Valve Ball Cemented Carbide Coating
The importance of trim coatings：
Metal seated ball valves achieve sealing by metal to metal contact between the ball and seat. When “soft” metals of similar hardness slide against each other under even moderate pressure, galling occurs. Microscopic protrusions on the seating surfaces catch on each other, resulting in surface friction, heat buildup and plastic deformation. Typically the damage gets worse as the valve cycles until it becomes inoperable due to seizure. If no coatings are applied to the trim, galling will be visible almost as soon as the valve is cycled on the test bench.Once installed in service, the various effects of difficult media would increase wear rate exponentially.
Properly selected coatings reduce the friction between the ball and seats allowing for smooth sliding operation over many numerous cycles, minimizing damage and wear due to galling, abrasion, erosion, particle impact, cavitation and thermal swings. Reducing friction in the trim lowers the valve’s operating torque which has several advantages. Lower valve torque allows a smaller actuator to be used which is more economical, results in smaller envelope dimensions of the assembly, improves signal response in control service and simplifies selection of accessories to meet cycle speed targets and other special requirements
Selection of trim coatings
Valve body, ball and seat materials are selected based on factors such as pressure,temperature and chemical compatibility.These criteria must also be considered when choosing trim coatings. Improper selection can cause almost instant failure of the valve upon startup.
It is also important to note that properly selected trim coatings cannot make up for unsuitable base materials. Most coatings are porous to some degree and do not isolate the base material against the effects of corrosive media. Base material and coating must both be selected to meet the demands of the application.
Hard Chromium (HCr)
Hard Chromium is the standard coating for most valves and is suited to a wide range of applications in liquids and gases at moderate temperatures and pressures. Corrosion resistance of HCr is generally comparable with stainless steels.
HCr is not compliant with strong acids like hydrochloric acid (HCl), hydrofluoric acid (HF) or sulfuric acid (H2SO4), and it should not be used with seawater, wet chlorine or other media with high chloride content. Nickel Boron (NiBo) Nickel Boron performs very well in high
temperature and high pressure applications. It is resistant to damage from erosion, cavitation and thermal shocks. NiBo is ideal coating for steam service, catalyst handling, slurry services and coal gasification. NiBo has limited corrosion resistance and is not recommended for use with acids and wet chlorine. NiBo coating is available on austenic stainless steels like CF8M (316 SS) balls up to 24” size.
Tungsten Carbide (WC-Co)
Tungsten Carbide is very resistant to wear from high cycle operations and erosion from abrasive catalysts, muds, slurries and powders. It is ideal for cryogenic applications, oxygen service and non-lubricative dry gas services. WC-Co is not suitable for use in corrosive services and condensates such as water, but generally performs well in hydrocarbon gases and liquids.