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CHAPTER 6: INDUSTRIAL VALVE - Coggle Diagram
CHAPTER 6: INDUSTRIAL VALVE
INTRODUCTION
Valve is one of the most vital instrument in the industry.
Important requirements
To ensure that the valve is efficiently doing its job in producing & maintaining optimal results in process, safety, economic & environment considerations.
Engineers, Operators, Maintenance technicians should know:
How to calculate flow through valves.
Factor to consider in specifying a valve for a specific application.
Materials used to make valves,where various designs should and shouldn't be used.
Valve maintenance & repair.
A basic knowledge of valve types & designs.
Definitions
Fluid
Any substance that can flow from one area to another when it subjected to a normal and sheer stress. Gases & liquids are both consider fluids.
On-Off Service
Valves that only operate in the fully open/fully closed positions are said to provide on-off service.
Valve
A mechanical device that is used in industrial piping to control the flow of fluid.
Throttling Service
Valves that control flow by partially restricting the passage of the fluid provide in what is called throttling service.
Pressure Drop
The loss of energy that occurs when a fluid encounters resistance passing through valve.
The greater resistance, the higher the pressure drop for a given flow rate.
A higher pressure drop means that more energy in the form of pumping/compressing, must be used to move the fluid through the piping.
VALVE FUNCTIONS & APPLICATIONS
Shut-off valves (block valves).
Function
To close off the flow.
Applications
A valve selected for this purpose should give a positive seal in the closed position & min resistance to flow when open.
Valve commonly used for this purpose: Gate, Plug & Ball valves
Control valves.
Function
To regulate flow.
Applications
A valve selected for this purpose should be capable of giving smooth control over the full range of flow, from fully open to closed.
Valve commonly used for this purpose: Glove valve, Butterfly valve ( often used for gas and vapour)
The careful selection & design of control valves is important: good flow control must be achieved, whilst keeping the pressure drop as low as possible
The valve must also be sized to avoid the flashing of hot liquids and the supercritical flow of gases and vapours.
Non-return valves are used to prevent back-flow of fluid in a process line. They do not normally give an absolute shut-off the reverse flow.
TYPES OF VALVES
On-Off Service
2) Plug
Quick opening (one-quarter turn), very popular in the process industry
Process very little restriction
The valve is positioned by turning a lever
Commonly design for fuel gas piping systems, low pressure situations, slurry & lubricant services, on/off services, low temperature services & multiport operation.
Plug is made out of a solid piece that has as opening through it.
Maintenance:
Should never used in throttling
Most plug valve should be limited to temperature under 480 F
3) Ball
The ball has opening / port which allows fluids to pass through.
Ball valves are quick opening.
A ball valve is similar in design & operation to a plug valve
The ball is highly polished which makes it uniform and smooth. As a result, there is very little friction between the ball and the seat rings when the valve is operated.
Because friction is not a problem in ball valves, they do not require lubrication.
Shouldn't be used for throttling services.
Ball valves are not designed for excessive temperature since the seats are made of plastic-coated material that tends to breakdown under high temperature.
1) Gate
Used in the industry & good choice for on-off service & infrequent operation.
Position:
Closed: Gate is placed across the pipe's opening
Open: Gate is raised completely out of the flow line.
In fully open position,the gate causes very little resistance to flow. The low resistance to flow means that very little energy is lost / wasted as fluids pass through the valve.
This type of valve is used where flow rate are not restricted and should not used to throttle flow for extended periods.
The largest part of the valve is the body which can be connected to the process piping by flanges, threaded connections / welding.
Design for various process condition such as corrosive, high & low temperature services.
Gate valve materials & application:
Stainless steel gate valve
Corrosive, high T & P
Alloy gate valve
High T & P
Bronze gate valve
Low T & P
Brass gate valve
Low T & P
Cast iron gate valve
Water, lubricant & low P steam
Maintenance:
When closing a gate valve, operator should take care not to damage valve seats.
Temperature changes, close a valve too quickly & pipe expansion can cause a valve to warp.
Use lubricant to moving parts
Throttling Service
1) Globe
The disc of a globe valve does not slide along the seating surface. Hence, all contact between the disc & the seats stops as soon as flow begins
This globe valve is preferred to use for services that require frequent valve operation.
The higher turbulence & pressure drop mean that more energy is required to move fluids through an open globe valve.
Can be connected to the process piping by flanges, welding & threaded connections.
Materials & application
Stainless steel glove valve
Corrosive, high T & low P
Alloy globe valve
High T & P
Bronze globe valve
Low T & P
Brass globe valve
Low T & P
Cast iron globe valve
Water, lubricant, & low P
In a globe valve, the flow path changes in direction compared to gate valve that the flow through is in a straight line.
This change of flow causes turbulence, and also results in a higher pressure drop across the valve.
2) Butterfly
Physically is smaller than the other valves
Quick opening. One-quarter turn for open/close the valve
Flow rate is adjusted by changing the angle of the disc. Flow rate is greater when the disc is parallel to the pipe wall.
Designed to be operated at low temperature & low pressure.
Used for throttling & on/off services
Commonly found in cooling water & heat exchanger system
3) Check valve
It limits backflow but not considered tight shutoff
Check valve design
2) Lift check valve
Ideal for system in which flow is fluctuate
Recommended fro vapor services
More durable than swing check
Designed to close when flow is reverses
Has a disc that rests on the seat when flow is idle and lifts when is active
1) Swing check valve
Has a hinged disc that slams when flow reverses
Typically used in industries
3) Ball check valve
Has a ball-shaped disc that rest on the seat
The ball is down when flow is idle an dup when flow is active
Designed to close when flow reverses
Ideal for system in which flow rates fluctuate / the fluid contains some solid
Durable as a list check and more durable than swing check.
Check valve is designed to prevent reverse flow & avoid possible contamination / damage to equipment
Material & application
Alloy check valve
High T & P
Bronze check valve
Low T & P
Brass check valve
Low T & P
Cast iron check valve
Water, lubricant & low P
Stainless steel check valve
Corrosive, high T & low P
Diaphragm Valves
When the valve stem is moved down, the compressor pushed on a flexible diaphragm & stop the flow through the valve.
When the valve stem is raised, the diaphragm moves upwards and flow through the line starts
The stem is attached to the compressor
Diaphragm valve is the best valve for handling sticky / corrosive fluids or other substances that require high purity.
This valve does not have a plug / disc. Instead, it uses a flexible diaphragm to control flow
Diaphragm Valve design
1) Weir Diaphragm Valve
Has a weir (a dam) located in the body of the valve
There is a large pressure drop across the body
2) Nonweir Diaphragm Valve / Straight-through-diaphragm valve
Pressure drop across the valve depends on the position of the diaphragm