5 July, 2013 REPORT ON GENERAL VISIT MADE TO BENGAL ENERGY LIMITED
By K.K.Parthiban / Venus Energy Audit System The visit was made for to study the problems faced with respect to steam temperature and decide on the right operating procedure for better heat recovery from the coke oven flue gas. The visit was made between 1st and 4th of July 2013. Following are the summary of findings and recommendations prepared after the review of data, drawings, si te visit and various discussions held with operation and maintenance team. 1. REVIEW OF BOILER DESIGN PARAMETERS PARAMETERS The superheater surfacing is to be checked for minimum gas flow for which the boiler is designed. The boiler is designed for a minimum gas flow of 93000 nm3/h. This is nearly 80% of the MCR gas flow considered. The steam generation for this gas flow has to be 38.8 TPH nett considering 1.5% blow down. See calculation presented in photo photo 1 of annexure. Some boilers are found to generate mostly 34 TPH. Considering a gas outlet temperature of 178 deg C and a blow down of 1.5%, the gas flow must be 81482 Nm3/h. See photo 2 in annexure. This is about 70.9 % of the MCR gas flow. We can say this is the lowest gas flow for which SH surfacing should be designed for. Normally SH control range is specified based on the minimum gas flow conditions expected. The thermal performance is checked up to SSH to know whether provided SH surface is OK or not. See annexure. With the normal fouling heat transfer factor of 0.002 the surfacing is OK. The dilution of gas by air infiltration from main stack cap is not considered.
Photo 3 is on the performance of the screen tubes. The screen tubes can cause a temperature drop of 30 deg C. Photo 4 shows a program print out on how much would be the heat transfer coefficient to gas radiation in the waterwall above the PSH. The HTC is around 36.35 kcal/hm2C. Photo 5 shows the heat balancing and the resultant gas temperature at PSH inlet. The gas temperature would be 844 deg C. Photo 6 shows the PSH performance. The steam outlet temperature will be 422 deg C. This matches with the present performance with inlet gas temperature of 840 deg C. Photo 7 shows the cavity radiation performance in the waterwall space between PSH and SSH. The HTC is estimated to be 24.11 kcal/m2hC. Photo 8 shows that by heat balancing the inlet gas temperature to SSH would be 724.6 deg C. Photo 9 shows the thermal performance of the SSH with inlet gas temperature of even 714 deg C. The steam temperature that can be achieved is 515 deg C.
Practically after refractory lining the radiant waterwall section, the inlet gas temperature to PSH is presently maintained at 840 deg C. The SSH outlet steam temperature is close to 505 deg C when the steam generation is around 34 TPH.
Lowest load in some some boilers At the time of visit, the load was even 26 TPH. This case is very rare. Since not all the boilers will be on low load, SH surfacing need not be sized for such low load. 2. BOILER REFRACTORY REFRACTORY MODIFICATION The data provided by Thermax engineer was taken up for analysis. In boiler no 630 before refractory application the temperature drop between boiler inlet and SH inlet is 955-717= 238 deg C. After refractory application the same is 955-832 = 123 deg C. Basically the SH inlet gas temp is improved by refractory application to design requirement of 840 deg C. The LMTD has contributed for the steam temperature rise. Once the refractory lining is done, there cannot be much temperature drop between boiler inlet and SH inlet. The screen HTA can absorb heat to cause a temperature drop of 30 deg C. The heating surface is only 26 m2. The gas temperature will drop further by exposed roof. This is estimated to be about 10 deg C. Hence the gas temperature difference at WHRB inlet to PSH inlet should be max 40 deg C. See photo 13 for the calculation. The excess temperature drop is an indication of air mixing inside the radiant chamber and diluting the gas temperature. Instead of 40 deg C, the gas temperature drop is seen to be 123 deg C. As per present surfacing, the gas temperature at PSH inlet has to be 840 deg C, without which the steam temperature can not be achieved. The steam temperature is further governed by any additional fouling. At the time of visit, the extent of dilution was seen to be 90 to 100 deg C. See photo 14. 3. EFFECT OF PRESSURE PRESSURE PART CLEANING Ash constituents There seems to be some fouling taking place in the superheater section. The ash chemical analysis of the coal used at oven is presented below. It contains Na, K and P. There is no physical transportation of the ash to boiler. But alkali such as Na, K and P can transport in molecular form. These molecules condense as alkali sulphates / alkali phosphates over the boiler tube. The deposit analysis must be done and given to fire side chemical suppliers. It may be possible to control the deposition by fire side additive. Economiser fouling It is learnt that the economiser also had fouling. There are two reasons for this. The operation of the economiser with low feed water temperature at any time would have caused a deposit of products. Here also the deposits should be given for analysis to understand the cause.
