Selected Question And Answer About Boiler

drum level control goes haywire We are having 100 tones FBC boilers. Frequent problem observed is that whenever there is sudden load change the boiler drum level control goes haywire leading to tripping of boiler and turbine on drum level low or high. Our drum level control is 3-element control in auto mode. Answer: The most common fault with a three-point level controller is the steam flow transmitter. Loosen the impulse pipes and cleanse the holes into the measuring orifice. Question:   Firing light crude oil Can a Main Boiler built to fire 380 cst HFO, be fired with light crude oil directly from the cargo-tanks? Answer: A completely new fuel system is required, from deck to the burner rails of the boiler. To prevent any possibility of gasses leaking from flanges, there have to be ducting enclosing the entire fuel-system with forced draft fans that vent 30 times the volume of the trunking to the outside. Also there have to be a burner hood to be constructed all over the burner roof, equally vented. Naturally there have to be new burners and so is the burner management. There are two main contractors who are capable and willing to carry this out: HAMWORTHY-Combustion Engineering in UK and SAACKE in Germany. Question:   Seawater in the boiler If the boiler had to be operated with seawater what would be the result. Answer: The salinity will rise rapidly since the salt remains in the boiler while the water boils off. Salt will son precipitate and accumulate on the bottom and also on the heating surface where it, just as boiler-scales, inhibit the heat transmission to the water and causes the metal to overheat and in worst case burst. You may also get foam in the boiler that will cause difficulties to maintain the water level and water droplets might follow with the steam, causing problems with turbines and engines. It is very dangerous to operate a boiler with salt in it, and you have to control the salt concentration by frequently blowing off from the bottom of the boiler and form the water surface to keep the salinity below 9.5% (boilermakers and classification societies may recommend other values). It would also be a good measure to reduce the capacity of the boiler. After this emergency operation it would be wise to open up the boiler for inspection since seawater further accumulation of scales. In the old days some ships sailing on lakes used the lakes water as make up water for their boilers, but even that water caused problem with salt in the boilers although it is supposed to be fresh-water. Question:   Heavy fuel oil viscosity Heavy fuel oil viscosity is defined in the standards as the viscosity at 100°C yet the oils are often described in terms of their nominal 40 or 50°C temperature viscosities. e.g. a G35 oil (35cst at 100°C) is often described as anything from a 350cst to 390cst oil. Refineries blend to control the viscosity at 100°C. Testing aboard ship or in boiler hosues appears to test the viscosity at 50°C. Because of the variation in quality of the residual oil and distillate that make up a heavy fuel oil, it is difficult to make a good correlation between the 50°C and 100°C measurements. My question is, how is the blending of oil in the terminals or on fuel barges controlled? and how reliable is this in achieving the required viscosity? Answer: The values you get from my Fuel Oil Calculation program are normally sufficient for firing a boilers heavy fuel oil burner. For a diesel engine on the other hand, I assume that an automatic viscosity-controller would be indispensable. Question:   Heating up a fire-tube boiler Is there a minimum temperature that a fire-tube boiler should reach before going to a high fire state to prevent tubes from leaking? Answer: The important thing is to heat up the boiler slowly so all parts of the boiler expand just as much. The leaks occur when some part expands more, or less, than the rest of the boiler. You will be on the safe side if you slowly heat up the boiler to, or almost to, normal operation pressure before you start high firing. Question:   Combustion air preheating Please tell me how air inlet temperature affects boiler efficiency. What are the benefits of air preheating? Answer: The combustion air will be heated to the flame temperature. This heating cost money. If you have some waste heat to be used for preheating the combustion air it will pay. Question:   Water in the heavy fuel oil Is it possible to overheat heavy fuel oil thus causing any water in it to turn to steam and cause problems at the pump and burner?. Answer: Yes it is. The temperature of the heavy fuel oil is very often 130°C to 150°C and water introduced to that temperature would immediately evaporate into steam. When boiling it expands about 1590 times. The situation might be dangerous since the safety valves not are designed for steam. This kind of problem is very likely to occur when you change fuel oil tank and some water from a poorly drained pipe mixes with the heavy