5. Boiler Interlocks and Burner Management System_upload

Boiler Interlocks and Burner Management system

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Basics • • • •

Interlock means to fit or fasten together Developed using electrical contacts Contacts are wired to accomplish logical tasks Nomenclature :

• The “normal” means status of contacts when a switch is under a condition of minimum physical stimulus. • E.g. For a momentary-contact pushbutton switch, this would be the status of the switch contact when it is not being pressed. 2 H.G.Dholakia

India Boiler Dot Com

Basics • Industry uses discrete input (process sensors) and output devices for interlocks • Discrete means having two states : ON and OFF • “normal” definitions for various discrete sensor types: – • Hand switch: no one pressing the switch – • Limit switch: target not contacting the switch – • Proximity switch: target far away – • Pressure switch: low pressure (or even a vacuum) – • Level switch: low level (empty) – • Temperature switch: low temperature (cold) – • Flow switch: low flow rate (fluid stopped)

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Discrete Input Devices • Hand Switches : Symbol

• Limit Switches Symbols :

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Discrete Input Devices • • • •

Proximity Switches : Non contact sensor that senses closeness of the object Uses Magnetic, Electric or Optical means Inductive proximity switches sense the presence of metallic objects using a high-frequency magnetic field. • Capacitive proximity switches sense the presence of non-metallic objects using a high-frequency electric field. • Optical switches detect the interruption of a light beam by an object. • Similar to Limit switch except the switch symbol is enclosed by a diamond shape, indicating a powered (active) device 5 H.G.Dholakia


Proximity Probe

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Proximity Switch Mounting

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Discrete Input Devices • Pressure Switches : diaphragm, bellows or bourdon tube as sensor • A glass bulb partially filled with mercury or a microswitch is used as the electrical switching element Mercury

Contacts

• Advantages of mercury tilt switches include immunity to switch contact degradation from harmful atmospheres (oil mist, dirt, dust, corrosion) as well as safety in explosive atmospheres . • Disadvantages include the possibility of intermittent electrical contact resulting from mechanical vibration, as well as sensitivity to mounting angle 8 H.G.Dholakia


Discrete Input Devices - Pressure Switches • Pressure Switch Symbol

Sample Pressure Switches

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Discrete Input Devices - Level Switches • Level Switches detect level of liquid or solid (granules or powder) in a vessel. • Types of level switches : – Float – Vibrating fork – Ultrasonic – Capacitance, etc. • Level Switch Symbol

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Discrete Input Devices • Temperature Switch: • Uses Metal bulb filled with fluid • Expansion of fluid due to temperature actuates switch as in case of pressure switch • Electronic switches employ RTD or Thermocouple sensors

• Symbol :

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Discrete Input Devices • Flow Switch : • Uses Paddles as the flow sensing element

• Symbol

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Discrete Output Devices • The Discrete Output Device has only two states : – ON and OFF or – OPEN and CLOSE or – ENERGISED and DE- ENERGISED etc. • Typical Output Devices : – ON /OFF Valves

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Discrete Output Devices • Fluid Powered Systems :

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Discrete Output Devices • Example of Fluid Powered System

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Discrete Output Devices

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Discrete Output Devices • Solenoid Valve Actuators : • It is a coil of wire that produces a magnetic force when electrically energised. • Movable iron called Armature moves under influence of magnetic field.

• 2 - Way Solenoid Valve

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Discrete Output Devices • 3 – Way Solenoid Valve :

• Symbols of Solenoid for P&ID :

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Discrete Output Devices • Control Relays : • It is Electrical Switch actuated by an electromagnet coil. • It switches electrical contacts when coil is energised or de-energised. • Types of Relays : – SPST : Single Pole Single Throw - 1 NO or 1 NC – SPDT : Single Pole Double Throw - 1 NO , 1 NC – DPDT : Double Pole Double Throw - 2 NO, 2 NC

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Discrete Output Devices • Industrial Relay has clear plastic case and multi pins for connection. DPDT Relay

