TYPES OF VALVESDOE-HDBK-1018/2-93ValvesGlobeValveDiskand StemConnectionsGlobe valves employ two methods for connecting disk and stem: T-slot construction and disknut construction. In the T-slot design, the disk slips over the stem. In the disk nut design, thedisk is screwed into the stem.GlobeValveSeatsGlobe valve seats are either integral with or screwed into the valve body. Many globe valveshave backseats. A backseat is a seating arrangement that provides a seal between the stem andbonnet. When the valve is fully open, the disk seats against the backseat. The backseat designprevents system pressure from building against the valve packing.GlobeValveDirectionof FlowFor low temperature applications, globe and angle valves are ordinarily installed so that pressureis under the disk. This promotes easy operation, helps protect the packing, and eliminates acertain amount of erosive action to the seat and disk faces. For high temperature steam service,globe valves are installed so that pressure is above the disk. Otherwise, the stem will contractupon cooling and tend to lift the disk off the seat.BallValvesA ball valve is a rotational motion valve that uses a ball-shaped disk to stop or start fluid flow.The ball, shown in Figure 12, performs the same function as the disk in the globe valve. Whenthe valve handle is turned to open the valve, the ball rotates to a point where the hole throughthe ball is in line with the valve body inlet and outlet. When the valve is shut, the ball is rotatedso that the hole is perpendicular to the flow openings of the valve body and the flow is stopped.Most ball valve actuators are of the quick-acting type, which require a 90 turn of the valvehandle to operate the valve. Other ball valve actuators are planetary gear-operated. This typeof gearing allows the use of a relatively small handwheel and operating force to operate a fairlylarge valve.Some ball valves have been developed with a spherical surface coated plug that is off to one sidein the open position and rotates into the flow passage until it blocks the flowpath completely.Seating is accomplished by the eccentric movement of the plug. The valve requires nolubrication and can be used for throttling service.ME-04Rev. 0Page 18
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