Piping and Instrumentation Diagrams Tutorials on Flow and Level Control

In this article, we will study Flow and Level control through P&ID Diagrams Generally, we all know about the terms P&ID, Flow, and Level.

In most process industries and in power plants the flow and level control of the Fluid, Steam, or Gas is very important.

A control system is employed to control the flow and level of the fluid in the Container or tank.

Identification of Pipe Line Number in P&ID

The Flow and level control system is described as shown below.

  1. The flow control is based upon flow measurement through an orifice plate or orifice meter, the fluid flow is controlled by the flow control valve.

  2. The flow can be controlled automatically through a level control loop by adjusting the set point in the flow controller as required.

  3. The flow controller compares the actual tank level with a given set point and commands the control valve to regulate the flow in proportion to it.

  4. If the tank level is raised above the limit, then the set point must be lowered. Or if the level reduces below the limit, then the set point must be increased.

  5. The orifice meter measures the flow and the flow transmitter transmits the measured value as an electrical signal (4-20mA) to a controller in a central control room.

  6. The secondary device called the flow indicator indicates the flow rate on the controller.

  7. The electronic controller will send an electrical signal (4-20mA) to an I-P converter.

  8. An I-P converter will send the proportional pneumatic signal to the control valve actuator.

  9. The tank level is measured through a level transmitter with a local indication on the transmitter and the same value is displayed at the level controller in the control room.

  10. The level control instrumentation in the control room allows the operator to activate High-Level Alarms (LAH) and Low-Level Alarms (LAL) with respect to levels high or low.

  11. The container must have gauge glass local sight glass for local indication of existing tank level.

  12. The level controller sends the level signal to the flow controller to adjust the set point for the flow control valve.

Piping & Instrumentation Diagrams Tutorials on Pressure Control

Developing the Piping and Instrumentation Diagram

To develop the P&ID for flow and level control, we must be familiar with each and every standard set of instrument symbols used for representing P&ID.

The project or instrumentation engineer must have enough knowledge about various instruments and their control functions encountered in most instrumentation systems.

To develop a Piping & Instrumentation Diagram the following points must be noted.

  1. An empty bubble without any solid line indicates the instrument is field mounted.

  2. The bubble with a solid line indicates that the instrument is control room mounted.

  3. The first alphabet of tag numbers represents the process variable.

  4. The second and third alphabet followed by the first alphabet of the tag number represents the functions performed by that instrument for a particular process variable.

  5. The number followed by the alphabet represents the logical numerator.

Development of P & ID for Flow and Level control.

Refer to the P&ID diagram shown below.

  1. A Flow Control Valve FCV 100 is required to regulate and control fluid flow.

  2. The set point is adjusted at Flow Indicator and Controller FIC 100 by a level control loop.

  3. The loop number for the flow Control Loop and Level Control Loop is 100 and 101.

  4. The set point of the Flow Indicator and Controller FIC 100 can be adjusted by Level transmitter LT 101 and Level Indicator and Controller LIC 101 make up the level control loop.

  5. The Flow Transmitter built in conjunction with the orifice plate transmits the measured electrical 4-20mA signal to the controller. It also has a digital indicator for the local display of the measured value in the plant.

  6. The controller in the control room also has an indicator function for displaying the flow rate.

  7. The Flow Controller FIC 100 sends a 4 - 20 mA electrical signal to the Current to Pneumatic (I-P) converter.

  8. The Current to the Pneumatic converter will send the proportional pneumatic signal to the valve actuator to regulate the flow through Flow Control Valve FV 100.

  9. For level measurement, the tank or large container must have Level Indicator and Transmitter LIT 101 with a local indicating function at the field side.

  10. This Level Indicator and Transmitter LIT 101 sends a 4 - 20 mA electrical signal to the level controller located in the control room.

  11. The Level Indicator and Controller LIC 101 compare the measured value (tank level) with the set point and send the control signal to Flow Control Valve to control the flow.

  12. The level instrumentation system is built with alarm facilities Level Alarm High (LAH 101), and Level Alarm Low (LAL 101) to indicate the high levels and low levels of the tank.

Let us understand some basic concepts

What is Flow Control?

  • Flow control is any attempt to alter and handle the action of a fluid flow in a controlled manner to improve the system’s performance and efficiency.

  • Flow control is the management of fluid flow between two or more tanks or containers. The discharge may be through gravity or through some external sources like electric pumps.

  • The term flow control is a mechanism to regulate the flow and pressure of the fluid in a closed pipeline.

What is a level control valve?

A level control valve is also known as an altitude control valve. These valves will respond to changes in the height of fluid in a large storage cell. Considering a toilet flush as an example this consists of a set of ball cock, the flush cycle is actuated automatically by emptying or filling the tank

Why is level control important?

The level measurement determines the actual position of the fluid level relative to the top or bottom of the storage tank or silo. Level control is essential to assure the safety of the industrial process, maintain efficiency, and increase profitability

How does a level control system work?

The level control system uses a displacer connected with the controller to sense the level of liquid inside the tank. The displacer is fixed in the process fluid. When the fluid level increases the displacer rises to the maximum set point. The level controller will send a pneumatic signal to the control valve to open.

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