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Application of RCM for safety considerations in a steel plant
Reliability Engineering and System Safety 78 (2002) 325–334 www.elsevier.com/locate/ress
Short Communication
Application of RCM for safety considerations in a steel plant V.S. Deshpande*, J.P. Modak Department of Mechanical Engineering, P.C.E&A, Nagpur University, Nagpur, Maharashtra 440 019, India Received 27 May 2002; accepted 7 September 2002
Abstract Any operation or process done on machine or its parts to enhance the efficiency of machine before or after the breakdown is called maintenance. A concern may be said to be successful over the years, when it runs non-interrupted, maintains a smooth production flow consistently and at optimum productivity levels. Plant can achieve productivity up to a satisfactory level by proper maintenance work. The maintenance system may be categorized as either ‘Planned’ or ‘Unplanned‘. The classic maintenance problems in modern industries are insufficient pro-active maintenance, frequent problem repetition, erroneous maintenance work, sound maintenance practices not institutionalized, unnecessary and conservative PM, sketchy rationale for PM actions, lack of traceability/visibility for maintenance program, blind acceptance of OEM inputs, PM variability between like/similar units, paucity of predictive maintenance applications. Hence it was necessary to develop an appropriate methodology to develop strategies and programmatic approach to deal with such problems. The reliability centered maintenance offers the most systematic and efficient process to address an overall programmatic approach to the optimization of plant and equipment maintenance In this paper, the concept of RCM has been applied to process of vacuum degassing/vacuum oxygen decarburising (VD/VOD) in steel melting shop of a medium scale steel industry. Safety consideration is the significant aspect for selection of the system. By systematically applying the RCM methodology, failures, failure modes are analyzed. To preserve the system function, preventive maintenance tasks such as inspection/checking, lubrication, cleaning, adjustment, replacement, are allotted for various failure modes. RCM based preventive maintenance schedule for the system is formulated and compared with the company’s existing preventive maintenance schedule. For subsystems such as ejectors, water-ring pumps and the RLC/ladle car/furnace, existing maintenance schedule is conservative. For oxygen lance and cooling pumps, maintenance frequency is to be increased to quarterly from yearly in the present schedule. For condensers bimonthly schedule is recommended as compared to none, whereas for cooling pumps the schedule remains unchanged. This reveals that RCM based tasks need not necessarily increase the frequency but can, retain or even decrease the frequency of maintenance based on functional priorities. RCM can also recommend the additional maintenance tasks. Application of RCM based PM schedule requires use of age exploration technique and attitudinal changes. q 2002 Published by Elsevier Science Ltd. Keywords: RCM; Safety; Maintenance schedule; PM; Steel industry
1. Introduction Any operation or process done on machine or its parts to enhance the efficiency of machine before or after the breakdown is called maintenance. A concern may be said to be successful over the years, when it runs non-interrupted, maintains a smooth production flow and consistently and optimum productivity levels. This is not possible only with * Corresponding author. Address: Plot No. 120, Telecomnagar, Nagpur, Maharashtra 440022, India. Tel.: þ 91-712-263822; fax: þ 91-7104-37681. E-mail address: [email protected] (V.S. Deshpande), [email protected] (V.S. Deshpande). 0951-8320/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0 9 5 1 - 8 3 2 0 ( 0 2 ) 0 0 1 7 7 - 1
the installation of fully automated and sophisticated machinery. No plant can achieve productivity up to a satisfactory level without proper maintenance work. By the effective application of maintenance work, the gap between achieved and achievable productivity can be minimized. An efficient maintenance schedule not only reduces the probability of breakage of machine elements or shutdown of machines, hampering the scheduled production, but at the same time such a function enhances the efficiency and accuracy of the production machines, lengthening their life span with usual reliability [1]. The maintenance system may be categorized as either ‘Planned’ or ‘Unplanned’ as shown below.
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operational and economic criteria. RCM employs a system perspective in its analysis of system functions, failures of functions and prevention of these failures [6]. 2.1. Evolution of reliability centered maintenance
1.1. Preventive maintenance Preventive maintenance is undertaken before the interruption of production and major breakdown. It is defined as the planned maintenance of plants and equipment in order to prevent or minimize breakdowns and depreciation rates. It is the procedure followed in most of the concerns to maintain desirable and reliable operating conditions of equipment and machinery. 1.2. Current maintenance scenario The classic maintenance problems in modern industries are insufficient pro-active maintenance, frequent problem repetition, erroneous maintenance work, sound maintenance practices not institutionalized, unnecessary and conservative PM, sketchy rationale for PM actions, lack of traceability/visibility for maintenance program, blind acceptance of OEM inputs, PM variability between like/ similar units, paucity of predictive maintenance applications. Hence it was necessary to develop an appropriate methodology to develop strategies and programmatic approach to deal with such problems. The reliability centered maintenance offers the most systematic and efficient process to address an overall programmatic approach to the optimization of plant and equipment maintenance [2].
2. Reliability centered maintenance Various formal definitions of RCM are as follows. 1. RCM is a system consideration of system functions, the way functions can fail, and a priority based consideration of safety and economics that identifies applicable and effective PM tasks [3]. 2. It is a process used to determine the maintenance requirements of any physical asset in its operating context [4]. 3. A process used to determine what must be done to ensure that any physical asset continues to fulfill its intended functions in its present operating context [5]. 4. RCM is a method for developing and selecting maintenance design alternatives based on safety,
RCM is a unique tool used by reliability, safety and/or maintenance engineers for developing optimum maintenance plans that define requirements and tasks to be performed in restoring or maintaining the operational capability of a system or equipment. The concept of RCM was developed in the early 1970s by the commercial airline industry Maintenance Steering Group and was endorsed by the Air Transport Association, the Aerospace Manufacturers Association and the US Federal Aviation Administration. This new maintenance philosophy was designated MSG-1, updated as MSG-2 and most recently, in a revised form, MSG-3. While originally structured to meet the needs of the airline industry, RCM is adaptable to a vast range of maintenance—important areas including nuclear power plants, weapon systems and products. The effectiveness of RCM as a mechanism for producing cost-optimized maintenance programs has been proven during more than a decade of development and use within the commercial airline industry and the Department of Defense [7]. Only recently has its value been recognized and implemented as an effective tool for establishing maintenance requirements for nuclear power plants [8,9]. 2.2. RCM terminology RCM focuses on the maintenance of system rather than equipment operation. Significant terms in RCM method are system, subsystem, functional failure, and failure mode. 1. System. System is the overall plant or plant subsection that has been identified for RCM analysis. 2. Subsystem. It is an assembly of equipment and/or components that together provide one or more functions and can be considered a functionally separate unit within the system. 3. Functional failure. Every subsystem performs certain function. Functional failures describe how failures of each can occur. 4. Failure mode. A failure mode identifies each specific equipment related condition that can cause the loss of the subsystem’s functions. These terms are called the functional components of RCM, and are interrelated as shown in Fig. 1. The terminology defined above has been used throughout the process of formulation of an effective schedule. This is the first basic step in the application of the RCM approach. The actual implementation of the RCM method involves five steps.