fuel oil. Question:   Oil showing in the water level gauge glass Whilst on your engine room rounds, you discover oil showing in the water level gauge glass of an auxiliary boiler. Describe the remedial actions you would take, explaining why such actions must not be delayed. Answer: Stop the burner immediately. Oil present, even small quantities, in boiler water will cause foaming and moisture carry-over. It also forms a heat insulating film, sometimes a carbonized layer, over tubes or shell surfaces. Even a very thin layer may result in tube or plate material failure due to overheating. The oil manifests itself by forming an oily ring inside the water gauge glasses, at the water level. Question:   Emergency low boiler water level You are an officer on watch, & finds the boiler water-level gauge glass to be empty & the burner firing...What is your course of action? (Assuming the gauge glass to be clear & good working order) Answer: Normally a boiler is provided with two independent sensors for emergency low water level burner cut-outs. So this would never happen. However, if it does, don’t take any chances! Shut off the burners immediately! Before you start raising the level in the boiler you have to find out if any part of the furnace walls has been overheated. If you raise the level over a glowing steel-wall then the boiler might produce more steam than the safety valves can handle and a nasty explosion would be the result. A quick test to see if it is safe to put water into the boiler is to temporarily close the steam cock on the gauge glass. If the level rises to the top of the glass, it means that there is still a water level in the water leg, which is also over the highest heat exchange surface in a firetube boiler. The water rises because of the vacuum caused in the glass with condensing steam. Question:   Differential pressure transmitters for the steam drum level Way is the high pressure leg of the transmitter connected to the water side and the low-pressure leg connected to the steam side? Answer: The signal from the transmitter ought to increase when the water level raises and decreases when the level falls. Furthermore the signal shall be zero, and give impulse to stop the burner, in case of transmitter malfunction, power failure or cable breakdown. Both requirements will be fulfilled if the transmitter is mounted with the high pressure measuring point connected below the lowest water level and the low pressure measuring point connected above the highest water level. The output will increase when the level is raised. To compensate for the water column in the reference leg the output signal's zero-point has to be elevated. This is the common method. If the transmitter is swapped, with the low pressure side to lower end and the high pressure side to upper end, then the signal will decrease when the level is raised. This signal can be used to control the level as well, but the signal can not be used to stop the burner for emergency low level in case of power failure or cable breakdown. This system requires an extra sensor to trip burner at emergency low water level. One can of course use the emergency high water level alarm to stop the burner, but this is not correct. The emergency high water level shall stop the feed water pump and whenever applicable stop the steam turbine, but not the burner. Question:   Fluctuating boiler water level The feed water control valve is fully open and the water levels fluctuate at normal boiler load. Answer: Check if: the control valve really is fully open by means of the hand-manoeuvre device. all stop valves in the line are fully open. the suction filter to the feed water pump is satisfactory clean. the feed water pump discharge pressure is sufficient. the feed water control valve pressure drop is normal. (>=2 bar or >=30 psi) If all these are fund to be in order, then you should recalculate the control valves KV-value (CV-value). Under-sized control valves do exist, even though over-sized control valves more often cause malfunction of automatic control loops. The tuning of the controller(s) is dependent on the system you have, but do not even try to tune the control loops until the above criteria are fulfilled. Question:   The burner starts and stops very often The burner starts and stops very often, sometimes every second minute. An alarming temperature-raise has been observed in the combustion air fans electric motor. Answer: Increasing the burners turn down ratio would be a nice solution, but it's not always possibly. Run the burner in minimum load, i.e., prevent the burner from increasing the load just after the burner start. Install a five to ten minutes' time-delay in the fan-motor stop function. Then the fan will continue to run during the shortest burner stops and the combustion air fan motor will get a little rest from the start current. Question:   Most likely source of errors In which part of a boiler control system is it most likely to get a failure. Answer: When you have problem with