Control Valve -
The control valve commonly named the final control element of control contains a pneumatic device that convert the control signal from the controller in action regulating the process fluid flow.
Valves makes approximately 15% of the total expenditure for the materials and equipment for the refinery or chemical processing industry.
Valves are used to handle variety of liquids,gases,steam,air etc…
Category of valves
ON-OFF Services - Gate,Ball,Diaphragm,Plug,Butterfly valves.
THROTTLING SERVICES - Globe,Butterfly,Diaphragm,Pinch
NON-REVERSE FLOW - Check valves
Gate Valves
The Gate valves are characterized by a siding gate which is moved by the stem perpendicular to the direction flow.
Application - It is used for on-off application.Suited for high temperature and pressure use with variety of fluids.They are not primarily used for slurries,viscous fluid etc…
Advantages of Gate Valve - Low pressure drop when fully open and tight sealing
Disadvantages of Gate Valve - Causes vibration,seat disc wear in partial open condition.Slow response characteristics and require large actuating force.
Globe Valve
In globe valves disc or plug is moved on or off the seat.The seat opening is directly proportional to the travel of the plug.It is short stem travel,high seating capacity,large pressure drop and high flow controllability.
Application - It is used primarily for throttling purposes. It may be considered a general purpose flow control valve high temp application.
Advantages of Globe Valve - Faster to open or close,most reliable form of seating,throttling to control the flow to any desired degree,positive shut-off.
Disadvantages of Globe Valve - Seat disc wear in partial open conditions.
Plug Valves
It consist of body,plug and cover. The plug is tapered or cylindrical.In the open position,the bore in the plug connects the inlet and outlet ends of the valve providing straight line flow.
Application - The plug valves are extensively used in refinery,petrochem and chemical industries and for general purpose involving on off services.
Advantages of plug valve - Normally small in size,require less headroom and available in wide range of materials.They provide tight shut off,quick opening and low pressure drop.
Disadvantages of plug valve- Plug valve may be subjected to galling
Ball Valves
It is improvisation of the plug valve.It is basically a ported sphere in a housing.The seat matching the ball is circular so that the seating stress is circumferentially uniform.The seats are usually made up of PTFE which is inert to all chemicals,has low coefficient of friction and resiliency.
Application - It is used for flow and pressure control and shut off and for corrosive fluids,slurries,normal liquid and gases.For high temp and pressure.
Advantages of Ball Valve - Low pressure drop shut-off,quarter turn operation,easy to maintain,low torque.They are small in size and low in weight.
Disadvantages of Ball valve - PTFE seats are subjected to extrusion is the valve is used for throttling. Fluid trapped in the ball in the closed position may cause problem of build up of vapor pressure and corrosion.
Application - Large flow control,Low pressure applications where leakage is unimportant.
Application - Used for corrosive liquids at low temp and pressure.