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Fig. 1. Interrelationships of RCM functional components.
2.3. Steps in RCM The implementation of the RCM method involves the following five steps [6]. 1. Define system and subsystem boundaries. The system is divided into mutually exclusive subsystems with separate, non-overleaping boundaries. Everything that crosses these interfaces is identified. The artificial boundary helps to ensure that all of the important equipment necessary for system function is included in the analysis. 2. Define subsystem interface, functions and functional failures. Each system has ‘in interfaces’, indicating what comes into the subsystem, and ‘out interfaces’, indicating what goes out of the subsystem. The in interfaces are transformed to the out interfaces by the functions of the subsystem. These functions and how they can fail are enumerated. 3. Define failure modes for each functional failure. Specific equipment and component failure that cause each functional failure are identified. This is the most detailed level of functional decomposition. It must be performed accurately and completely because it is from these identified characteristics that maintenance tasks will be determined. 4. Categorize maintenance tasks. This deals with the allotment of criticality classes (C.C.) to each of the failure modes. For each of the failure mode, a series of questions are asked. Based on the answers, criticality classes are established for each failure mode. The set of questions is called a decision tree and is shown in Fig. 2. 5. Implement maintenance tasks. The last step of the RCM method is to group tasks. It is a very powerful procedure that measures the effectiveness of the maintenance program. In this step, the maintenance tasks that will be implemented to address the problems identified by each failure mode are developed. 2.4. RCM based maintenance schedule During the implementation, the first three steps of the RCM method are applied initially. These are the steps of
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dividing the system into subsystems and classifying there in interfaces and out interfaces. The in interfaces are transformed to the out interfaces by the functions of the subsystem. The functions and the corresponding functional failure of each subsystem are listed. Considering each functional failure, failure modes for the individual failure are enumerated. The spreadsheet for these steps is prepared. In the fourth step a series of yes/no questions are asked for each failure mode. This is done using the RCM decision tree shown in Fig. 2. This allots critically classes to each failure mode. For example, the safety of the people and the plant is given first priority and this is given class. A. Next the operational capability of the system is put to question. If the failure affects the operation, then it is categorized under the class B, or else it is under class C. If the occurrence of the failure is not visible to the operator, it is classified under the class D. After the allotment of criticality classes, the tasks that will be performed for the maintenance to avoid failure are classified and specified for the failure modes. The criticality classes and the tasks are then entered into the spreadsheet and fresh sheets are prepared to establish the correlation between the failure mode and the maintenance tasks. This approach is used for formulation of maintenance schedule. 2.5. Numbering system To simplify the tracking of a large volume of highly structured data, the use of an index system is very helpful. This index or numbering system is helpful in quickly identifying items as subsystems, functional failures and failure modes and their relationships. The index hierarchy for the functional dependencies of a system is based on the use of seven digits. The left most digit identifies the system. The next two digits identify the subsystem number. Digit 4 identifies functional failure of the subsystem. The last digit refers to failure modes of a functional failure. Illustrative example is as follows: 1000000: system 1 1020000: subsystem #2 of system 1 1020100: functional failure #1 of subsystem #2 of system 1 1020101: failure mode #1 of functional failure #1 of subsystem #2 of system 1
3. Scope of this article The RCM methodology has already been applied to electric arc furnace (EAF), a sub system of same industry [10]. The main reason for selection of EAF as an appropriate system for application of RCM concept was considerable maintenance cost. In this paper based on the consideration of safety, the subsystem is selected. Accordingly vacuum
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Fig. 2. RCM decision tree.
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degassing/vacuum oxygen decarburising (VD/VOD) system of this industry becomes most important. Hence the details follow regarding application of RCM to VD/VOD system. The findings revealed substantial difference between the policy of maintenance observed presently and what it should be as a result of application of RCM.
4. Application of RCM
Fig. 3. SMS layout (SCHEMATIC).
The RCM approach has been applied to a steel-producing unit situated in MIDC, Nagpur. The unit produces cast steels, alloy steels and stainless steels in wide variety of grades and sizes. It also produces special grade steels including ball bearing steel, lead free cutting steel and forging quality steel which have their application in the automobile industry. All the units of the plant have been laid out in sequential arrangement according to technological interrelationship, so as to ensure uninterrupted flow of raw materials, molten iron, hot ingot, etc. as well as the disposal of metallurgical waste, slag, etc. Since this is a continuous process industry, preventive maintenance plays an important role in smooth running of the plant. Preventive maintenance optimization by proper planning and scheduling of various tasks therefore becomes significant area for improvement. Steel industry basically consists of two main divisions: 1. Steel melting shop (SMS): in this blooms are prepared from scrap. 2. Rolling mill shop (RMS): in this required section such as round bars, rectangular bars, square sections are rolled from billets.
In the present case RCM concept has been applied to SMS. SCHEMATIC of SMS is as shown in Fig. 3. SMS consists of following major equipment (Table 1) [10]. 1. Electric arc furnace (EAF). In this, the scrap material as per the specification of the final product is melted. The capacity of the furnace is 22 – 23 tons. This molten metal is sent to LRF or AOD as per the requirement. 2. Argon oxygen decarburising (AOD). In this process the argon and oxygen is introduced in the furnace. With the help of oxygen and argon the carbon is maintained inside the molten metal. This process is done according to the demand of the customer. From this process the metal is sent to the LRF. 3. Ladle refining furnace (LRF). In this molten metal is refined using an electric arc. This molten metal is sent to VD/VOD or Con-Cost. 4. Vacuum degassing (VD) and vacuum oxygen decarburising (VOD). In this process impure gases and unwanted carbon are removed (VD) or kept at a fixed amount in the metal using vacuum and oxygen (VOD). After this the molten metal is sent to the Con-Cost.
Table 1 Equipment details—SMS S. no.