Lowest load in some some boilers At the time of visit, the load was even 26 TPH. This case is very rare. Since not all the boilers will be on low load, SH surfacing need not be sized for such low load. 2. BOILER REFRACTORY REFRACTORY MODIFICATION The data provided by Thermax engineer was taken up for analysis. In boiler no 630 before refractory application the temperature drop between boiler inlet and SH inlet is 955-717= 238 deg C. After refractory application the same is 955-832 = 123 deg C. Basically the SH inlet gas temp is improved by refractory application to design requirement of 840 deg C. The LMTD has contributed for the steam temperature rise. Once the refractory lining is done, there cannot be much temperature drop between boiler inlet and SH inlet. The screen HTA can absorb heat to cause a temperature drop of 30 deg C. The heating surface is only 26 m2. The gas temperature will drop further by exposed roof. This is estimated to be about 10 deg C. Hence the gas temperature difference at WHRB inlet to PSH inlet should be max 40 deg C. See photo 13 for the calculation. The excess temperature drop is an indication of air mixing inside the radiant chamber and diluting the gas temperature. Instead of 40 deg C, the gas temperature drop is seen to be 123 deg C. As per present surfacing, the gas temperature at PSH inlet has to be 840 deg C, without which the steam temperature can not be achieved. The steam temperature is further governed by any additional fouling. At the time of visit, the extent of dilution was seen to be 90 to 100 deg C. See photo 14. 3. EFFECT OF PRESSURE PRESSURE PART CLEANING Ash constituents There seems to be some fouling taking place in the superheater section. The ash chemical analysis of the coal used at oven is presented below. It contains Na, K and P. There is no physical transportation of the ash to boiler. But alkali such as Na, K and P can transport in molecular form. These molecules condense as alkali sulphates / alkali phosphates over the boiler tube. The deposit analysis must be done and given to fire side chemical suppliers. It may be possible to control the deposition by fire side additive. Economiser fouling It is learnt that the economiser also had fouling. There are two reasons for this. The operation of the economiser with low feed water temperature at any time would have caused a deposit of products. Here also the deposits should be given for analysis to understand the cause.
Faster cooling Faster cooling of boiler with air or cold water cause sticky deposits. This is seen with boilers operating with sulfur containing gases. When the boiler is shut, it has to be allowed for natural cooling only. Only when the boiler is depressurised by natural cooling, ID fan can be used for further cooling. Pumping of cold water is not advised. Section wise fouling monitoring It is advised to monitor section wise temperature rise over a period. On shut down the deposits should be photographed and samples have to be analysed. The alkali phosphates can work as glue. Use of sonic horns / air blasters Sonic horns / air blasters are used for loose dust. If the dust can be blown by compressed air, it can be removed by sonic horns / air blasters. Steam soot blowers Steam operated soot blowers are not provided since the dust load is less than 50 mg/Nm3. It was learnt that removal of the final layer of deposits was difficult even with high pressure water jet. Steam operated soot blower cause erosion as well. Fouling factor for superheater design design The trend of deposition is very slow. It is learnt that the deposition occurs very slowly. If the deposits is adherent then the fouling factor has to be considered in sizing the superheater surfacing. There is a space of 1.6 m between the two SH. It is possible to add one more loop to account for fouling. fouling. 