a boiler control system you should keep in mind that most faults occur outside the control cubicle, but on the other hand, your problem might not be among the most common. Statistically calculated faults in control systems. Transmitters and sensors 40 % Actuators 25 % Controllers 10 % Loss of electric power 5 % Others 20 % Question:   Open steam valves slowly Why has a steam valve, or at least a big steam valve, to be opened slowly? Answer: If you have a one liter of water standing in the pipe just after the valve and open the valve too fast, then you will get a projectile of one kg rushing down the pipe. At next valve, bend or other obstacle the speed of the water mass will be converted into pressure. You can hardly imagine the damage this energy can cause. Thermal stress is an other reason to be very cautious and drain out water carefully when you open a steam valve. A large steam valve ought to have a small by-pass valve to simplify preheating of the pipe. Question:   Low viscosity fuel for high viscosity burners Our boiler plant is designed to be fired with heavy fuel oil of 590 cSt @ 50°C. Would it be safe to run it on Gas Oil of 7 cSt @ 40°C? Answer: Well, this is a question for the burner maker. Don't forget to ask them too. It is outmost important that the fuel oil heaters are by-passed when firing Gas Oil or Diesel Oil. These fuels must never be heated above their flash-point. For a steam atomized burner you have to switch over to compressed air atomizing since light fuels must not be atomized by means of steam. This burner will probably fire the fuel without any problem since they normally are designed for fuel viscosity between 10 and 20 cSt. A rotary cup burner, on the other hand, might give some problem with the flame since this atomizing method normally require a fuel viscosity of about 25 to 60 cSt. In the event of failed ignition, gas-oil is more likely to cause an explosive mixture than heavy oil. However, a dangerous situation should never occur if the procedure for such failed ignition is followed. Upon two consecutive failed ignition attempts, a thorough investigation into the cause should be sought and the furnace correctly purged inline with the manufacturers and ruling classification society’s requirements. N.B.   7 cSt @ 40°C  =  5.5 cSt @ 50°C. The viscosity’s of lighter fuels are normally given at 40°C and the viscosity of heavier fuels at 50°C, 80°C, 100°C or even higher temperature. Question:   Fuel Oil Ignition Quality, CCAI What is the CCAI of a fuel oil? Answer: The CCAI, the Calculated Carbon Aromaticity Index, is a measure of the Fuel Oils Ignition Quality. The value can be calculated using the following, ISO 8217 Ann.C, formula: CCAI = D-81-141 Log10 Log10( VK+0. 85)-483 Log10((T+273)/323) where D = density (kg/m³) at 15°C VK = kinematic viscosity (mm²/s) at temperature T (°C) Question:   Composite Boilers What is the inherent problem in Composite Boiler? Answer: There are different types of composite boilers. Normally one part of the boiler is heated by means of a fuel oil burner and the other part is heated by the exhaust gases from a diesel engine. Heating of one part of a boiler at the time often causes thermal stress that may lead to leakage. One single composite boiler does not fulfill normal requirement of redundancy when the steam is used for essential service purpose. Question:   Steam valves open causing a sudden large load I have two 82,000 lb/hr natural gas fired boiler that is designed for 300 psig and operating at 205 psig (saturated). The boilers serve a large campus with numerous buildings, each with an integrated building management control system. Due to an unresolvable characteristic of the building control system, occasionally all of the building steam valves open causing a sudden large load on the boilers that lasts for 20 to 30 minutes. The demand for steam is not real in that no heat is actually required by the buildings. When this condition occurs we have a serious water carryover problem. My question is how can we maintain boiler pressure and water level while either ignoring or controlling the sudden false load. Our combustion control system is a PLC based system, metered/cross limited air-fuel ratio , three element drum level and oxygen trim. Shall we try to correct thorugh the control algorithm or add backpressure control valves. I look forward to any advise. Answer: The steam capacity doesn’t seem to be sufficient to supply all the fully open control valves. First of all recalculate the control valves. Over-sized control valves are very common cause of problems. Question:   Transport superheated steam Is it possible to transport superheated steam of the order of 45 t/h at 30 bar pressure and temp of 300 deg from a aux boiler to a distance of 1.5 Km? Answer: Calculate with a velocity of 15 m/s (49 ft/s). To avoid water hammering the pipe-line should slop slitely downwards in the steam flowing direction. To start up the line you will need a drain valve on every 30 m (100 ft).