Equipment
Function
Capacity (MT)
Main parts
1
Electric arc furnace
Melting shop
20
2
Ladle refining furnace
Refining and temperature increase
20
3
Argon oxygen decarburising
Reduction of carbon content
20
4a
Vacuum degassing
Removal of impure gases
20
4b
Vacuum oxygen decarburising
Reduction of carbon content
20
5
Continuous casting
Casting of blooms
20
Electrode, electrode holder, electrode cooler, roof furnace shell, water cooled panel, electrode lifting cylinder, roof lifting cylinder, electrode pressing cylinder, door lifting cylinder Electrode, electrode holder, roof, electrode holding arm, electrode lifting cylinder, roof lifting cylinder, electrode pressing cylinder Top cone, trunnion, bottom cone, dishend, tuyers pipe Watering pumps, steam ejectors, water cooled top lance Watering pumps, steam ejectors, water cooled top lance Withdrawal, DC motor operated rollers, mould oscillation mechanism, withdrawal, cooling system, gearbox, electromagnetic stirrer
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5. Con-cost. In this preparing mould different shapes and sizes makes the final product, which is sent to the rolling mill shop. 4.1. System selection—vacuum degassing/vacuum oxygen decarburising (VD/VOD) The reason for selection of the system is the safety considerations. The molten metal is maintained at very high temperatures, i.e. about 1700 8C. Besides the process is maintained and operated by the maintenance personnel themselves. Both the VD and the VOD processes are done at the same time. Therefore its maintenance requires due care and attention. 4.2. System description Vacuum degassing/vacuum oxygen decarburising (VD/VOD) is as shown in Fig. 4. Vacuum degassing (VD). Vacuum degassing is the process of removal of unwanted gases from the molten metal. These gases formed during the refining process, cause impurities and hence have to be removed. The main components of the system are four ejectors (E1, E2, E3, E4), three condensers (C1, C2, C3), two water-ring pumps (M2, M4), two dewatering pumps and cooling pumps. After the refining process, the furnace is moved on the trolley to the VD station. Here a conical dome is lowered on to the ladle. It consists of an oxygen lance, which is used to inject oxygen in the furnace during the VOD process. The process is explained in three stages. Stage 1. The water-ring pumps, also called the vacuum pumps are switched on in the first stage. In these pumps, a water-ring is formed with the help of an impeller which rotates centrifugally and removes all the air from the pumps, creating a vacuum. The vacuum achieved here is 90 mbar. After this the ejector E4 is switched on and due to the vacuum, gases from the metal in the furnace are pulled towards the ejector. This is because of the difference in pressure created. The ejectors consist of convergent – divergent nozzles to create low pressure. In these ejectors there is an inlet for steam at high velocity. When gases enter the ejector they are carried to condenser C3 by the steam. The vacuum achieved here is 71 –72 mbar. Stage 2. In this stage, the ejector E3 is switched on. This further pulls the dissolved gases from the furnace creating a vacuum of 10 –15 mbar. Next the ejector E2 is switched on and this further pulls more gases. The vacuum achieved here is 2– 7 mbar. Finally E1 is switched on and this is the strongest ejector pulling out all the remaining gases from the metal. When a pressure of 1 mbar is achieved it indicates that all the gases has been removed and the metal is purified. The ejectors E1, E2 and E3 send the gases and steam to condensers C1 and C2. Stage 3. In the final stage, in condensers the cold water from the cooling pumps is sprayed on the steam and gases.
Hence, condensation takes place and the water formed is drained out through the dewatering pumps. In this way, the unwanted gases are removed from the liquid metal. Vacuum oxygen decarburising (VOD). In the process of removal of carbon from the liquid metal, some metals like stainless steel, require to be freed from carbon completely, hence they have to undergo VOD process. In this process, the same set up as the VD process is used, the only difference is that an oxygen lance fitted on the dome is activated. The purpose of the oxygen glance is to inject the oxygen into the furnace that would react with the carbon present, forming carbon dioxide. The carbon dioxide released is removed by creating vacuum as in the VD process To create vacuum, water-ring pumps are used. The ejector E4 is switched on and it sucks out as much carbon as it can and sends to condenser C3 by steam. Next ejector E3 is switched on followed by E2 and then E1. In this way maximum carbon dioxide is removed and sent to condensers. In the condensers, water from the cooling pumps is spread on the mixture of steam and carbon dioxide, thus condensing it. Water is drained out through the dewatering pumps. The VOD process for stainless steel takes 2 h to remove all the carbon whereas in the VD process it takes 15 min to remove the gases from alloy steel and it takes 30 min to remove gases from ball bearing steel. The oxygen lance consists of three concentric pipes. The inner most pipe is the convergent – divergent pipe used to inject oxygen in the ladle. It is made up of stainless steel. The middle pipe is the partition between outer and inner pipes. It carries water for cooling and after cooling the warm water is removed to the outer pipe. The height of the oxygen lance is kept about 1 m above the level of the metal in the furnace, to avoid the oxygen from getting ignited by heat in the furnace. 4.3. Existing preventive maintenance schedule of VD/VOD Existing preventive maintenance schedule of VD/VOD is as follows 1. 2. 3. 4. 5. 6.
Ladle car checking—monthly. Ejectors checking—monthly. KDP-100 pumps checking—monthly. Water-ring pump/vacuum pump checking—monthly. Oxygen lance checking—yearly. Hydraulic power pack—quarterly. (1) EJECTORS CHECK POINTS Ejector’s nozzles (E1, E2, E3, E4, E4a). Ejector’s funnel cone (E1, E2, E3, E4, E4a). Ejector’s body (E1, E2, E3, E4, E4a). Steam control valves. PROCEDURE:
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Fig. 4. VD/VOD process.
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Clean the ejector before starting VD campaign. Check the nozzle cone. Check steam flow as per design capacity. (2) CONDENSERS (C1, C2, C3) CHECK POINTS: Condenser cooling nozzle. Control valves. PROCEDURE: Clean condenser by opening of main hole covers. Clean cooling water spray nozzle. Check water flow control valves and do required adjustment. (3) OXYGEN LANCE: CHECK POINTS Copper pipe (inner). Mild steel seamless pipe (other). Nozzle electrolytic copper. Stuffing box. Inlet, outlet connections. PROCEDURE: Checking of leakage by pressure testing. Hydraulic testing (25 kg/cm2). (One spare oxygen lance is kept ready). (4) WATER-RING PUMPS CHECK POINTS: Rotors Rotor shaft Bearing Mechanical sealing Pulley and v-belt Glands/stuffing box PROCEDURE: Check bearing and greasing in bearing. Check stuffing box and glands. Vacuum developed by water-ring pump (100 mbar). (5) COOLING PUMPS CHECK POINTS: KDP-100 mechanical sealing pumps. DSM3—40 Hp pumps for cooling system. Oxygen lance cooling pumps. PROCEDURE: To check Oil level of bearing lubrication chambers. Mechanical seal. Discharge flow of pump. Check valves and control valves. Cooling water and discharge temperature. Water inlet pressure in the cooling header.
