Is an automatic controlled boiler an explosion risk? An easy way to find out if your boiler control system lights up the first burner safely Safely shut off the fuel supply to the burner before the test. Power up the boiler control system Start the burner Start a stopwatch when the combustion air damper has reached its maximum position. Stop the stopwatch when the combustion air damper begins to close again, and note the purge time. The air in the furnace should be changed at least five times during the prepurge period. Find the funace volume and the fan capacity from the documentations and calculate the purge time needed. The time must never be less than fifteen seconds even if your calculation says so Start the stopwatch again when the automatic fuel oil shutoff valves open Stop the stopwatch when the fuel oil shutoff valves close and you get a flame failure or misfiring alarm, and note the trail-for-ignition time If the time you get is more than fifteen seconds, then you must not ignite the burner ever, until the time has been adjusted. Five seconds is a relevant trail-for-ignition time, but different classification societies specify different maximum time. Get the correct maximum time from the rules of the actual N. B. This shut off delay is only allowed during trail-for-ignition. when you got at flame failure during norma firing the fuel oil valves must shut off instantly Some further checks to improve the safety The fuel oil flow during light-up must not exceed 20% of the full load flow, but in burners with limited turndown ratio the burners minimum load has to be acce A corrupt flow transmitter signal may cause sever problems therefore When purging the furnace whit air prior to light-up the position of the combustion air damper should be confirmed by means of a limit switch rather than relying only on the air flow transmitters signal. At burner light-up the position of the fuel oil control valve and the combustion air damper should be confirmed by means of limit switches rather than relying only on the flow transmitters'signals. You should of course use the transmitters' signals, but they ought to be confirmed to be reasonable by means of limit witches Direct the light from a flashlight onto the flame scanner sensors, when the burner is off, to confirm that the auto-check-function works correctly and you get an alarm. If you get any other action, such as opening of the fuel valves, then your system needs a thorough improvement. 4. Using the igniter during the post-purge of the last burner's lance(or a single burner's lance)has some disadvantages. Upon reset of the system, after a flame failure, the igniter will start firing before the fumace has en properly purged with air, which will cause impending risk of furnace explosions. Consider the following: A well-tried method is to purge the fuel line and the burner-lance slowly to let the fuel continue to burn, without igniter support, until the lance is empty. the burner -lance at all is an other method What ever you do, secure that the igniter not under any circumstances starts before the furnace Is an automatic controlled boiler an explosion risk? An easy way to find out if your boiler control system lights up the first burner safely Safely shut off the fuel supply to the burner before the test
Is an automatic controlled boiler an explosion risk? An easy way to find out if your boiler control system lights up the first burner safely. Safely shut off the fuel supply to the burner before the test. 1. Power up the boiler control system. 2. Start the burner. 3. Start a stopwatch when the combustion air damper has reached its maximum position. 4. Stop the stopwatch when the combustion air damper begins to close again, and note the purge time. The air in the furnace should be changed at least five times during the prepurge period. Find the furnace volume and the fan capacity from the documentation’s and calculate the purge time needed. The time must never be less than fifteen seconds even if your calculation says so. 5. Start the stopwatch again when the automatic fuel oil shutoff valves open. 6. Stop the stopwatch when the fuel oil shutoff valves close and you get a flame failure or misfiring alarm, and note the trail-for-ignition time. If the time you get is more than fifteen seconds, then you must not ignite the burner ever, until the time has been adjusted. Five seconds is a relevant trail-for-ignition time, but different classification societies specify different maximum time. Get the correct maximum time from the rules of the actual classification society. N.B. This shut off delay is only allowed during trail-for-ignition. When you got at flame failure during normal firing the fuel oil valves must shut off instantly. Some further checks to improve the safety 1. The fuel oil flow during light-up must not exceed 20% of the full load flow, but in burners with limited turndown ratio the burners minimum load has to be accepted. 2. A corrupt flow transmitter signal may cause sever problems therefore: When purging the furnace whit air prior to light-up the position of the combustion air damper should be confirmed by means of a limit switch rather than relying only on the air flow transmitter's signal. At burner light-up the position of the fuel oil control valve and the combustion air damper should be confirmed by means of limit switches rather than relying only on the flow transmitters' signals. You should of course use the transmitters' signals, but they ought to be confirmed to be reasonable by means of limit switches. 