CASE STUDIES OF STEAM SYSTEM AUDIT IN SOME PLANTS By K.K.Parthiban, Venus Energy Audit System Introduction This article outlines the outcome of auditing of steam system conducted in process Industries. The details of modifications / improvements carried out may be of use to other boiler users. During the audit the operating staff had exchanged all their experiences and this had resulted in benefit of their plant. Case 1: Steam system auditing in a process industry Interaction & listing of the problems A detailed interaction with various level personnel in the plant resulted in a long list of the problems that could be addressed in order to improve the plant efficiency. The list was quite exhaustive and it included the finer details of the problems as experienced. The studies included both the boiler and the entire steam & condensate system of the plant. For photos refer annexure. The problem, causes & Remedial actions Problem Cause Remedial action Gauge glass There was excess chemical dosage. thinning down. The boiler water ph used to go up 11.8. The normal ph recommended was 9.5 to 10.5. See photo 3 & 4. Replacement at an interval of 10 days. See photo 1. Excess condensate drainage at the first trap itself along the steam line. Frequent choking of the flue tubes in the boiler. Air choking preheater On inspection of the boiler steam drum internal, it was found that there was scope for turbulence of boiler water at the level gauge tapping. See photo 2 Excess chemical dosage has led to foaming inside the drum. See photo 6. Steam risers did not have proper baffles to prevent direct passage of water to drier box. See photo 5 & 7. Wood containing sodium / potassium deposit in the tube sheet at the first pass entrance. Such boilers need precollection chamber & reduction flue gas temperature to less than 650 deg C before shell. See photo 7 & 8. Boilers when fired with high moisture fuels experience flue gas condensation in APH tubes. Simultaneously it leads to ash sticking to the moist surface and leads to choking. See photo 11. The plant personnel were educated on the purpose of chemical dosing. The antiscalant dosage was reduced as the plant has very little make up water. Proposed for baffle box in front of water level gauge. Steam separator baffles added to reduce turbulence inside the steam drum. See figure 1. The chemical dosage was restricted to maintain the boiler water ph to 10.5 Extensive modification carried out in the drum as per figure 2. Suggestion was given for an air operated hose for cleaning the tube sheet on line. This was implemented and the benefits obtained. See figure 3 & photo 9 &10. Steam coil preheater is the right solution. When the air is preheated to 65 deg C, the condensation of water can be avoided. But for this plant pressure jet water
Corrosion of ducts Excess consumption Improvement process performance. fuel in Outside air ingress is seen in to APH causing condensation of flue gas moisture. See photo 12. Boilers with biomass firing need insulation right up to chimney inlet to prevent moisture condensation in the ducts. See photo 13 & 14. Auto air vents were seen installed in the steam piping. These were not maintained. The steam was mistaken as air by the operating staff. See photo 15. So many valves were not insulated at all. See photo 16 & 17 Steam loss was very high at the traps. See 18. Wrong trap arrangement was seen at several places. Traps are not located for easy inspection. See photo 19. cleaning was advised. Tubes fail periodically. It is necessary to understand the periodicity of replacement & plan spare. The inspection door needed strengthening. Every time any inspection door is opened only fresh seal rope must be provided. It was advised to insulate all the ducts up to Chimney inlet and arrest all ingress. Advised to close the auto air vent. Air vents are provided only in heat exchangers. These are again required if there is a possibility of air trapping. Valves are to be insulated with removable pad insulation. This brings down the radiation loss to surrounding. Traps at unnecessary places were closed down. Wherever the traps are required and the traps had started passing steam, the bypass lines were cracked optimally to bring down the loss. Right trap arrangement was given. Wrong orientation of traps was corrected. Regular inspection of traps is a must. For auditing IR camera & ultrasonic probe come in handy. See photo 19 & 20. Case 2: Audit of a coal fired unit in a paper mill The customer had requested for an audit of their plant to bring down the coal consumption. The boiler was originally a husk fired. Later due to fuel price escalations, customer switched over to coal firing most of the year. The problem, causes & Remedial actions Problem Cause Remedial action High coal Vibratory feeder used in place of For coal feeders only rotary / drag consumption rotary feeder. See photo 22. chain feeder to be used. The oxygen level was so much hunting, ( see figure 24, 25 & 26) hat Customer was advised about the art of fuel feed regulation. The CO
the fuel did not burn in the furnace resulting in high carbon in bed ash itself. See photo 23. The surface temperature audit by IR camera proved that lot of scope exists in reducing the heat loss to ambient. See photo 27, 28 & 29. Process side heat loss was also high. In the paper industry, the drying cylinders meant for drying the paper was without the end radiation shields. ppm exceeded 5000 ppm. Insulation was advised to this customer. The surface temperatures between the case with radiation shield & the case without the shield were shown to client. One can see the IR camera outputs at the annexure. See photo 30 & 31. Case 3: Audit of a biomass fired boiler in a paper mill The customer had requested for an operational audit of their boiler to bring down the fuel consumption and to address the dust complaint from nearby factories. The boiler was originally a wood fired boiler. Later due to non availability of wood the boiler was converted to FBC firing with external refractory furnace walls. The biomass fuels used include rice husk, tamarind shell, ground nut shell, coffee husk. The problem, causes & Remedial actions Clinker in bed. formation ID fan capacity inadequate Chimney failure Dust from factory. complaint adjourning The fuels used have ash fusion temperatures as low as 750 deg C. There was clinker formation above the bed on the wall due to high free board temperature excursions & due to presence of uncooled walls. It was seen that the air / flue gas ducting is designed in a haphazard manner. See photo 34. It was seen that the ID fan is provided with a damper right at the fan inlet. Such obstructions bring down the fan performance. See photo 33. This is a common phenomenon in many plants. The cause is the high moisture content in fuel & low exhaust temperature. Though the ash content is less, due to improper combustion, improper ash collection system combined with condensation of water vapor has caused carbon particulate. Furnace is not provided with over-fire air arrangement. High volatile fuels The combustion temperatures were adjusted for a lower bed temperature. The biomass of fouling nature needs waterwall furnace enclosure. Only flue gas recycling is to be done now to reduce the furnace temperature. Ducting reengineering was advised to bring down the draft loss. Customer was advised to remove the flap from the damper and to provide multi flap damper in the MDC outlet duct. The exhaust temperature at chimney top should be over 110 deg C. This can be improved by insulating the gas ducting up to chimney inlet. Customer was advised for rotary ash feeder at all ash collection points. In addition unconventional dust traps as seen in photo 32 & 33 were added. Customer was advised to incorporate secondary air system.