4. DRAFT AND BOILER BOILER OPERATION Draft effect and ID ID fan A natural draft is created at coke oven stack because of the hot gases. The coke oven stack is designed to create the necessary draft by natural draft created by the chimney. The height and diameter of the CO stack are selected to create the required draft at Coke oven. At the foot of the stack the draft is more. As we go up in the height the draft available reduces. Suppose at the foot of the stack the draft created is 40 mmWC at 31.5 m, the draft will be 40*31.5/54 = -23.3 mmWC. As long as the draft maintained by ID fan at boiler inlet point is -23 mmWC, the air would not enter the stack. As the negative draft is increased, cold air would start coming in to the boiler. We must maintain the same draft as the coke oven would have operated using natural draft. The study conducted by SS energy shows that at an rpm of 780 and above, the ambient air is coming in to the boiler. See the table above. The O 2 was measured from a tapping made about 1 m above the
tapping in Coke oven stack. At 695 rpm, the gas had filled the stack above boiler tapping point. At 780 rpm, air has filled the space, which means beyond an rpm the air infiltration starts increasing. See the figure below for the understanding. Variation in gas flow and operators control During the discussion it was informed that the kiln gas flow remains constant. It is not the case at present. This can be found that the ID fan rpm is not kept constant by the operators as they were told to maintain – 40 mmWC at boiler inlet. As such the ideal set point has to be -23 mmWC. When the gas flow is less, the draft loss at stack is less. Then part of the gas would leave the coke oven stack as the available head is more. Operators have to find the optimum rpm at which the oxygen in economiser outlet does not rise beyond 8.5%. This oxygen is said to be 8.5% because the coke oven might be maintaining the same in order to maintain complete combustion of the gases and at the same time they maintain Coke oven temperature at 1050 deg C or so. Even if the gas temperature can be maintained higher it is good for boiler. But it may not be good for the life of the refractory. Boiler can accept 1050 deg C. Using difference between the coke oven gas temperature to boiler inlet gas temperature as a control At the time of visit, boiler 633 was at 971 deg C and boiler inlet gas temperature was at 930 deg C. The difference was about 41 deg C. By decreasing the ID fan rpm, the boiler inlet gas temperature could be raised to 937 deg C. This meant that the outside air infiltration is being minimised. The rpm was at 830 and the steam generation was at 26.9 TPH. The economiser outlet gas temperature came down from 181 deg C to 174 deg C. This means that the infiltration of air is reducing. Further exercise is advised to narrow down the temperature drop. The steam temperature at PSH may go up beyond 430 deg C. As such the PSH outlet steam piping and headers are of P11 material. There is good margin available for PSH outlet header. The IBR calculation page was checked. Photo 16 shows the IBR calculation provided by the boiler maker. Can the Oxygen in flue gas be used for control? It is recommended to install oxygen analyser at boiler outlet. The operator can decide the ID fan rpm based on the Oxygen level. The CO ppm level at the maximum can be 100 ppm. The oxygen at CO oven inlet must be matched. It is advised to do sampling of the gas at CO duct when all the ovens are operating at optimum point. 5. MEASUREMENT OF GAS FLOW AT ECONOMISER AND VARIATION IN STEAM OUTPUT Batch operation of the coke oven and variation in heat content
At a given instance the flue gas flow can be measured by pitot and the gas input to the boiler can be calculated. Accordingly the instantaneous steam generation can be calculated from inlet gas temperature and outlet gas temperature. However the steam generation cannot be assumed to be constant over the 48 hrs. As it was explained by MD in the discussion only the total steam generated per cycle should match. Hence gas flow versus steam generation will not be correct. In fact the gas flow must follow the rate at which volatile matter is released in the coke oven. Is the measured inlet gas temperature correct? Beyond 725 rpm or so, the outside air seems to mixing more into the boiler. Otherwise also