Control & Instrumentation

by SKV Energy Power Plant Consultants In India

The instrumentation and control system is designed for minimum local manning and operator attention.


 The basic need of the Instrumentation is for safe operation and efficient functioning of the plant.


 This can be achieved by installing optimum number measurement, indicating, transmitting and controlling instruments for all required parameters of the plants (e.g. Pressure, Level, temperature, flow etc.) 

  • Sensor

•Also often called Primary Element.

•Acquires information about the status of the process variables.

•Typical examples: 

 RTD/thermocouples (for temperature measurements), Capacitance type cells (for liquid level /draft /pressure measurements), etc.

  • Controller

•The Brain or Heart Of the control system (the decision maker).

•It is the hardware element with Built-in capacity for performing the only task requiring some form of Intelligence.

•Typical examples: 

 Electronic controllers and digital computers are used as controllers.

  • Transmitter

•Secondary Element.

•Responsible for passing the information acquired by the sensor to the controller and sending the controller decision to the final control element.

•Measurement and control signals may be transmitted or as electrical signals.

•Typical examples:

 Electrical transmitters.

  • Final control element

•Have the task of actually implementing the control command issued by the controller on the process.

•Typical examples: 

 Control valve, variable speed motors, electric motors etc.

•The importance of these components is that they perform the three basic operations that must be present in every control system:

 1. Measurement: 

 Measuring the variable to be controlled is usually done by the combination of sensor and transmitter.

 2. Decision:

 Based on the measurements and the set point, the controller must then decide what to do to maintain the variable at its desired value.

 3. Action:

 As a result of the controller's decision, the system must then take action. This is usually accomplished by the final control element.

Basic Parameters

Measurement & Control of the following Parameters are essential for Instrumentation






Pressure measurements are one of the most common measurements required in the boiler. These range from very low -1500mmwc Draft to very high steam pressure 150 kg/cm2.That includes different types of media like steam, water, fuel oil, air, and gas, each with varying degrees of accuracy and reliability. 

•Pressure varies depending on altitude above sea level, weather pressure fronts and other conditions. The measure of Pressure is, therefore, relative, and pressure measurements are stated as either gauge or absolute. A gauge pressure device will indicate zero Pressure when bled down to atmospheric pressure (i.e., gauge pressure is referenced to atmospheric pressure). Gauge pressure is denoted by a (g) at the end of the pressure unit [e.g., kPa (g)]. 

•Absolute Pressure includes the effect of atmospheric pressure on the gauge pressure. It is denoted by an (a) at the end of the pressure unit [e.g., kPa (a)]. An absolute pressure indicator would indicate atmospheric Pressure when completely vented down to the atmosphere - it would not indicate scale zero. 

•Absolute Pressure = Gauge Pressure + Atmospheric Pressure 

•The majority of pressure measurements in a plant are gauged.

Pressure measuring devices

•The common pressure measuring devices used in IJT boilers are

•Diaphragm, capsule gauges for low-pressure measurement 

•Bourdon tube gauges for medium and high Pressure 

•Smart type Pressure Transmitter with the required range 


•Pressure Gauges are installed on-site using accessories as follows: 




• 2-way manifold

 These are protective devices for Pressure measuring instruments from surges & pulsations. These devices are used as per application requirements. 

Temperature Measurement

•Temperature is measured by the following:

•Temperature Gauge:

1. Mercury in Steel type

2. Bi-Metallic

3. Gas Filled

•Temperature Sensors:

1. RTD

2. Thermocouple

Drum Level 

Drum Level Control Method

single Element

three Element

 Shrink & Swell Effect

 Shrink: When Steam Flow Decreases    

         Drum Level Decreases 

 Swell :  

When Steam Flow Increases

         Drum Level Increases

Basic Elements

drum Level 

steam Flow

Fred Water Flow


•Transmitters are generally used of 2 wire type. This is the most widely used method for transmitter connections. There are three basic elements in this loop, namely Power Supply, the transmitter and the receiving instrument. They are connected in series, and the transmitter acts as a current regulator in the series circuit. The current in the series circuit changes with respect to changes in process parameters. This simplifies cabling and reduces erection and cable costs.

Control Valve


 Being the Final Control Element in a system is not an easy job. 

 To start with, Control Valves are blamed for any and all problems that crop up in the process. 

 Control valves are subjected to corrosion, high velocity, cavitations, flashing liquids, cryogenic temperatures, abrasion, high temperatures and thermal shock. 

 Control Valve are expected not only to throttle along through all this but most likely; you are also being asked to act as a block valve and shut off tight.

Boiler spare parts 

 Control Valve 

 Leakage Classifications

•Class I: 

 Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made.


•Class II:

 Intended for double-port or balanced single-port valves with a metal piston ring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the test fluid. Allowable leakage is 0.5% of the rated full open capacity.


•Class III:

 Intended for the same types of valves as in Class II. 

 Allowable leakage is limited to 0.1% of rated valve capacity.


•Class IV:

 Intended for single-port and balanced single-port valves with extra-tight piston seals and metal-to-metal seats. The leakage rate is limited to 0.01% of rated valve capacity.

••Class V:

 Intended for the same types of valves as Class IV.

 The test fluid is water at 100 psig or operating Pressure. Leakage allowed is limited to 5 X 10 ml per minute per inch of orifice diameter per psi differential.


•Class VI:

 Intended for resilient-seating valves.

 The test fluid is air or nitrogen. Pressure is the lesser of 50 psig or operating Pressure. 

 The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches.

troubleshooting of boiler

Basic Control Loops in BOILER

Pressure Control

  •  W header pressure
  • Furnace Draft
  • Combustion Control (Main Stem Pressure)
  • Soot Blower
  • De-aerator

Level control

  • Drum Level
  • Single Element
  • Three Element
  • De-aerator
  • CBD

Temperature Control

Super Heated Steam

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About SKV Energy Freshman     Power Plant Consultants In India

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Joined APSense since, October 25th, 2021, From Noida, India.

Created on Jul 26th 2022 00:12. Viewed 159 times.


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