Analysis of failure in boiler industry--a case study. Introduction For any Boiler industry to generate power, apart from the main equipments, some auxiliary equipments are also needed to support them for the generation of power. The equipments for the processes like fuel supply, air supply, ash removal, water supply are pulverizing mills conveyors, induction fans, forced draught fans, water circulation systems, etc. The pulverizing mill is used for converting the raw lignite into fine powder. This process is termed as pulverization. Pulverization is done in order to improve the utilization of the fuel efficiently. This helps in reducing the unburnt fuel in the boilers

Construction Details of Pulverizing Mills The mill consists of a hollow shaft on which discs with suspended hammers are provided. Spacers are provided to maintain the distance between the discs. The hammer holders are fixed upon the holder's pins that pass through holes in the discs. The hammers in turn are fixed to hammer holder pins. The hammers are made of high wear resisting material such as 13% of manganese steel. The mill housing is of welded structure. The inner surface of the housing is faced with inner plates to protect the walls against adhesive effect of lignite and hammer is about 30mm which increases with wear on the hammers and wear plates. The side walls of housing are provided access for replacing the worn out hammer and holders. The mill rotor is mounted on the double row self adjusting roller bearings. A water jacket assembly is fitted in the bearing housing. Cooling water flows from the free end to motor end through a metal tube and comes out through the hollow space of the rotor, there by cooling the rotor which is subjected to the heat of gas and mixture during the fact. Each mill is provided with emergency button nearby for stopping the mill in case of emergency.

Identification of failures in the industry During the operation of pulverizing mills, different types of mill break downs, outages occur due to many reasons. Some are inevitable, for example, Failure due to wear and tear and some are accidental, for example failure due to foreign material entry. Pulverizing mill break downs has a great significance in the operation of a boiler unit as it leads to a considerable drop in steam generation and power generation. Some common failures in pulverizing mills of the industry are also discussed.

Identified Failures (1) Hammer throw off. (2) Bearing Failure. (3) Rotor shaft failure. (4) Wear plate failure. (5) Shield plate failure. (6) Over loading of mills. (7) Knocking of bearings. Hammer Throw Off This refers to a failure which occurs due to dislodging of hammers from its position. It is a chain/cascade process and dislodging of one hammer leads to hitting the other hammer and there by the damage is much with in a short time span. Hammer throws off Normally, hammer throw off occurs due to entry of foreign material with lignite. When the foreign material hit the hammer it is dislodged from its position and leads to a cascade process of hammer damage. Some times worn out hammers, holders also happen to be the reason for hammer throw off. Consequences of Hammer Throw: If the mill is not stopped immediately, hammer throw off can cause much damage to many parts of the mill namely, hammer, bearings, rotor, shafts, insulation plates and leads to imbalance of rotor. Bearing Failure: There are two bearings at both the ends of the rotor(one is free and the other is coupling end). Any damage to the bearings will lead to stoppage of mill and needs minor replacement. Rotor Shaft Failure This is a major failure in mills. Normally rotor shaft failure occurs due to shearing of the shaft. The failure generally originate from the key hole area of the shaft and propagate to other parts of the shaft and finally the whole shaft is sheared off. Though the rate of occurrence is much less, the failure leads to stoppage of mill for up to 2 days. Wear Plate Failure Wear plate is fastened to the base plate or mother plate. Due to failure of fasteners, which fix the wear plate with mother plate, rub with hammers and lead to over loading of mill or hammer throw off. Shield Plate Failures: Due to continuous exposure of high temperatures welding joint failure occurs in the plate and that leads to overloading of mills. The temperature in the mill shaft is to be strictly adhered to avoid plate failures.

Overloading of Mills: This is another type of performance failure which leads to stoppage of mill. Various factors are responsible for overloading of mills. They are worn out hammers, more air ingress and choke in pulverized fuel burner. Knocking on Bearings" This is an indication of imbalance of rotor or failure of bearings. The mill has to be stopped and inside of mill are to be inspected to find out the reason for knocking. Water line failure Though the possibility of failure is remote, the consequences of the failure are very serious leading to mill shaft(rotor shaft) failure. Methodology Adopted The methodology used in this article is Failure Mode and Effect Analysis(FMEA). It is used to identify the potential failure modes for a product or process, to assess the risk associated with those failure modes, to rank the issues in terms of importance and to identify and carry out corrective actions to address the most serious concerns.

