Types of Control Valves - Application,Advantages and Disadvantages

instrumentation

#1

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
Valve classification - category
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 Advantages and Disadvantages of Gate Valve
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 Advantages and Disadvantages of Globe valve
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 Advantages and Disadvantages of Plug Valve
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 Advantages and Disadvantages of Ball Valve
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.


#2

Butterfly Valves

They are rotary valves with a disc rotating at right angles within a pipe section body.

The stem passes through the disc is supported by both ends of the body.

The shaft is ensured to the disc either by bolts or pins.Rotating stem through 90 degrees.Fully opens or closes the valves.
Application Advantages and Disadvantages of Butterfly
Application - Large flow control,Low pressure applications where leakage is unimportant.

Advantages of Butterfly Valve -
Simple,Compact form,Quick opening,Good control,large flow,low pressure drop, and low weights and costs.

Disadvantages of Butterfly Valve-
No tight shut off

Diaphragm Valves

The flexible diaphragm is connected to a support member known as compressor,which represents the closure member.When the valve is opened the diaphragm is lifted out of the flow path and the fluid has a smooth streamlined flow.When the valve is closed the diaphragm is tightly seated against the seat in the body known as the wier. The diaphragm is of NEOPRENE.
Application Advantages and Disadvantages of Diaphragm valves
Application - Used for corrosive liquids at low temp and pressure.

Diaphragm Valves Advantages -
The diaphragm completely keeps the working parts in isolation from the process fluids.Simple in construction,easier to operate and maintain.

Disadvantages of Diaphragm valves-
Not for high temp and pressure.