Boiler Fuel Feed Simulation for Bunker C

sjknight

Member
Join Date
Nov 2005
Location
St. John's
Posts
7
I am doing a tech project at school and our topic is a boiler fuel feed simulation for the transportation of bunker C from a storage facility to the boiler. Has anyone ever worked in a plant where they have implemented one of these systems? I would like to know about the heat tracing, level control and temperature control. I am hoping to use a PID controller for the level and flow. Any advice would be greatly appreciated.
 
Stationary boilers frequently use multiple PID loops for controi.
Level control is known as drum level, flow control of fuel is usually done with fuel/air ratio control, timmed by excess oxygen on the bigger boilers.

Drum Level Control.
The steam drum is an integral part of a boiler.
The vessel’s primary function is to
provide a surface area and volume near the top of the
boiler where separation of steam from water can
occur. It also provides a location for chemical water
treatment, addition of feedwater, recirculation water,
and blowdown.

Blowdown removes residue and maintains a specified
impurity level to reduce scale formation. Because
these functions involve the continual addition and loss
of material, the water-steam interface level is
critical.Low level affects the recirculation of water to
the boiler tubes and reduces the water treatment
effectiveness. High level reduces the surface area
and can lead to water and dissolved solids entering
the steam distribution system.

The objective of the drum level control system is to
maintain the water-steam interface at the specified
level and provide a continuous mass balance by
Types of Drum Level Control. There are 3 basic
types of drum level control systems: single element,
two element and three element. Their application
depends upon specific boiler size and load changes.

The Single Element System is the simplest
approach. It measures level and regulates feedwater
flow to maintain the level. This is only effective for
smaller boilers supplying steady processes which
have slow and moderate load changes.

The Two Element System uses two variables, drum
level and steam flow to manipulate the feedwater
flow. Steam flow load changes are fed forward to the
feedwater valve providing an initial correction for load
changes. The steam flow range and feedwater flow
range are matched so that a one pound change in
steam flow results in a one pound change in
feedwater flow. This system is adequate for load
changes of moderate speed and magnitude and can
be applied to any size boiler. It does have two
drawbacks. It cannot adjust for pressure or load
disturbances in the feedwater system and cannot
adjust for phasing interaction in the process because
only the relatively slow responding drum level is
controlled.

The Three Element System adds a third variable,
feedwater flow rate to manipulate the feedwater
control valve. This system provides close control
during transient conditions because the two
controllers minimize phasing interaction present in the
two element approach. The feedwater control assures
an immediate correction for feedwater disturbances.
The drum level control provides trimming action. This
system can handle large and rapid load changes and
feedwater disturbances regardless of boiler capacity.
This approach is required on multiple boilers having a
common feedwater supply. It is ideal for plants with
both batch and continuous processes where sudden
and unpredictable steam demand changes are
common.

Fuel-Air Ratio Control..
A fuel-air metering control system is essential
for efficient combustion in boilers.

There are 3 basic types of Fuel-Air Ratio
control systems: series metered system, parallel
metered system, and cross limiting system.

The Series Metered System is fairly common where
load changes are not large or common. Both fuel and
air are metered. The steam pressure controller
regulates the fuel flow which is measured, linearized
and then is ratioed and used as the remote set point
to the air flow controller. This positions an air damper
to maintain the specified ratio between the fuel and
air.

This system is adequate for near steady state
conditions. However lags in response to load changes
can result in temporary smoking, incomplete
combustion and fuel-rich conditions.

The Parallel Metered System operates the fuel and
air control controllers in parallel from a setpoint
generated by the steam pressure controller. The
setpoint signal is ratioed before being used as the
setpoint to the air controller to establish the fuel-air
proportions. This system relies on similar responses
from both controllers to prevent improper fuel-air
mixtures. This system is best applied to processes
which experience relatively slow load changes.

The Cross Limiting System is used when large or
frequent load changes are expected.This is a
dynamic system which helps compensate for the
different speed of response of the fuel valve and air
damper. It prevents a “fuel-rich” condition and
minimizes smoking and air pollution from the stack.

The system is also known as a lead-lag system.
When demand increases, a low selector blocks then
increase forcing the air flow signal to become the
setpoint to the fuel flow controller. A high selector
passes the increase to the airflow controller’s
setpoint. This means fuel flow can not increase until
air flow has begun to increase, i.e. air increase leads
fuel increase.

When demand decreases, the low selector passes
the signal to the fuel flow controller setpoint: while the
high selector passes the fuel flow signal to the air flow
controller setpoint. This means flow can not decrease
until the fuel flow begins to drop hence air decrease
lags fuel decrease. This means a fuel rich condition
is avoided, regardless of the direction of load change.
Oxygen Trim. Automatic oxygen trim of the fuel-air
ratio is used to reduce excess air and thereby excess
oxygen to nearly stoichiometric combustion efficiency.
Too much air results in energy lost up the stack.
Insufficient air results in loss of heat generation and
increased pollution due to incomplete fuel
combustion.

A certain amount of excess air is required to insure
that complete combustion occurs within the
combustion chamber and to compensate for delays in
fuel-air ratio control action during load changes.
In addition to improved efficiency lower excess air
helps reduce corrosion and air pollution by minimizing
formation undesired gases.
 

Similar Topics

Hello, I have problem i'm working on boiler plc, but i get to the problem that i can't solve myself. I have problem with material gate. I need to...
Replies
2
Views
335
HI All : We have a boiler that can produce up to 75000 lb/hour, this boiler has two CompactLogix Plcs from Allen Bradley , one is used for burner...
Replies
10
Views
3,854
hi everyone im using OPERATOR PANEL SPOT PLUS 150 35W 24VDC AUTOMATA ID42003 (as hmi system) and automata optispark controller to control and...
Replies
8
Views
2,394
All, My client is looking to have me calculate the boiler feed water differential pressure in kPa to and actual flowrate in m3/hr. Once the...
Replies
2
Views
1,482
Good day Gents, Does atmospheric pressure have an influence of a boiler steam pressure.
Replies
31
Views
8,470
Back
Top Bottom