07-12-2012, 01:15 PM
Essential Boiler Instrumentation and Controls
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Abstract
Many of the low pressure, old sugar plant boilers are working negligible or no
instrumentation. The modern day high pressure boilers which are provided with
optimised heat transfer areas for its rated duty require some of the important operating
parameters to be controlled closely to maintain a steady output. Thought instrumentation
is a very vast subject to be discussed, this paper highlights a few of the important
instrumentation and control loops that are required for safe, economic and reliable
working of the boilers.
Introduction
While it is theoretically possible to operate a boiler with manual control the operator must
maintain a tedious, constant which for disturbances and variations of parameters. Time is
needed for the boiler to respond to a correction and this lead to over correction with
further upset to the boiler. An automatic controller once properly tuned will make the
proper adjustment quickly to minimise upsets and will control the system more accurately
and reliably.
Instrumentation systems are provided for the boiler to achieve the following:
1. To measure the actual values of different parameters for which the boiler is designed.
2. Safe working range of the different parameters are maintained.
3. To monitor one or more variables at a time and provide input for automatic control.
4. In case of operator failure to take remedial action for an upset condition, it protects the
boiler by alarms and trippings.
5. To provide data on operating conditions before failure of the equipment for analysing
the failure.
Special care is required to be taken while selecting the instrumentation for the boiler.
Simple auto control loops, avoiding duplication of instruments and selecting proven
instruments with the correct operating range forms the basis of the reliable
instrumentation. There are many instruments and control loops that are provided for a
boiler. Based on the feed backs collected from various operating sugar plants a few of the
important protection and controls which are essential are listed below:
Drum level high & low protection
The water level in the steam drum has to be maintained within the desired limits. The
reduction of drum level below the low level mark will cause tube failures and level above
the high level mark will cause water carry over to the turbine. Hence it is essential to
have reliable direct level gauge on both ends of the drum. A low level trip to shut down
the boiler in case the water level falls below the allowable limit and a very high level trip / alarm has been suggested for all boilers. At present it is noticed that these contacts for
the indication / alarm has been taken from the drum level controller, which is not
preferred. Due to many reasons if the controller is isolated during operation of the boiler
the protection is not available. It is always recommended that a separate level switch be
provided for eater level low and high alarm and trip.
Furnace temperature probe
The new series of the boilers require controlled start up and loading programme. Most of
the manufacturers provide a start up curve which generally gives the rate of pressure
increase in the boiler to be followed.
During start up of the boiler adequate steam flow is not established through the
superheater. Hence high rate of heat input if provided to the boiler furnace, will result in
high furnace outlet temperature of the flue gas causing over heating and tube failure. It is
advisable to control the rate of heat input into the boiler and maintain the flue gas
temperature leaving the furnace at around 540o C.
A furnace temperature probe is a high temperature thermocouple surrounded by a jacket
of a water. Cooling water can be circulate through the jacket to protect the thermocouple
from over heating. Once adequate steam flow is established through the superheater there
would not be any limitation on the rate of fuel firing.
Combustion Control System
The main object of automatic boiler combustion control is to regulate automatically, the
heat input to a boiler in terms of fuel and air supplied in relation to eat output or steam
demand. This should be implemented as efficiently as possible in terms of combustion
quality and furnace stability. Combustion air supplies are to be adjusted with change in
boiler loads. Steam pressure is taken as the indication for boiler load as the steam
pressure is taken as the indication for boiler load as the steam pressure decreases in
increasing load and vice versa. Hence variations in steam pressure is detected and the
supply of the fuel and air adjusted accordingly. (Refer Fig.6)
General
While selecting the various elements for the instrumentation care should be taken to
select the correct range. A well engineered system / configuration plays a major role in
the availability of the instruments. Staffing of the sugar plant maintenance crew with
trained and qualified instrument mechanics is essential for calibration and maintenance of
the equipment.