Selected Question And Answer About Boiler 2

Question:   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. For conversion of viscosity and density at one temperature to viscosity and density at an other temperature I recommend  The FUEL OIL Calculation Program. 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) The CCAI-calculation can easily be carried out by means of   The FUEL OIL Calculation Program. 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: No problems. 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 100 ft.