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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?
- Complete isolation
- Throttle
TYPES OF VALVES:
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Basically the valve types are classified into 8 types there are,
- Screw down valve
- Parellel slide valve
- Gate valve
- Check valve
- Ball valve
- Butterfly valve
- Diaphragm valve
- Bellow seal valve
1).SCREW DOWN VALVE:
- It uses threaded spindle to control valve position
- There are four types of valve seat and geometry
- Mitre valve
- Flat valve
- Globe valve
- 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.
- 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,
- Swing check valve
- Split check
- 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
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