The main reason for the leakage inside the
valve is the damage to the steam valve seal. There are many reasons for the failure of
valve seal, among which the failure of the sealing pair composed of valve core and valve seat is the main reason.
There are many factors that affect the damage of the valve seal surface, including mechanical wear and high-speed erosion caused by incorrect selection, erosion of medium, various corrosion, impurities jamming, selection and heat treatment process of valve core and valve seat materials, and deformation of sealing pair caused by water hammer.
Electrochemical erosion occurs due to factors such as mutual contact of sealing surfaces, contact of sealing surfaces with closure bodies and valve bodies, concentration difference of medium, oxygen concentration difference, etc., which generate potential difference and cause electrochemical erosion, leading to erosion of the sealing surface on the anode side. Chemical erosion of the medium occurs when the medium near the sealing surface chemically reacts with the sealing surface without generating current, causing erosion of the sealing surface.
Erosion and cavitation of the medium are the results of the wear, flushing, and cavitation of the sealing surface when the medium is active. At a certain velocity, floating fine particles in the medium come into contact with the sealing surface, causing local damage. When the medium is in high-speed motion, it directly flushes the sealing surface, causing local damage. When the medium flows and partially vaporizes, gas bubbles burst and impact the surface of the sealing surface, causing local damage. The alternating action of erosion and chemical erosion of the medium strongly corrodes the sealing surface. Damage caused by improper selection and operation mainly manifests as the valve not being selected according to the operating conditions, using a shut-off valve as a throttle valve, resulting in excessive pressure and rapid or inadequate closure, causing erosion and wear of the sealing surface.
Poor machining quality of the sealing surface mainly manifests as cracks, pores, and inclusions on the sealing surface, caused by improper selection of surfacing and heat treatment specifications and poor control during surfacing and heat treatment processes. Excessive or insufficient hardness of the sealing surface is caused by incorrect material selection or improper heat treatment. Non-uniform hardness and poor corrosion resistance of the sealing surface are mainly caused by blowing the underlying metal onto the surface during surfacing, diluting the alloy composition of the sealing surface.
Improper installation and inadequate maintenance often lead to abnormal operation of the sealing surface. Valves operating with issues prematurely damage the sealing surface. Sometimes, rough handling and excessive closing force are also causes of sealing surface failure, but they are often difficult to detect and assess.
Impurities jamming is a common problem caused by welding slag and excess gasket material not cleaned during steam pipeline welding, as well as scale deposits shedding from the steam system. If a 100-mesh steam filter is not installed before the control valve, sealing surface damage due to jamming can easily occur.
It can be concluded that the causes of sealing surface damage can be categorized into two types: intentional damage and application-related damage. Intentional damage is caused by factors such as poor design, manufacturing defects, improper material selection, incorrect installation, poor usage, and inadequate maintenance. Application-related damage is the wear and tear occurring during normal valve operation, resulting from inevitable erosion and cavitation of the sealing surface by the medium. Preventing damage effectively can reduce losses and prolong service life. Regardless of the type of damage, selecting the appropriate steam valve, strictly following the installation, configuration, and commissioning procedures outlined in the user manual, and conducting regular maintenance are crucial for extending valve life and reducing leaks caused by sealing surface damage.