Industry News

Discuss the reason of valve sealing surface damage

2021-07-19
Valves are pipeline accessories used to open and close pipelines, control flow, adjust and control the parameters (temperature, pressure and flow) of the conveying medium. From the simplest shut-off valve to the various valves used in extremely complex automatic control systems, there are many varieties and specifications. The nominal diameter of the valve ranges from extremely small instrument valves to industrial pipeline valves with a diameter of up to 10m. It can be used to control the flow of various types of fluids such as water, steam, oil, gas, mud, various corrosive media, liquid metals and radioactive fluids. The working pressure of the valve can be from 0.0013MPa to 1000MPa ultra-high pressure, and the working temperature can be c-270℃ ultra-low temperature to 1430℃ high temperature. The valve sealing surface is often damaged during the use of the valve, and the damage will cause internal leakage. Why does this happen? There are many aspects to understand here, let's take a look together.
Mechanical damage, the sealing surface will be damaged by scratches, bruises, crushing, etc. during opening and closing. Between the two sealing surfaces, under the action of high temperature and high pressure, atoms permeate and seep each other, resulting in adhesion. When the two sealing surfaces move to each other, the adhesion is easily torn. The higher the roughness of the sealing surface, the more likely this phenomenon will occur. The sealing surface of the valve will be scratched and crushed during the closing process of the valve and the valve clack during the re-seat process, causing local wear or indentation on the sealing surface.
Erosion of the medium is the result of wear, washing and cavitation on the sealing surface when the medium is active. When the medium is at a certain speed, the floating fine particles in the medium interfere with the sealing surface, causing local damage. The high-speed moving medium directly washes the sealing surface and causes local damage. When the medium is mixed and partially vaporized, air bubbles and blasts are generated. Impact the surface of the sealing surface, causing local damage. The erosion of the medium and the alternate action of chemical erosion will strongly etch the sealing surface.
Electrochemical corrosion, the contact between the sealing surfaces, the contact between the sealing surface and the sealing body and the valve body, the concentration difference of the medium, the oxygen concentration and other reasons, will produce a potential difference, electrochemical corrosion will occur, and the sealing surface of the anode will be corroded.
The chemical corrosion of the medium, the medium near the sealing surface does not produce current, the medium directly chemically interacts with the sealing surface, corroding the sealing surface. Improper installation and poor maintenance have caused the sealing surface to work abnormally, and the valve runs ill, which prematurely damages the sealing surface.
Damage caused by improper selection and poor operation. The main manifestation is that the valve is not selected according to the working conditions, and the shutoff valve is used as a throttle valve, which leads to excessive closing specific pressure and closing too fast or not tightly, causing the sealing surface to be eroded and worn. The processing quality of the sealing surface is not good, mainly manifested in the defects such as cracks, pores and ballast on the sealing surface, which are caused by improper selection of surfacing and heat treatment specifications and poor manipulation during surfacing and heat treatment. The sealing surface is too hard. Or too low, due to the wrong material selection or improper heat treatment. The sealing surface has uneven hardness and is not resistant to corrosion, mainly because the underlying metal is blown to it during the surfacing process, which dilutes the alloy composition of the sealing surface. of. Of course, there are also design issues.