Direct Acting Control Valves & Reverse Acting Control Valves

The terms direct and reverse are frequently used when discussing control valves, positioners, and controllers when writing control stories for complex loops, although the definitions of Direct and reverse seem pretty straightforward.

Direct action is said to occur when the measured variable (also called the process variable PV)Increases, production increases. When the measured variable increases, the opposite action is called production decreases.

A control valve is a valve that regulates fluid flow by changing the size of the flow passage in response to a control signal. This allows for direct control of flow rate and, as a result, process variables including pressure, temperature, and liquid level. A control valve is referred to as a final control element in automatic control language.

Electrical, hydraulic, or pneumatic actuators are commonly used to open or close automatic control valves. Valve positioners are typically used with modulating valves, which can be set to any position between fully open and totally closed, to ensure the valve achieves the required degree of opening.

Air-actuated valves are often employed due to their simplicity, as they simply require compressed air, whereas electrically-operated valves require additional cabling and switchgear, and hydraulically-actuated valves necessitate high-pressure hydraulic fluid supply and return lines.

Traditionally, pneumatic control signals were based on a pressure range of 3-15psi (0.2-1.0 bar), or, more recently, an electrical signal of 4-20mA for industrial or 0-10V for HVAC systems. Electrical control now frequently incorporates a Smart communication signal superimposed on the 4-20mA control current, allowing the health and verification of the valve position to be signaled back to the control station.

Actuators are classified as either direct acting or reverse acting, with some configurations shown in the diagram below. Increased pneumatic pressure delivered to the diaphragm lifts the valve stem in a reverse acting actuator (in a normally seated valve, this opens the valve and is known as air to open).

An increase in the pneumatic pressure supplied to the diaphragm extends the valve stem in a direct-acting actuator (for a typically seated valve, this closes the valve and is referred to as air to close).

Safety factors influence which valve action is used. When the pneumatic supply fails, it may be advantageous to have the valve fail fully open. It may be c in another application.

Listed below are three cases whose combination leaves a young engineer confused.

Case I: The condition of failure of the control valve i.e. if damage to the valve occurs is defined as

When the air supply/power/signal causes the valve to open, the valve is said to have a fallopian (FO) failure action condition and if the valve is defined when there is a loss of air supply/ If the valve is closed due to power/signal then the valve is said to be with fail-closed (FC) failure condition.

Case II: Positioner of control valve which can be set by direct or reverse action.It depends on the type of valve actuator you choose. I can’t see it as a separate matter from Casie I. Once you have selected the type of actuator, you need to match the positioner action.

Case III: Controller can be set as Direct in DCS/PLC related to Control Valve Loop Or acting in reverse. I can’t see the point in this case.

Why we are talking About Controllers and Control Valves?

An automatic control valve is made up of three basic sections, each of which comes in a variety of styles and designs:

  • The valve actuator, such as a ball or butterfly valve, moves the valve’s modulating element.
  • Valve positioner – This device guarantees that the valve has opened to the proper degree. Friction and wear issues are no longer an issue.
  • The modulating element, a plug, and the globe are all included within the valve body.

In the case of an air-operated valve, there are two alternative control actions:

  • “Air or current to open” – As the control signal value increases, the flow limitation reduces.
  • “Air or current to close” – As the control signal value is increased, the flow limitation increases.

Failure of safety modes can also be possible:

  • Failure of an air or control signal to shut “- If the actuator loses pressurized air, the valve closes under spring pressure or with backup power.
  • Failure of the air or control signal to open “- If the actuator loses compressed air, the valve opens under spring pressure or with backup power.

A positioner’s primary duty is to give pressurized air to the valve actuator, allowing the valve stem or shaft to correspond to the setpoint from the control system. When a valve requires throttling, positioners are often utilized. A positioner uses position input from the valve stem or shaft to open and close the valve and supplies pneumatic pressure to the actuator.

The positioner must be attached to the control valve assembly or close to it. Positioners are divided into three types based on the type of control signal, diagnostic capability, and communication protocol: pneumatic analog, digital, and hybrid.

Direct Acting Control Valves & Reverse Acting Control Valves

A pneumatic/spring valve’s fail-safe mode is determined by the actions of both the actuator and the valve body.

A direct-acting actuator for sliding-stem valves pushes down on the stem as pressure rises, whereas a reverse-acting actuator pulls up on the stem as pressure rises.

Direct-acting sliding-stem valve bodies open up when the stem is lifted, while reverse-acting valve bodies shut off (close) when the stem is lifted.

By simply matching the right actuator and body types, a sliding-stem, pneumatically actuated control valve can be either air-to-open or air-to-close.

As indicated in this picture, the most typical combinations combine a direct-acting valve body with either a reverse – or direct-acting valve actuator:

Reverse-acting valve bodies may also be used, with opposite results:

The left-hand illustration shows a reverse-acting gate valve body that is open, with fluid flowing around the stem and the wide plug sitting well below the seat region.

Because reverse-acting valve bodies are more complicated to build than direct-acting valve bodies, they are used in fewer control valve applications.

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