if the draft is crossed above a certain point, the cold air would enter in to the boiler. At present the radiant chamber is lined with 70 mm thick refractory. Practically only 50 deg C should be the drop between the WHRB inlet gas temperature and that of PSH inlet gas temperature. The difference between the gas inlet temperatures at the PSH inlet gas temperature has gone up after the shut down. The draft at WHRB inlet must be too high. Hence it is possible the boiler inlet gas temperature is not correctly sensed by the thermocouple. In the boiler 630, a new thermocouple is added downstream of the first thermocouple it is seen that there is a drop of 25 deg C between the two. The air must be travelling at top part of duct. Slowly it must be mixing up in the duct and the mixing must be complete inside the boiler. It is advised to locate a new thermocouple at the middle of gas entry just in front of the screen tube. Can the gate at CO stack be kept full open when the ID fan is in operation? If the Coke oven was operated using CO stack, the gate might have been full open, because the draft created by the coke oven stack might be just sufficient. When ID fan was connected to the system, the available draft is higher than what the CO stack would produce. Ideally draft at CO should be directly controlled by ID fan by the CO operators. At present the CO outlet gate is throttled. Hence the draft at WHRB inlet has to be kept little more. This little more itself can exceed the natural draft at 31.5 m and reverse the air flow. Is there air infiltration from the Fabric joints? The fabric joint flanges are provided with asbestos rope. Over a period it hardens and after cyclic heating it
breaks down. It can develop air leaks. It is advised to seal weld the flanges. The fabric is replaced in situ and never the flanges are required to be removed. The same is recommended in the other two fabric joints. Why the steam generated is reduced by air infiltration? The heat provided by the Coke oven is unnecessarily used for heating the infiltration air from 30 deg C to 180 deg C. If the infiltration air is reduced, the heat would be transferred to water to produce steam. Infiltration air is the key for variation in steam to coal ratio. Day generation versus coal consumed
The day steam generation is not uniform in a boiler. The heat input from the Coke oven is varying / air infiltration levels are varying. Only then the steam flow every day from the boiler to boiler is varying. Hence the gas flow must be varying over time. It is advised to generate trend graphs of the steam generated to understand the steam generation cycles versus CO operation. Depending on the number of charges at coke oven, there would be spikes of steam generation. Hence the steam generation versus coal generation must be computed between cycles and not the days. Perhaps steam generated per cycle (48 hrs / 44 hrs) must be compared. 6. LOSS OF HEAT FROM CASING AND EFFECTIVENESS OF INSULATION Loss of heat from pressure parts and casings in boiler It is advised to check the insulation condition by thermography camera. As long as the surface temperature is less than 60 deg C, the losses of heat will hardly be 0.4% of the total heat. Loss of heat from hot gas duct from coke oven to WHRB The loss of heat from the 10 m hot duct connecting the boiler is expected to be 0.95%. See calculation attached in annexure. Since the duct between CO outlet duct to boiler duct will be 40 m long in total, we can say 4% of the heat is lost between the Coke oven and the WHRB. The gas temperature drop between temperature measurement point at CO outlet and WRHB inlet should not be more than 32 deg C. It is advised to optimise the temperature loss by reducing the rpm and increasing the gate opening. This will reduce the air infiltration from cap. Infiltration flow is a function of the cap sealing. This can vary from boiler to boiler. Hence best installation would be one with smallest difference between CO oven & PSH inlet gas temperatures. Needless to say, that the new thermocouple shall be provided close to boiler inlet.
