What are valves?
Valves are equipment that are designed to control fluids, they can open, close, regulate, or isolate. They can be as small as fractions of an inch and as large as 360 inches. They can be as simple as the copper valve found in a hardware store and as complex as custom-made valves in exotic alloys for nuclear reactors.
Valves are one of the oldest products known to man, they are found in many of our everyday activities such as turning on the water in the sink.
Main parts of a valve
Each type of valve has its own design features which govern its main parts, however there are 4 main components that every valve regardless of its type. These parts are the stem, the closing membrane, the body, and the seat.
- The stem: Part that transfer the energy from the operator to the valve closing membrane to move the valve
- Closing membrane: Part of the valve that is used to block the fluid flow
- Body: Pressure containing envelope
- Seat: Part of the valve that is normally located between the closing member and the body to achieve the shut off rating of the valve
Classification according to the application
Depending on its function valves can be classified as:
- Isolation (ON/OFF) valves
- Control Valves
Isolation (ON/OFF) valves
Isolation or ON/OFF valves: Used in those applications where the only function of the valve is to isolate the fluid flow, it is understood then that these valves are either completely open or closed.
They are used to isolate sections in application such as maintenance and repair and are located so that the isolated areas do not cause inconveniences to other service areas. Usually located to isolate pumping equipment or control valves.
Control Valves
Control Valves: Used in those applications where the function of the valve is to control the fluid by generating a pressure drop through the incorporation of a restriction that changes the turbulence of the fluid, thus regulating flow, outlet pressures or changing the direction of the fluid. The selection of a control valve is based on the calculation of the flow coefficient required to generate the control conditions at maximum, minimum and normal operating conditions, it is recommended that the valve is selected to modulate is these ranges within 30% to 80% of his career.
Where do we use valves in industrial applications?
In industrial process valves are commonly used to control the process as pressure reducing and reliving valves, to control flow as balancing valves, drain valves, shut off maintenance valves or as safety devices as emergency shutdown valves and in many other applications.
Valve Construction
Because control valves are designed to modulate and be partially open their internal parts are especially susceptible to damage due to fluids that are either corrosive or contain abrasive materials for this reason the valve construction materials are essential to fit the application. Most control valves will have a hard coating applied to the most critical areas of their internal parts, such as chrome carbide or tungsten carbide overlays.
Temperature is an important factor when selecting the type of control valve to use , normally valves sealing parts, such as the seats, are made of resilient materials due to its cost and sealing properties, these valves are commonly known as resilient seated valves. However resilient materials present temperature limitations, in applications where the temperature is high, they might no longer be a suitable solution. For example, crude oil entering the refinery is first distilled in the crude distillation section and the crude preheat furnace temperature range is typically 370-380°C (698 – 716°F). In this application a metal seated control valve might be needed to ensure the integrity of the sealing materials and the shut off rating of the valve. Low temperatures can also present a challenge when selecting the right materials for a valve, in temperatures lower than -29C standard grades of carbon steel such as ASTM A216 WCB might no longer suitable, in those cases low temperature carbon steels such as ASTM A352 LCC are required to ensure the integrity of the valve body at the designed working pressure under low temperature conditions. In cryogenic applications stainless steel materials such as ASTM A316 are needed.
How to Choose the right valve?
There are many valves options to choose from such as globe valve, full port, reduce port, V-port and segmented ball valves, gate valves knife gate, sleeve pinch valves, butterfly valves, plug valves, needle valves, check valves and many more. So how do you choose a valve for your application?
First, you should understand what you want the valve to do. Valves are used in functions such as Isolation, direction control, , flow control, pressure reduction, pressure relief, excess-flow protection, and pressure protection.
When selecting a valve, it is critical to match the type of valve with the application. As mentioned, the characteristics of the fluid; temperature, composition, pressure are critical to the selection of the right materials. To select the right valve understanding the function of the valve is crucial. To control clean fluids such as drinking water or oils, valves with internal cavities such as v-ported ball valves or globe valves are suitable since the risk of getting material built up in the internal parts of the valves that could latter affect the ability to shut off is minimal. However, for applications with a percentage of solids such a tailings, valves with minimal internal contact areas or valves with internal components are less susceptible to damage due to erosion are needed, such as hard coated knife gate valves or pinch valve .
- Ball Valves
- Plug Valves
- Butterfly Valves
- Globe Valves
- Gate Valves
- Knife gate Valves
- Control knife gate Valves
- Check Valves
- Pinch Valve
On/Off Isolation Valve Typical Use Table
[wptb id=24887]
Control Valve Typical Use Table
[wptb id=24889]
Valve Actuators
Valves need energy to be operated, the energy required depends directly on the design and the pressure differential across the valve. Depending on whether this power source is provided by the effort of the operator or by an external source, the valves can be classified as;
Manuals: Valves that depend on the effort of a human to be operated. Depending on the torque required, manual valves come with a lever or handwheel coupled directly to the stem or with a gearbox that provides a torque multiplier for larger diameter valves or operating pressure
Automated: Valves that depend on an external power source to be operated. This external power is used by equipment called actuators.
Learn more about selecting a valve actuator and the type of valve actuators available.
Pneumatic actuators: Use air pressure to provide the force required to operate the valve.
Hydraulic actuators: Use pressure from a non-compressible fluid to provide the force required to operate the valve, usually oil. Usually used in applications where the force or torque required to operate the valve, or the operating speed is very high, or in applications where the valve is in remote locations.
Electric actuators: Use electric power to provide the force or torque required to operate the valve. Usually used in applications where pneumatic or hydraulic pressure is not available or in applications where data collection on valve operation is required.
Electrohydraulic actuators: Use electrical power to drive an oil pump and operate a hydraulic actuator that provides the force or torque required to operate the valve. Usually used in applications where pneumatic or hydraulic pressure is not available and in applications where it is necessary for the valve to have a fail-safe position in case of loss of electrical power. Depending on the force or torque required, a valve can be operated using stored energy in the form of batteries. Very common in the mining and oil company.
