Page 219 - Root Cause Failure Analysis

Control Valves 207

In addition, it must provide a relatively laminar flow with minimum pressure drop
in the fully open position. When evaluating valves, the following criteria should be
considered: capacity rating, flow characteristics, pressure drop, and response char-
acteristics.

Capacity Rating
The primary selection criteria of a control valve is its capacity rating. Each type of
valve is available in a variety of sizes to handle most typical process-flow rates. How-
ever, proper size selection is critical to the performance characteristics of the valve
and the system where it is installed. A valve’s capacity must accommodate variations
in viscosity, temperature, flow rates, and upstream pressure.

Flow Characteristics
The internal design of process-control valves has a direct impact on the flow charac-
teristics of the gas or liquid flowing through the valve. A fully open butterfly or gate
valve provides a relatively straight, obstruction-free flow path. As a result, the product
should not be affected. Refer to the previous section on valve configuration for a dis-
cussion of the flow characteristics by valve type.

Pressure Drop
The control-valve configuration affects the resistance to flow through the valve. The
amount of resistance, or pressure drop, will vary greatly, depending on type, size, and
position of the valve’s flow-control device (i.e., ball, gate, or disk). Pressure-drop for-
mulas can be obtained for all common valve types from several sources.

Response Characteristics
With the exception of simple, manually controlled shutoff valves, process-control
valves generally are used to control the volume and pressure of gases or liquids within
a process system. In most applications, valves are controlled from a remote location
through the use of pneumatic, hydraulic, or electronic actuators. Actuators are used to
position the gate, ball, or disk that starts, stops, directs, or proportions the flow of gas
or liquid through the valve. Therefore, the response characteristics of a valve are
determined, in part, by the actuator. Three factors critical to proper valve operation are
response time, length of travel, and repeatability.

Response Time Response time is the total time required for a valve to open or close
to a specific set-point position. These positions are fully open, fully closed, and any
position in between. The selection and maintenance of the actuator used to control
process-control valves have a major impact on response time.

Length ofTravel The valve’s flow-control device (Le., gate, ball, or disk) must
travel some distance when going from one set point to another. With a manually oper-
ated valve, this is a relatively simple operation. The operator moves the stem lever or
handwheel until the desired position is reached. The only reasons why a manually