Automatic Control Valves

Pressure_Management_Valve_Type3

An automatic control valve is a regulating valve for the flow or the pressure (or both) of a fluid in a pipeline system. Automatic control valves consist of a main valve (usually in a globe valve design) and a self-actuated pilot valve. The self-actuated pilot valve controls the opening and closing of the main valve in response to the pressure condition in the pilot valve piping system.

Automatic control valves do not need an external power source as the fluid’s pressure is sufficient for operating the control valve via the pilot valve. The pilot valve represents a spring-loaded pressure relief valve that controls the main valve by the fluid being discharged. The pilot valve can be replaced by other control devices such as solenoid valve, electric actuators, pneumatic actuators or hydraulic actuators.

Some of the purposes for using an automatic control valves is to protect pipes and pumps from pressure surges, prevent transients, dissipate excess energy, mitigate the risk of cavitation and perform other crucial functions. It is a very important component of any hydraulic system. Automatic control valves usually respond to signals generated by independent instruments such as flow meters, temperature sensors or pressure gauges or other devices.

Many types of automatic control valves have been produced over the years across different industries. The design and operational features of automatic control valves may be summarized as follows:

Operating Functions Media to be Controlled (liquid / gas / steam / mixture)Pressure Drop
  • Control (modulating)
  • On/Off (in some cases)
  • Density
  • Temperature
  • Chemical composition (if corrosive)
  • Pressure drop across the valve
  • Operating pressure in the system
FlowValve Size 
  • Minimum flow to be controlled
  • Normal operating flow
  • Maximum flow
  • Nominal pipe size
  • Pressure class
  • End connections type
 

The design of an automatic control valve depends on these specific requirements.

Pressure_Relief_Sustaining Valve 2

Pressure Sustaining (Retaining) & Relief Valve (PSV)

The term control valve refers more to the function than the style of the valve. It can refer to any valve that serves to regulate the steady-state flow or pressure in a system. Of more interest are control valves used to control the flow without creating undesirable transients, excess cavitation, or head loss and able to function under all expected flow conditions.

The design of a particular control valve depends upon the specific system requirements and the need to provide a suitable solution. Control valves can be grouped into two main categories:

Valves which control the flowing volume by a reciprocating linear motion of a valve plug attached to a stem:

  • Globe valves (e.g. hydraulic-automatic diaphragm control valves)
  • Cage trim valves (e.g. plunger or needle valves)

Valves which control the flowing volume by means of rotating disk or sphere:

  • Butterfly valves
  • Plug valves
  • Ball valves