7. CORROSION OF ECONOMISER Operation at low economiser water inlet temperature It is learnt that at one point of time low feedwater temperature was maintained between the economiser inlet to reduce the boiler exit temperature. Since the gas contains sulfur, it is advised not to reduce the water temperature below 135 deg C. Corrosion can be identified by pitting. Tubes at cold end must be brushed and inspected for pits. Acid dew point is normally around 125 deg C. 8. TIME FOR START UP Boiler with superheater in front At present the radiation chamber is covered with refractory. Hence there is only superheater at first. We need to restrict the gas inlet temperature to 600 deg C till steam pressure rises. If radiation chamber was open to heat pick up, the steam generation would be faster. Start with hot water Always start with hot water. Steam generation will be instantaneous. It puts less strain to SH. Circulation would develop immediately. After hydrotest water must be drained off to DM tank. Lay required piping from boiler to DM plant for this purpose. 9. OPTIMUM PARAMETERS FOR MAIN STEAM AT BOILER Turbine heat rate The turbine heat rate decides the optimum boiler pressure / temperature. Calculate turbine heat rate for various conditions and decide. Each boiler will have its own back pressure based on the line pressure losses. Steam temperature is decided by the gas inlet temperature to the WHRB and the gas quantity. The superheater coils are widely pitched and is directly viewing the inlet hot gases. Hence radiation pick up will be more as the inlet gas temperature is raised. Steam pipe diameter A steam pipe diameter is selected to keep the pressure drop optimum. Thus the material cost will be less. If the diameter is on the higher side, the pressure drop will be lesser. There is no other effect to turbine. 10. METHOD OF FLUE GAS MEASUREMENT Gas flow measurement at CO outlet / boiler outlet Ideally at both places it should be measured. Measuring at hot condition would not be practical as the Pitot cannot be held due to heat conduction. At boiler outlet the gas flow can be measured. The gas flow can be easily measured by averaging pitot at economiser outlet. As the small amount of dust is likely to plug the port of averaging pitot tube, provision must be kept for easy withdrawal and cleaning / compressed air cleaning on line. In case the gas flow is steady from CO, then this flow reading will be steady.
Pitot readings and flow calculations The flow calculation made by plant personnel from pitot readings is correct. The steam generated calculation is wrong as the WHRB inlet gas temperature is considered for steam generation. PSH inlet gas temperature + 50 deg C may be the actual gas inlet temperature after the fresh air mixing. 11. CHECK POINTS FROM BOILER PERFORMANCE RELATED TO COAL CHARGED AT COKE OVEN Steam flow / feed flow measurements between the cycle start to cycle end It is advised to monitor steam and feed flow readings with respect to charging from first oven to recharging of the first oven in the next cycle. This will be meaningful and consistent. Day report will not be correct as the booking of coal may fall in the next day. Check how the weight of the coal consumed is calculated. It should be a direct measure of what is fed. 12. CAUSES OF HIGH FLUE GAS TEMPERATURE AT EVAPORATOR OUTLET AND REMEDY The gas duct arrangement at evaporator and economiser is not good. The evaporator duct size is 6.2 m high x 3.2 m wide. The economiser duct size is 3.2 m high x 6.2 m wide. The length of inter connecting duct is 2.8 m. This will not distribute the gas evenly both at evaporator and at economiser. It is advised to carry out CFD analysis and arrive at the baffle plate configuration suitable for both for minimum gas flow of 83000 nm3 /h and a maximum of 115000 nm3/h. This work is generally done by IIT – industrial services. 13. CAUSES OF ROOF TUBE FAILURE It was informed that the roof tube had failed at two boilers at same locations. The exact tube location detail is not available. The failed tube can be seen in photo 17 & 18 in annexure. Tube is seen to have failed due to long term overheating locally. Localised overheating can be due to stagnation of dirt at the bend. That particular tube seemed to be the central tube of the manhole frame. It is recommended to cover the bend portion completely by 300 mm wide – 70 mm thick refractory. In future the failures can be avoided. 14. EXTERNAL SCALE DEPOSITS AND REMEDY This is already discussed. The scale sample and scale nature will decide the type of the cleaning required. Chemicals can be used to prevent adhesion of the scale. Soot blowers can be used if they are not strongly adherent to tube. If it a foulant and thickness is minimum, then heating surface has to be increased as necessary. 15. MAIN STEAM TEMPERATURE DROP BETWEEN THE BOILERS AND TURBINE Measurement error For steam temperature measurement and for < 600 deg C, RTD is to be used for measurement. At present K type thermocouple is used which is practically used for a maximum temperature of 1400
deg C. There is no transmitter used. Directly cable is taken to DCS. There can be mV drop. Disconnect the cable and measure mV at thermocouple. Measure the mV at cable end (before DCS) and compare. Local mercury in steel thermometer Mercury in steel thermometers should be provided to measure the temperature locally. It is learnt that new tapping has to be made. The range of the thermometer can be 600 deg C. Steam loss in steam line It is seen that the steam traps are kept in service in the main steam line. Superheated steam lines do not cause condensation. It is the trap piping that condenses the steam and creates the trap action. Heat being removed from the piping can cause temperature drop in main steam. In the case of black out, drains shall be opened just prior to restarting. Drains shall be opened when the steam is first charged to cold piping. Heat lost in steam piping
The temperature of the insulating cladding of the piping is reported to be 55 deg C. The heat lost over the entire steam piping is calculated and the revised steam temperature is found out. The temperature drop in main steam piping is thus only 8 deg C. 16. REPLACEMENT OF INSULATION This is not required as the skin temperature is reported to be 55 deg C. It is advised to measure the temperature with IR camera to locate the sources of heat loss. Nowadays rigid insulation mattresses are available either from mineral wool or from glass wool. One such product installed in another plant for loss of temperature is shown in photo 16 of annexure. Similar products are available in LRB
mattress too. 17. IMPACT OF AIR INGRESS AT DIFFERENT ZONES OF BOILER Air ingress is a heat loss. Instead of heating the water / steam the heat is transferred to air. Thus it is a loss. It is seen that the all the air ingress points are already sealed. The oxygen mapping taken by SS energy confirms that there is no air ingress. The oxygen levels are same right from the boiler inlet to economiser outlet for all boilers.