3. Direct the light from a flashlight onto the flame scanner sensors, when the burner is off, to confirm that the auto-check-function works correctly and you get an alarm. If you get any other action, such as opening of the fuel valves, then your system needs a thorough improvement. 4. Using the igniter during the post-purge of the last burner's lance (or a single burner's lance) has some disadvantages. Upon reset of the system, after a flame failure, the igniter will start firing before the furnace has been properly purged with air, which will cause impending risk of furnace explosions. Consider the following: A well-tried method is to purge the fuel line and the burner-lance slowly to let the fuel continue to burn, without igniter support, until the lance is empty. Not purging the burner-lance at all is an other method, but it requires a stand-by heating of the tubing and the lance to keep the fuel sufficiently heated to be floating. What ever you do, secure that the igniter not under any circumstances starts before the furnace has been properly purged with air. Is an automatic controlled boiler an explosion risk? An easy way to find out if your boiler control system lights up the first burner safely. Safely shut off the fuel supply to the burner before the test
Power up the boiler control system Iner Start a stopwatch when the combustion air damper has reached its maximum position Stop the stopwatch when the combustion air damper begins to close again, and note the purge time. The air in the furnace should be changed at least five times during the prepurge period Find the fumace volume and the fan capacity from the documentations and calculate the purge time needed. The time must never be less than fifteen seconds even if your calculation says so Start the stopwatch again when the automatic fuel oil shutoff valves open. Stop the stopwatch when the fuel oil shutoff valves close and you get a flame failure or misfiring alarm, and note the trail-for-ignition time If the time you get is more than fifteen seconds, then you must not ignite the burner ever, until the time has been adjusted. Five seconds is a relevant trail-for-ignition time, but different classification societies specify different maximum time. Get the correct maximum time from the rules of the actual N B. This shut off delay is only allowed during trail-for-ignition. When you got at flame failure during normal ring the fuel oil valves must shut off instantly. Some further checks to improve the safety The fuel oil flow during light-up must not exceed 20% of the full load flow, but in burners with limited turndown ratio the burners minimum load has to be accepted. A corrupt flow transmitter signal may cause sever problems therefore When purging the furnace whit air prior to light-up the position of the combustion air damper should be confirmed by means of a limit switch rather than relying only on the air flow transmitters signal At burner light-up the positio fuel oil control valve and the combustion air damper should be confirmed by means of limit switches rather than relying only on the flow transmitters signals.You should of course use the transmitters signals, but they ought to be confirmed to be reasonable by means of limit witches Direct the light from a flashlight onto the flame scanner sensors when the burner is off, to confirm that the auto-check-function works correctly and you get an alarm. If you get any other action, such as opening of the fuel valves, then your system needs a thorough improvement Using the igniter during the post-purge of the last burners lance(or a single burner's lance)has some disadvantages. Upon reset of the system, after a flame failure, the igniter will start firing before the furnace has been properly purged with air, which will cause impending risk of furnace explosions. Consider A well-tried method is to purge the fuel line and the burner-lance slowly to let continue to burn, without igniter support, until the lance is empty Not purging the burner-lance at all is an other method, but it requires a stand-by heating of the lubing and the lance to keep the fuel sufficiently heated to be floating What ever you do, secure that the igniter not under any circumstances starts before the fumace has been properly purged with air Safety with marine boilers steam, maintain design conditie steady steaming, secure the boiler units and detect promptly malfunctions and failures. The automatic control arrangement on a shipboard boiler is divided into two parts
1. Power up the boiler control system. 2. Start the burner. 3. Start a stopwatch when the combustion air damper has reached its maximum position. 4. Stop the stopwatch when the combustion air damper begins to close again, and note the purge time. The air in the furnace should be changed at least five times during the prepurge period. Find the furnace volume and the fan capacity from the documentation’s and calculate the purge time needed. The time must never be less than fifteen seconds even if your calculation says so. 5. Start the stopwatch again when the automatic fuel oil shutoff valves open. 6. Stop the stopwatch when the fuel oil shutoff valves close and you get a flame failure or misfiring alarm, and note the trail-for-ignition time. If the time you get is more than fifteen seconds, then you must not ignite the burner ever, until the time has been adjusted. Five seconds is a relevant trail-for-ignition time, but different classification societies specify different maximum time. Get the correct maximum time from the rules of the actual classification society. N.B. This shut off delay is only allowed during trail-for-ignition. When you got at flame failure during normal firing the fuel oil valves must shut off instantly. Some further checks to improve the safety 1. The fuel oil flow during light-up must not exceed 20% of the full load flow, but in burners with limited turndown ratio the burners minimum load has to be accepted. 