Reference Manual

1−2
Valve Types and Characteristics
The control valve regulates the rate of fluid flow as
the position of the valve plug or disk is changed by
force from the actuator. To do this, the valve must:
D Contain the fluid without external leakage.
D Have adequate capacity for the intended
service.
D Be capable of withstanding the erosive,
corrosive, and temperature influences of the
process.
D Incorporate appropriate end connections to
mate with adjacent pipelines and actuator
attachment means to permit transmission of
actuator thrust to the valve plug stem or rotary
shaft.
Many styles of control valve bodies have been
developed. Some can be used effectively in a
number of applications while others meet specific
service demands or conditions and are used less
frequently. The subsequent text describes popular
control valve body styles utilized today.
Globe Valves
Single-Port Valve Bodies
Single-port is the most common valve body style
and is simple in construction. Single-port valves
are available in various forms, such as globe,
angle, bar stock, forged, and split constructions.
Generally, single-port valves are specified for
applications with stringent shutoff requirements.
They use metal-to-metal seating surfaces or
soft-seating with PTFE or other composition
materials forming the seal.
Single-port valves can handle most service
requirements. Because high pressure fluid is
normally loading the entire area of the port, the
unbalance force created must be considered when
selecting actuators for single-port control valve
bodies. Although most popular in the smaller
sizes, single-port valves can often be used in NPS
4 to 8 with high thrust actuators.
Many modern single-seated valve bodies use cage
or retainer-style construction to retain the seat ring
cage, provide valve plug guiding, and provide a
means for establishing particular valve flow
Figure 1-1. Single-Ported Globe-Style Valve
Body
W7027-1
characteristics. Retainer-style trim also offers ease
of maintenance with flow characteristics altered by
changing the plug. Cage or retainer-style
single-seated valve bodies can also be easily
modified by a change of trim parts to provide
reduced-capacity flow, noise attenuation, or
cavitation eliminating or reducing trim (see
chapter 4).
Figure 1-1 shows one of the more popular styles of
single-ported or single-seated globe valve bodies.
They are widely used in process control
applications, particularly in sizes NPS 1 through
NPS 4. Normal flow direction is most often flow-up
through the seat ring.
Angle valves are nearly always single ported, as
shown in figure 1-2. This valve has cage-style trim
construction. Others might have screwed-in seat
rings, expanded outlet connections, restricted trim,
and outlet liners for reduction of erosion damage.
Bar stock valve bodies are often specified for
corrosive applications in the chemical industry
(figure 1-3), but may also be requested in other
low flow corrosive applications. They can be
machined from any metallic bar-stock material and
from some plastics. When exotic metal alloys are
required for corrosion resistance, a bar-stock valve
body is normally less expensive than a valve body
produced from a casting.
High pressure single-ported globe valves are often
found in power plants due to high pressure steam
(figure 1-4). Variations available include