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Burner Management System Overview
The Burner Management System designed by Pegler Automation, Inc. is a PLC (Programmable Logic Controller) based system
which replaces the relays of the past. All Pegler Automation, Inc. Burner Management Systems are designed to meet or exceed the requirements of the Code of Federal Regulations (46 CFR 62 – Vital Automation), American Bureau of Shipping Part 4 section 11 and NFPA 85.
The Burner Management system monitors the presence or absence of the forced draft fan running and verifies air flow is being produced, ensures the water level in the boiler drum is satisfactory, and verifies that there is at least one flame in the furnace. The absence of any of these four permissives will trip the boiler. Other boiler trip functions include operator trip, unsuccessful burner shutdown, or a bad flame check on the only burner in service.
The Burner Management alarm system provides for a “first out” cause of trip allowing operators to readily identify the reason for an unexpected boiler trip condition.
The Burner Management System controls the air registers, igniters and fuel oil valves for each burner, as well as monitoring of the flame signals for each burner in service. If the flame quality degrades to 60% of maximum signal strength a poor flame alarm is issued to the alarm system. When a flame quality signal is below 40% for 4 seconds the burner will trip. The Burner Management System lights off the initial burner and cuts out burners automatically based on an operator selected sequence. It determines when burners need to be cutout based on the steam pressure signal from the ACC (Automatic Combustion Control) system. It also provides an interface to the ACC system to tell it when there is a state of change that can affect how the ACC will respond or perform.The brains of the Burner Management system are Allen-Bradley model SLC500 Programmable Logic Controllers (PLC).
The PLC essentially consists of a series of digital inputs from the field devices and digital outputs to the field devices and indicator lights.
- Field inputs
All of the burner management system field inputs (pressure switches, limit switches, flame detected contacts, etc) and control pushbuttons and selector switches provide a ground or “voltage low” to the input modules.
The input modules are sixteen channel input devices which have small rectangular LED’s to indicate whether a input is on of off. When the light is on the field device is closed.
- Field outputs
The field outputs include all solenoid valves, indicator lights and relay coils. The output modules switch 24 VDC from the buss to the field device or indicator. The output modules are sixteen channel relay output devices which have small rectangular LED’s to indicate whether a output is on or off. When the light is on the field device is energized.The Burner Management system controls the forced draft fan inlet vane position during boiler purging, burner lightoff, and low fire
mode.
When the Burner Management System sends a signal for Purging, the inlet vanes of the Forced Draft Fan will open 100%. When measured Air Flow exceeds 55%, the Air Flow Control Loop will send a "Purge OK" signal back to the Burner Management System.
When the Burner Management System sends a signal for Air Flow to Light Off position, the inlet vanes of the Forced Draft Fan will open 10% providing sufficient air for lightoff of the initial burner.
When the Burner Management System sends a signal for Air Flow to Low Fire position, the inlet vanes of the Forced Draft Fan will go to approximately the 1% open position providing sufficient air for warm up firing.
Since the number of burners that can be online varies, a means to compensate for this variable has to be programmed into the air flow controller. The Burner Management System is programmed to notify the Air Flow Controller how many burners are online. In the event of a failure, there is a manual switch on the Burner Management cabinet to select the number of burners online. The variable is compensated by using a gain that manipulates the output of the Air Flow transmitter so that the Air Flow controller thinks that the percentage of Air Flow is the same as the percentage of the firing rate. This is an automatic function about which the operator does not need to be concerned. The compensated process input is shown as "PV" on the air flow controller display.
The basic burner management sequence of operation when placing a burner in service is:
- Close air register and prove closed
- Extend ignitor and prove both power to the ignitor transformer and ignitor inserted.
- Verift atomizing steam pressure is satisfactory
- Open burner oil valve and prove open
- Establish flame and prove with flame detector
- Open air register after three seconds and prove open
- Retract ignitor after trial for ignition period ends (10 seconds)
If a burner does not light off, observe the sequence of operation and determine what is not happening. The problem is most likely with the operation that is supposed to happen prior to what is observed not happening.
The burner will trip if a flame is not established after the trial for ignition period ends, or if there is a loss of flame, the air register is not open, or the oil valve is not open.
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