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. 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.

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. Back to the question in the headline, is an automatic controlled boiler an explosion risk? In automatic mode and properly adjusted: No In manual mode, skillfully operated: No In automatic mode and not properly adjusted: Yes In manual mode, not skillfully operated: Yes

Fail-safe systems Any predictable failure must result in a safe situation. How to make a control loop fail-safe Example: A simple control loop for pressure control of a steam boiler with one oil fired burner. Control Valve  An electro-pneumatic Control Valve for the fuel oil to the burner should close upon control air failure as well as missing current signal. Pressure Transmitter   The Pressure transmitter has normally a direct output signal, that is, the signal increases on raising pressure. Normally the current signal will never be lower than 4 mA, but if it does, the Controller should immediately close the burners fuel oil control valve Controller   Any internal fault in the Controller must initiate closing of the fuel oil control valve.   How to make a relay fail-safe A relay is almost fail-safe since it is very likely to break the circuit when it is malfunctioning. However, using two relays will increase the reliability considerably. This can be done in different ways. One method is to wire the relays, A and B, as shown on the picture. The system cannot be reset unless the pressure switch (PS) makes and both relays work correctly, but the circuit between terminal 1 and 2 will break when PS breaks even if only one relay works rightly.   How to make a computerized control system fail-safe An output from a computer will become either high or low when it fails. There are some methods using the computer to check its own outputs. Feeding back an output’s signal to an input will enable the computer to check if the output is what it is supposed to be, but the system as a whole will not be fail-safe. The CPU or any other vital part might break down and then the check of the outputs is out of order. The only way to make a computer system fail-safe is to use an other computer to check all the functions. It is, of course, possible to check all the functions by means of hard wiring and relays, but who wants to do that?

Beware of fuel oil in the marine steam boilers feed water 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 feed water 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. One can say that you will get fewer buckets for your bucks. N.B. ppm=parts per million. 1 ppm=1kg per 1000 metric tons or 1 ppm=2 lb. per 1000 short tons. An oily ring inside the water gauge glasses at the water level is a serious warning.

Harmful Substances in Boiler Feed Water. The mineral and organic substances present in natural water supplies vary greatly in their relative proportions, but are principally comprised under the carbonates, sulphates and chlorides of lime, magnesia and sodium, iron and aluminum salts, silicates, mineral and organic acids, and the gases oxygen and carbon dioxide. Scale is formed from the carbonates and sulphates of lime and magnesia, and from the oxides of iron, aluminum and silicon and it will result in: Reduction in the boiler efficiency because of the decreased rate of heat transfer. Overheating and burning of tubes resulting in tube failure. Scales are dangerous long before it reaches this thickness. A very thin scale can cause tube failure due to overheating. Scale has about one-fortyeighth of the heat conductivity of steel. A scale thickness of about 1 mm (0.04") can be sufficient to reduce the heat transportation to a dangerous point; when the water inside the tube cannot receive and carry away the heat fast enough from the tube metal to keep its temperature below its fusion temperature, resulting in the tubes "burning-out".

Feedwater heating has a number of advantages: Using an open or direct contact feedwater heater, where the water is raised to near-steam temperature is the most effective, the cheapest and the most convenient system of de-aeration. Heating boiler feed water by means of exhaust steam effects a saving of about 1% of fuel for each 6ºC rise in the temperature of the feed water. Consequently, heating the water from 10° to 99ºC saves about 14% fuel. Besides the fuel saving, heating the feed water increases the actual steaming capacity of the boilers. Using surplus of exhaust steam, above that required for heating the boiler feed water, to heat buildings will realize additional economies. If all of the exhaust steam from engines or turbines can be utilized for heating during the heating season, it will pay to shut down the condenser. The first and most important use for the exhaust steam is to heat the feed water, since all of the heat of the exhaust steam so utilized returns undiminished to the boilers. Some other valuable advantages: Lesser temperature difference at the feed water inlet to the steam boiler reduces thermal stress in the material. Higher feed water temperature reduces the shrinks and swell in the steam drum and simplifies the water level control.