Table 2 Spreadsheet for interfaces, functions, functional failures for subsystems 10000 Ejectors In interface: steam Out interfaces: steam, impure gases Functions: Generate high velocity steam by using convergent nozzle Extract gases from the ladle and eject it to condensers. Create low pressure as compared to ladle pressure Send gases and steam to condensers Functional failures: 10100 Blockage of nozzles or nozzle getting released 10200 Failure in creating vacuum 10300 Failure of steam valves 10400 Leakage in joints and gaskets 20000 Condensers In interfaces: steam and gases from ejector, cooling water from cooling pumps Out interfaces: mixture of water and gases in condensed form Functions: Spray cold water on steam and gases Control the speed of flow of water Condense steam into water and remove through outlet Functional failures: 20100 Blocking of nozzles or nozzle getting released 20200 Jamming of control valves 30000 Oxygen lance In interfaces: Oxygen and cold water Out interface: hot water Functions: To eject O2 into furnace Functional failure: 30100 Oxygen leakage 30200 Cold water shortage 30300 Blockage of nozzles 40000 Water-ring pumps (vacuum pumps) In interface: water Out interface: water Functions: Remove the air and create vacuum Maintain the measure of vacuum Functional failure: 40100 Failure in formation of water-ring 50000 Cooling pumps In interfaces: cold water Out interfaces: cold water Functions: Provides cold water to condensers, roof, oxygen lancing, ladle Functional failure: 50100 Failure in the flow of water 50200 Failure of the suction pipe 60000 Roof lifting cylinder/ladle furnace/ladle car In interfaces: molten metal, argon gas, oil Out interfaces: impure gases Functions: To lift the roof of ladle To store the molten metal To carry the furnace to workstation Functional failure: 60100 Cylinder failure 60200 Lifting piston failure 60300 Gear failure of ladle car
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Stuffing box and glands, greasing of bearing, and fill grease if required on bearings.
Table 3 Spreadsheet for failure modes and its correlation Index
Description
333
C.C. I
L
C A
R
4.4. RCM applied to VD/VOD process 10000 Ejectors 10100 Blockage of nozzles 10101 High velocity of steam 10102 Irregular cleaning of nozzle cones 10103 Uncontrolled flow of steam
C B C
10200 Failure in creating vacuum 10201 Failure of water-ring pumps
B
6
10300 Failure of steam valves 10301 Bursting of diaphragm valves 10302 Solenoid coil fails
A B
1 3
A A B
p
10400 Leakage in joints and gaskets 10401 Acidity of steam 10402 Acidic gases from ladle 10403 Gasket failure 20000 Condensers 20100 Blocking of nozzles or nozzles getting released 20101 High velocity of water 20102 Impurities in water 20103 Irregular cleaning of nozzle cones 20104 Uncontrolled flow of water
1
1 3 4 5
4
RCM method is applied to the system of the VD/VOD process. The various subsystems are ejectors, condensers, oxygen lance, water-ring pumps, cooling pumps, roof lifting cylinder. The individual subsystems with their respective interfaces, functions and functional failures are as shown in spreadsheet in Table 2. Since there is only one system under study, the first digit is allocated to the subsystem. The failure modes for each of functional failure for each subsystem, are as shown in spreadsheet in Table 3. These spreadsheets also consist of the allotment of criticality classes to each failure mode. This is done by putting each failure mode under the series of questions shown in the decision tree. The analysis also revealed that the maintenance tasks for the system are of five types. These tasks are directly related to the failure modes that have been entered in the spreadsheet. The tasks are:
4
C
30200 Cold water shortage 30201 Failure of cooling pumps
A
50200 Failure of the suction pipe 50201 Foot valves leakage 60000 Roof lifting cylinder/ladle furnace/ladle 60100 Cylinder failure 60101 Oil pressure built up by pump 60102 Jamming of pressure release valve 60200 Lifting piston failure 60201 Guide roller failure 60202 Bearing failure 60300 Gear failure of ladle car 60301 Gear box failure 60302 Gear coupling failure and lack of greasing 60303 Oil shortage in gear box
2
6 2
20200 Jamming of control valves 20201 Lack of lubrication 20202 Rare usage 30000 Oxygen lance 30100 Oxygen leakage 30101 Leakage of joint (Gasket)
30300 Blockage of nozzles/sleeve 30301 Irregular cleaning of nozzle cones 40000 Water-ring pumps (vacuum pumps) 40100 Failure in formation of water-ring 40101 Leakage involute casing and impeller 40102 Rotor jamming 40103 Lack of lubrication in moving parts 40104 Bearing failure 50000 Cooling pumps 50100 Failure in the flow of water 50101 Blockage in pipes 50102 Impeller damage 50103 Leakage in pipe 50104 Lack of lubrication
1
I: inspection/checking L: lubrication C: cleaning A: adjustment R: replacement
6 2
4 4
B
4
B B D B
4 4 3 1
B B C D
2 3 6 12
C car
3
C B
1/3 2
B B
1 1
B C
1
D
1 1 1 6 6
Suitable tasks were allotted to each failure mode. Thus a correlation of the failure modes was established with the pertinent maintenance tasks. Based on the data collected, the maintenance schedule for the system is now formulated. In the spreadsheet of the ejector, for example, the frequencies of the tasks to be performed are indicated. These frequencies indicate the number of times the tasks has to be performed in a year. All the people, from the maintenance manager to the operator on the shop floor, were consulted to obtain frequencies. Each of them was asked the question—‘If the maintenance of the part was not done how often would it fail in an year?’ Based on the answers obtained, the frequency of the tasks was decided upon and assigned to each failure mode. In the spreadsheet of the ejectors it is seen that the maximum number of times a part has to be maintained is six, which is for water-ring pumps. So, if the maintenance of ejectors as a whole is considered, it can be said that it should be done six times in a year, i.e. every two months.
5. Conclusion The maintenance tasks for all the subsystems were decided and a schedule was prepared. This was then compared with the company’s existing preventive maintenance schedule. The comparison is shown clearly in Table 4.
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Table 4 Maintenance task comparison Subsystems
Existing schedule
RCM-based schedule
Ejectors Condensers Water-ring pumps Cooling pumps Oxygen lance RLC/ladle car/furnace
Monthly N.A. Monthly Monthly Yearly Monthly
Bi-monthly Bi-monthly Quarterly Monthly Quarterly Bi-monthly
For subsystems such as ejectors, water-ring pumps and the RLC/ladle car/furnace, existing maintenance schedule is conservative. For the ejectors, water-ring pumps and the RLC/ladle car/furnace, the frequency of the maintenance need not be monthly. For oxygen lance and cooling pumps maintenance frequency is to be increased to quarterly from yearly in the present schedule. For condensers bimonthly schedule is recommended as compared to none, whereas for cooling pumps the schedule remains unchanged. This reveals that RCM based tasks need not necessarily increase the frequency but can, retain or even decrease the frequency of maintenance based on functional priorities. RCM can also recommend the additional maintenance tasks. The correctness of the frequencies could be argued. The company has been following its schedule for several years and in the present schedule certain changes are suggested. Application of a totally new schedule would have to face resistance from a lot of people in the company. To find a solution to this situation, another technique known as age exploration (AE) technique can be used. AE is a proven technique that can be employed to predict more accurately the correct task interval. In AE method during the initial PM task/overhaul, the entire condition of the system along with its components and parts is meticulously recorded where aging and wear out are thought to be possible. If the inspection does not reveal any wear or aging sign the initial interval is increased by 10% (or more) and continued until on one of the overhauls the incipient signs of aging or wear out are observed. At this point AE is to be stopped and backed of by 10% and this can be defined as correct task interval. It can be best explained by an example. For the
ejectors, as the company cannot shift from monthly to bimonthly directly, it can use this technique to arrive at the right interval. According to this technique, the company should first increase the interval by one week. If there are no breakdowns or problems arising, it can further increase it by another week. This process can be continued till a stage is reached where increasing the interval further would result in major breakdowns or damages to the system. At this point it can stop or perhaps back off by one week. This interval obtained would be more accurate than any theoretical value and can be applied to the other subsystems to obtain accurate tasks intervals for all. Very often it has been observed and experienced that the checklist as per the existing PM schedule (daily checklist) is not attended resulting in corrective maintenance forced outage. This situation again needs to be altered. RCM methodology can be applied to manufacturing facility for establishing the preventive maintenance schedule based on safety as well as economical consequences.
