Many different types of valves are used in industrial applications worldwide. a valve can be defined as:
“A generic name for a device with a moveable feature that opens and closes a passageway in order to allow, prevent or control the flow of fluids.”
Valves can be categorized into the following basic types:
(Gate valves - Globe valves - Ball valves - Plug valves - Diaphragm valves - Butterfly valve - Needle valves - Check valves - Relief and safety valves - Clapper valve - Thermal expansion valve - Sampling valves)
1.Gate valves
Gate valves are used when a straight-line flow of fluid and minimum restriction is desired. Gate valves are so named because the part that either stops or allows flow through the valve acts somewhat like the opening or closing of a gate and is called, appropriately, the gate. The gate is usually wedge shaped. When the valve is wide open, the gate is fully drawn up into the valve, leaving an opening for flow through the valve the same size as the pipe in which the valve is installed.
Therefore, there is little pressure drop or flow restriction through the valve. Gate valves are not suitable for throttling purposes since the control of flow would be difficult due to valve design and since the flow of fluid slapping against a partially open gate can cause extensive damage to the valve. Except as specifically authorized, gate valves should not be used for throttling.
- Advantages:
- No resistance to flow when open
- Little pressure drop
- Good sealing when closed
- Little/no leakage
- Disadvantages:
- Flow changes nonlinearly with stem travel
- Vibration/cavitation when partially open
- Subject to wear
- Repair work is difficult
2.Globe valves
Globe valves are probably the most common valves in existence. The globe valve derives its name from the globular shape of the valve body. However, positive identification of a globe valve must be made internally because other valve types may have globular appearing bodies. Globe valve inlet and outlet openings are arranged in several ways to suit varying requirements of flow.
- Advantages:
Throttling and - regulating flow - Less seat leakage than Gate Valve.
- Disadvantages:
- High head-loss due to flow direction changes
- Dynamics can create pulsation and damage trim/packing/actuators
- Noisy in high pressure applications
- Valves can be very heavy/large in size for a given application.
3.Ball valves
Ball valves, as the name implies, are stop valves that use a ball to stop or start the flow of fluid. The ball performs the same function as the disk in the globe valve. When the valve handle is operated to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet. When the valve is shut, which requires only a 90-degree rotation of the hand-wheel for most valves, the ball is rotated so the hole is perpendicular to the flow openings of the valve body, and flow is stopped.
Most ball valves are of the quick-acting type (requiring only a 90-degree turn to operate the valve either completely open or closed), but many are planetary gear operated. This type of gearing allows the use of a relatively small hand-wheel and operating force to operate a fairly large valve. The gearing does, however, increase the operating time for the valve. Some ball valves contain a swing check located within the ball to give the valve a check valve feature.
Ball valves are normally found in the following systems aboard ship: seawater, sanitary, trim and drain, air, hydraulic, and oil transfer.
Ball valves are normally found in the following systems aboard ship: seawater, sanitary, trim and drain, air, hydraulic, and oil transfer.
Advantages:
- Less expensive
- Low maintenance costs
- Low torque
- Quick action on/off
- Compact
- No lubrication
- Tight sealing
Disadvantages:
- Relatively poor for throttling
- Throttling leads to seat erosion
4.Butterfly valve
The butterfly valve may be used in a variety of systems aboard ship. These valves can be used effectively in freshwater, saltwater, and chill water systems aboard ship. The butterfly valve is light in weight, relatively small, relatively quick-acting, provides positive shut-off, and can be used for throttling.
The butterfly valve has a body, a resilient seat, a butterfly disk, a stem, packing, a notched positioning plate, and a handle. The resilient seat is under compression when it is mounted in the valve body, thus making a seal around the periphery of the disk and both upper and lower points where the stem passes through the seat. Packing is provided to form a positive seal around the stem for added protection in case the seal formed by the seat should become damaged.
To close or open a butterfly valve, turn the handle only one quarter turn to rotate the disk 90°. Some larger butterfly valves may have a hand-wheel that operates through a gearing arrangement to operate the valve. This method is used especially where space limitation prevents use of a long handle.
Butterfly valves are relatively easy to maintain. The resilient seat is held in place by mechanical means, and neither bonding nor cementing is necessary, Because the seat is replaceable, the valve seat does not require lapping, grinding, or machine work.
Advantages:
- On/off as well as throttle/regulate
- Easily/quickly operated
- Good for large flow/low pressure applications due to saving in weight/size/cost
- Good for slurries/suspended solids.