Steam boiler water level sensors Differential pressure measuring transmitter. This method uses normally a differential pressure measuring system with "wet leg", that is, the impulse pipes to the positive measuring point below the lowest water level and the impulse pipe to the negative measuring point above the highest water level, are both filled with water. The differential pressure signal is normally elevated so the output signal corresponds to the level in the boiler. 0% of water in the level glass correspond to 4 mA (3 psi) and 100% corresponds to 20 mA (15 psi). The measuring result is insensitive to foam in the boiler drum since the measuring method is based on the weight of the water.

Capacitance measuring electrode transmitter. An insulated electrode is placed in the boiler and the length should be sufficient to reach the water at all acceptable levels. The insulated electrode and the boiler work as a capacitor with the water and the steam as a variable dielectric medium. The capacitance of the ”capacitor” is converted into a current signal. 0% of water in the level glass corresponds to 4 mA and 100% corresponds to 20 mA. The measuring result is rather sensitive to foam in the boiler and deposits on the electrode influences negatively on the measuring results. This method is relatively new as level control in steam boilers although it has been used for a long time for dry substances such as grains in warehouses.

Conductivity measuring electrode level switch. An electrode is placed in the steam dome and the length of the electrode decides the level where the alarm shall occur. A low voltage alternating current passes through the water in the boiler to the electrode and the circuit breaks when the water level falls below the end of electrode. This method is mainly used for high and low level alarms. It is also used, in small size boilers, to start and stop the feed water pumps. Deposit on the electrode is not a problem. Normal degree of coverage does not disturb the measuring results. The measuring result is sensitive to dense foam in the boiler. It’s normally one electrode for emergency low water level alarm, or two electrodes if also high level alarm is required. This water level sensing method is also common on small boilers for start and stop of the feed water pumps. Up to six electrodes can be used in one electrode stand.

Level float switch. Level float switches have been used for decades as level switches in boilers, but nowadays electrodes have replaced them. Moving parts are hard to check in boilers and deposit on the hinge may seriously disturb the function. The measuring result is rather insensitive to foam in the boiler.

Level float transmitter. A level displacer (a float heavier than the water) is suspended to a spring balance by means of a stainless steel cable or a rod. When semi-submerged the weight of the displacer changes when the water level varies and the change in weight is converted into a level signal that corresponds to the level in the boiler. 0% of water in the level glass correspond to 4 mA (3 psi) and 100% corresponds to 20 mA (15 psi). This level measuring method is not so very common onboard ships since the measuring result is very influenced by the ship’s movement. Ashore, on the other hand, it works very well.

Steam 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).

Add to the monthly maintenance list: Check the process impulse piping between the steam outlet line and the steam flow transmitter. Fur deposit is very common in the orifice’s measuring holes. Check the process piping between the steam drum and the water level transmitter. Condensate has to accumulate in the tubing before the transmitter can work properly again. Draining the level transmitter that often wouldn’t be necessary if you have stainless steel tubing between the boiler drum and the level transmitter. N.B. the feed water flow controller has to be in manual mode during these procedures.

Two-boilers operation 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 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.

Hydrazine A colorless fuming liquid, N2H4, derived from diazoacetic acid: used as a reducing agent in organic synthesis and as a fuel in jet engines. Used in a boiler, to reduce oxygen in the system, the normal level of hydrazine reserve to be maintained in the water is between 0.1 and 0.3 ppm. The chemical reaction with oxygen can be expressed as: N2H4 + O2 ==> 2H2O + N2 Excess hydrazine, in a boiler, breaks down to produce ammonia that has beneficial effects in raising the pH of the steam/condensate system and thus affords some protection to the system by neutralizing any carbon dioxide. The reaction can occur in two ways: 3N2H4 ==> 4NH3+ N2 2N2H4 ==> 2NH3 + N2 + H2 Hydrazine should be handled with care since it is carcinogenic and allergenic.