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Ladder Diagram • Ladder diagrams (also called "ladder logic") are a type of electrical notation and symbology used to illustrate how electromechanical switches and relays are interconnected. • The two vertical lines are called "rails" and attach to opposite poles of a power supply, usually 120 volts AC. • L1 designates the "hot" AC wire and L2 the "neutral" (may be grounded) conductor. • Horizontal lines in a ladder diagram are called "rungs," each one representing a unique parallel circuit branch between the poles of the power supply. • Typically, wires in control systems are marked with numbers and/or letters for identification. • The rule is, all permanently connected (electrically common) points must bear the same label. 21 H.G.Dholakia


Ladder Diagram Symbols

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Ladder Diagram Rails Rung

Rung Number

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Ladder Diagram • Digital Logic Function : • Standard Binary Notation : 0 for de-energised, 1 for energised On Rung, 2 NO contacts are in parallel. Lamp comes ON if either contact A or B is actuated . This is OR Logic function

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Ladder Diagram • AND Logic Function : • On ladder, 2 NO contacts are wired in series. • Lamp glows only if contacts A AND B both are actuated.

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Ladder Diagram • NOT Logical Function : • Use of NC contact instead of NO contact in ladder results in logical inversion. • Now, the lamp energizes if the contact is not actuated, and de-energizes when the contact is actuated.

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Ladder Diagram • NAND Logical Function : (NOT + AND) • When OR function receives inverted input , the resultant output will be NAND function as per DeMorgan’s Theorem. • The lamp will be energized if either contact is unactuated. It will go out only if both contacts are actuated simultaneously.

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Ladder Diagram • NOR Logical Function : ( NOT + OR) • When AND function receives inverted input through NC contacts, resultant output is NOR function. • Lamp is ON if both inputs are NOT actuated. Lamp goes OFF if any of the input is actuated

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Ladder Diagram • Exclusive OR TOP NOT/AND

logic Function :

GATE

BOTTOM NOT/AND GATE

2 PARALLEL RUNGS FORM OR GATE

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Ladder Diagram • OUTPUT INVERSION : • We have seen INPUT is inversed by using NC contact in place of NO contact. • For OUTPUT INVERSION a Relay having NC contact is required. Control Relay

STOP

CR1

3

RUN

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Ladder Diagram v/s Logic Function • From the earlier discussion we can conclude : • Parallel contacts are equivalent to an OR gate. • Series contacts are equivalent to an AND gate. • Normally-closed contacts are equivalent to a NOT gate (inverter). • A relay must be used to invert the output of a logic gate function, while simple normally-closed switch contacts are sufficient to represent inverted gate inputs.

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Permissive and Interlock • The practical example of foregoing is given below

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Basic Boiler Interlocks • PURGE INTERLOCK – Prevents fuel from being admitted to an unfired furnace until the furnace has been thoroughly air purged. • LOW AIR FLOW INTERLOCK OR FAN INTERLOCK – Fuel is shut off upon loss of air flow or combustion air fan or blower. • LOW FUEL SUPPLY INTERLOCK – Fuel is shut off upon loss of fuel supply that would otherwise result in unstable flame conditions. • LOSS OF FLAME INTERLOCK – All fuel is shut off upon loss of flame in the furnace, or fuel to an individual burner is shut off upon loss of flame to that burner. H.G.Dholakia


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Basic Boiler Interlocks • FAN INTERLOCK – Stops forced draft upon loss of induced draft fan. • LOW WATER INTERLOCK (OPTIONAL) – Shuts off fuel on low water level in boiler drum. • HIGH COMBUSTIBLES INTERLOCK (OPTIONAL) – Shuts off fuel on highly combustible content in the flue gases.

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Boiler Burner Management What is a BMS? • A Burner Management System is defined as the following: – A Control System that is dedicated to boiler safety, operator assistance in the starting and stopping of fuel preparation and burning equipment, and the prevention of mis-operation of and damage to fuel preparation and fuel burning equipment. 1 1. From NFPA 8501 “Standard for Single Burner Boiler Operation”

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Burner Management Objectives • Sequence burner through safe start-up • Insure a complete pre-purge of boiler • Supervise safety limits during operation • Supervise the flame presence during operation • Sequence a safe shutdown at end of cycle • Integrate with combustion control system for proper fuel and air flows

