What are the basic functions and types of valves?

How are Valves Classified?

There are five main valve functions in industrial fluid systems that you should know: on/off, flow control, directional flow, overpressure protection, and excess flow control.
On/Off. This is the most basic type of valve function. On/off valves start or stop the flow of fluid. Despite the basic nature of the valve function, there are many different types of on/off valves to choose from.

Ball valves:

Ball valves provide reliable shutoff by using a handle to rotate a ball within the valve to block or allow the flow of fluid through the valve.

Plug valves:

Plug valves work similarly, minus the ball. They are operated using a handle that spins a plug within the valve to either block or allow fluid flow.

Bellows valves:

Bellows valves feature a taller cylindrical stem with an expanding and contracting bellows used to restrict flow to precise levels as a handle is spun, pressing an actuator onto the bellows.

Diaphragm valves:

Diaphragm valves operate similarly to bellows valves, but instead of an actuator constricting a bellows in the valve stem, the actuator presses down on a flexible diaphragm to restrict flow.
Flow control. These types of valves help adjust the flow through a system, providing functionality from simple flow regulation to fine metering. Needle valves are most commonly used to control or throttle flow. These valves are controlled by turning a handle to finely control an actuator inside the valve stem that ends in a needle that blocks flow through the valve as it is lowered.

Directional flow:

This valve function ensures that flow is headed in the desired direction, and these types of valves can change the direction of flow in the system. Check valves and multi-port ball valves are commonly used for directional control. Check valves prevent backflow, ensuring fluid continues flowing in only one direction. Mult-port ball valves can provide on-off functionality like other ball valves, but allow an operator to direct fluid flow in different directions based on the direction in which a handle is turned.

Overpressure protection:

This valve function prevents system pressure buildup beyond a prescribed limit. Relief valves and rupture discs are the most common types of valves used here. Relief devices are highly important in any system that operates under pressure, acting as a last line of defense against overpressure conditions that could lead to a blowout. Relief valves can be calibrated to open automatically at a predetermined pressure level or gradually as pressure increases. Bleed valvesOverpressure protection valve symbol and purge valves are also used to bleed, vent, or drain fluid to allow safe maintenance or operation of fluid systems

Excess flow control:

Excess flow valves can also have a role in system safety, helping contain uncontrolled release of system media by activating when flow through the valve increases to a

predetermined value:

Should an excess flow condition occur downstream, the valve’s poppet rapidly moves to the tripped or fully forward position, stopping most of the uncontrolled release of system media to minimize fluid escape. Excess flow valves then reset when pressure equalizes.