The gate keepers of any industrial process fluid flow, these devices are responsible for or aid in the control of fluid flow. They are mechanical devices that can be interacted with directly or indirectly. Directly the operator can physically/manually operate the valve to open, close or throttle the fluid flow passing within. Indirectly the valve can be actuated by pneumatic or hydraulic means or by the use of electric servo or stepper motors remotely.
Typically many new automated industrial plants tend to use high amounts of automation to remotely control process equipment, in this case valves, centrally.
Valves typically have 3 basic components:
The Actuator which is responsible for positioning the throttling element within the valve body.
The valve body which houses all the components that facilitate the throttling and plugging of the fluid flow. It is through this body that fluid flow occurs.
Other accessories that are not needed for the valve to function but provide useful feedback or allow for greater nuance in the control of the device.
Valve control mechanisms are either linear or rotary, linear meaning that the throttling element moves in a linear fashion, while in rotary the element rotates to achieve throttling.
Gate, diaphragm and globe valves are all linear in nature, most suitable for high pressure applications, and can throttle small flow rates. They tend to offer fairly unrestricted flow through the valve body, and as such low pressure drops across the component, as with gate valves.
Diaphragm valves have a diaphragm upon or to which a force is placed and this stretches the diaphragm into closing the valve. Can be used for fluid flows with small solids and have diaphragms that can withstand up to 50psi of pressure. In most cases the pressure drop across the valve is negligible, however these valves may have a weir that the diaphragm rests on when closed that when open may cause a pressure drop when open.
Globe valves are the most common types of gate valves due to their characteristic globular valve body shape. This is not a positive means of identification though the internal components should be observed and noted before such a determination is made. This type of valve is used extensively in industrial settings for flow regulation.
Rotary valves tend to have a higher capacity and the ability to handle slurries and abrasives. Chief among these types valves is the butterfly valve, which is widely used in throttling systems due to its quick acting nature, high recovery rate and lightweight. This type of valve does leave the seat exposed to fluid flow, as such these seals are mounted mechanically and can be replaced.
Ball valves use a ball mechanism with a hole through it to allow or deny fluid flow. Handle operated it only requires a 90deg. rotation to fully open or close the valve. When closed the axis of the hole in the ball is perpendicular to the fluid stream, and parallel when open. The are used in high temperature instances and for drain and dump systems.
Plug valves like ball require only 90deg. to fully open or close. It functions much like the ball, however it does not have a ball within the valve body but a plug cylindrical or tapered, with a hole through it. Can be used for general service.
There are also specialty valves that ensure fluid flow in only one direction, they are reflux, check or non-return valves. They can be as simple a mechanisms as a flap or spring loaded flapper that will only be actuated by or at a certain pressure. These are commonly seen on the outlet side of pumps. They are open when pumping but closed once pumping is done.
Relief valves are usually automatic and function to relieve either pressure and or fluid above a certain threshold. They function with out external input, being triggered by some characteristic of a process, typically pressure, to open the valve and then resets itself so long as the characteristic returns to or falls below a threshold. Is many cases they are spring operated and the adjustment of this spring determines the characteristic of the value opening and closing.