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BMS Design Standards • Each Burner Management System should be designed in accordance with the below listed guidelines to control and monitor all sequences of the start-up and shutdown of the burner n National Fire Protection Association (NFPA 8501 /8502 or others) n Industrial Risk Insurers (IRI) n Factory Mutual loss prevention guidelines o Each burner management system should be designed to accomplish a safety shutdown in the event of an unsafe condition. (FAIL SAFE)

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BMS Design Standards • National standards - typical requirements – Governs safety system design on virtually all boilers (regardless of the process to be used to combust the fuel) – Requires the separation of the Burner Management System from any other control system – Requires the use of a hardwired backup tripping scheme for microprocessor based systems – Requires that a single failure NOT prevent an appropriate shutdown

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BMS Definitions • Furnace Explosions – “Ignition of accumulated combustible mixture within the confined space of a furnace or associated boiler passes, ducts, and fans that convey gases of combustion to the stack”1 – Magnitude and intensity of explosion depends on relative quantity of combustibles and the proportion of air at the time of ignition 1. From NFPA 8502 “Prevention of Furnace Explosions / Implosions in Multiple Burner Boilers”

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BMS Definitions • Furnace Explosions can occur with any or a combination of the following: – Momentary loss of flame followed by delayed reignition – Fuel leakage into an idle furnace ignited by source of ignition (such as a welding spark) – Repeated Light-off attempts without proper purging – Loss of Flame on one Burner while others are in operation – Complete Furnace Flame-out followed by an attempt to light a burner 1. From NFPA 8502 “Prevention of Furnace Explosions / Implosions in Multiple Burner Boilers” 42 H.G.Dholakia


BMS Definitions • Furnace Implosions – More common in large Utility Boilers – Caused by any of the following: • Malfunction of equipment regulating boiler gas flow resulting in furnace exposure to excessive induced draft fan head capability • Rapid decay for furnace gas temperature and pressure due to furnace trip

1. From NFPA 8502 “Prevention of Furnace Explosions / Implosions in Multiple Burner Boilers” 43 H.G.Dholakia


BMS Basic Definitions • Common Terminology – Supervised Manual • Manual Burner Lightoff with Interlocks – Automatic Recycling (Single Burner Only) • Automatic Burner Start and Stop based on preset operating range (ie.. Drum pressure) – Automatic Non Recycling (Single Burner Only) • Automatic Burner Start and Stop based on Manual command to start.

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BMS Functions • The BMS is typically designed to perform the following functions: • Prevent firing unless a satisfactory furnace purge has first been completed. • Prohibit start-up of the equipment unless certain permissive interlocks have first been completed. • Monitor and control the correct component sequencing during start-up and shut-down of the equipment. • Conditionally allow the continued operation of the equipment only while certain safety interlocks remaining satisfied. • Provide component condition feedback to the operator and, if so equipped, to the plant control systems and/or data loggers.

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BMS Functions • Provide automatic supervision when the equipment is in service and provide means to make a Master Fuel Trip (MFT) should certain unacceptable firing conditions occur. • Execute a MFT upon certain adverse unit operating conditions.

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BMS • Furnace Purge : • Before Fuel Firing is Permitted, following purge requirement must be satisfied • 1. Drum level within operating range (not high, not low) •

2. Instrument air header pressure within operating range

•

3. Fan is in service

•

4. Purge airflow capable of a minimum of 70% of the full load airflow established through the unit.

•

5. All flame scanners reading "No Flame“ 47

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BMS – Furnace Purge (contd) •

6. Natural gas block valves are proven closed

•

7. Fuel oil block valves are proven closed

•

8. Air dampers are in the fully open position

•

9. Natural gas, or fuel oil, header pressure upstream of block valve is satisfactory

• 10. Pilot gas header pressure is satisfactory •

11. Burner Control System is energized

•

12. A "No Master Fuel Trip condition" state is established 48

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BMS – Furnace Purge sequence • Once all of the above conditions are satisfied, indicator light glows as "PURGE PERMISSIVES" on the operator control console. • START PURGE switch is enabled by the system logic. Activating the START PURGE control. • Timed furnace purge cycle begins as indicated by a light illuminating "PURGE IN PROGRESS". • Purge time is set for at least 8 air changes in the furnace • At the end of purge cycle, – The "PURGE COMPLETE" light will illuminate – The boiler trip circuit shall be ready for reset, and so indicated by a " RESET MFT " light. – The damper control shall position the inlet damper for light-off, approximately 10% to 30% airflow.

