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How to extend the Service Life of Control Valves?

How to extend the Service Life of Control Valves?

Control valves play a very important role in the automatic control of modern factories, where production depends on the correct distribution and control of the flowing media. These controls, whether it is the exchange of energy, pressure reduction or simple filling of containers, need to be done by control valves.

1 . Work with wide openings

Let the control valves work on the maximum opening as much as possible at the beginning, such as 90%. In this way, air turbidity, erosion and other damage occurs on the head of the spool.

With the destruction of the spool, the flow rate increases, the corresponding valve and then close a little, so that the continuous destruction, gradually closed, so that the entire spool all fully utilised, until the spool root and sealing surface damage, can not be used.

At the same time, large openings work throttle clearance, erosion weakened, which is more than the beginning of letting the valve in the middle of the openings and small openings on the work to improve the life of more than 1-5 times.

2. Reduce the valve resistance ratio

Decrease S, that is, increase the loss of the system in addition to the regulator valve, so that the distribution of the pressure drop on the valve to reduce the flow through the regulator valve, will inevitably increase the regulator valve opening, at the same time, the valve on the pressure drop decreases, so that the cavitation, erosion is also weakened.

Specific methods are: set up after the valve orifice plate throttling consumption pressure drop; close the pipeline series of manual valves, to the regulating valve to obtain a more ideal work opening. At the beginning of the valve selection in a small opening workers, the use of this method is very simple, convenient and effective.

3. Decrease the calibre, increase the working degree of opening

By reducing the calibre of the valve to increase the working opening, specific methods:

① Change a small calibre of the valve, such as DN32 into DN25;

② Valve body does not change, replace the spool valve seat with a small seat diameter.

4 . Transfer the damage location

The damage is serious from the key position to the secondary location to protect the spool and seat sealing surface and throttling surface. The service life of the valve can be improved.

5. Change flow direction

Flow open to the spool opening direction, cavitation, erosion mainly in the sealing surface, so that the spool root and spool valve seat sealing surface soon suffered damage; flow closed to the closing direction, cavitation, erosion in the throttle after the seat sealing surface below, to protect the sealing surface and the spool root, prolonging the life of the valve.

Note: flow open to flow closed will also have a jump phenomenon (regulating valve open), in the regulation process there is the influence of vortex, so that the control system can not be adjusted smoothly. The use of this method must be careful and comprehensive consideration.

6. Change to special materials

In order to resist cavitation (destroying shapes such as honeycomb-shaped dots) and scouring (streamlined dregs), can be changed to resist cavitation and scouring of special materials to manufacture throttle parts.

Such special materials are 6YC-1, A4 steel, Stellet, carbide and so on. For corrosion resistance, can be changed to a more corrosion-resistant, and have certain mechanical properties, physical properties of the material.

This material is divided into non-metallic materials (such as rubber, PTFE, ceramics, etc.) and metal materials (such as Monel, Hastelloy, etc.) two categories.

7. The correct choice of valve structure

The advantages of Control valves in the process of use are:

(1) Agile action, can be completed in time to regulate the command;

(2) And large cylinders with the use of a larger moment to promote;

(3) In the harsh working environment, the performance is more stable, can operate normally;

(4) High safety performance.

The normal operation of the control valve and the agility of the work will have a direct impact on the production quality and efficiency.

Therefore, it is particularly important to analyse and solve the countermeasures for the influencing factors of the failure of the control valve in the process of using.

II. Common failures and their influencing factors

1. Jamming

Card blocking is a frequently occurring problem in the control valve, often occurring in the new operation of the system and after overhauling the re-commissioning period. This is due to the pipeline internal welding slag, iron filings and other impurities in the throttle port and guide parts of the gathering and blocking phenomenon.

After the occurrence of jamming, it will make the medium circulation is not smooth or control valve overhaul packing is too tight, so that the friction becomes larger, resulting in a small signal does not act, the big signal action over the phenomenon.

2. Control valve leakage

Control valve leakage generally includes internal leakage, packing leakage, spool and seat deformation caused by leakage.

(1) Valve leakage, is due to the length of the valve stem is not appropriate, the length of the valve stem of the air-open valve is too long, so that the valve stem upward or downward distance is too short, resulting in the existence of a gap between the valve spool and valve seat, not completely in contact with the valve, resulting in valve leakage phenomenon.

(2) Packing leakage, packing into the stuffing box, will apply axial pressure to the regulating valve. Due to the plastic deformation of the packing, radial pressure will be generated, and the valve stem contact is close, but the contact is not uniform.

In the working process of the control valve, the valve stem and packing will undergo axial movement. In the high temperature, high pressure environment, by the influence of high permeability fluid medium, easy to filler leakage.

(3) Spool and seat deformation leakage, erosion and impact, so that the deformation, over time, will produce the spool and seat mismatch phenomenon, there is a gap between them, and produce shut-off and leakage.

3. Control valve oscillation

Control valve oscillation is caused by insufficient spring stiffness of the control valve, the control valve output signal is not stable and produce sharp changes, the frequency of the control valve and the system frequency is the same, the pipeline and the base of the violent vibration.

There is also improper selection of control valves, control valves in small openings work, resulting in dramatic flow resistance, flow rate and pressure changes, when this change exceeds the stiffness of the control valve, so that the stability of the control valve to reduce the serious cases of control valve oscillation.

