Page 214 - Root Cause Failure Analysis

CONTROL VALVES

Control valves can be broken into two major classifications: process and fluid power.
Process valves control the flow of gases and liquids through a process system. Fluid-
power valves control pneumatic or hydraulic systems.

PROCESS
Process-control valves are available in a variety of sizes, configurations, and materials
of construction. Generally, this type of valve is classified by its internal configuration.

Configuration
The device used to control flow through a valve varies with its intended function. The
more common types are ball, gate, butterfly, and globe valves.

Ball
Ball valves (see Figure 17-1) are simple shutoff devices that use a ball to stop and
start the flow of fluid downstream of the valve. As the valve stem turns to the open
position, the ball rotates to a point where part or all of the hole machined through the
ball is in line with the valve-body inlet and outlet. This allows fluid to pass through
the valve. When the ball rotates so that the hole is perpendicular to the flow path, the
flow stops.

Most ball valves are quick-acting and require a 90" turn of the actuator lever to fully
open or close the valve. This feature, coupled with the turbulent flow generated when
the ball opening is only partially open, limits the use of the ball valve. Use should be
limited to strictly an ordoff control function (Le., fully open or fully closed) because
of the turbulent flow condition and severe friction loss when in the partially open
position. These valves should not be used for throttling or flow control.

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