2. A corrupt flow transmitter signal may cause sever problems therefore: When purging the furnace whit air prior to light-up the position of the combustion air damper should be confirmed by means of a limit switch rather than relying only on the air flow transmitter's signal. At burner light-up the position of the fuel oil control valve and the combustion air damper should be confirmed by means of limit switches rather than relying only on the flow transmitters' signals. You should of course use the transmitters' signals, but they ought to be confirmed to be reasonable by means of limit switches. 3. Direct the light from a flashlight onto the flame scanner sensors, when the burner is off, to confirm that the auto-check-function works correctly and you get an alarm. If you get any other action, such as opening of the fuel valves, then your system needs a thorough improvement. 4. Using the igniter during the post-purge of the last burner's lance (or a single burner's lance) has some disadvantages. Upon reset of the system, after a flame failure, the igniter will start firing before the furnace has been properly purged with air, which will cause impending risk of furnace explosions. Consider the following: A well-tried method is to purge the fuel line and the burner-lance slowly to let the fuel continue to burn, without igniter support, until the lance is empty. Not purging the burner-lance at all is an other method, but it requires a stand-by heating of the tubing and the lance to keep the fuel sufficiently heated to be floating. What ever you do, secure that the igniter not under any circumstances starts before the furnace has been properly purged with air. Safety with marine boilers Marine boiler plants require adequate control systems to raise steam, maintain design conditions for steady steaming, secure the boiler units and detect promptly malfunctions and failures. The automatic control arrangement on a shipboard boiler is divided into two parts:
Safety system that controls that all values are within the predetermined limits and give automatic alarm if some of them are not, and also initiate an automatic burner trip in case of a dangerous situation parameters for water level pressure control, fuel oil temperature control, blowdown control, superheat temperature control etc. The combustion control system maintains constant steam pressure by controlling the flow of air and oil to the ols transmit the air and oil loading simultaneously but with a slight lag between air and oil, so that with an increased boiler load the air will lead the oil, and on a decrease in the boiler load the oil will lead the air. Such an arrangement makes it possible to minimize the emission of smoke during maneuvering. All the classification societies have special requirement for marine applications due to the environment and the fact that one can't escape from an accident nor get service when the ship is sailing at sea just have to work. Beware of fuel oil in the marine st eam boilers feedwater system The marine steam boilers on board in ships no than they were in the old days. However, the marine steam boilers are still important parts of the system especially if the main engines are running on heavy fuel oil that need to be heated. I don t want to go through the entire instruction book for the marine boiler but just point out one important matter. As we all know, a very thin layer of oil on the surface of the boiler tubes or any of the direct heated surfaces of a boiler might cause local overheating of the material and possible damage to the boiler. What we maybe not know is how very small quantity of oil it requires to get a hotwell will manage that, but a small crack in a tube may cause you an even more serious problem. a tiny oil leakage giving some 15 to 25 ppm of oil in the boiler feedwater would not be visible, the water is still limpid. Yet this apparently insignificant oil pollution might causes sever danger to the boiler. If a boiler has a capacity of 20 tons of steam per hour at full load and the feed water is polluted with 25 ppm of oil, then it will accumulate approximately 12 kg oil in the steam drum per day. a bucket of oil in the boiler every day; I suggest buy you It will cost you, but it might save you from some future cost of repairs N B ppm= parts per million. I ppm= l kg per 1000 metric tons. In a two-boiler system it's often a problem to keep the slave boiler at operation pressure when the steam demand is low his problem does not appear when the vessel is loading or unloading in a harbor since those operations normally need oth boilers. On the other hand, when the ship is at sea and only one oil-fired boiler is used then the slave boiler tends to ool down far below the required stand by conditions Different methods have been used to solve this problem. Installing steam heating coils in the bottom of the boiler is one