18. POLICY OF ID FAN RPM This is already covered against point no 4. ID fan rpm should be such that the temperature difference between the Coke oven outlet and the flue gas inlet to WHRB. New thermocouple should be installed just at inlet of screen tubes. Once the oxygen analysers installed, the dilution can be prevented easily by following O2 since the response will be better with O2. 19. SOOT BLOWERS This point is covered under point 3. Soot blowers will not be useful for removal of adherent alkali layer. As such the dust load is less large accumulations of dust will not be present. 20. SEALBOX FILLING MATERIAL Superheater seal box cannot be filled with refractory as it will prevent thermal expansion of the SH coils. Only ceramic blankets / boards are used at the hotter side and is backed up by the insulation mattress. If hot spots are noticed, ceramic blankets are recommended for the complete seal box. 21. MINOR MODIFICATIONS IN BOILER Support for flue gas inlet duct It was seen that the boiler flue gas duct is lifted up along with boiler. Usually a spring support is
provided here. It is learnt that spring supports are provided for one boiler only. See photo 22 in annexure. Hopper baffle plates Evaporator hoppers and economiser hoppers can be provided with gas baffles to prevent gas by passing at bottom. See photo 23 in annexure. Roof for horizontal economiser duct It was seen that roof cladding gets blown due to wind. A self (duct) supported light weight roof may be provided to prevent rain water ingress to insulation. This area will collect rain and lead to corrosion of economiser top casing. Curved bends The ducts in ID fan area are seen with abrupt 90 deg bend and abrupt Ts. Smooth bends will help in reducing the power. 22. DIFFERENCES IN TOTALISED BOILER STEAM FLOW & TG STEAM FLOW Range of calibration of transmitters Range of calibration can be changed for sensitivity. If the steam generation of the boiler is not exceeding 50 TPH, then calibrate to 55 TPH flow. Correction for specific volume changes Mass flow of steam is dependent on the steam pressure and steam temperature. When it varies the mass flow needs to be corrected. Check if this is inbuilt in the indicated flow. The feed flow will be fairly accurate as its specific volume does not change. Arrangement of condensing pots for main steam line Condensing pots for steam flow nozzles shall be located at the same level of the piping or above the piping. It is learnt that at present the pots are below the steam lines. 23. DRUM LEVELS ARE BEING MAINTAINED BY THE 30% FCV ITSELF Specification of the feed pump It is seen the FCV inlet pressure is around 105 kg/cm2 whereas the steam drum pressure is at 68 kg/cm2. The feed pump specification was not available for comments. Pressure developed by the pump can be reduced by fluid coupling / VFD. VFDs are available nowadays for HT motors too. The pump and motor suppliers concurrence is required for this. Fluid coupling is a good choice for power saving as well. Stage removal is possible in the pump. KSB will ask for the pump to be sent to works for this purpose. This will be the most economical solution. During the shutdown time, the pump may be sent for changes. Or else procure one new pump and fit on the existing base and send remaining pumps one by one for changes.
