In discussions with suppliers of valves (16" side monted on/of ball valve, type with trunnion mounted ball valve, direct action), there is no clear answer to if there should be a defined interval for overhaul of these valves.
There is only a description of periodical functional tests of the valves.
Does anyone know if there is a requirement in API or equial for such overhaul/classing interval for such valve?
Depends on fluid properties and corrosiveness, pressure, temperature, velocity and erosion, installed location and the frequency and service, i.e. if used for throttling, don't expect it to last long.
Velocity + sand = possibility of erosion
Partially Open = erosion, cutting, wash out of lubrication, sand trapped in seals, ring wear.
Heat = deterioration of elastomerics
Water + sulfur + oil = possibility of sulfuric acids and internal corrosion.
If the above is assumed to have minimum effect, check yearly. Moderate, check every 6 months. If severe, check every 3 months.

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