, anti-cavitation valve, corrosion resistant valve.
9. Decrease stroke to improve diaphragm life
For the two-position regulator, when the action frequency is very frequent, the diaphragm will soon be broken in the up and down folding, the damage position is often in the circumference of the tray.
The simplest and most effective way to improve the life of the diaphragm is to reduce the stroke. So the reduced travel is going to be 1/4dg. If the valve of dgl25, its standard stroke is 60mm, can reduce to 30mm, shorten 50%.
In addition, the following factors can be considered:
The pressure of membrane chamber should be reduced as far as possible under the condition of opening and closing. Improve the tray and diaphragm fitting finish.
（Ⅱ）The valve plug prevention (stuck) method
1. The cleaning method
The welding slag, rust and residuum in the pipeline are blocked or stuck in the throttle orifice, guiding position and the balance hole of the lower valve cover, causing the spool surface, guiding surface to produce strain and scratch, sealing surface to produce indentation, etc. This often occurs in the new operation system and the initial operation after overhaul. This is the most common breakdown.
In this case, must be removed for cleaning, get rid of slag, such as sealing surface damage should also be grinding; At the same time, open the bottom plug to flush away any residue that has fallen into the lower cover from the balance hole and flush the line. Before put into operation, let the regulating valve fully open, the medium flow after a period of time into normal operation.
2. External flushing method
For some easy to precipitate, containing solid particles of the medium with ordinary valve regulation, often in the throttle orifice, guide plug, can be under the cover of the bottom plug outside flushing gas and steam.
When the valve is blocked or stuck, open the external gas or steam valve to complete the flushing without moving the regulating valve and make the valve run normally.
Install pipe strainer method
For small diameter control valve, especially the ultra-small flow control valve, its throttling gap is particularly small, medium can not have a little slag.
In this case, it is best to install a filter on the pipe in front of the valve to ensure the smooth passage of the medium.
With the use of positioner regulator, positioner work is not normal, its gas throttle plug is the most common fault.
Therefore, when working with positioner, it is necessary to deal with the air source. The usual method is to install the air filter pressure reducing valve in the air source pipeline before the positioner.
4. Increase throttling gap method
Solid particles such as the medium or washed off in pipe welding slag and rust due to pass the throttle mouth such as congestion, jam, such as fault, can convert the throttle gap bigger throttle piece - throttling area of a window, open the valve core and the sleeve, rather than the circumference of a circle because of the throttling area of concentration distribution, fault can easily be ruled out.
If it is single, two - seat valve can be changed to the plunger - shaped spool "V" shape of the spool, or change to sleeve valve.
For example, a chemical plant had a two-seat valve that was often stuck, and the problem was solved immediately after a sleeve valve was recommended.
5. Medium flushing method
Using the medium of its own erosion energy, erosion and take away easy precipitation, easy to plug things, so as to improve the valve blocking function.
Common methods include:
Use as a flow closure;
Adopt streamlined valve body;
The orifice is placed in the place where the erosion is most serious.
6. Straight through to angular method
The direct flow is inverted S flow, the flow path is complex, the upper and lower cavity dead zone, for the precipitation of the medium provides a place. Angular connection, medium as through 90℃ elbow, erosion performance, small dead zone, easy to design into a flow line. Therefore, the use of a straight through valve to produce a slight blockage can be changed to Angle valve use.
（Ⅲ） Pneumatic control valve fault analysis
1. The regulating valve does not operate
Reason 1: No air source or too small air source pressure
First check whether the air source (instrument air) is unobstructed, whether the air source pressure meets the service requirements of the valve.
Reason 2: Air source, no output signal air pressure
For mechanical controller or positioner, the mechanical pressure controller or positioner should be more.
For the control valve of the mechanical controller, check whether the signal collection pipeline valve of the medium on the installation pipe of the control valve is fully open or leakage is serious. If there is any abnormality, it should be dealt with in time.
Reason 3: the gas pressure of the output signal is normal and still does not move.
Check whether the diaphragm of pneumatic diaphragm actuator has serious air leakage phenomenon, if air leakage, should timely replace the diaphragm and related seals.
If the diaphragm is intact and there is no air leakage, the main valve spool should be stuck with the bushing and seat, the main valve should be disassembled to check, clean up debris.
Valve stem bending deformation is serious, should disassemble check the main valve, according to the inspection situation replacement treatment.
Signal air source pipeline leakage, check treatment leakage point.
Signal amplifier failure or improper adjustment, the main air source did not enter the diaphragm cavity through the amplifier, should be adjusted or replaced in a timely manner.
2. When the regulating valve works, there will be adjustment shock and control instability。
Reason 1: the air source and signal pressure are stable, but the work of the regulating valve is still unstable.
Check air tightness of pneumatic diaphragm actuator. Carefully check the drive push rod seal whether there is slight leakage phenomenon, if there is leakage should be disassembled and replace the seal; Disassembly check whether the diaphragm is scratched, stab and caused by minor air leakage phenomenon, such as air leakage should be replaced diaphragm.
Carefully check whether there is a gap between the positioner and the main valve connection. Replace or tighten the relevant connector between the positioner and the main valve.
Use soapy water to carefully check whether there is slight air leakage in the signal trachea road.
The adjustment of signal air source amplifier is unreasonable. Adjust the balance screw of signal air source amplifier.
Control valve movement mechanism (including the main valve and pneumatic diaphragm actuator) resistance is too large, should be dismantled the main valve, check and replace abnormal components, including spool, seat, stem, stem seal and drive push rod.
Check whether the pressure balance spring in the pneumatic diaphragm actuator is damaged, check whether the spring has fatigue deformation, if there is abnormal should replace the pressure balance spring.