A.    Fuel Oil header The Fuel Oil header should be a standing up relatively large pipe with a deaeration pipe to a higher level than the fuel oil service tanks; some times it's connected to the top of the diesel oil service tank. B.     Fuel Oil filters Clean the Fuel Oil filter frequently. If the filters are clogged the fuel oil might vaporize in the pump and cause damage. A differential pressure meter across the filter would be a good help to have the filters cleaned in time. C.     Shut off valves Shut off valves for cleaning of the filters. The valves should normally be open. D.    Fuel Oil pumps Each pump should have sufficient capacity to run both boilers on maximum load. Normally one pump is running and the other is standing-by. The stand-by pump should start upon low pressure in the process line rather than being started upon a pump motor failure. E.     Shut off non return valves These valves have to be non-return type, or a shut off valve and a non-return valve. The valves should always be open on both pumps, so the stand-by pump can start automatically. F.     Fuel Oil heater The fuel oil heater is a heat exchanger that must have a safety relief valve. Always install a safety relief valve if a fuel oil volume can be shut up and heated. G.    Burners 1.     Change over valve, Heavy Fuel Oil to Diesel Oil The three-way valve ought to be provided with a limit switch that breaks the electric circuit and shut off the fuel oil heating valve, in the control loop 3 , avoiding the diesel oil from being heated over its flashpoint. 2.     Fuel oil pressure control The pressure control loop adjusts the fuel oil pressure by means of the pressure controller and the fuel oil return valve. The pressure is measured after the fuel oil heater to get accurate pressure to the burners. The return valve before the fuel oil heater recycles cold fuel to protect the pumps from overheating. 3.     Fuel oil temperature control The temperature control loop adjusts the fuel oil temperature by means of the temperature controller and the steam inlet control valve. The temperature controller's set point should be set to assure an adequate fuel oil viscosity for the actual burners. Sometimes the control valve is installed in the condensate outlet line. This requires a smaller control valve and the control function works even better. However, this installation is for some reason not so very common. 4.     Fuel oil flow control On large boilers the fuel oil flow controller gets its setpoint from the burner management system or the airflow controller to achieve a proper air-fuel mixture. On smaller boilers the fuel oil flow control valve often is connected to the combustion air damper by means of a metal rod or wire. In both cases the steam pressure in the boiler sets a suitable fuel oil flow. 5.     Fuel oil shut of valves Almost every classification society and other authorities requires two shut of valves for the fuel oil, mechanically in series and electrically in parallel. 6.     Fuel oil recirculation valve This valve does not always exist, but if it does, the valve should open when the fuel oil shut of valves closes. The recirculation has advantages and disadvantages. ·         The advantage is that it keeps the fuel oil line warm when the burner is off. ·         The disadvantages is that it will, in the long run, heat up the fuel oil passing through the oil pump that might cause problem with cavitations. Leading the recirculation fuel oil back to the fuel oil service tank wouldn't be wise. The fuel oil's temperature often exceeds 120ºC and water that might occur at the bottom of the tank will vaporize and in worst-case cause damage to the tank. 7.     Fuel oil drain valve Some authorities require a drain valve to be installed between the shut off valves. The burner management program mustn't open the drain valve before the shut off valves are totally closed and it should close the drain valve before the shut off valves open.

