24-04-2012, 12:51 PM
boiler control system
Boiler Instrumentation and Controls.pdf (Size: 256.46 KB / Downloads: 57)
INTRODUCTION
Instrumentation and controls in a boiler plant encompass an
enormous range of equipment from simple industrial plant to the complex
in the large utility station.
The boiler control system is the means by which the balance of
energy & mass into and out of the boiler are achieved. Inputs are fuel,
combustion air, atomizing air or steam &feed water. Of these, fuel is the
major energy input. Combustion air is the major mass input, outputs are
steam, flue gas, blowdown, radiation & soot blowing.
CONTROL LOOPS
Boiler control systems contain several variable with interaction
occurring among the control loops for fuel, combustion air, & feedwater .
The overall system generally can be treated as a series of basic control
loops connected together. for safety purposes, fuel addition should be
limited by the amount of combustion air and it may need minimum limiting for flame stability.
Combustion controls
Amounts of fuel and air must be carefully regulated to keep excess
air within close tolerances-especially over the loads. This is critical to
efficient boiler operation no matter what the unit size, type of fuel fired or
control system used.
Feedwater control
Industrial boilers are subject to wide load variations and require
quick responding control to maintain constant drum level. Multiple element
feed water control can help faster and more accurate control response.
DIFFERENT TYPES OF COMBUSTION CONTROLS
There are three general types of combustion control schemes used
today: They are series, parallel & series-parallel controls.
In series control, variations in steam header pressure(the master
control signal) cause a change in combustion air flow which in turn results
in a sequential change in fuel flow. This type of control is limited to small
boilers having relatively constant steam load & burning fuel.
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.
DISADVANTAGES
1. Boilers require quick responding controls.
2. Level of the water in the boiler must be kept above 50% of height.
CONCLUSIONS
The various goals of boiler control includes:
1. To minimize excess air
2. To minimize blowdown
3. To minimize steam pressure
4. To measure efficiency
5. To find when to perform maintenance