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BMS – Furnace Purge sequence • PURGE COMPLETE light remains for 10 minutes within which Operator has to RESET MFT else PURGE COMPLETE goes off and entire purge cycle has to be repeated. • Upon pressing RESET MFT button, MFT RESET light comes on and boiler control system is ready for main flame start up sequence.

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Purge Interlocks BOILER TRIPPED

AND

PURGE / RESET PB START-UP TIMER

START FD FAN PERMISSIVES SATISFIED: - MAIN FUEL VALVES CLOSED -

NO FLAME PRESENT FD FAN RUNNING MINIMUM AIR FLOW SWITCH MADE WATER LEVEL SATISFACTORY ATOMIZING MEDIUM ON FUEL SUPPLY PRESSURE NOT LOW

AND

ENERGIZE FUEL RELAY NOT

AND

PURGE SIGNAL TO CCS

PURGE AIR FLOW SWITCH MADE

AND

FD DAMPER IN FULL OPEN POSITION

PURGE TIMER SET

PURGE COMPLETE YES

REMOVE PURGE TO CCS H.G.Dholakia


NO

SYSTEM TRIP

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BMS – Main Flame Start up sequence • Lighting off of Pilot flame to Main Flame typically follows automated sequence. • The sequence starts when START BOILER button is pressed. ( can be terminated by STOP BOILER button). • Pre–Requisite for Pilot Flame Light-Off – For Pilot Igniter • MFT relay reset • Pilot gas header pressure normal – For Natural Gas • All of the above mentioned for the pilot igniter • Natural gas pressure normal • Natural gas control valve is in light-off position

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BMS – Main Flame Start up sequence • For Fuel Oil – All of the above mentioned for the pilot igniter – Oil gun is in place in the burner – Oil pressure is normal – Fuel oil atomizing interlocks are satisfied – Fuel oil atomizing medium is provided to the burner – Oil control valve is in light-off position • Other Conditions: – No MFT condition after purge – All flame scanners report no flame – All natural gas, or all fuel oil, block valves shown closed – All air dampers are in light-off position H.G.Dholakia


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BMS – Main Flame Start up sequence • Pilot Flame Sequence : • • • •

Fuel vent valves get closed Fuel Block valve opens For 10 seconds the igniter transformer energised. If flame scanner proves flame, main flame sequence continues. • Pilot flame failure initiates shutdown. • Additional attempts are permitted within 10 minutes after furnace purge completion

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Igniter Interlocks PURGE COMPLETE

AIR DAMPER IN LOW FIRE POSITION

AND

FUEL VALVE IN LOW FIRE POSITION

ENERGIZE IGNITER AND IGNITER HEADER VALVES

10 SECOND DELAY

10 SEC PILOT TRIAL FOR IGNITION

TIMER COMPLETE FLAME PROVEN

NOT

AND

SYSTEM TRIP PERMIT FOR MAIN FLAME H.G.Dholakia


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BMS – Main Flame Start up sequence • Main Flame Light off : • Upon pilot flame establishment, header block valve opens for selected fuel. • Timer for gas = 15 secs, FO = 20 Secs. • 5 sec before time out pilot fuel system stops and main flame proving checked. • Successful main flame detection brings boiler in NORMAL RUN condition. • Under “No Flame” condition Boiler trip is activated that requires fresh furnace purge.

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Main Flame Interlocks IGNITER TIMER COMPLETE

FLAME PROVEN

AND ENERGIZE MAIN FUEL VALVES 10 SEC MAIN FLAME TRIAL

TIMER COMPLETE NOT

AND

DE-ENERGIZE IGNITION COMPONENTS

RELEASE TO MODULATE TO CCS H.G.Dholakia


SYSTEM TRIP

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Boiler Master Fuel Trip • Following is list of Conditions causing Boiler trip. • Boiler trip calls for Furnace purge before restart up • Fuel Oil System – Excessive steam pressure. – Low water level. – Low fuel pressure. – Low oil temperature. – Loss of combustion air supply. – Loss of flame. – Loss of control system power. – Loss of atomizing medium, if used. 58 H.G.Dholakia


Boiler Master Fuel Trip • For natural gas: – Excessive steam pressure or water temperature. – Low water level. – High or low gas pressure. – Loss of combustion air supply. – Loss of flame. – Loss of control system power. • Upon trip Boiler returns to pre purge state

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Timing diagram for typical multifuel burner lightoff sequence.

