TYPES OF VALVES
Globe Valve Disk and Stem Connections
Globe valves employ two methods for connecting disk and stem: T-slot construction and disk
nut construction. In the T-slot design, the disk slips over the stem. In the disk nut design, the
disk is screwed into the stem.
Globe Valve Seats
Globe valve seats are either integral with or screwed into the valve body. Many globe valves
have backseats. A backseat is a seating arrangement that provides a seal between the stem and
bonnet. When the valve is fully open, the disk seats against the backseat. The backseat design
prevents system pressure from building against the valve packing.
Globe Valve Direction of Flow
For low temperature applications, globe and angle valves are ordinarily installed so that pressure
is under the disk. This promotes easy operation, helps protect the packing, and eliminates a
certain 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 contract
upon cooling and tend to lift the disk off the seat.
A 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. When
the valve handle is turned to open the valve, the ball rotates to a point where the hole through
the ball is in line with the valve body inlet and outlet. When the valve is shut, the ball is rotated
so 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 valve
handle to operate the valve. Other ball valve actuators are planetary gear-operated. This type
of gearing allows the use of a relatively small handwheel and operating force to operate a fairly
Some ball valves have been developed with a spherical surface coated plug that is off to one side
in 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 no
lubrication and can be used for throttling service.