Figure : Composite photograph showing the failure

Data Collection After collecting the datas for 6 months from July to December and the total equipment outages (in hrs) are tabulated below. Results and Discussions

Suggestions The following suggestions were made after analyzing the failures in the boiler industry. (1) Magnetic separators must be installed at intake side of the mill to avoid the entry of foreign particles into the mills. This reduces the problem of hammer throw off. (2) The conveyor belt speed must be reduced from 9.3m/sec to an optimum speed of 4.2m/sec in order to separate foreign materials from lignite. Because slower speed increases the contact of lignite with magnetic separator. (3) Width of the belt over the conveyor should be increased from 900mm to 15600mm as increased width of the belt ensures greater contact of lignite with magnetic separator. (4) Inspection of bearings must be done at regular intervals to avoid bearing knocking at free and coupling end of the mill. Also proper lubrication must be done to avoid damages due to friction. (5) The cooling water line must be coated with paint to avoid the corrosion of the pipe lines. Also the replacement of cooling pipe lines at regular intervals of time must be carried out. (6) The shaft material must be surface hardened to increase the fatigue strength. This must be done to avoid the fatigue failure. (7) The shaft key material must be replaced with C55 Mn75 whose tensile strength is 700-850 N/[m.sup.2], where as the existing one is made of C50 and its tensile strength is 650 N/[mm.sup.2]. (8) Preventive maintenance schedule is to be modified and its frequency must be increased. (9) The hollow shaft must be replaced with splined shaft to avoid shaft puncture.

PIPELINE RUPTURE, LIQUID BUTANE RELEASE, AND FIRE

Case study :

Operating conditions of the pipeline :

Fluid : liquid butane

Temperature : 25 c

Material of Pipeline : steel

Diameter : 8 – 10 inch

Schematic illustration of the pipeline

Figure of the pipe failure (Pipe section containing 12.5-inch rupture )

Figure : Composite photograph showing corroded area at center of rupture

Laboratory analysis results:

Pipe surface samples were acidic with a pH of 5 to 6, Sulfides were present in small amounts, Sulfate-reducing bacteria were present in insignificant amounts, Anaerobic acid-producing bacteria were present in small amounts (100 bacteria/ml), and Aerobic acid-producing bacteria were “strongly present” (10,000 bacteria/ml).

Conclusions Findings 1. The corrosion damage found during the 1996 postaccident inspection indicated that rapid corrosion had occurred on the pipeline since the 1995 internal inspection. 2. It is unlikely that the pipeline corrosion damage near the rupture location was underreported by the 1995 internal inspection. 3. Stray currents did not contribute to the corrosion observed on the pipeline. 4. Inadequate corrosion protection at the rupture site and at numerous other locations on the pipeline allowed active corrosion to occur before the accident. 5. Because cathodic protection levels were inadequate, the stress cracks that existed in the coating created areas in which rapid corrosion could occur. 6. Disbonded tape coating most likely created locally shielded areas on the pipe that prevented adequate cathodic protection current from reaching its surface, creating other areas in which rapid corrosion could occur. 7. Although Koch’s records contained information that cathodic protection levels were inadequate and that active corrosion was occurring on its pipeline system before the accident, the conditions went uncorrected. 8. The tape coating on Koch’s entire pipeline may have tape cracking and disbondment. 9. Because no overall requirement exists for operators to evaluate pipeline coating condition, problems similar to those that occurred on Koch’s pipeline could occur on other pipelines. 10. This accident illustrates the continuing need for performance measures for adequate cathodic protection on liquid pipelines. 11. Stress cracks and disbonded tape coating on the pipe created areas where microbial corrosion could potentially occur. 12. The contribution of microbes to the corrosion damage cannot be accurately determined because of inadequate sampling and testing techniques. 13. The format and content of the public education bulletin mailed by Koch before the accident did not effectively convey important safety information to the public.

14. Koch’s distribution program for its public education materials before the accident was inadequate. 15. Requirements for the content, format, and periodic evaluation of public education programs can help pipeline operators ensure that their programs are effective.