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Ignitors • The gas fired igniter is a device whose flame is designed to ignite a main burner. • A high voltage is generated from an input voltage (mains voltage), which generates an ignition spark at the gas nozzle. • The resulting flame generates a flame signal via the flame rod. This flame signal is amplified in the ionization flame monitor and enables the main burner. • The flame is monitored by the flame rod, which must be immersed in the direction of the flame. • Alternating voltage is applied to this flame rod. • The burning flame creates an electrically conductive connection to the igniter earth and at the same time acts as a rectifier for the ionization current. 61 H.G.Dholakia


Ignitors • This direct current signal is measured and amplified in the ionization flame monitor. • The amplified ionization current activates the flame relay with one SPDT contact (special version) and the flame signal output.

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Ignitors • An ion is a charged atom that has either gained an electron to become negatively charged (anion) or has lost an electron to become positively charged (cation). • The energy released during a combustion process will cause electrons to be knocked loose from an atom, resulting in a positively charged particle and a free electron. • This ionization, if monitored properly, can be used to generate a safe and reliable indication of a flame.

Normal combustion: • CH4 + 2O2----->CO2 + 2H2O

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Ignitors • There is sometimes an intermediate step in which a uniform proportion of the molecules in this reaction do the following • CH4 + 2O2----->C++ O2 + 2H2O + e- ----->CO2 + 2H2O • The number of ions produced is greatest where the chemical reaction is the strongest. • If the air-fuel ratio is optimal, the reaction will be the strongest, and more free ions and electrons will be produced. • Since the electrons are so much lighter than the ions, the electrons travel much faster and move away from the burner mouth toward the tip of the flame much more quickly than the heavier ions. • This leaves a greater concentration of positively charged ions in the area near the burner mouth than free electrons. 65 H.G.Dholakia


Types of Flame Scanners • Infrared (IR) Detectors – Single Burner Applications – More Suitable with Oil Burning Flames • Ultra-Violet (UV) Detectors – Multiple Burner Applications – More Suitable for Gas Burners and Combination Gas / Oil Burners

• Self Check Scanners – Flame Signal is interrupted at set intervals to verify proper operation of scanner

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Flame Scanners  Flame scanners are a crucial part of a boiler's safety system. Their primary responsibility is to identify potential dangerous "flame out" conditions where ignition has ceased and continued addition of fuel could cause an explosion.  Because of the flame scanners importance, they must be extremely reliable and rugged  The monitoring can be performed by the combination of a flame sensor (also flame scanner) that transforms characteristic properties of the flame into an electrical signal, with a control unit that provides the flame signal and ensures error free operation.  Alternatively these two parts are combined in one compact flame monitor

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Flame Scanners  Flame detection is the technology for detecting flames, using a flame detector.  Flame detectors are optical equipment for the detection of flame phenomena of a fire.  There are two types :  Flame detector for the detection of a fire in a fire alarm system.  Flame scanner for monitoring the condition of a flame in a burner

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Flame Scanners • The optical flame detector is a detector that uses optical sensors to detect flames. • There are also ionization flame detectors, which use current flow in the flame to detect flame presence. • Some use thermocouple as flame detectors.

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Flame Scanners ULTRAVIOLET An ultraviolet (UV) sensor is often sensitive for radiation in the 185 to 260 nm range. This frequency range is the least sensitive for natural background radiation sources like cosmic radiation and especially sunlight. The sunlight is, in the higher frequencies, absorbed by almost all vapors and gases; especially by ozone and smoke but also by an oil or grease film on the window of a flame detector. Almost every fire radiates UV light, and the UV sensor is a good all round flame detector.

