In the automatic control of modern chemical plants, regulating valves play a very important role. The production of these plants depends on the correct distribution and control of flowing media. These controls, whether it is energy exchange, pressure reduction, or simple container feeding, all require certain final control components to complete.
Tips for extending service life
1
Large opening operation
Let the regulating valve work at a large opening as much as possible from the beginning, such as 90%. In this way, damage such as gas turbidity and erosion occurs on the valve core head.
As the valve core is damaged and the flow rate increases, the corresponding valve is closed a little more. This continues to damage and gradually closes, making the entire valve core fully utilized until the root and sealing surface of the valve core are damaged and cannot be used.
At the same time, the large opening work has a large throttle clearance and weakened erosion, which increases the service life of the valve by more than 1-5 times compared to working at the middle and small opening from the beginning. If a chemical plant adopts this method, the service life of the valve has been increased by two times.
2
Reduce aperture and increase working opening
By reducing the valve diameter to increase the working opening, the specific methods are:
Change to a valve with a smaller caliber, such as DN25 instead of DN32;
The valve body remains unchanged, replace the valve core and valve seat with a small valve seat diameter. During a major overhaul of a chemical plant, the throttling component dg10 was replaced with dg8, which doubled the service life.
3
Reduce aperture and increase working opening
By reducing the valve diameter to increase the working opening, the specific methods are:
Change to a valve with a smaller caliber, such as DN25 instead of DN32;
The valve body remains unchanged, replace the valve core and valve seat with a small valve seat diameter. During a major overhaul of a chemical plant, the throttling component dg10 was replaced with dg8, which doubled the service life.
4
Transfer damage location
Transfer severely damaged areas from critical locations to secondary locations to protect the sealing and throttling surfaces of the valve core and valve seat. It can improve the service life of the valve.
5
Change flow direction
The flow opening type flows towards the opening direction of the valve core, and cavitation and erosion mainly act on the sealing surface, causing damage to the root of the valve core and the sealing surface of the valve seat quickly;The flow closed type flows towards the closed direction, and cavitation and erosion act below the sealing surface of the valve seat after throttling, protecting the sealing surface and the root of the valve core, and extending its service life.
Attention: After changing the flow opening to flow closing, there may also be a jumping phenomenon (when the regulating valve is opened), which may have the effect of vortices during the adjustment process, making the control system unable to adjust smoothly. This method must be adopted with caution and comprehensive consideration.
6
Use special materials instead
To resist cavitation (such as honeycomb shaped small dots) and erosion (streamlined small grains), special materials that are resistant to cavitation and erosion can be used to manufacture throttling components.
This special material includes 6YC-1, A4 steel, Stellite, hard alloy, etc. To resist corrosion, materials that are more resistant to corrosion and have certain mechanical and physical properties can be used instead.
This material is divided into two categories: non-metallic materials (such as rubber, PTFE, ceramics, etc.) and metallic materials (such as Monel, Hastelloy, etc.).
7
Correct selection of valve structure
By changing the valve structure or selecting valves with longer service life, the goal of improving service life can be achieved, such as selecting labyrinth valves, multi-stage valves, anti cavitation valves, corrosion-resistant valves, etc.
The advantages of regulating valves during use are:
(1) Agility in movement, able to complete various adjustment commands in a timely manner;
(2) Paired with large cylinders, it has a large torque driving force;
(3) Stable performance in harsh working environments and able to operate normally;
(4) High safety performance.
The normal operation and agility of the regulating valve will have a direct impact on production quality and efficiency.
Therefore, it is particularly important to analyze and solve the factors that affect the failure of the regulating valve during use.
Common faults and their influencing factors
1
Jamming
The problem that often occurs in regulating valves when they are stuck often occurs in newly put into operation systems and during periods of major repairs and re operation. This is due to the accumulation of impurities such as welding slag and iron filings inside the pipeline at the throttle and guide parts, resulting in blockage.
After a blockage occurs, it can cause the medium to flow poorly or the regulating valve to be filled too tightly during maintenance, resulting in increased friction and the phenomenon of small signal not acting or large signal acting excessively.
2
Leakage of regulating valve
Leakage of regulating valves generally includes internal leakage, packing leakage, and leakage caused by deformation of valve core and valve seat.
(1) Valve leakage is caused by the improper length of the valve stem, which is too long for an air valve to open. This results in a short distance between the valve stem and the valve seat, resulting in gaps and incomplete contact between the valve core and valve seat, resulting in valve leakage.
(2) If the packing leaks, installing the packing into the packing box will exert axial pressure on the regulating valve. Due to plastic deformation of the packing, radial pressure is generated and it is in close contact with the valve stem, but the contact is uneven.
During the operation of the regulating valve, the valve stem and packing will undergo axial movement. In high-temperature and high-pressure environments, it is easy to cause packing leakage due to the influence of high permeability fluid media.
(3) The deformation, leakage, erosion, and impact of the valve core and valve seat can cause them to deform. Over time, there will be a mismatch between the valve core and valve seat, and there will be gaps between them, resulting in lax closure and leakage.
3
Control valve oscillation
The reasons for the oscillation of the regulating valve are insufficient spring stiffness of the regulating valve, unstable output signal of the regulating valve causing sharp changes, the frequency of the regulating valve being the same as that of the system, and severe vibration of the pipeline and base.
There is also improper selection of regulating valves, which can cause severe changes in flow resistance, flow rate, and pressure when the regulating valve is operated with a small opening. When this change exceeds the stiffness of the regulating valve, the stability of the regulating valve is reduced, and in severe cases, the oscillation of the regulating valve occurs.