References [1] Sushil Kumar Shrivastava. Industrial maintenance management. New Delhi: S. Chand & Company Ltd; 1998. [2] Smith AM. Reliability centred maintenance. New York: McGrawHill; 1993. [3] Rausand M. Reliability centred maintenance. Reliab Engng Syst Safety 1998;60:121–32. [4] Moubray, John. Reliability centred maintenance. Oxford: Butter Worth– Heinmann Ltd; 1991. [5] Agrawal VK, Gandhi OP. Reliability-centred maintenance. Proceedings of SERC School on RAM, IIT Delhi, India; April 14–25 1997. p. 145–52. [6] Jones RB. Risk-based management. Mumbai, India: Jaico Publishing House; 1997. [7] Anderson AT, Neril L. Reliability-centred maintenance management and engineering methods. Amsterdam: Elsevier; 1990. [8] Kuehn Steven E. Reliability centred maintenance trims nuclear plant costs. Power Engng 1992;August. [9] Fox Barry H, Synder Melvin G, Smith Anthony M. Reliability centred maintenance improves operations at TMI nuclear plant. [10] Deshpande VS, Modak JP. Application of RCM to a medium scale industry. Reliab Engng Syst Safety 2002; in press.
Short Communication
Application of RCM for safety considerations in a steel plant V.S. Deshpande*, J.P. Modak Department of Mechanical Engineering, P.C.E&A, Nagpur University, Nagpur, Maharashtra 440 019, India Received 27 May 2002; accepted 7 September 2002
Abstract Any operation or process done on machine or its parts to enhance the efficiency of machine before or after the breakdown is called maintenance. A concern may be said to be successful over the years, when it runs non-interrupted, maintains a smooth production flow consistently and at optimum productivity levels. Plant can achieve productivity up to a satisfactory level by proper maintenance work. The maintenance system may be categorized as either ‘Planned’ or ‘Unplanned‘. The classic maintenance problems in modern industries are insufficient pro-active maintenance, frequent problem repetition, erroneous maintenance work, sound maintenance practices not institutionalized, unnecessary and conservative PM, sketchy rationale for PM actions, lack of traceability/visibility for maintenance program, blind acceptance of OEM inputs, PM variability between like/similar units, paucity of predictive maintenance applications. Hence it was necessary to develop an appropriate methodology to develop strategies and programmatic approach to deal with such problems. The reliability centered maintenance offers the most systematic and efficient process to address an overall programmatic approach to the optimization of plant and equipment maintenance In this paper, the concept of RCM has been applied to process of vacuum degassing/vacuum oxygen decarburising (VD/VOD) in steel melting shop of a medium scale steel industry. Safety consideration is the significant aspect for selection of the system. By systematically applying the RCM methodology, failures, failure modes are analyzed. To preserve the system function, preventive maintenance tasks such as inspection/checking, lubrication, cleaning, adjustment, replacement, are allotted for various failure modes. RCM based preventive maintenance schedule for the system is formulated and compared with the company’s existing preventive maintenance schedule. For subsystems such as ejectors, water-ring pumps and the RLC/ladle car/furnace, existing maintenance schedule is conservative. For oxygen lance and cooling pumps, maintenance frequency is to be increased to quarterly from yearly in the present schedule. For condensers bimonthly schedule is recommended as compared to none, whereas for cooling pumps the schedule remains unchanged. This reveals that RCM based tasks need not necessarily increase the frequency but can, retain or even decrease the frequency of maintenance based on functional priorities. RCM can also recommend the additional maintenance tasks. Application of RCM based PM schedule requires use of age exploration technique and attitudinal changes. q 2002 Published by Elsevier Science Ltd. Keywords: RCM; Safety; Maintenance schedule; PM; Steel industry
1. Introduction Any operation or process done on machine or its parts to enhance the efficiency of machine before or after the breakdown is called maintenance. A concern may be said to be successful over the years, when it runs non-interrupted, maintains a smooth production flow and consistently and optimum productivity levels. This is not possible only with * Corresponding author. Address: Plot No. 120, Telecomnagar, Nagpur, Maharashtra 440022, India. Tel.: þ 91-712-263822; fax: þ 91-7104-37681. E-mail address: [email protected] (V.S. Deshpande), [email protected] (V.S. Deshpande). 0951-8320/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0 9 5 1 - 8 3 2 0 ( 0 2 ) 0 0 1 7 7 - 1
the installation of fully automated and sophisticated machinery. No plant can achieve productivity up to a satisfactory level without proper maintenance work. By the effective application of maintenance work, the gap between achieved and achievable productivity can be minimized. An efficient maintenance schedule not only reduces the probability of breakage of machine elements or shutdown of machines, hampering the scheduled production, but at the same time such a function enhances the efficiency and accuracy of the production machines, lengthening their life span with usual reliability [1]. The maintenance system may be categorized as either ‘Planned’ or ‘Unplanned’ as shown below.
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operational and economic criteria. RCM employs a system perspective in its analysis of system functions, failures of functions and prevention of these failures [6]. 2.1. Evolution of reliability centered maintenance
1.1. Preventive maintenance Preventive maintenance is undertaken before the interruption of production and major breakdown. It is defined as the planned maintenance of plants and equipment in order to prevent or minimize breakdowns and depreciation rates. It is the procedure followed in most of the concerns to maintain desirable and reliable operating conditions of equipment and machinery. 1.2. Current maintenance scenario The classic maintenance problems in modern industries are insufficient pro-active maintenance, frequent problem repetition, erroneous maintenance work, sound maintenance practices not institutionalized, unnecessary and conservative PM, sketchy rationale for PM actions, lack of traceability/visibility for maintenance program, blind acceptance of OEM inputs, PM variability between like/ similar units, paucity of predictive maintenance applications. Hence it was necessary to develop an appropriate methodology to develop strategies and programmatic approach to deal with such problems. The reliability centered maintenance offers the most systematic and efficient process to address an overall programmatic approach to the optimization of plant and equipment maintenance [2].
