Lets start this session with some overview on Control Valves.
A power operated device, which modifies the fluid flow rate in a process control system. It consists of a valve connected to an actuator mechanism that is capable of changing the position of a flow controlling element in the valve in response to a signal from the controlling system.
Mainly control valves, which are used for the flow control (with some turn down) are Globe valve
Before we start on valve sizing part, lates start with the main terminology used for the Control Valves
TRIM: The internal parts of a valve which are in flowing contact with the controlled fluid.
Examples are the plug, seat ring, cage, stem and the parts used to attach the stem to the plug.
The body, bonnet, bottom flange, guide means and gaskets are not considered as part of the
Closure member: A movable part of the valve which is positioned in the flow path to modify the rate of flow through the valve.
Plug: A cylindrical part which moves In the flow stream with linear motion to modify the flow rate and which may or may not have a contoured portion to provide flow characterization.
Seat ring: A part that is assembled in the valve body and may provide part of the
flow control orifice. Seat Ring also can be an integral part of the body or cage material or may be
constructed from material added to the body or cage.
Cage: A part in a globe valve surrounding the closure member to provide alignment and
facilitate assembly of other parts of the valve trim. The cage may also provide flow
characterization and/or a seating surface for globe valves and flow characterization for some
Globe valve plug guides: The means by which the plug is aligned with the seat and held stable throughout its travel. The guide Is held rigidly in the body or bonnet.
Stem guide: A guide bushing closely fitted to the valve stem and aligned with the seat.
Disadvantage: Higher pressure drops and minor cavitation can excite vibrational modes that are
very destructive and can result in valve failure.
Post guide: Guide bushing or bushings fitted to posts or extensions larger than the valve stem
and aligned with the seat.