• The Main Failures of the Valve

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The Main Failures of the Valve

  • Jul 22, 2022
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A marine valve is a common mechanical product in marine products. The failure of valves directly affects the normal operation of system equipment, and even leads to catastrophic consequences. Therefore, the types of valve failures are sorted out and analyzed to find out common failure modes and causes. It is very necessary to conduct a reliability test assessment on the stress source of failures of valves, and its effect on improving the reliability of the valve is also very obvious.
 
Types of failures of valves
The valve plays an important role in cutting off and throttling the medium in the pipeline where it is located. Various failures will occur for the valve when it is used. When a serious failure occurs, the valve will stop running. The specific failures include shell parts, drive parts of valve discs, bellows and fasteners being damaged. After a minor failure, the valve can continue to be used according to the functional requirements, but the efficiency is reduced. According to the failure type, the failures are divided into damage, imbalance, looseness, degradation, blockage and leakage. The common failures of the valve are shown in Table 1.
 
Main failures of valves
The analysis of failures of valves is as follows: The main causes of damage such as rupture and wear are the action of internal and external forces, strength and wear caused by long-term use, which can be attributed to material strength and long-term wear. Aging, blocking and other imbalances are caused by improper design and operation except for scouring. The main reason for leakages of sealing is caused by media impurities, media corrosion, cavitation, environmental factors, and improper design, which eventually lead to leakages. In summary, in addition to materials, design and process problems, the failure caused by the medium is the most commonly seen, mainly including corrosion, overpressure, leakages and other problems, followed by the damage caused by the long-term use of the valve and the strength under the action of internal and external forces. Leakages, corrosion and rupture are also the main forms of failures of valves and the focus of reliability test design research.
 
Table 1 A summary of common failures of valves
Failures Reasons for the failures Examples
Cracking
 		 Valve parts are cracked with visible gaps due to insufficient strength. Cracking is caused by fatigue, external shock and vibration. Cracking of rubber parts and external leaks
Wear
 		 Excessive working pressure during the operation of the valve causes adhesive wear on the valve stem and valve sleeve made of the same material, resulting in abrasive particles, and serious wear on the thread after a long time. Fatigue wear, contamination and overpressure wear also happens. Wear on the valve stem and valve sleeve thread, wear on the fastener and internal and external leakages
 
Incorrect throttling and pressure regulation Scouring and the scale does not meet the requirements.
 		 The throttle disc is severely scoured, and the scale is not at the zero position or there is no scale.
Being stuck Rust and the pipeline fluid contains impurities. Contaminants are stuck, and temperatures are too high. The valve stem is corroded.
Adherency
 		 There are contaminants, loss of lubrication, air entrapment, excessive humidity, structural disturbance. The pressure drop is unstable and the operation is slow.
Looseness and falling
 		 Valve fasteners and connectors lose their proper tightening force. Failures occur due to excessive fatigue. Flange bolts, rivets and other connectors are loose and the air pressure is unstable.
Corrosion, rust and aging Corrosion is caused by internal and external water or other media, internal and external corrosion, harsh atmospheric environment and media. It is also caused by long-term wear, aging and aging caused by high temperature and high humidity.
Leakages at the valve body, packing and flange sealing surface		 The valve shaft is corroded and the rubber parts are aged. The valve paint is not up to standard, and the medium contains water.
External leakage It is caused by external scratches, breaking of valve bodies, deformation of flanges, unqualified material selection and improper installation.
Improper connections, weakened body sealing, ambient temperatures, sinking of seats, damaged installation and surface.		 External leakage of valve bodies, packing and sealing surfaces of flanges.
The flange connection is not good and the sealing of the valve bonnet and body is weakened.
Internal leakages They are caused by welding or casting defects, improper material selection, improper heat treatment and other defects, and problems such as reduced preload force due to poor sealing.
Corrosion, erosion impurities, damage to soft seals and valve seats, insufficient preload and wear.		 Leakage between valve bodies and diaphragms, valve discs and valve seats.
Corroded valve discs and damage to valve seats and soft sealing
Leakage Leakages are caused by abrasion between seal pairs.
Corrosion of the medium, impurities in the medium and frequent switching use
Cavitation, structural design defects, human causes, etc.		 After opening and closing several times, the sealing pair is scratched.
The sealing surface is damaged and the medium is corroded.
Cavitation causes corrosion and damage to components.
Sealing Damage to surfaces, mounting wear, embrittlement, distortion, dynamic instability, cavitation, design and process issues.
 		 When the medium passes through the shut-off valve, the flow resistance increases and the pressure decreases. When the medium reaches the saturated vapor pressure of the medium, cavitation occurs.
Blockage There is a foreign object in the valves and pipes, which prevents the liquid or gas from flowing or does not flow smoothly. Blockage of the control line, such as the blockage of the pilot valve pulse pipes
Overpressure The working pressure exceeds the design pressure.