Several Forms of Valve Failure

Since the reform and opening up, the valve industry has developed rapidly and made great progress in manufacturing capacity and complete sets of technologies. As an integral part of the pipeline system, the working state of the valve greatly affects the normal operation of the system. There are many kinds of failure events in the use of the valve. Valve failure refers to the phenomenon that the valve can't fulfill its intended function due to its structural damage and its inability to adapt to working conditions during service. In order to reduce the probability of valve failure and the possibility of accidents, it is necessary to learn lessons from historical accidents and find out the essence of valve failure, so as to inspire designers to make corresponding improvements to valve components and ensure that valve products have sufficient stability and reliability to obtain more stable performance and better quality, which has certain significance for ensuring the safety of pipeline system.
 
Forms of valve failure
 
1. Fracture

Plastic failure refers to the fracture failure form in which obvious plastic deformation occurs at the fracture position before the valve parts break. Liu Guoyong observed the macroscopic characteristics of the fracture surface of the valve stem and found that the fracture surface had obvious plastic deformation, uneven surface and ratchet-shaped distribution of fracture edges. Tian Chen and others observed the microscopic characteristics of the plastic fracture surface of the valve stem of the high-pressure main steam valve by scanning electron microscope and found that the fracture surface showed plastic dimple characteristics.
 
Brittle fracture failure refers to the failure form that valve parts suddenly break without obvious plastic deformation at the fracture position. Wenxiang Xu observed the macroscopic characteristics of brittle fracture failure of the valve stem of the high-pressure main steam valve and found that the fracture surface was relatively flat, with no fatigue source, and the whole fracture surface was rough, with cracks spreading along with the thinnest position. Zhou Liqin observed the microscopic characteristics of brittle fracture of the valve stem of the electric gate valve and found that the fracture surface was lamellar and river-like. Especially when the environmental medium is corrosive, stress corrosion cracking failure will occur in valve components. Stress fracture failure refers to the failure mode in which valve components are broken under the combined action of tensile stress and corrosive medium.
 
Fatigue failure refers to the fracture failure form of valve parts after a period of the cycle under the action of alternating stress. Zhou Shaoxin and others analyzed the macroscopic characteristics of fatigue fracture failure of distribution valve spring and found that the angle between fracture surface and longitudinal direction was 45, and the section could be divided into crack source area, crack propagation area and instantaneous fracture area, in which the surfaces of crack propagation area and crack source area were basically smooth, while the surfaces of instantaneous fracture area were rough. Xie Jinhong analyzed the microscopic morphology of fatigue fracture failure of butterfly valve butterfly plate and found that there were steps in the crack source area and fatigue striations in the crack propagation area. There are many parts in the valve that have broken and failed. Among them, the valve stem breaks most frequently, so it is necessary to monitor the deformation of the valve stem. In addition, from the above research, it can be found that the characteristics of fracture surfaces are important clues to distinguish different fracture failure forms of valves.
 
2. Corrosion

Corrosion failure refers to the failure forms of valve components that lose their original functions due to the action of corrosive medium, which can be divided into gap corrosion failure and galvanic corrosion failure. Gap corrosion failure refers to the failure form caused by corrosion at the gap of the valve structure. Wang Pei and others observed the macroscopic characteristics of crevice corrosion on the sealing surface of the globe valve disc and found that pits of different sizes appeared at the bottom of the corrosion area, while the corrosion outside the corrosion area was light.
 
Galvanic corrosion failure refers to the failure form that two metal parts with different electrochemical properties in the valve directly contact to produce a potential difference, which makes the corrosion of metal with high potential slow down and that of metal with low potential accelerate. Yuan Jun observed the characteristics of galvanic corrosion failure caused by electroplating coating on the valve stem of the hot air valve and found that there were precipitates on the fracture surface, and the local area was fine honeycomb, and the coating fell off in flake and spot. By SEM analysis of the corrosion products, it was found that there were Cl and S elements. The damage of galvanic corrosion failure is stronger than that of crevice corrosion failure, and it often occurs in the valve stem with a corrosion-resistant metal coating. At present, it is not enough to analyze the characteristics of different corrosion failure precipitation products, and it is impossible to distinguish the specific corrosion medium that affects the corrosion characteristics. In the future, research work should be considered from this direction.
 
3. Leakage

A leakage is a failure form in which the sealing structure loses its ability to stop fluid flow after being destroyed. The damage to the sealing surface structure is very great. There are many forms, which can be divided into fracture failure of flange and valve seat sealing surface, failure of surfacing layer and failure of sealing element from the failure position.
 
The fracture failure of the sealing surface has the same characteristics and failure reasons as the first one. At the same time, fluid abrasive wear will also cause leakage, and its characteristics and failure reasons are the same as those previously, so they will not be described here. It is a common measure to improve the wear resistance and corrosion resistance of valves to form a surfacing layer on the surface of parts. The failure of the surfacing layer refers to the failure form of leakage caused by the falling off of the metal layer on the surface of valve parts. Wang Hao analyzed the failure characteristics of the surfacing layer on the sealing surface of the check valve and found that the surface crack runs through the whole surfacing layer in a macroscopic state, and the crack fracture has a grayish-brown oxide layer and cracks along the grain boundary direction in microscopic state.
 
Seal failure refers to the failure form of valve leakage caused by the deformation or damage of packing, gasket, sealing ring, etc. Dai Xiaoying and others observed the macroscopic characteristics of O-ring failure and found that the sealing ring was deformed and dents were visible on the surface. There are many parts of the valve where leakage failure occurs, and the characteristics of sealing element failure are often not easy to identify, which is very important for different types of leakage. The classification of effects has not been unified. In the future, it is an important research direction to develop new identification methods to classify and identify leakage failures.
 
4. Wear

Wear refers to the wear of valve components in the process of repeated contact with fluid, which changes the surface state of components. A failure form that eventually loses its original function. Wear failure can be divided into abrasive wear failure and corrosion wear failure. Abrasive wear failure means that the flowing medium in the valve contains solid impurities such as particles, and the solid impurities have no effect on the valve during the flowing process.
 
The surface of parts is washed to form damage and failure. Wu Xiangqing and others analyzed the characteristics of abrasive wear failure of the spherical valve core. In a macroscopic state, the gullies on the surface of parts show a regular distribution of bright white areas, and the gullies are basically parallel with approximately the same spacing. Microscopically, it shows the lamellar morphology and surface peeling phenomenon left by grinding and rolling. Corrosion failure means that when the flowing medium in the valve is corrosive medium, the channel structure is corroded and worn and the solid particles are mixed.
 
Failure forms of common destruction. Wang Yaqing and others analyzed the macroscopic characteristics of corrosion and wear failure on the surface of the gate valve stem, and found that gully-shaped corrosion and wear grooves appear on the surface. The difference from corrosion failure lies in that there are no corrosion products on the surface of the parts with corrosion wear failure because the corrosion products are taken away during the flow and scouring process of the fluid. Guofu observed the microscopic characteristics of corrosion and wear failure of the valve core of the high-pressure coal water slurry valve, and found that there were local plastic deformation areas and extensive cutting and sliding areas on the worn surface.
 
At present, the macroscopic features of abrasive wear failure and corrosion wear failure are not clear, and they are all grooves left by wear. In the future, we should deeply compare and analyze the differences between macroscopic and microscopic features of different.