5.Relief and safety valves
Safety Valve is a one type of valve that automatically actuates when the pressure of inlet side of the valve increases to a predetermined pressure, to open the valve disc and discharge the fluid ( steam or gas ) ; and when the pressure decreases to the prescribed value, to close the valve disc again. Safety valve is so-called a final safety device which controls the pressure and discharges certain amount of fluid by itself without any electric power support.
Safety Valve is mainly installed in a chemical plant, electric power boiler, gas storage tank, preventing the pressure vessels from exploding or damaging.
Function of the safety valve :
1. "Nozzle" inside the Safety Valve starts to receive a higher pressure from the inlet side of the valve.
2. When the pressure becomes higher than the set pressure, "Disc" starts to lift and discharge the fluid.
3. When the pressure decreases until the predetermined pressure, the force of the spring closes "Disc".
Role of the spare parts
Nozzle --- Pressure Entrance
Disc ----- Lid
Spring --- Pressure Controller
6.Needle valves
Needle valves have a long, tapered, needle-like point that is used to make relatively fine adjustments in the amount of fluid flow. They are sometimes used as component parts for other valves because the needle allows for a gradual change in the size of the fluid flow opening. It’s Good for fine adjustment throttling.
7.Check valves
Check valves are used in systems employing gate valves because they prevent the reversal of flow in the piping system and there is a low pressure drop across the valve. The pressure of the fluid through the system opens the system, while the weight of the check mechanism will close the valve if the flow is reversed. It’s main function to Prevent reverse flow.
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4.Butterfly valve
The butterfly valve may be used in a variety of systems aboard ship. These valves can be used effectively in freshwater, saltwater, and chill water systems aboard ship. The butterfly valve is light in weight, relatively small, relatively quick-acting, provides positive shut-off, and can be used for throttling.
The butterfly valve has a body, a resilient seat, a butterfly disk, a stem, packing, a notched positioning plate, and a handle. The resilient seat is under compression when it is mounted in the valve body, thus making a seal around the periphery of the disk and both upper and lower points where the stem passes through the seat. Packing is provided to form a positive seal around the stem for added protection in case the seal formed by the seat should become damaged.
To close or open a butterfly valve, turn the handle only one quarter turn to rotate the disk 90°. Some larger butterfly valves may have a hand-wheel that operates through a gearing arrangement to operate the valve. This method is used especially where space limitation prevents use of a long handle.
Butterfly valves are relatively easy to maintain. The resilient seat is held in place by mechanical means, and neither bonding nor cementing is necessary, Because the seat is replaceable, the valve seat does not require lapping, grinding, or machine work.
Advantages:
- On/off as well as throttle/regulate
- Easily/quickly operated
- Good for large flow/low pressure applications due to saving in weight/size/cost
- Good for slurries/suspended solids.
5.Relief and safety valves
Safety Valve is a one type of valve that automatically actuates when the pressure of inlet side of the valve increases to a predetermined pressure, to open the valve disc and discharge the fluid ( steam or gas ) ; and when the pressure decreases to the prescribed value, to close the valve disc again. Safety valve is so-called a final safety device which controls the pressure and discharges certain amount of fluid by itself without any electric power support.
Safety Valve is mainly installed in a chemical plant, electric power boiler, gas storage tank, preventing the pressure vessels from exploding or damaging.
Function of the safety valve :
1. "Nozzle" inside the Safety Valve starts to receive a higher pressure from the inlet side of the valve.
2. When the pressure becomes higher than the set pressure, "Disc" starts to lift and discharge the fluid.
3. When the pressure decreases until the predetermined pressure, the force of the spring closes "Disc".
Role of the spare parts
Nozzle --- Pressure Entrance
Disc ----- Lid
Spring --- Pressure Controller
6.Needle valves
Needle valves have a long, tapered, needle-like point that is used to make relatively fine adjustments in the amount of fluid flow. They are sometimes used as component parts for other valves because the needle allows for a gradual change in the size of the fluid flow opening. It’s Good for fine adjustment throttling.
7.Check valves
Check valves are used in systems employing gate valves because they prevent the reversal of flow in the piping system and there is a low pressure drop across the valve. The pressure of the fluid through the system opens the system, while the weight of the check mechanism will close the valve if the flow is reversed. It’s main function to Prevent reverse flow.
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