Valves - Types, Applications and Selection of valves

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VALVES - SELECTION OF VALVES, TYPES AND ITS APPLICATIONS

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WHAT IS THE VALVE?

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• A valve is a device that controls not only a flow, but also the rate, the volume and the pressure, direction of liquids, gases, toxic gases, slurries through a pipeline chute or passageway.

REASONS FOR FITTINGS?

1. Complete isolation
2. Throttle

TYPES OF VALVES:

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Basically the valve types are classified into 8 types there are,

1. Screw down valve
2. Parellel slide valve
3. Gate valve
4. Check valve
5. Ball valve
6. Butterfly valve
7. Diaphragm valve
8. Bellow seal valve

1).SCREW DOWN VALVE:

• It uses threaded spindle to control valve position
• There are four types of valve seat and geometry

1. Mitre valve
2. Flat valve
3. Globe valve
4. Hemispherical valve

2). PARELLEL SLIDE VALVE(PSV VALVES):

• It used a stream system where full flow and no control is required
• Parellel disc forming valves lids are kept in the contact with valve seat by a spring
• Tight seal to flow is maintained by differential pressure acting on valves seat and lids.

3). GATE VALVE:

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• It general service valve, it operates (open/close) by a disc or gate that slides down through the valve to block the flow.

RECOMMENDED USES:

• Fully open/close, non-throtting
• Infrequent operation
• Minimal fluid trapping in line
• These valves are often described in terms of sealing element. (Eg : solid wedge, parellel slide, parellel double disc and split wedge.

APPLICATIONS:

• Oil, gas, slurries, heavy liquids, stream, corrosive liquids.

ADVANTAGES:

• Gate valves opens or closes slowly, which prevents fluids hammer and subsequent damage to the piping system.
• They need long operation time since setting the valve to the fully open or closed position requires a handle to be turned a many times.
• Good choice for on / off service
• Bi directional
• Full flow low pressure drop

DISADVANTAGES:

• It not suitable for throttling application
• It is prone to vibration in the partially open state.
• It is more subject to seat and disc wear
• Repairs, such as lapping and grinding are generally more difficult to accomplish.

4). CHECK VALVE:

• Check valves are described as reflux, non-return, back pressure, retaining valve.
• The check valves are designed to prevent back flow. Fluid flow in the desired direction opens the valve while backflow forces the valve closed.
• Most check valves are based on either swing concept or lifting concept. Some common types are,

1. Swing check valve
2. Split check
3. Dual plate check

ADVANTAGES:

• Prevents backflow
• Maintains pressure
• Serve as backup system

Disadvantages:

• Can't be used with pulsating system
• Clsoing element may slam close causing damage and excessive wear.

5). BUTTERFLY VALVE:

• A flat plate is positioned in the center of pipe. The plate is connected to the Rotating handle which turns the plate either parellel or perpendicular to the flow.
• A tight seal obtained by a synthatic rubber fitted around the edge of the plate.
• Valve can be used with on actuator on remote.

RECOMMENDED USES:

• Fully open/close or throttling service
• Frequent operation

APPLICATIONS:

• Liquids, gases, slurries and liquids with suspended solids.

ADVANTAGES:

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• They are suitable for large valve application
• The maintenance cost are very low.
• Used with Chemical or corrosive media
• Compact and light weight design.
• Pressure drop across a butterfly valves is small.

DISADVANTAGES:

• difficult to clean
• Throttling limited to low differential pressure
• Potential for cavitation and choke.
• Unguided disc movement is affected by flow turbulence.

6). BALL VALVE:

• Valve consists of a ball having a hole through its center.
• Ball fitted between the two seats of synthatic material.
• Quick Operation by a Rotating handle.

USES:

• Used in a sea water and fuel system.
• Used only for isolation and opening.

ADVANTAGES:

• Maintains and regulates an high volume, high pressure and high flow.
• Low purchase and maintenance costs
• No lubrication required.
• Give tight sealing with low torque.
• Time of valve operation is minimised.
• Can be used for throttling service.
• Can handle fluids with suspended solids.

DISADVANTAGES:

• Have relatively poor throttling characteristics.
• Difficult to clean, leads to contamination.

7). DIAPHRAGM VALVE:

• Diaphragm valve is a linear motion valve that is used to start, regulate and stop fluid flow.
• The name is derived from its flexible disc which mates with a seat located in the open area at top of the valve body to from seal.

ADVANTAGES:

• Valve components isolated from the process fluid.
• Valves construction prevents leakage of the fluid without the use of a gland seal (packing).

DISADVANTAGES:

• The diaphragm becomes worn more easily and regular maintenance is necessary.
• This types of valves generally not suitable for very high temperature fluid and are mainly used on liquid systems.

8). BELLOW SEAL VALVE:

• This type of valve used on nuclear submarine.
• Used as stream stop / bypass valve
• Consists of welded valve boday with removable bonnet assembly.
• Valves sealing is achieved by bellows.

SELECTION OF VALVES:

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• The substance to be handled and required flow rate.
• The ability of valve to withstand the maximum working pressure and temperature.
• The ability of valve to resist attack by corrosion or erosion.
• Actuator requirements
• Maintenance and repair requirements