 

A.    Economizer section Preheats the circulating water before it enters the evaporator tubes. B.     Evaporator section Water evaporates and emulsion of steam and water flows back to the boiler. C.     Superheater section Superheats the steam for the turbine. D.    Heat exchanger Preheats the boiler feedwater. E.     Condenser The exhaust steam from the turbine and excess steam from the steam system condense and recycle. F.     Condensate pump G.    Cooling water pump Seawater. H.    Make-up water pump From softener unit. I.       Boiler feedwater pump 1.     Steam pressure control The pressure control loop adjusts the burner load according the steam demand. 2.     Water level control A simple control loop will do for a boiler with large amount of water and relatively small steam output. To minimize shrink and swell at start and stop of the burner it would be wise to have two setpoints for the water level. A lower level (abt. 40%) when the burner is stop and a higher (abt. 50%) when it's firing. 3.     Economizer inlet temperature control The feedwater is pre-heated in order to increase the efficiency of the plant. The circulating water to the exhaust gas boiler heats the feed water and the three-way valve on the inlet to the heat-exchanger controls the temperature. The economizer inlet temperature must never fall below 135°C to avoid corrosion on the economizer-tubes. 4.     Condenser pressure control An absolute pressure transmitter and a controller adjust the cooling-water to the condenser to protect the condensate from being cooled down more than necessary. 5.     Condenser level control The level controller actuates the condensate outlet control valve. 6.     Feedwater tank level control The level controller actuates the make up water control valve.

Burner fuel-oil-atomizing methods Pressure jet atomizer The pressure jet atomizer utilizes the supply pressure energy to atomize the fuel into a spray of finely dispersed droplets. Provided adequate fuel pressure is used, extremely good combustion results can be achieved. The fuel oil is fed into the swirl chamber by means of the tangential ports in the main atomizer body. An air core is set up due to the vortex formed in the swirl chamber; this results in the fuel leaving the final orifice as a thin annular film. This film of fuel has angular as well as axial velocity causing the fuel to develop into a hollow con as it discharges from the orifice. This type of atomizer has a poor turn down ratio. A small drop in atomizing pressure will result in a dramatic reduction in combustion performance. Two or three nozzles are sometimes fitted to achieve turn down, one in use on low fire, two on medium fire and three for full rate firing. A variation of the pressure jet atomizer is spill return or recirculating burner. The problems with this type of burner are that the atomizer has an increasing cone angle of the issuing spray as the burner is turned down with impingement on the furnace walls, being made possible and the additional problem of how to dispose of the returned hot oil if the burner has been running for prolonged periods on low fire. External mix steam atomizer or Steam-assisted pressure jet atomizer The steam-assisted pressure jet atomizer is designed to make full use of pressure jet atomization at high outputs and full use of blast atomization at low outputs. The atomizer consists of a conventional simplex type of pressure jet tip, around which is provided steam supply passage. Steam issues through an annular gap after being provided with the correct degree of swirl to ensure that the steam-exit angle matches the oil-spray cone angle. The steam-supply pressure remains constant throughout the complete turndown range of the burner. No mixing of fuel oil and steam occurs within the burner-gun and hence oil o~tput is unaffected by slight variations in the steam pressure. Internal mix steam atomizer The burner lance consists of two concentric tubes, a one-piece nozzle and a sealing nut. The media supplies are arranged so that the steam is supplied down the center tube and the fuel oil through the outer tube. Consequently, the steam space is completely isolated from the oil space. The steam atomizer consists of an atomizer body that has a number of discharge nozzles arranged on a pitch circle in such a way that each oil bore meets a corresponding steam bore in a point of intersection. Oil and steam (air) mix internally forming an emulsion of oil and steam at high pressure. The expansion of this mixture as it issues from the final orifice produces a spray of finely atomized oil. Oil burners with steam atomizing are tolerant to viscosity changes. In addition to this advantage, the steam atomized oil burners have better turn down, do not require high fuel oil pump pressures, and are frugal in the use of steam. Rotary cup atomizer The rotary cup is driven at high speed (about 5000 RPM) by an electric motor via a heavy-duty belt drive. The fuel oil flows at low pressure into the conical spinning cup where it distributes uniformly on the inner surface and throws off the cup rim as a very fine oil film. A primary air fan discharges the primary air concentrically around the cup, strikes the oil film at high velocity and atomizing it into tiny droplets.The rotary cup burner finds considerable use on packaged shell type boilers. These burners have good turn down ratio and they are rather insensitive to pollutants in the fuel oil. Low pressure air atomizer The principle is similar to that of the rotary-cup-atomizing, but the fuel is forced to rotate in a fixed cup by means of a forcefully rotating primary airflow. Their general construction makes them suitable for firing into chambers of hot brickwork, avoiding all the hazards of back radiation to oil burners of more delicate construction. Low pressure air atomizers imply air up to 1000 mm WG. (40 ins. WG.). Blue flame burner In the blue flame burner some of the hot gases from the flame are circulated back to the inlet where they mix with and heat the combustion air. The fuel oil is, during normal firing, introduced into the hot recirculating gases and vaporizes. When the hot flue gases and gaseous fuel mix with the combustion air it burns with a blue flame without visible smoke. This is the closest to stoichiometric combustion one can get with a commercial burner today. Oilon once made small size blue flame burners without ceramic material and only one fuel nozzle.

