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How a Pneumatic Displacer level sensor is used to Control Liquid Level

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Although we have covered the principle of operation of the displacer level sensor before, here we attempt to introduce you how a pneumatic displacer level sensor is applied in liquid level control in a process plant. The anatomy of a typical pneumatic displacer level sensor is exemplified by the fisher 25000 controller/displacer level sensor assembly:
Fisher 2500 Pneumatic Level Sensor. Photo Credit : Fisher

Principle of Operation of the Displacer Level Sensor/Controller 

Liquid Level Controller Using a Displacer Level Sensor

As shown above, changes in the level of liquid in the vessel whose level is being controlled exerts a buoyant force on a displacer which causes the rotation of a torque tube shaft. The rotation of the torque tube shaft is converted into a proportional pneumatic output signal by the pneumatic controller attached to the displacer level sensor. Typically, this pneumatic output is 3 – 15psig. The output signal from the controller drives a dump valve open to evacuate liquid from the vessel. When liquid level rises, the buoyant force on the displacer increases leading to increasing output from the controller if it is a direct acting controller. When the liquid level falls, the buoyant force on the displacer decreases resulting in decreasing output to the controller. If the controller is set at 25% of vessel level for example, then as soon as liquid in the vessel reaches 25%, the controller outputs a signal to completely open the dump control valve to release the liquid to a dump or safe area in the case of a hazardous liquid e.g hydrocarbon condensate.

How the Foxboro 43AP Pneumatic Controller Works

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The Foxboro 43AP pneumatic controller is a versatile process instrument controller that can be used to control pressure, temperature , flow and level. As with all process controllers, the Foxboro 43AP pneumatic controller continuously detects the difference between a process measurement and its set point, and produces an output air signal that is a function of this difference and the type of control. 
Controller Loop for a Pneumatic Controller
The output signal is transmitted to a control valve or other control device. The process measurement, set point, and output signal are indicated on the controller.

Foxboro Pneumatic Controller with Proportional, Reset and Derivative Actions and Automatic Manual Transfer System. Photo Credit: Foxboro
Principe of Operation of the 43AP Pneumatic Controller
The above schematic shows the basic internal parts of the mechanism of operation of a Foxboro 43AP pneumatic controller. The principle of operation of the device is explained below:

  1. A differential linkage measures difference between measurement pointer and setting index         positions as shown in the schematic above. This error signal moves proportioning lever.
  2. The proportioning lever pivots at its center on the end of a flat spring.
  3. This motion of the proportioning lever changes flapper nozzle relationship, causing relay to        establish an output pressure.
  4. This output pressure is fed back to the proportioning bellows, which acts through the                  proportioning lever to re-balance flapper nozzle.
As shown above, this particular controller model has reset bellows as well as a derivative tank. The reset bellows and tank assembly are used when measurement must be maintained exactly at control point -that is without “offset”. The derivative tank assembly is used to improve system response to a slow process.

Pressure Drop Regimes Across a Control Valve

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Control valves are critical elements in industrial process control. They are used for controlling various types of fluid. However as fluid passes across a control valve, the pressure regimes across the valve especially pressure drop changes with flow. A basic understanding of the pressure regimes across a control valve will help in the valve sizing process.

To accurately size a control valve, we must correctly predict the pressure drop across the valve from minimum flow to normal to maximum flow.

Typically as flow increases across a valve, the pressure drop across the valve reduces until it gets to the minimum allowable pressure drop across the valve at maximum fluid flow. As flow decreases, the pressure drop across the valve increases commensurately. These variations are illustrated in the diagram below:

As shown in the diagram above:
1. As flow increases across the valve , upstream pressure P1 drops
2. The pressure drop across the valve, ∆P, decreases as flow increases
3. At zero flow, ∆P is maximum and the downstream pressure P2 = 0
4. As flow increases, downstream pressure, P2 increases.
5. At maximum flow across the valve, ∆P is minimum