Wear and Corrosion Factors of Valves and Seats
Ball valves with metal seats provide sealing by metal to metal contact between the ball and seats.

Sliding of unprotected metals of similar hardness against each other leads to galling. Microscopic bulges on the valve surfaces catch on each other, resulting in high surface friction, heat buildup and plastic deformation. Typically the damage gets worse as the valve cycles until it becomes inoperable.

If no coatings are applied to the trim, galling will be visible almost immediately as the valve is cycled on the test bench. The real service media would increase the wear rate exponentially.

Appropriately selected coatings reduce the friction between the ball and seats allowing for smooth sliding operation over many cycles, minimizing wear due to galling, abrasion, erosion, particle impact and thermal shocks.

Reduced trim friction lowers the valve’s operating torque. Lower valve torque allows using a smaller, more economical actuator which results in smaller envelope dimensions of the assembly, improves signal response in control service and simplifies selection of accessories to meet cycle speed targets.

Surface Coatings Protecting Ball and Gate Valves against Wear
Material Selection for Valve Coatings
Tungsten Carbide is resistant to wear from high cycle operations and erosion from abrasive catalysts, muds, slurries and powders. It is ideal for valves in cryogenic applications, oxygen service and non-lubricative dry gas services. WC-CoCr performs well in hydrocarbon gases and liquids. Carbide coatings offer a low coefficient of friction for ease of valve operation.

Chromium Carbide (CrC) coatings are well suited to very high temperature gas or liquid applications and have excellent resistance to wear, erosion and corrosive media. It performs well in sour (H2S) hydrocarbon gas, high chloride waters, coal liquefaction, catalyst handling and geothermal brine.

Cobalt-Based alloys often referred as Stellites. Particular materials of the class are applied based on their individual properties and suitability for use on different types of parts such as seats, plugs, shafts and bearings.