Safety system that controls that all values are within the predetermined limits and give automatic alarm if some of them are not, and also initiate an automatic burner trip in case of a dangerous situation. Continuously control of the different parameters for water level control, steam pressure control, fuel oil pressure control, fuel oil temperature control, blowdown control, superheat temperature control etc. The combustion control system maintains constant steam pressure by controlling the flow of air and oil to the burner. The more advanced combustion controls transmit the air and oil loading simultaneously but with a slight lag between air and oil, so that with an increased boiler load, the air will lead the oil, and on a decrease in the boiler load the oil will lead the air. Such an arrangement makes it possible to minimize the emission of smoke during maneuvering. All the classification societies have special requirement for marine applications due to the environment and the fact that one can't escape from an accident nor get service when the ship is sailing at sea. Things just have to work. Beware of fuel oil in the marine steam boilers feedwater system The marine steam boilers on board in ships nowadays have become a less evident part of the engine room than they were in the old days. However, the marine steam boilers are still important parts of the system, especially if the main engines are running on heavy fuel oil that need to be heated. I don’t want to go through the entire instruction book for the marine boiler but just point out one important matter. As we all know, a very thin layer of oil on the surface of the boiler tubes or any of the direct heated surfaces of a boiler might cause local overheating of the material and possible damage to the boiler. What we maybe not know is how very small quantity of oil it requires to get a dangerous situation. A tube break in a fuel oil heat exchanger is very easy to detect, a simple detector in the hotwell will manage that, but a small crack in a tube may cause you an even more serious problem. A tiny oil leakage giving some 15 to 25 ppm of oil in the boiler feedwater would not be visible; the water is still limpid. Yet this apparently insignificant oil pollution might causes sever danger to the boiler. If a boiler has a capacity of 20 tons of steam per hour at full load and the feed water is polluted with 25 ppm of oil, then it will accumulate approximately 12 kg oil in the steam drum per day. A bucket of oil in the boiler every day; I suggest that you buy yourself a sensitive oil detector. It will cost you, but it might save you from some future cost of repairs. N.B. ppm = parts per million. 1 ppm = 1 kg per 1000 metric tons. Keeping a slave boiler pressurized. In a two-boiler system it’s often a problem to keep the slave boiler at operation pressure when the steam demand is low. This problem does not appear when the vessel is loading or unloading in a harbor since those operations normally need both boilers. On the other hand, when the ship is at sea and only one oil-fired boiler is used then the slave boiler tends to cool down far below the required stand by conditions. Different methods have been used to solve this problem. Installing steam heating coils in the bottom of the boiler is one
method and a sophisticated Common steam outlet start-and-stop method for the slave boiler's burner to keep the pressure at desired level is an other These installations will be Steam Steam unnecessary if you happened to have Water Water chaust gas economizer just connect the exhaust gas economizer to the slave boiler nstead of the master boiler. This aster a Lars Josefsson Boiler operation method will guaranty normal operation pressure on both boilers all the time at sea. The method has been used in many ships and the chief engineers are satisfied with the result. Boiler water level contro In a boiler drum exist water and steam at saturation pressure and saturation temperature. Furthermore, the water is mixed with steam bubbles in different sizes. If one opens the valve to a steam consumer then the pressure in the boiler drum falls and hence the steam bubbles expand and the water level rises despite the fact that the water mass actually decreases On the other hand Steam Water Feed Water cause the steam bubbles to Flow Controll pse and the water level falls Boiler Drum ⑩D Flow Transm Transm. when the water quantity actually increases in the steam drum @Lars Josefsson These shrinks and swell phenomena will complicate the control of the boilers steam drum level. For a boiler with large amount of water and relatively low steam production a single water level transmitter on the steam drum is sufficient for the level controller to maintain a level Far more sophisticated methods are required for boilers with hig production and relatively small water olume. the classic method works as follow The outlet steam-mass is measured and the inlet feed -water-mass is adjusted to the very same amount. The level transmitter is merely used to tune up the system so the water level lies within the limits Thus, the setpoint to the feed water flow controller is the sum of the outlet steam flow signal and the reversed water level controllers signal minus 12 mA(or 9 psi if it is a pneumatic system) BOILER DRUM LEVEL CONTROL Two and three element drum level control Enhanced three element drum level control Boiler drum level control is critical for both plant protection and equipment safety and applies equally to high and low levels of water within the boiler drum
