Control valves are constructed using various combinations of the valve actuator and valve body.
The failure modes in the control valve actuator is achieved by combining direct acting or reverse acting actuator and direct acting or reverse acting valve body.
The selection of valve actuator and valve body is done in such a way to protect a particular valve failure mode for any reason of the instrument air supply failure.
A diaphragm type actuator is a commonly used actuator to build a control valve for all industrial applications.
In the case of valve bodies, the diaphragm actuators are now categorized as direct acting and reverse acting.
What is a control valve?
The control valve is a valve installed in the fluid line to control the rate of fluid flow by varying the size of the flow passage in response to the control signal received by the actuator. The actuator in the control valve changes the position of the valve plug or disk.
The control valve allows direct control of the flow rate in the line. In automatic control language, a control valve is referred to as a final control element.
What is the valve body in the control valve?
The valve body is referred to as a shell. It is the outermost casing of the control valve.
What is the actuator in the control valve?
It is a device that produces a motion by converting energy and signals going into the system and the motion produced may be either rotary or linear.
The actuator in the control valve is an accessory installed with the control valve. The actuator provides power for the control valve operation the power may be pneumatic, hydraulic, or electric.
But the pneumatically operated control valve actuators are the most popular type of actuator used in industrial applications due to their low price and simplicity in construction compared to electric and hydraulic. In demanding service conditions, the pneumatically operated piston actuators provide high efficiency and high stem force output.
What are the modes of control valve failure?
The control valve failure scenario can be addressed by two modes of valve operation, one is Air to Close valve (ATC) and the other is Air to Open valve (ATO).
1. Air to Close Control valve (ATC)
An Air to Close (ATC) valve or fail open valve is normally an open type valve, it is built by combining a reverse acting actuator and a reverse acting valve body or a direct acting actuator and a direct acting valve body.
2. Air to Open Control valve (ATO)
An Air to Open (ATO) valve or fail close is normally a close type valve, it is built by combining a direct acting actuator and reverse acting valve body or reverse acting actuator and direct acting valve body.
Control valve failure modes and a combination of valve bodies and actuators:
The action of an actuator can easily be determined by the supply of air in the upper half or lower half of the valve housing.
The identification of direct acting or reverse acting valve body is not easily identifiable.
But with reference to the nameplate or flow sheet, it is easy to identify the action of the valve body such as reverse acting or direct acting.
The possible combinations of the valve body and valve actuators with their action and failure mode are shown in the below table.
|Valve Actuator||Valve Body||Valve Action||Failure Mode|
|Direct||Direct||Air to Close||Fail Open|
|Reverse||Reverse||Air to Close||Fail Open|
|Direct||Reverse||Air to Open||Fail Closed|
|Reverse||Direct||Air to Open||Fail Closed|
How do we classify the control valve actuators?
Here the control valve actuators are classified based on the mode of acting as direct acting and reverse acting.
During the discussion of control valves, the positioners, and actuators are employed in building a complex loop for a particular control network here the terms direct acting and reverse acting are considered.
Reverse Acting Control Valves:
For a normally seated valve when the pneumatic pressure applied to the diaphragm is increased, it lifts the valve stem and will open the valve which is referred to as Air to Open.
Direct Acting Control Valves:
For a normally seated valve when the pneumatic pressure applied to the diaphragm is increased, it extends the valve stem and will close the valve which is referred to as Air to Close.
Considering the safety issues the valve action can be dictated as
- When the pneumatic supply fails the control valve is desired to have the valve fail fully open.
- In another application, it may be considered better if the valve fails fully shut.
The failsafe mode of a pneumatic spring valve is a function of both the actuators and the valve body.
Sliding stem valve bodies are categorized into two groups one is direct acting and the other is reverse acting
- In the case of direct acting, the valve gets opened when the stem is lifted.
- For reverse acting the valve gets closed (shut off) when the stem is lifted.
In the case of sliding stem valves,
- When the pressure rises the direct acting actuator pushes the valve stem in the downward direction.
- When the pressure rises the reverse acting actuator pulls the valve stem in an upward direction
For sliding stem valves, the pneumatically actuated control valve can be made air to open or air to close by selecting the particular body type and actuator.
In most applications, the direct acting valve body is combined with reverse acting or direct acting valve actuators.
The construction of the reverse acting valve bodies is more complex than direct acting valve bodies,
Now let us see how these valves operate in the process industry for a particular application considering the hot well level control in the power plant:
- Recirculation valve called “Air to Open”:
This valve operates at a range of 2.0 kg/cm2 to 2.8 kg/cm2 this valve remains fully closed from 0.2 kg/cm2 to 2.0 kg/cm2 of air Pressure. This valve opens as air pressure rises beyond 2.0 kg/cm2. It gets fully opened as the air pressure increases from 2.0 kg/cm2 to 2.8 kg/cm2.
If the hot-well level drops below the setpoint SP > PV, then the P-I controller sends the proportional signal to the recirculation valve called the “Air to Open” to get open, and the water from the deaerator will flow back to maintain the hot well level.
- Main Control valve called “Air to Close”:
This valve operates at a range of 0.2 kg/cm2 to 1.0 kg/cm2 of air pressure, this valve remains fully open and starts closing as air pressure starts rising above 0.2 kg/cm2 and as the air pressure drops to 1.0 kg/cm2 now the valve starts closing and restricts the fluid flow.
If the hot well level rises above the setpoint SP < PV, then the P-I controller sends the proportional signal to the main control “Air to Close”: valve to get open and the excess water will flow to the deaerator and desired level in the hot well is maintained.