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A valve is a device that is used to control the volume of flow and pressure of fluids. Closing, opening, or partially opening a passage through which the fluid passes regulates this flow. Fluid can be water, gas, slurry, etc. Valves are made of different materials like cast iron, bronze, steel, aluminum, plastic, ceramic and other materials. Valves are used in areas of commercial applications, domestic and process applications, automobiles, hydraulic presses, medical equipment and many more.
Working of a Valve
In general, a valve has a plunger in the form of a rod, flap or a ball that is fixed to a shaft. The shaft and plunger assembly is housed in a casting through which the fluid passes. When the shaft is in one extreme position, the passage is open allowing the fluids to pass. At the other extreme position, the passage is blocked and the flow is stopped. The movement of the shaft and the mounted flaps or balls is calibrated. This helps precise control of liquid flow.
Types of Valves
Electronic/Electrical Valves: In these valves, movement of the flap or ball is controlled through digital electronics or electrical circuits. These valves have a precise control and reaction time, which allow quick regulation of liquid flow. Such valves are used in expensive automobiles, aircrafts, Space applications, ships, high risk chemical processing and other critical areas.
Non-Return Valves: These valves are used in applications like chemical processes, gas turbines, automotive and others where unidirectional flow is required. The valves open in only one direction. When pressure in the opposite direction is applied, the valves get closed.
Electromechanical Valves: Electro magnets control the opening and closing mechanism of the valves. These valves may have only full open or full close positions.
Mechanical Valves: Use mechanical energy for opening and closing mechanism. A set of levers, pulleys, linkages, springs, threaded shafts, etc. are used to operate the valve. Smaller valves may be operated manually by turning a wheel or using a lever or handle. They are used in domestic water supply lines, taps for water pipes, cooking ranges, fire-fighting equipment, etc.
Types of valves used in domestic and industrial applications are:
Ballcock Valves are used to control the level of water in a tank. The valve is connected to a float using a lever. The unit is placed at the top of the tank. The valve is connected to the input side of the water supply. The float rises with the water and forces the valve to shut off the valve when the maximum level is reached. It is used in water closets, water tanks and other low risk areas.
Ball Valves open when a ball handle is turned in a specific direction. A cavity is made in the center of the ball. Flow is allowed when the cavity is in-line with both the ends of the valve. Varying the movement of the handle controls the flow. The valve body is made of metal, ceramic, plastic or it can be chrome plated. Types of ball valves are explained below.
Full port ball valves give unrestricted flow in the open position since the ball hole and pipe size are the same.
Standard port ball valves have a ball with a smaller size, which restricts the flow.
Reduced port ball valves have the flow diameter and pipe diameter in ratio of 2:1.
Three-way ball valves have a T-shaped cavity. The flow can be made to pass through one side or both sides or stopped completely.
Bibcock or Sillcock Valves are used in applications like building hose connections, gardening, car washing, watering lawns, and others. It has a removable control knob that when closed, presses a plug of rubber against the water passage.
Butterfly Valves have circular plates attached to a shaft resembling wings of butterfly, hence the name. The shaft has a handle that can be turned so that the wings obstruct the water passage. The wings are immersed in the water and this causes a pressure drop. These valves are used in water supply systems for flow control.
Gate Valves are used in process industries for full open or full close systems. It has a rectangular or round gate fitted to a stem, which is enclosed in a threaded housing. The gate allows the flow when stem is raised and stops when it is lowered. It is used in high-pressure lines like steam turbines.
Globe Valves have a spherical body shape with two mating halves. A baffle with a central disc separating the two halves is placed in a housing. A screw handle controls the rate of flow. They are used in high frequency throttling applications. The whole assembly is mounted in a bonnet to make it leak proof.
Check Valves allow flow in only one direction. These are used in chemical process plants, water purification plants, petrochemical industries where the processed fluid is made to flow into a low-pressure area. These valves prevent reverse flow of the fluid and hence avoid contamination.
Solenoid Valves are used in machine tool hydraulics and heavy earthmoving equipment. It has a loop of wire, wrapped around a metallic core and when current is passed, it creates a magnetic field. The magnetic field is used to open or close plungers in flow lines to quickly open or close the flow.
The types of solenoid valves are:
Electromechanical - use electrical energy
Pneumatic - use compressed air
Hydraulic - use energy from pressurized fluids
Flow Control Valves are used to automatically maintain a constant flow rate. They can be controlled by a calibrated flow meter or temperature and pressure gages. The gages can be preset to the desired flow rate. When the actual flow rate varies from the preset value, the gages will adjust the flow. These devices are used in complex chemical and petrochemical plants where manual adjustment of flow is not feasible since there may be thousands of such valves.
Poppet Valves are used to control the flow of exhaust and intake gases in automobiles. These valves are controlled by the camshaft and other devices and need to be very accurate and durable over extended use at high temperatures.
Safety Valves are used as safety devices that open up to release the pressure when it reaches a preset value. They are used in domestic pressure cookers, steam pressure vessels and other appliances.
Considerations for Valves
Before making a choice of valve it is very important to understand the requirements carefully. A careful analysis on usage, purpose, installation and maintenance will help you make a perfect choice. Other factor that one might also want to consider is the cost. Your decision should be balanced on the requirements, utility and cost. Consider the following factors when selecting/installing valves:
The most important aspect is the fluid material. If it is water at normal flow pressure, then a little leakage may be tolerated but if you are controlling corrosive fluids and gases, the valve has to be very reliable. Build safety backups in case the primary valve fails. Also you may need to use corrosion resisting materials.
Check the maximum pressure and design the valve giving a sufficient safety margin. During accidents, the pressure can rise several times than the theoretical value.
Certain types of valves like butterfly or gate valves cause pressure drop. This has to be considered while designing the valve system.
Valves need to be regularly maintained, cleaned and inspected for leakages and seepage into the ground. Suitable bypass lines need to be built in to allow maintenance without interrupting the flow. The valves should turn freely at the specified pressure. If a greater pressure needs to be applied, then this would indicate a blockage in the line or that some part of the valve is rubbing against the pipeline.
Certain process industries always need to have a constant pressure maintained in the flow line even when the line is idle. Suitable return proof valves need to be built into the system.
If the valve is closed too rapidly, water hammer effect can occur. In this effect, trapped air globules can strike the pipe sides of the valve faces damaging them.