Basic controls for boilers

Basic Controls For BoilersMENTOR : PROFESSOR VIDITA TILVA

BY:

AASHEESH TANDON (12BIC044)

PAXAJ SHUKLA (12BIC056)

1. Introduction

How does a boiler works?

A boiler is a water containing vessel which transfers heat from afuel source (oil, gas or coal) into steam which is piped to a pointwhere it can be used to run production equipment, to sterilize, to provideheat, to steam-clean, etc.

1. Introduction

The energy given up by the steam is sufficient to convert it back into the form of water. When 100% of the steam produced is returned to be reused, the system is called a closed system.

1. Introduction

Since some processes can contaminate the steam, so it is not always desirable to feed the condensate back into the boiler. Asystem that does not return the condensate is called an open

system.

2. Basic type of boilers

The two main types of boilers are:

• Firetube

Fire or hot gases are directed through the inside of tubes within the boiler shell which are surrounded by water.

• Watertube

Fire or hot gases are directed to and around the outside of tubes containing water, arranged in a vertical position.

3. Why need boiler controls?

1. Increase uptime and availability

2. Reduce flue gas emissions

3. Maintain boiler safety

4. Control operating costs

4. Combustion control for boilers

A combustion control system is broken down into :

(a) fuel control

(b) combustion air control

4. Combustion control for boilers

There are three general types of combustion control schemes used today:

(a) series,

(b) parallel, &

(c) series-parallel controls.

4(A). Hardware Used In Combustion Control

(a) ON/OFF controls

(b) Positioning systems

(c) Metering control system

4.(B) Combustion Control Methods

Burner combustion control generally includes one or a combination of the following methods:-

Excess air regulation

Oxygen trim

Burner modulation

Air/Fuel cross-limiting

Total heat control

5. Feedwater Control For Boilers

Feedwater control is the regulation of water to the boiler drum.

Risks :

If the level is too high, may interfere with the internal operation of internal devices in the boiler drum, such as - flooding of steam purification equipment can occur.

If the level is too low, reduction in efficiency of the treatment and recirculation function, undesired pressure build up, over heating of boiler coils/tubes.

5.(A) Components Affecting Drum Water Level

steam bubbles

Drum pressure

‘swell‘ : surge in water level as a result of the drum pressure decreasing

'shrink‘ : decrease in water level as result of drum pressure increasing

5.(B) Single And Two Element Control Systems

Single Element :

controls feed water flow based on drum level

Response is very slow

Two Element :

uses steam flow changes as a feed forward signal

flow and level transmitters are summed by a computing relay

response is faster

5.(C) Three Element Control

Used when :

unstable feedwater system exists

if large unpredictable steam demands are frequent

5. Application Chart For Drum Level Control System

5.(C) Five And Seven Element Control

Five element control is better than Three Element control in terms of :

steam flow measurement is temperature compensated, &

drum level measurement is pressure compensated.

Seven Element control is better than Five Element control in terms of :

Transmitters for blow down flow

Transmitters for soot blower flow

5. Feedwater Control for Boilers

Advantages :

1. Multiple element feedwater control can help:

i. Faster response of systems.

ii. More accurate control.

iii. Maximum system stability.

2. Metering control system maintains combustion efficiency over wide load changes and over long period of time

3. Parallel combustion control can be used in any size of boilers

5. Feedwater Control for Boilers

Disadvantages :

1. Boilers require quick responding controls.

2. Level of the water in the boiler must be kept above 50% of height.

6. Other Control Functions

A. Furnace air and draft control

B. Steam temperature control

C. Steam pressure and load control

THE END