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Flame Scanners NEAR INFRARED :  A near Infrared(IR) sensor (0.7 to 1.1 μm) is especially able to monitor flame phenomena, without too much hindrance from water and water vapor.  Pyro electric sensors operating at this wavelength can be relatively cheap.  Multiple channel or pixel array sensors monitoring flames in the near IR band are arguably the most reliable technologies available for detection of fires.  Digital image processing can be utilized to recognize flames through analysis of the video created from the near IR images react to radiation having a wavelength of 800 nm or higher.  It is only the flickering of the flame which is analyzed. Constant radiation sources, such as the glowing of the furnace walls, are not detected as a flame. 74 H.G.Dholakia


Flame Scanners  Flames radiating in the UV range, but whose UV component is absorbed by dust, steam or other substances, can often also be monitored using infrared detectors.  The detection element of the Infrared Flame Detector consists of two pyroelectric sensors (sensor 1 and 2) and a silicon photo diode (sensor 3). • Flame most useful for its detection, is the electromagnetic radiation produced by it. This radiation covers the spectral range from infrared to far ultraviolet. Infrared and visible radiations, are functions of flame temperature and emissivity. • Since furnace and burner parts become heated by the flame, they become potential secondary sources of infrared and visible radiation, which must be discriminated against. 75 H.G.Dholakia


Flame Scanners  A UV/IR detector consists of an UV and single frequency IR sensor paired to form one unit.

 The two sensors individually operate the same as previously described, but additional circuitry processes signals from both sensors. This means the combined detector has better false alarm rejection capabilities than the individual UV or IR detectors.

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Flame Scanners  Strengths of the UV/IR detector are;  Virtually immune to false alarms  High speed response – under 500 milliseconds  Solar, welding, lightning, X-rays, sparks, arcs, and corona insensitive

 Limitations of UV/IR detector are;  Not recommended for non carbon fires  Some gases and vapors will inhibit detection due to blinding of the UV sensor

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Flame Scanners  Since the UV/IR detector pairs two sensor types, it will typically only detect fires that emit both UV and flickering IR radiation. UV detectors will respond to virtually all fires including hydrocarbon (liquids, gases, and solids), metals (magnesium), sulfur, hydrogen, hydrazine and ammonia. IR detectors typically only respond to hydrocarbon fires.  Since the IR detector is not sensitive to burning metals, ammonia, hydrogen and sulfur the combined unit will not respond to these fires.  The detector is suitable for applications where hydrocarbon fires are likely and other sources of radiation may be present (X-rays, hot surfaces, arc welding). They maintain constant protection while arc welding takes place. The UV/IR detectors are highly reliable with fast response times and low propensity to false alarms. 78 H.G.Dholakia


Logic Diagram BMS OPERATE

TOTAL FLAME FAILURE

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Logic Diagram MFT RESET

FG MAIN VLV / FG VENT VLV

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Logic Diagram LDO

MAIN

VLV / LDO RECIRC. VLV

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Logic Diagram FO MAIN VLV/ FO RECIRC. VLV

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Logic Diagram LDO BURNER VALVE OPEN AND LDO TIME OVER

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Logic Diagram NO.1

BNR (FG) SEQUENCE

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Logic Diagram IGNITOR

VLV / IGNITOR VENT VALVE

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Simplified BMS Interlock narration

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Exercise - Interlocks L1

Explain how this interlock functions and name the input / output in logic diagram

OHL

1

OHH

2

CR2

CR3

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L2

4

3

CR2

CR3

4

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OHL

OHH

Exercise on Interlock Overhead Tank

O H H

Design of scheme O H L

Design Interlock for following application : Pump starts only when OHL is reached, if UGL is NOT activated

U G L

U G H

Pump stops only when OHH is reached or UGL is reached whichever is earlier. Pump starts when UGH is activated irrespective of status of OHL. Underground Tank

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Exercise on Interlock UGL CR1

UGH CR2

OHL CR3

OHH CR4

CR3.1

CR1.1

CR4.1 CR5

CR5.1

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Logic Diagram DURING PURGE & PURGE COMPLETE

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Logic Diagram DURING PURGE & PURGE COMPLETE

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5. Boiler Interlocks and Burner Management System_upload

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