1) definition and classification by function

Engineering - Valves · 05 Dec 2018

1] Valves: Mechanical devices specifically designed to direct, start, stop, mix, or regulate the flow, pressure, or temperature of a working fluid. Here is a type of valves - ball valves. ¹

[Fig 1. Brass ball valve with a red handle]

2] Classification for the functions

On-Off Valves	Nonreturn Valves	Throttling Valves
Gate valve	All of the check valves	acts as on-off valves
Plug valve		Pressure regulator
Ball valve		Automatic control valves
Pressure-relief valve
Tank-bottom valve

[Table 1. Classification for the functions of each valve]

On-off valve
- Used to start or stop the flow of the medium through the process.
- A majority of them are operated by hands. [like Fig. 1.]
  - Some of them are automated with the addition of actuator.
- Pressure-relief valve: self-actuated when a preset pressure is surpassed.
  - Direct-loaded pressure relief valves²
  - Piloted pressure relief valves
  - Relief valve - used for liquid
  - Safety valve - used for gas
Nonreturn Valves
- Allow the fluid to flow only in the desired direction.
  - All check valves are non-return valves.
    - Lift check
    - Swing check
    - Tilting disc
    - Diaphragm
[Fig 2. Check valve]
- Used to prevent backflow of fluid - which can cause the damage to equipment
  - Pump(in liquid application)
  - Compressor(in gas application)
  - Process systems that have varying pressures, which must be kept separate.
Throttling Valves
- Used to regulate the flow, temperature, or pressure of the service.
- Can move to any position within the stroke of the valve or hold that position
  - Including the full-open or full-closed positions.
  - So they can act as on-off valve also. [Superset of On-off valve]
- Commonly hand-operated / some are equipped with actuators or actuation systems.
  - Actuators provide greater thrust and positioning capability, as well as automatic control.
- Pressure regulators - vary the valve’s position to maintain constant pressure downstream.
- (Automatic) control valve: commonly used to describe valves that are capable of varying flow conditions to match the process requirements.
  - Equipped with the actuators to achieve automatic control.
Final Control Elements within a Control Loop
- Control valve: the most commonly used final control element
- Needed to provide the power and accuracy to control the flowing medium to the desired service conditions.
- Part of the control loop - applied to the feedback control

Valve Handbook, 2nd ed., p.1. ↩
Valve Selection Handbook, 5th ed., p.2 ↩