Frequent boiler feed pump failure Conclusion need a good turbulence above the bed, particularly in over bed feed system. This is required to burn off the unburnt gases. The feed pump suction piping is with mistakes that take away the NPSH. See photo 35, 36 & 37. Seeing the CO values in the flue gas at shell outlet the over-fire air has to be adjusted. Customer was advised to remove the thermocouple & flow meter immediately. If problem persists, globe valve shall be replaced with gate valve. Generally in all the plants there remains a good scope for fuel savings & for improving the steam generating & distributing system. There are numerous cases where plant engineers & operators have incorporated many improvements. The users could bring out their achievements in this boiler meet in the further years.
Photo 1: Gauge glass seen eroded. This failure took place every week. Photo 2: Gauge glass located close to risers. This causes turbulence in water level gauge. Figure 1: Photo 3: Excess chemicals have led to whitish deposit seen in the shell above the tubes. Photo 4: Rear tube sheet seen with whitish deposit. If hardness salts are present, tubes would be seen with deposits.
Photo 5: A baffle box provided at one end of the drum did not have level equalizer. The steam water mixture would have to climb over the box. Photo 6: Steam separator seen with foaming mark indicating the drum was under turbulent water level. Figure 2: Photo 7: Risers do not have a cap. Thus the water can go directly to the steam pipe nearby. Photo 8: Inlet of the I pass tubes at shell. A view after cleaning. Old deposits could be seen.
Figure 3: Photo 9: Compressor lined up for ash deposit removal. Photo 10: Cleaning ports in the inspection door. Compressed air line is taken out of port. Photo 11: Fresh air ingress to APH causes gas condensation leading to corrosion & choking. Photo 12: Improper APH inspection door seating leads to cold ingress.
Photo 13: Shell intermittent hopper (not insulated?)with improper flange seating allowing air ingress. Photo 14: MDC cones seen coated with condensed ash deposits. Insulation is advised. Photo 15: Unnecessary air vent in main steam line. At high points, only a vent is required for air release during hydrotest. Photo 16:Uninsulated valves result in heat loss. Photo 17: Uninsulated valves result in heat loss. Photo 18: Steam seen gushing out of trap. There were traps downstream & hence this was closed.
Photo 19: In accessible traps not suited for regular check. Photo 20: Auditing equipment Infrared camera for identifying defective trap, bearings & electrical panels. Photo 21: Energy audit equipment ultrasonic probe for various application Photo 22: Vibratory feeder being used for coal feeding in an FBC boiler. Boiler designed for husk. Photo 23: Unburnt coal seen in the bed ash itself. It is not practically adjust required coal feed rate with vibratory feeder. The LOI ranged to 2% in this case. Photo 24: The Oxygen analyzer is provided to ensure Oxygen is available for combustion. If load is varying then the minimum Oxygen has to be 3.5%..
Photo 25 Photo 26 Photo 27: Uninsulated areas in a bi-drum boiler Photo 28: Almost at all sealbox the temperatures run over the stipulated ambient + 20 deg C. Max temperature is 197 deg C. Photo 29: Close to the bottom drum, in the finned area, temperature is seen to be maximum of 202 deg C. Photo 30: In a paper industry, the drying cylinders are not covered by many. This one is without a cover. We may see the peak temperature is 154 deg C.
Photo 31: This cylinder is with a cover. These drying cylinders rotate at a speed. Naturally the convection heat loss will be predominant as compared to radiation mode heat loss. Photo 32: Ideas to reduce dust emission in small boilers Photo 33: A way to reduce dust emission. Further at all ash collection points rotary feeders added. Photo 34. Haphazard ducting adds up to higher draft loss. Customer had gone ahead with replacement of fans. Photo 35, 36 & 37 show the mistakes made in feed pump suction piping that the frequently the pumps fails to pump due to cavitation. Thermocouple in the line, flow meter & globe valve in pump suction line creates loss of NPSH.