4
The regulating valve does not operate
The failure phenomena of the regulating valve not operating include:
(1) No signal, no air source. The reason for this is that the air source is not turned on or the air source is malfunctioning, the air duct is blocked, or the pressure reducing valve fails due to blockage, resulting in a compressor malfunction light;
(2) There is an air source but no signal. The reason for this is that the locator malfunctions due to air leakage caused by the rupture of the bellows, or the converter malfunctions;
(3) The locator has no air source. The reason for this is that the filter and pressure reducer are blocked and fail;
(4) The locator has an air source but no output. The cause of the malfunction is due to blockage in the throttle hole of the locator;
(5) There is a signal but no action. The cause is the valve core falling off; The valve stem is bent or broken.
5
The action of the regulating valve is unstable
The reason for the instability of the regulating valve is:
(1) The compressor capacity is too small, causing the pressure reducing valve to malfunction, resulting in unstable gas source pressure;
(2) The regulator is unstable, and the time constant of the control system is not appropriate, resulting in unstable signal pressure;
(3) The friction resistance of the valve stem during movement is high, causing blockage during contact;
(4) The ball valve of the amplifier in the locator is not tightly closed due to dirt and wear, and when the air consumption increases, it will produce output oscillation
6
Slow action of regulating valve
The pneumatic membrane is damaged and leaked during operation, causing the valve stem to become sluggish when acting unilaterally;
The weak response performance of the locator can lead to sluggish action of the regulating valve;
There are obstructions such as rust inside the regulating valve, causing it to act slowly in axial motion;
The poor quality of the packing or the tight filling of the packing increases the frictional resistance, which leads to the phenomenon of slow action of the regulating valve.
Fault Analysis of Regulating Valve
1
What should I do if the regulating valve gets stuck?
After the regulating valve is stuck, it is necessary to quickly open and close the regulating valve, and use fluid medium to flush the blockage away; You can also use a pipe wrench to clamp the valve stem, and under the action of external signal pressure, the valve stem can be swayed by positive and negative force, causing the valve core to flash past the clamp.
If the blockage phenomenon cannot be solved at this time, the external pressure and driving power can be appropriately increased to repeatedly move. If the action cannot be restored, professional personnel need to disassemble the control valve.
2
How to deal with the leakage of the regulating valve?
After adjusting the valve for internal leakage, it is necessary to adjust the length of the valve stem appropriately to ensure that the valve stem length is suitable and no internal leakage occurs; After the leakage of the packing, in order to make the packing convenient, it is necessary to chamfer the top of the packing box and add a corrosion-resistant and small gap metal protective ring at the bottom of the packing box. The contact surface between the metal protective ring and the packing should not be inclined to prevent the packing from being pushed out due to medium pressure.
The surface of the contact part between the stuffing box and the packing should be precision machined to improve surface smoothness and reduce packing wear.
Flexible graphite should be chosen as the best filler. The internal flexible graphite has high gas sealing performance, low friction, low wear after long-term use, and is easy to repair, with high pressure resistance and heat resistance.
When the valve core and valve seat are deformed or leaked, the material of the valve core and valve seat should be strictly selected, and materials with high corrosion resistance should be selected. Products with defects such as pits and sand holes should be removed to ensure a high sealing smoothness of the valve core and valve seat. When the valve core and valve seat are severely worn or damaged, a new valve should be replaced.
3
How to handle the oscillation of the regulating valve?
When the oscillation of the regulating valve is relatively mild, the stiffness of the spring can be increased, such as selecting a spring with stiffness for the regulating valve; When pipelines and foundations vibrate violently, support should be added to eliminate vibration interference; The frequency of the valve is the same as the system frequency, and the regulating valve needs to be replaced.
4
What if the regulating valve does not operate?
When the regulating valve does not operate, adjust the pressure reducing valve to reach the specified air source pressure; Check the leakage points of the locator joint box pipeline, tighten or replace the joints and pipelines; Check the amplification hole and throttle hole of the locator and repair them.
5
How to handle the unstable operation of the regulating valve?
The action of the regulating valve is unstable, and it is necessary to inspect the signal pipeline, joints, sealing rings, and other places and eliminate leakage; Find loose parts and tighten them; Add lubricating oil to components with high friction.
6
How to handle the slow action of the regulating valve?
If the regulating valve acts slowly, check the components with high friction and reassemble them; Adjust the packing gland and replace the packing again.
How to maintain the regulating valve?
If the regulating valve malfunctions during use, it will pose a huge threat to the normal and stable operation of the system. Therefore, maintenance of the regulating valve is required to reduce the occurrence rate of regulating valve failures.
1
Passive maintenance
When the regulating valve malfunctions, it is necessary to conduct maintenance on the regulating valve, identify the cause of the malfunction, and carry out timely maintenance.
2
Preventive maintenance
Regularly clean the regulating valve, maintain its hygiene and ensure the integrity and durability of all components. Regularly flush the pipeline, pay attention to blockages and garbage inside the pipeline, and regularly inspect the fixed connectors of the regulating valve to ensure the anti-corrosion performance and lubrication of the components.
When installing the regulating valve, reduce the phenomenon of hard pulling and bending during installation, reduce pressure, and reduce deformation.Maintain a distance between the regulating valve and the compressor to reduce vibration from the source. Regularly inspect the regulating valve, closely monitor its operation, check for leaks in the packing, and regularly add lubricating oil to the packing.
3
Predictive Maintenance
Obtain operational information of the regulating valve through intelligent instruments or other diagnostic equipment, and enhance the regulating quality and diagnostic function of the regulating valve. Through the diagnosis of intelligent instruments or other diagnostic equipment, problems with regulating valves can be identified in a timely manner and solutions can be found for maintenance. Increase the development and use of intelligent valve positioners.
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