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 -
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Low pressure drop shut-off, quarter turn operation, easy to maintain, low torque.
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They are small in size and low in weight.
Disadvantages of Ball valve -
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PTFE seats are subjected to extrusion is the valve is used for throttling.
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Fluid trapped in the ball in the closed position may cause problem of build up of vapor pressure and corrosion.
Butterfly Valve
This type of valve contains a circular disc with the same diameter as the valve body. It is rarely used in the fully open position due to debris between the body and the disc; in this case, it must be opened & cleaned excessively. This valve experiences an excessive amount of packing leakage.
Application of Butterfly valves
- Butterfly valves regulate air and water flow in HVAC systems.
- They control water and fluid flow in water treatment plants.
- Butterfly valves handle corrosive fluids in chemical plants.
- They regulate cooling water & other fluids in power plants.
- Butterfly valves control oil, gas, & other hydrocarbon flow in pipelines and refineries.
Advantages of Butterfly valves-
- Butterfly valves can be opened and closed fast to control flow.
- Their tiny shape makes them ideal for space-constrained conditions.
- Butterfly valves are cheaper than other valves.
- When fully open, they have low pressure drop, which helps fluid systems run efficiently.
- Butterfly valves are straightforward to install & maintain, minimizing downtime and costs.
Disadvantages of Butterfly valves-
- Butterfly valves can have lower pressure ratings than gate valves.
- Butterfly valves can control on/off, however they may not be good at throttling.
- In high-pressure (or) high-temperature conditions, sealing may be an issue depending on design and use.
- Butterfly valves may be eroded and damaged by cavitation.
- Materials must be compatible with the fluid being handled since some substances or conditions might induce corrosion or degradation.
Diaphragm Valve
In this kind of valves there is no contact between the medium and moving parts, that keeps it from oxidizing material, so this valve can be made of cheap metals. There is diaphragm between the valves and its cover.
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 of Diaphragm Valves - Used for corrosive liquids at low temp and pressure.
Advantages of Diaphragm Valves -
- 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.
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