Valve specifications The valves are seen with discharge pressure of 97 kg/cm2 as well. See photos 24annexure. At no point of time this pressure is required in service. Due to this valve trim will not be suitable for higher travel. Normally valves have pressure drop from 2.5 kg/cm2 to 12 kg/cm2 only, which is based on the pump discharge pressure at various flows. 24. TEMPERATURE CONTROL FOR EXTRACTION LINE AND DESH Specification of TCV Extraction line TCV will call for larger pressure drop. It is to be given with multistage orifice by the valve manufacturer. DESH TCV is wrongly specified. See specification attached in photo 25 in annexure. This can be used once the pump discharge pressure is reduced. 25. WATER CHEMISTRY Blow down It is learnt that the CBD needs to be kept open continuously and thus HP dosing is continuous. CBD is being given to control silica. Silica can be kept in boiler water at 2.5 ppm for this pressure. Silica can rise due to chemicals. Please check this out. Steam loss out of the system will lead to concentration of silica. Stopping of steam traps will considerably reduce the steam loss. It is seen that the deaerator venting is already minimum. Any other passing of valves must be arrested. PO 4 and pH in boiler water The PO4 level can be reduced to 4-5 ppm to avoid problems related to concentration of chemicals in evaporator bundle, which has low flow circuits. The pH of the boiler water can be 9.4 to 9.6. Only in the case of condenser tube failure we need to boost PO 4 to avoid low pH situation. Drum internal condition It is required to observe the steam drum internal surface for magnetite layer formation. See photo 26 & 27 in annexure showing two steam drums. The best drum is shown in photo 26. If this surface is not achieved, problems can develop at evaporator bundles. Corrosion products can accumulate at evaporator bundles and its supply pipes. Assess mechanical carry over It is advised to check sodium in boiler water and in saturated steam. Even trace of PO4 found in saturated steam is an indication of mechanical carry over. If mechanical carry over is present, the boiler water silica has to be lowered and the blow down losses will be more. CONCLUSION
1. The useful heat from Coke oven should not be diluted by excess air both and coke oven end and at boiler end. Maintaining gas temperature close to 1050 is advised. 2. The gate at coke oven outlet shall be at the same level, when the CO stack is used for operation.
3. 4.
5. 6.
7. 8. 9.
Its opening can be increased step by step, if the temperature difference between the Coke oven outlet and WHRB inlet is more than 40 deg C. By reducing the pressure drop at gate, natural draft at WRHB inlet can be increased and thus infiltration can be reduced. The draft at CO outlet and temperature indications shall be duplicated at boiler DCS for better regulation of the ID fan rpm. O2 analysers shall be added at boiler outlet and CO outlet. Maintaining a constant O 2 will help in achieving highest steam generation. Heat carried away by ambient air can be reduced. Once installed O2 has to be minimised subject to gas temperature limit of 1050 deg C, which otherwise can affect the refractory life. Add thermocouples just after the screen tubes at mid point to know the gas temperature. Coke oven outlet and boiler outlet temperature difference shall be minimised by the operators by adjusting the ID fan rpm. If this helps to improve the steam temperature then consider partial removal of refractory. Add one loop of SSH coils to take care of fouling. This decision has to be taken after stabilising the boiler operation as per point 6. The coke oven cap design shall be good to minimise the ingress flow. The sealing detail of the new cap and existing caps may be sent to undersigned for review. Measure the draft above WHRB tapping at a time when Coke oven is operating with its stack. This is the value to be maintained by ID fan to minimise the infiltration / loss of useful heat.
Feedback / complete log sheet may be sent to undersigned in a tabulated form giving the O 2 at boiler outlet, draft at WHRB inlet, temperature difference between Coke oven outlet and WHRB inlet. The above three can be the control parameters. We have to find out which gives the best result.
K.K.Parthiban
ANNEXURE
Photo 1: Steam generated with 93000 nm3/h considering 1.5% blow down.