2. Reliability centered maintenance Various formal definitions of RCM are as follows. 1. RCM is a system consideration of system functions, the way functions can fail, and a priority based consideration of safety and economics that identifies applicable and effective PM tasks [3]. 2. It is a process used to determine the maintenance requirements of any physical asset in its operating context [4]. 3. A process used to determine what must be done to ensure that any physical asset continues to fulfill its intended functions in its present operating context [5]. 4. RCM is a method for developing and selecting maintenance design alternatives based on safety,
RCM is a unique tool used by reliability, safety and/or maintenance engineers for developing optimum maintenance plans that define requirements and tasks to be performed in restoring or maintaining the operational capability of a system or equipment. The concept of RCM was developed in the early 1970s by the commercial airline industry Maintenance Steering Group and was endorsed by the Air Transport Association, the Aerospace Manufacturers Association and the US Federal Aviation Administration. This new maintenance philosophy was designated MSG-1, updated as MSG-2 and most recently, in a revised form, MSG-3. While originally structured to meet the needs of the airline industry, RCM is adaptable to a vast range of maintenance—important areas including nuclear power plants, weapon systems and products. The effectiveness of RCM as a mechanism for producing cost-optimized maintenance programs has been proven during more than a decade of development and use within the commercial airline industry and the Department of Defense [7]. Only recently has its value been recognized and implemented as an effective tool for establishing maintenance requirements for nuclear power plants [8,9]. 2.2. RCM terminology RCM focuses on the maintenance of system rather than equipment operation. Significant terms in RCM method are system, subsystem, functional failure, and failure mode. 1. System. System is the overall plant or plant subsection that has been identified for RCM analysis. 2. Subsystem. It is an assembly of equipment and/or components that together provide one or more functions and can be considered a functionally separate unit within the system. 3. Functional failure. Every subsystem performs certain function. Functional failures describe how failures of each can occur. 4. Failure mode. A failure mode identifies each specific equipment related condition that can cause the loss of the subsystem’s functions. These terms are called the functional components of RCM, and are interrelated as shown in Fig. 1. The terminology defined above has been used throughout the process of formulation of an effective schedule. This is the first basic step in the application of the RCM approach. The actual implementation of the RCM method involves five steps.
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Fig. 1. Interrelationships of RCM functional components.
2.3. Steps in RCM The implementation of the RCM method involves the following five steps [6]. 1. Define system and subsystem boundaries. The system is divided into mutually exclusive subsystems with separate, non-overleaping boundaries. Everything that crosses these interfaces is identified. The artificial boundary helps to ensure that all of the important equipment necessary for system function is included in the analysis. 2. Define subsystem interface, functions and functional failures. Each system has ‘in interfaces’, indicating what comes into the subsystem, and ‘out interfaces’, indicating what goes out of the subsystem. The in interfaces are transformed to the out interfaces by the functions of the subsystem. These functions and how they can fail are enumerated. 3. Define failure modes for each functional failure. Specific equipment and component failure that cause each functional failure are identified. This is the most detailed level of functional decomposition. It must be performed accurately and completely because it is from these identified characteristics that maintenance tasks will be determined. 4. Categorize maintenance tasks. This deals with the allotment of criticality classes (C.C.) to each of the failure modes. For each of the failure mode, a series of questions are asked. Based on the answers, criticality classes are established for each failure mode. The set of questions is called a decision tree and is shown in Fig. 2. 5. Implement maintenance tasks. The last step of the RCM method is to group tasks. It is a very powerful procedure that measures the effectiveness of the maintenance program. In this step, the maintenance tasks that will be implemented to address the problems identified by each failure mode are developed. 2.4. RCM based maintenance schedule During the implementation, the first three steps of the RCM method are applied initially. These are the steps of
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dividing the system into subsystems and classifying there in interfaces and out interfaces. The in interfaces are transformed to the out interfaces by the functions of the subsystem. The functions and the corresponding functional failure of each subsystem are listed. Considering each functional failure, failure modes for the individual failure are enumerated. The spreadsheet for these steps is prepared. In the fourth step a series of yes/no questions are asked for each failure mode. This is done using the RCM decision tree shown in Fig. 2. This allots critically classes to each failure mode. For example, the safety of the people and the plant is given first priority and this is given class. A. Next the operational capability of the system is put to question. If the failure affects the operation, then it is categorized under the class B, or else it is under class C. If the occurrence of the failure is not visible to the operator, it is classified under the class D. After the allotment of criticality classes, the tasks that will be performed for the maintenance to avoid failure are classified and specified for the failure modes. The criticality classes and the tasks are then entered into the spreadsheet and fresh sheets are prepared to establish the correlation between the failure mode and the maintenance tasks. This approach is used for formulation of maintenance schedule. 2.5. Numbering system To simplify the tracking of a large volume of highly structured data, the use of an index system is very helpful. This index or numbering system is helpful in quickly identifying items as subsystems, functional failures and failure modes and their relationships. The index hierarchy for the functional dependencies of a system is based on the use of seven digits. The left most digit identifies the system. The next two digits identify the subsystem number. Digit 4 identifies functional failure of the subsystem. The last digit refers to failure modes of a functional failure. Illustrative example is as follows: 1000000: system 1 1020000: subsystem #2 of system 1 1020100: functional failure #1 of subsystem #2 of system 1 1020101: failure mode #1 of functional failure #1 of subsystem #2 of system 1
3. Scope of this article The RCM methodology has already been applied to electric arc furnace (EAF), a sub system of same industry [10]. The main reason for selection of EAF as an appropriate system for application of RCM concept was considerable maintenance cost. In this paper based on the consideration of safety, the subsystem is selected. Accordingly vacuum
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Fig. 2. RCM decision tree.
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degassing/vacuum oxygen decarburising (VD/VOD) system of this industry becomes most important. Hence the details follow regarding application of RCM to VD/VOD system. The findings revealed substantial difference between the policy of maintenance observed presently and what it should be as a result of application of RCM.
4. Application of RCM
Fig. 3. SMS layout (SCHEMATIC).
The RCM approach has been applied to a steel-producing unit situated in MIDC, Nagpur. The unit produces cast steels, alloy steels and stainless steels in wide variety of grades and sizes. It also produces special grade steels including ball bearing steel, lead free cutting steel and forging quality steel which have their application in the automobile industry. All the units of the plant have been laid out in sequential arrangement according to technological interrelationship, so as to ensure uninterrupted flow of raw materials, molten iron, hot ingot, etc. as well as the disposal of metallurgical waste, slag, etc. Since this is a continuous process industry, preventive maintenance plays an important role in smooth running of the plant. Preventive maintenance optimization by proper planning and scheduling of various tasks therefore becomes significant area for improvement. Steel industry basically consists of two main divisions: 1. Steel melting shop (SMS): in this blooms are prepared from scrap. 2. Rolling mill shop (RMS): in this required section such as round bars, rectangular bars, square sections are rolled from billets.