method and a sophisticated start-and-stop method for the slave boiler’s burner to keep the pressure at desired level is an other. These installations will be unnecessary if you happened to have an exhaust gas economizer. Just connect the exhaust gas economizer to the slave boiler instead of the master boiler. This operation method will guaranty normal operation pressure on both boilers all the time at sea. The method has been used in many ships and the chief engineers are satisfied with the result. Boiler Water Level Control In a boiler drum exist water and steam at saturation pressure and saturation temperature. Furthermore, the water is mixed with steam bubbles in different sizes. If one opens the valve to a steam consumer then the pressure in the boiler drum falls and hence the steam bubbles expand and the water level rises despite the fact that the water mass actually decreases. On the other hand, pumping in cold feed water will cause the steam bubbles to collapse and the water level falls when the water quantity actually increases in the steam drum. These shrinks and swell phenomena will complicate the control of the boilers steam drum level. For a boiler with large amount of water and relatively low steam production a single water level transmitter on the steam drum is sufficient for the level controller to maintain a level with acceptable variation. Far more sophisticated methods are required for boilers with high steam production and relatively small water volume. The classic method works as follow: The outlet steam-mass is measured and the inlet feed-water-mass is adjusted to the very same amount. The level transmitter is merely used to tune up the system so the water level lies within the limits. Thus, the setpoint to the feed water flow controller is the sum of the outlet steam flow signal and the reversed water level controllers signal minus 12 mA (or 9 psi if it is a pneumatic system). BOILER DRUM LEVEL CONTROL Two and three element drum level control Enhanced three element drum level control Boiler drum level control is critical for both plant protection and equipment safety and applies equally to high and low levels of water within the boiler drum
The purpose of the drum level controller is to bring the drum up to level at boiler start-up and maintain the level at constant steam load. a dramatic decrease in this level at constant steam load. a dramatic decrease in this level may uncover boiler tubes, allowing them to become overheated and damaged. An increase in this level may interfere with the process of separating moisture from steam within the drum, thus reducing boiler efficiency and carrying The functions of this control models can be broken down into the following Operator adjustment of the setpoint for drum level Compensation for the shrink swell effects Automatic control of drum level Manual control of the feedwater valve Bump-less transfer between auto and manual modes Indication of drum level and steam flow Indication of feedwater valve position and feedwater flow Absolute/deviation alarms for drum level The Three main options available for drum level control are The simplest but least effective form of drum level control This consists of proportional signal or process variable(Pv) coming from the drum level transmitter. This signal is compared to a setpoint and the difference is a deviation value This signal is acted upon by the controller which generates corrective action in the form of a proportional output The output is then passed to the boiler feedwater valve, which then adjusts the lev vel of feedwater flow into the boiler drum Only one analogue input and one analogue output required Can only be applied to single boiler /single feedpump configurations with relatively stable loads since there is no relationship between drum level and steam-or feedwater flow Possible inadequate control option because of the swell effect Two element drum level control
The purpose of the drum level controller is to bring the drum up to level at boiler start-up and maintain the level at constant steam load. A dramatic decrease in this level at constant steam load. A dramatic decrease in this level may uncover boiler tubes, allowing them to become overheated and damaged. An increase in this level may interfere with the process of separating moisture from steam within the drum, thus reducing boiler efficiency and carrying moisture into the process or turbine The functions of this control models can be broken down into the following Operator adjustment of the setpoint for drum level Compensation for the shrink & swell effects Automatic control of drum level Manual control of the feedwater valve Bump-less transfer between auto and manual modes Indication of drum level and steam flow Indication of feedwater valve position and feedwater flow Absolute/deviation alarms for drum level The Three main options available for drum level control are Single element drum level control The simplest but least effective form of drum level control This consists of proportional signal or process variable (PV) coming from the drum level transmitter. This signal is compared to a setpoint and the difference is a deviation value This signal is acted upon by the controller which generates corrective action in the form of a proportional output. The output is then passed to the boiler feedwater valve, which then adjusts the level of feedwater flow into the boiler drum Notes: Only one analogue input and one analogue output required Can only be applied to single boiler / single feedpump configurations with relatively stable loads since there is no relationship between drum level and steam- or feedwater flow Possible inadequate control option because of the swell effect Two element drum level control