Photo 2: The gas flow for 34 TPH steam generation is 81482 kg/h. This is about 70.9 % of the design gas flow. The SH surfacing should be designed for this.
Photo 3: Performance of the screen tubes is predicted above for a gas flow of 81482 nm3/h – 103042 kg/h.
Photo 4: The heat transfer coefficient at radiant chamber is expected to be 36.35 kcal/hm3deg C.
Photo 5: Radiant waterwall above the PSH will cool the gas to 844.34 deg C. There will be a steam generation of nearly 6782 kg/h from radiant section.
Photo 6: The PSH outlet steam temperature should be 422 deg C for steam flow of 34000 kg/h.
Photo 6: The cavity above PSH would have a heat transfer coefficient of 24.11 kcal/hm2deg C.
Photo 8: The cavity between PSH and SSH would drop the gas temperature to 724.6 deg C.
Photo 9: The SSH will give main steam temperature of 519 deg C incase no DESH is envisaged.
Photo 10: Before shut down the screen drop is seen to be 946-821 = 125 deg C. After shut down the temperature drop across screen is seen to be 939-800 = 149 deg C. First of all screen does not drop the temperature this high. It can be the effect of air dilution. The above is for boiler no 633.
Photo 11: The above is the temperature profile of boiler 631 before and after shut down. The temperature drop across screen before shut down is 1033-898.5 = 134.5 deg C. After shut down, this is seen to be 966-833 = 133 deg C. Again screen does not drop the gas temperature this much. There can be air dilution effect.
Photo 12: The above is the temperature profile of boiler 630 before and after shut down. The temperature drop across screen before shut down is 956-832 = 124 deg C. After shut down, this is seen to be 936-805 = 131 deg C. Again screen does not drop the gas temperature this much. There can be air dilution effect. The dilution is governed by the draft at boiler inlet. Higher the draft higher will be the dilution. Higher the ID fan rpm, higher will be the air ingress from CO stack cap.
Photo 13: The gas cooling due to present refractory lining in four walls except the roof will be to an extent of 10 deg C only. At the time of visit the picture of gas temperature profile and ID rpm were as below.
Photo 14: The above shows there is about 135 deg C to 154 deg C whereas it should be only 40 deg C due to screen tube and the roof panel exposed to heat pickup.
Photo 15: The main steam line temperature drop can be around 8 deg C only if the surface temperature is 55 deg C.
Photo 16: The O/L header of PSH and DESH are all of P11 material.
Photo 17: The failed roof tube shows long term overheating sign.
Photo 18: The failed location is shown. The swelling is at the beginning of the bend. It is localised and hence it is not a complete starvation case.
Photo 19: The central tubes at manhole header would have less flow. Hence the construction dirt / dirt ingress during shut down / corrosion products in boiler water can accumulate at the roof bends of these tubes and cause over heating failure.
Photo 20: The riser at roof panel is made with a loop. This creates air lock and retard circulation.
Photo 21: Photo shows the rigid insulation used for steam piping at another plant. This will not sag due to its weight. The product is sold by dynaweldindia.com.
Photo 22: boiler inlet gas duct is refractory lined and it is a cantilever duct. It should be supported by springs to avoid tube / fin cracks.
Photo 23: Gas baffles are to be provided in ash hoppers for a depth of 1 m and to the full width. It is to be welded to the hopper directly. This will prevent gas bypassing. Same is to be provided for evaporator hopper.
Photo 24: The FCV and TCV are specified for a 3 kg/cm2 pressure drop as well. Up to 21% of valve travel the control valve will work better. When the pump is provided with fluid coupling all FCV and TCV would work better.
Photo 25: The above is the TCV specification. The outlet pressure for design is given as 97.44 kg/cm2. This is wrong. The inlet pressure also is 105 kg/cm2 actually. This will make the regulation difficult. It is advisable to reduce the pump discharge pressure and go for lower DP valve to save power.
Photo 26: The photo shows a good drum with the magnetite layer. This is what is to be achieved by the proper pH.
Photo 27: The photo shows a bad drum with corrosion products in the boiler water. Take photograph of the steam drums and compare the treatment effect.