In the present case RCM concept has been applied to SMS. SCHEMATIC of SMS is as shown in Fig. 3. SMS consists of following major equipment (Table 1) [10]. 1. Electric arc furnace (EAF). In this, the scrap material as per the specification of the final product is melted. The capacity of the furnace is 22 – 23 tons. This molten metal is sent to LRF or AOD as per the requirement. 2. Argon oxygen decarburising (AOD). In this process the argon and oxygen is introduced in the furnace. With the help of oxygen and argon the carbon is maintained inside the molten metal. This process is done according to the demand of the customer. From this process the metal is sent to the LRF. 3. Ladle refining furnace (LRF). In this molten metal is refined using an electric arc. This molten metal is sent to VD/VOD or Con-Cost. 4. Vacuum degassing (VD) and vacuum oxygen decarburising (VOD). In this process impure gases and unwanted carbon are removed (VD) or kept at a fixed amount in the metal using vacuum and oxygen (VOD). After this the molten metal is sent to the Con-Cost.
Table 1 Equipment details—SMS S. no.
Equipment
Function
Capacity (MT)
Main parts
1
Electric arc furnace
Melting shop
20
2
Ladle refining furnace
Refining and temperature increase
20
3
Argon oxygen decarburising
Reduction of carbon content
20
4a
Vacuum degassing
Removal of impure gases
20
4b
Vacuum oxygen decarburising
Reduction of carbon content
20
5
Continuous casting
Casting of blooms
20
Electrode, electrode holder, electrode cooler, roof furnace shell, water cooled panel, electrode lifting cylinder, roof lifting cylinder, electrode pressing cylinder, door lifting cylinder Electrode, electrode holder, roof, electrode holding arm, electrode lifting cylinder, roof lifting cylinder, electrode pressing cylinder Top cone, trunnion, bottom cone, dishend, tuyers pipe Watering pumps, steam ejectors, water cooled top lance Watering pumps, steam ejectors, water cooled top lance Withdrawal, DC motor operated rollers, mould oscillation mechanism, withdrawal, cooling system, gearbox, electromagnetic stirrer
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5. Con-cost. In this preparing mould different shapes and sizes makes the final product, which is sent to the rolling mill shop. 4.1. System selection—vacuum degassing/vacuum oxygen decarburising (VD/VOD) The reason for selection of the system is the safety considerations. The molten metal is maintained at very high temperatures, i.e. about 1700 8C. Besides the process is maintained and operated by the maintenance personnel themselves. Both the VD and the VOD processes are done at the same time. Therefore its maintenance requires due care and attention. 4.2. System description Vacuum degassing/vacuum oxygen decarburising (VD/VOD) is as shown in Fig. 4. Vacuum degassing (VD). Vacuum degassing is the process of removal of unwanted gases from the molten metal. These gases formed during the refining process, cause impurities and hence have to be removed. The main components of the system are four ejectors (E1, E2, E3, E4), three condensers (C1, C2, C3), two water-ring pumps (M2, M4), two dewatering pumps and cooling pumps. After the refining process, the furnace is moved on the trolley to the VD station. Here a conical dome is lowered on to the ladle. It consists of an oxygen lance, which is used to inject oxygen in the furnace during the VOD process. The process is explained in three stages. Stage 1. The water-ring pumps, also called the vacuum pumps are switched on in the first stage. In these pumps, a water-ring is formed with the help of an impeller which rotates centrifugally and removes all the air from the pumps, creating a vacuum. The vacuum achieved here is 90 mbar. After this the ejector E4 is switched on and due to the vacuum, gases from the metal in the furnace are pulled towards the ejector. This is because of the difference in pressure created. The ejectors consist of convergent – divergent nozzles to create low pressure. In these ejectors there is an inlet for steam at high velocity. When gases enter the ejector they are carried to condenser C3 by the steam. The vacuum achieved here is 71 –72 mbar. Stage 2. In this stage, the ejector E3 is switched on. This further pulls the dissolved gases from the furnace creating a vacuum of 10 –15 mbar. Next the ejector E2 is switched on and this further pulls more gases. The vacuum achieved here is 2– 7 mbar. Finally E1 is switched on and this is the strongest ejector pulling out all the remaining gases from the metal. When a pressure of 1 mbar is achieved it indicates that all the gases has been removed and the metal is purified. The ejectors E1, E2 and E3 send the gases and steam to condensers C1 and C2. Stage 3. In the final stage, in condensers the cold water from the cooling pumps is sprayed on the steam and gases.
Hence, condensation takes place and the water formed is drained out through the dewatering pumps. In this way, the unwanted gases are removed from the liquid metal. Vacuum oxygen decarburising (VOD). In the process of removal of carbon from the liquid metal, some metals like stainless steel, require to be freed from carbon completely, hence they have to undergo VOD process. In this process, the same set up as the VD process is used, the only difference is that an oxygen lance fitted on the dome is activated. The purpose of the oxygen glance is to inject the oxygen into the furnace that would react with the carbon present, forming carbon dioxide. The carbon dioxide released is removed by creating vacuum as in the VD process To create vacuum, water-ring pumps are used. The ejector E4 is switched on and it sucks out as much carbon as it can and sends to condenser C3 by steam. Next ejector E3 is switched on followed by E2 and then E1. In this way maximum carbon dioxide is removed and sent to condensers. In the condensers, water from the cooling pumps is spread on the mixture of steam and carbon dioxide, thus condensing it. Water is drained out through the dewatering pumps. The VOD process for stainless steel takes 2 h to remove all the carbon whereas in the VD process it takes 15 min to remove the gases from alloy steel and it takes 30 min to remove gases from ball bearing steel. The oxygen lance consists of three concentric pipes. The inner most pipe is the convergent – divergent pipe used to inject oxygen in the ladle. It is made up of stainless steel. The middle pipe is the partition between outer and inner pipes. It carries water for cooling and after cooling the warm water is removed to the outer pipe. The height of the oxygen lance is kept about 1 m above the level of the metal in the furnace, to avoid the oxygen from getting ignited by heat in the furnace. 4.3. Existing preventive maintenance schedule of VD/VOD Existing preventive maintenance schedule of VD/VOD is as follows 1. 2. 3. 4. 5. 6.
Ladle car checking—monthly. Ejectors checking—monthly. KDP-100 pumps checking—monthly. Water-ring pump/vacuum pump checking—monthly. Oxygen lance checking—yearly. Hydraulic power pack—quarterly. (1) EJECTORS CHECK POINTS Ejector’s nozzles (E1, E2, E3, E4, E4a). Ejector’s funnel cone (E1, E2, E3, E4, E4a). Ejector’s body (E1, E2, E3, E4, E4a). Steam control valves. PROCEDURE:
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Fig. 4. VD/VOD process.