The fault phenomenon of the regulating valve does not operate include:

(1) No signal, no air source. The reason is that the gas source is not open or gas source failure, air duct blockage or pressure reducing valve failure due to blockage, compressor failure lights;

(2) Air supply, no signal. The reason for this is that the positioner fails due to air leakage caused by bellows rupture, or the converter fails;

(3) No air supply to the positioner. The reason is that the filter, pressure reducer clogging phenomenon and failure;

(4) The positioner has air supply but no output. The reason for this is that the positioner throttle hole is clogged and fails;

(5) Signal, no action. The reason is that the valve spool is detached and the valve stem is bent or broken.

5. Unstable operation of control valve

The reason for the unstable action of the control valve is:

(1) Compressor capacity is too small, resulting in malfunctioning of the pressure reducing valve, which produces unstable pressure of the gas source;

(2) The regulator is unstable, and the time constant of the control system is not suitable, thus producing the phenomenon of unstable signal pressure;

(3) The frictional resistance of the valve stem in the movement is large, and the blocking phenomenon occurs in the contact process;

(4) The ball valve of the amplifier in the positioner is not closed tightly by the wear and tear of dirt, and the output oscillation will be generated when the air consumption is particularly increased.

6 . Sluggish action of the regulating valve

Pneumatic diaphragm breakage leakage in operation, so that the valve stem in the unilateral action is sluggish;

Weak response performance of the positioner can lead to sluggish action of the regulating valve;

There are rust and other obstructions in the regulating valve, blockage occurs, making the regulating valve sluggish in axial movement;

Poor quality of packing or packing filling too tight, so that the friction resistance increases, which leads to the phenomenon of sluggish action o

III. Failure solutions of control valve

1. How to deal with the jamming of the control valve?

Control valve jamming problems, to quickly open and close the valve, the use of fluid media will be blocked run; you can also use a pipe wrench to clamp the valve stem, under the action of the external signal pressure, positive and negative force will be flared stem, so that the spool flashes through the card.

At this point, if you can not solve the jamming phenomenon, you can increase the appropriate external pressure and drive power, repeated movement, if you can not return to action, you need to be a professional control valve disassembly processing.

2. How to deal with control valve leakage?

After the internal leakage of the control valve, the length of the valve stem should be adjusted appropriately, so that the length of the valve stem is suitable and no more internal leakage; after the leakage of the packing, in order to make the packing convenient, it is necessary to chamfer the top of the stuffing box, and add the corrosion-resistant, small-gap metal protection ring at the bottom of the stuffing box, and the contact surface between the metal protection ring and the packing can not be made to make the bevelled surface, so as to prevent the filling from being launched by the pressure of the medium.

Stuffing box and filler contact part of the surface should be finishing, can improve the surface finish, reduce filler wear.

Packing to choose the best flexible graphite, within the flexible graphite gas sealing high, low friction, wear and tear after long-term use of small, and easy to repair, with high pressure resistance and heat resistance.

Spool and seat deformation leakage, to strictly select the material of the spool and seat, to choose high corrosion resistance materials, to the existence of pockmarks, trachoma and other defects in the product to be eliminated, so that the spool and seat have a high degree of sealing finish, when the spool and seat wear and tear, damage is serious, to replace the new valve.

3 . How to deal with control valve oscillation?

Control valve oscillation, when the oscillation is relatively slight, you can increase the stiffness of the spring, for example, control valve selection of the stiffness of the spring; when the pipeline, the base of the violent vibration, to increase the support to eliminate vibration interference; the valve frequency and the system frequency is the same as the regulating valve to be replaced.

4. How to do when control valve does not operate?

When the regulating valve does not operate, the pressure reducing valve should be adjusted so that the pressure reaches the specified pressure of the gas source; the leakage point of the positioner joint box pipeline should be checked, and the joints and pipelines should be tightened or replaced; the enlarged holes of the positioner should be checked, and the throttle holes should be repaired.

5. How to deal with unstable action of control valve?

If the action of control valve is unstable, it is necessary to check the signal pipeline, joints, sealing ring and other places and eliminate leakage; find the loose parts and tighten them; add lubricant to the parts with large friction.

6. How to deal with slow action of control valve?

Regulating valve action is slow, check the friction parts, reassembly; adjust the packing gland, and replace the packing.

Regulating valve failure in the use of the process, the normal and stable operation of the system will produce great harm, so the requirements of the regulating valve maintenance, reduce the incidence of regulating valve failure.

1. Passive maintenance

When the regulating valve failure occurs, the regulating valve should be overhauled to identify the cause of the failure, and timely maintenance.

2. Preventive maintenance

Regular cleaning of the control valve, to maintain the health of the control valve and the integrity of the parts of the durable, regular flushing of the pipeline, pay attention to the blockage of the pipeline and rubbish, etc., the fixed joints of the control valve for regular inspection, to ensure that the parts of the anticorrosion properties of the good lubrication.

When installing the regulating valve, reduce the hard pulling and bending phenomenon in the installation, reduce the pressure and reduce the deformation. Maintain the distance between the control valve and the compressor to reduce vibration from the source of vibration. Regular inspections of the control valve should be carried out, to pay close attention to the operation of the control valve, check whether the packing is leaking, and regularly add lubricant to the packing.

3. Predictive maintenance

Through the intelligent instrument or other diagnostic equipment to obtain the operating information of the control valve, to strengthen the adjustment quality and diagnostic function of the control valve. Through the diagnosis of intelligent instrumentation or other diagnostic equipment, the problems of the control valve can be found in a timely manner, and find timely solutions for maintenance. Increase the development and use of intelligent valve positioners.

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