Sludge firing A few words about firing sludge in a boiler furnace. Is the sludge a waste just to get rid of or is it a valuable fuel? Well, it’s both. It’s waste oil that you want to get rid of, but it contains energy that you ought to utilize. The sludge has to be conditioned to form a smooth emulsion with between 20% and 30% of water. Circulating the content of the sludge service tank with a large pump and, if necessary, add some water will be a useful method. The percentage of water might be measured by means of a capacitive electrode in the pipe downstream the circulation pump. The measuring method has some disadvantages, but it works. The temperature of the emulsion should be adjusted to achieve the viscosity recommended by the burner maker, but it mustn't exceed the boiling point of the water. It’s normally difficult to fire pure sludge; it has to be mixed with normal fuel oil. The sludge/fuel oil ratio depends on the sludge quality. With a really poor sludge you can only fire about 10% sludge. With a better, well-mixed sludge the ratio could be higher, but the burner has to be continuously supervised, since it’s very difficult to get a homogeneous sludge-mixture and the air demand may change suddenly. There is no problem to fire sludge with this method. The problem is that sludge contains a great portion of ash and nonburnable sediments that accumulates on the surfaces of the furnace. Increased soot-cleaning is strongly advised.

Smoke Density Monitor It is very easy to achieve a smokeless fire without any special equipment for supervision. On the other hand, to achieve a smokeless fire and economic combustion will be a bit harder. The air supply has to be slightly in excess of the theoretical requirements. That is, the combustion air flow is reduced almost to the smoke limit. A plant that never shows traces of smoke at the chimney is not burning the fuel efficiently. There are four instruments that will help you to achieve the optimal combustion: 1.     Smoke Density Monitor.   High smoke density indicates uneconomical combustion and it might also cause penalty from the environmental protecting authorities. 2.     O2 - Analyzer.   High O2 content indicates heating of unnecessary high quantity of combustion air. 3.     CO - Analyzer.  High CO content indicates unburned hydrocarbons. This is not only uneconomical, the CO is also harmful to mans health. Besides, mixed with air, CO might be an explosion risk. 4.     CO2 - Analyzer.  Low CO2 content indicates poor combustion, but it doesn’t tell if more or less air is required. The most important instrument, the Smoke Density Monitor, happened to be the cheapest of them and you can’t do without it if you are trying to optimize the combustion of your burners. The equipment is very simple. A light beam is sent across the flue duct, from a light emitter to a light receiver. An electronic unit monitors the opacity. It indicates 0% if there is no black smoke present and 100% if the light beam is totally absorbed by the smoke. An alarm activates if the smoke density exceeds the preset limit. Some advises: The smoke density meters light emitter and light receiver are normally scavenged by sealing air to keep them free from soot. Therefore it would be vise to install these units higher up in the flue duct than the O2-analyzer to prevent the measured O2 value from being influenced by the smoke density meters sealing air flow. Check and clean the glasses of the smoke density meters light emitter and the light receiver regularly. The smoke density alarm should be overridden during the burners start and stop sequences, since the opacity normally increases before the flame has stabilized. Never adjust the O2 content all the way down to zero unless you have facilities to check the CO content. It might be worthwhile to spend a few minutes studying the diagram below.