Following are critical parts of globe type control valve and their information
A. Plug stem
Plug stem is the potion of valve which connects the actuator steam and the valve. The guiding action from actuator is translated to valve plug. Plug stem takes all the force of opening and closing of valve and is prone to damage early. Hence, proper plug stem selection must.
Packing is the portion surrounding the plug stem and this prevents the fluid from leaking. As this is surrounding the plug stem, this is one item which needs regular inspection and likely change-out during planned shutdown or turnaround works.
Bonnet provides the enclosure to the valve plug stem and packing. This upper part of valve body.
D. Valve plug
Valve plug is the portion which opens and closes to regulate the control valve function. Type of valve plug is critical avoid leakage from the valve.
Cage is between the valve plug and the body of the valve. Type of the cage determines the leakage from the valve plug to the bonnet. To meet the desired leakage class, cage plays critical role.
Seat is the locations where valve plug or disc is rests in close position. The soft or hard seal is based on the pressure, temperature and type of fluid to be handled. Soft seats gives less leakage rates where as hard seats gives higher leakage.
Trim is the combination of the valve plug or disc, cage and seat. The trim of the control valve decided rated Cv, leakage class and valve characteristics. Trim is part of control valve, which can be customized based on the requirements.
H. Valve body
Valve body is the lower part of valve which is holds the pressure of fluid and is most heavy part of the valve. Valve body is connected to the bonnet and provides housing to valve internals.
Following figures given details of valve body parts -
Further opening of a control valves is controlled via its actuator. Read this details post on how actuators work and understand how they are constituted.