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Butterfly valves central disc should ordinarily be mounted with the spindle horizontal since this allows trash in the pipe invert to be swept clear as the valve is closed. Where the spindle is vertical solids can lodge under the disc at the spindle and cause damage to the seal. Disc position indicators are useful and strong disc stops integral with the body should be specified, so that the operator can feel with certainty when the disc is fully closed or fully open. The disc can be considered as an aerofoil, where greater energies are applied on the upper side than on the lower side.
Butterfly valves are generally favoured because it costs less than other valve designs, and are a lighter in weight so they need less maintenance.