Feed Water Flow Vdve The two-element drum level controller can best be applied to a single drum boiler where the feedwater is at a The two elements are made up of the following: Level Element: a proportional signal or process variable(Pv) coming from the drum level transmitter. This signal is compared to a setpoint and the resultant is a deviation value. This signal is acted upon by the controller which generates corrective action in the form of a proportional value Steam Flow Element: a mass flow rate signal (corrected for density is used to control the feedwater flow, giving immediate corrections to feedwater demand in response to load changes Any imbalance between steam mass flow out and feedwater mass flow into the drum is corrected by the level controller This imbalance can arise from Variations in feedwater supply pressure Leaks in the steam circuits Tighter control of drum level than with only one element Steam flow act as feed forward signal to allow faster level adjustments Can best be applied to single boiler / single feedpump configurations with a constant feedwater pressure Three-element drum level control Steam The three-element drum level control is ideally suited where a boiler plant consists of multiple boilers and multiple feedwater pumps or where the feedwater has variations in pressure or flow The three-elements are made up of the following Level Element Steam Flow Element: corrects for unmeasured disturbances within the system such as Boiler and superheater tube leaks Feedwater Flow element: responds rapidly to variations in feedwater demand, either from the Steam flow rate feedforward signal Feedwater pressure or flow fluctuations
The two-element drum level controller can best be applied to a single drum boiler where the feedwater is at a constant pressure. The two elements are made up of the following: Level Element : a proportional signal or process variable (PV) coming from the drum level transmitter. This signal is compared to a setpoint and the resultant is a deviation value. This signal is acted upon by the controller which generates corrective action in the form of a proportional value. Steam Flow Element: a mass flow rate signal (corrected for density) is used to control the feedwater flow, giving immediate corrections to feedwater demand in response to load changes. Any imbalance between steam mass flow out and feedwater mass flow into the drum is corrected by the level controller. This imbalance can arise from Blowdown variations due to changes in dissolved solids Variations in feedwater supply pressure Leaks in the steam circuits. Notes: Tighter control of drum level than with only one element Steam flow act as feed forward signal to allow faster level adjustments Can best be applied to single boiler / single feedpump configurations with a constant feedwater pressure Three-element drum level control The three-element drum level control is ideally suited where a boiler plant consists of multiple boilers and multiple feedwater pumps or where the feedwater has variations in pressure or flow.. The three-elements are made up of the following Level Element & Steam Flow Element: corrects for unmeasured disturbances within the system such as Boiler blowdown Boiler and superheater tube leaks Feedwater Flow element: responds rapidly to variations in feedwater demand, either from the Steam flow rate feedforward signal Feedwater pressure or flow fluctuations
In order to achieve optimum control, both steam and feedwater flow values should be corrected for density No he three-element system provides tighter control for drum level with fluctuating steam load. Ideal where a system suffers from fluctuating feedwater pressure or flow More sophisticated level of control required Additional input for feedwater flow required Enhanced three element drum level contro The enhanced three-element drum level control module incorporates the standard three element level components ith the following improvements The three-element mode is used during high steam demand. The two-element mode is used if the steam flow measurement fails and the module falls back to single element level control if the feedwater flow measurement should fail or if there is a low steam demand The drum level can be derived from up to three independent transmitters and is density compensated for pressure within the boiler drum Tighter control through a choice of control schemes Drum level maintained on failure of steam or feedwater flow measurements This module introduces an additional level control loop
In order to achieve optimum control, both steam and feedwater flow values should be corrected for density Notes: The three-element system provides tighter control for drum level with fluctuating steam load. Ideal where a system suffers from fluctuating feedwater pressure or flow More sophisticated level of control required Additional input for feedwater flow required Enhanced three element drum level control The enhanced three-element drum level control module incorporates the standard three element level components with the following improvements The three-element mode is used during high steam demand. The two-element mode is used if the steam flow measurement fails and the module falls back to single element level control if the feedwater flow measurement should fail or if there is a low steam demand. The drum level can be derived from up to three independent transmitters and is density compensated for pressure within the boiler drum. Notes: Tighter control through a choice of control schemes. Drum level maintained on failure of steam or feedwater flow measurements. This module introduces an additional level control loop