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Clean the ejector before starting VD campaign. Check the nozzle cone. Check steam flow as per design capacity. (2) CONDENSERS (C1, C2, C3) CHECK POINTS: Condenser cooling nozzle. Control valves. PROCEDURE: Clean condenser by opening of main hole covers. Clean cooling water spray nozzle. Check water flow control valves and do required adjustment. (3) OXYGEN LANCE: CHECK POINTS Copper pipe (inner). Mild steel seamless pipe (other). Nozzle electrolytic copper. Stuffing box. Inlet, outlet connections. PROCEDURE: Checking of leakage by pressure testing. Hydraulic testing (25 kg/cm2). (One spare oxygen lance is kept ready). (4) WATER-RING PUMPS CHECK POINTS: Rotors Rotor shaft Bearing Mechanical sealing Pulley and v-belt Glands/stuffing box PROCEDURE: Check bearing and greasing in bearing. Check stuffing box and glands. Vacuum developed by water-ring pump (100 mbar). (5) COOLING PUMPS CHECK POINTS: KDP-100 mechanical sealing pumps. DSM3—40 Hp pumps for cooling system. Oxygen lance cooling pumps. PROCEDURE: To check Oil level of bearing lubrication chambers. Mechanical seal. Discharge flow of pump. Check valves and control valves. Cooling water and discharge temperature. Water inlet pressure in the cooling header.
Table 2 Spreadsheet for interfaces, functions, functional failures for subsystems 10000 Ejectors In interface: steam Out interfaces: steam, impure gases Functions: Generate high velocity steam by using convergent nozzle Extract gases from the ladle and eject it to condensers. Create low pressure as compared to ladle pressure Send gases and steam to condensers Functional failures: 10100 Blockage of nozzles or nozzle getting released 10200 Failure in creating vacuum 10300 Failure of steam valves 10400 Leakage in joints and gaskets 20000 Condensers In interfaces: steam and gases from ejector, cooling water from cooling pumps Out interfaces: mixture of water and gases in condensed form Functions: Spray cold water on steam and gases Control the speed of flow of water Condense steam into water and remove through outlet Functional failures: 20100 Blocking of nozzles or nozzle getting released 20200 Jamming of control valves 30000 Oxygen lance In interfaces: Oxygen and cold water Out interface: hot water Functions: To eject O2 into furnace Functional failure: 30100 Oxygen leakage 30200 Cold water shortage 30300 Blockage of nozzles 40000 Water-ring pumps (vacuum pumps) In interface: water Out interface: water Functions: Remove the air and create vacuum Maintain the measure of vacuum Functional failure: 40100 Failure in formation of water-ring 50000 Cooling pumps In interfaces: cold water Out interfaces: cold water Functions: Provides cold water to condensers, roof, oxygen lancing, ladle Functional failure: 50100 Failure in the flow of water 50200 Failure of the suction pipe 60000 Roof lifting cylinder/ladle furnace/ladle car In interfaces: molten metal, argon gas, oil Out interfaces: impure gases Functions: To lift the roof of ladle To store the molten metal To carry the furnace to workstation Functional failure: 60100 Cylinder failure 60200 Lifting piston failure 60300 Gear failure of ladle car
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Stuffing box and glands, greasing of bearing, and fill grease if required on bearings.
Table 3 Spreadsheet for failure modes and its correlation Index
Description
333
C.C. I
L
C A
R
4.4. RCM applied to VD/VOD process 10000 Ejectors 10100 Blockage of nozzles 10101 High velocity of steam 10102 Irregular cleaning of nozzle cones 10103 Uncontrolled flow of steam
C B C
10200 Failure in creating vacuum 10201 Failure of water-ring pumps
B
6
10300 Failure of steam valves 10301 Bursting of diaphragm valves 10302 Solenoid coil fails
A B
1 3
A A B
p
10400 Leakage in joints and gaskets 10401 Acidity of steam 10402 Acidic gases from ladle 10403 Gasket failure 20000 Condensers 20100 Blocking of nozzles or nozzles getting released 20101 High velocity of water 20102 Impurities in water 20103 Irregular cleaning of nozzle cones 20104 Uncontrolled flow of water
1
1 3 4 5
4
RCM method is applied to the system of the VD/VOD process. The various subsystems are ejectors, condensers, oxygen lance, water-ring pumps, cooling pumps, roof lifting cylinder. The individual subsystems with their respective interfaces, functions and functional failures are as shown in spreadsheet in Table 2. Since there is only one system under study, the first digit is allocated to the subsystem. The failure modes for each of functional failure for each subsystem, are as shown in spreadsheet in Table 3. These spreadsheets also consist of the allotment of criticality classes to each failure mode. This is done by putting each failure mode under the series of questions shown in the decision tree. The analysis also revealed that the maintenance tasks for the system are of five types. These tasks are directly related to the failure modes that have been entered in the spreadsheet. The tasks are:
4
C
30200 Cold water shortage 30201 Failure of cooling pumps
A
50200 Failure of the suction pipe 50201 Foot valves leakage 60000 Roof lifting cylinder/ladle furnace/ladle 60100 Cylinder failure 60101 Oil pressure built up by pump 60102 Jamming of pressure release valve 60200 Lifting piston failure 60201 Guide roller failure 60202 Bearing failure 60300 Gear failure of ladle car 60301 Gear box failure 60302 Gear coupling failure and lack of greasing 60303 Oil shortage in gear box
2
6 2
20200 Jamming of control valves 20201 Lack of lubrication 20202 Rare usage 30000 Oxygen lance 30100 Oxygen leakage 30101 Leakage of joint (Gasket)
30300 Blockage of nozzles/sleeve 30301 Irregular cleaning of nozzle cones 40000 Water-ring pumps (vacuum pumps) 40100 Failure in formation of water-ring 40101 Leakage involute casing and impeller 40102 Rotor jamming 40103 Lack of lubrication in moving parts 40104 Bearing failure 50000 Cooling pumps 50100 Failure in the flow of water 50101 Blockage in pipes 50102 Impeller damage 50103 Leakage in pipe 50104 Lack of lubrication
1
I: inspection/checking L: lubrication C: cleaning A: adjustment R: replacement
6 2
4 4
B
4
B B D B
4 4 3 1
B B C D
2 3 6 12
C car
3
C B
1/3 2
B B
1 1
B C
1
D
1 1 1 6 6
Suitable tasks were allotted to each failure mode. Thus a correlation of the failure modes was established with the pertinent maintenance tasks. Based on the data collected, the maintenance schedule for the system is now formulated. In the spreadsheet of the ejector, for example, the frequencies of the tasks to be performed are indicated. These frequencies indicate the number of times the tasks has to be performed in a year. All the people, from the maintenance manager to the operator on the shop floor, were consulted to obtain frequencies. Each of them was asked the question—‘If the maintenance of the part was not done how often would it fail in an year?’ Based on the answers obtained, the frequency of the tasks was decided upon and assigned to each failure mode. In the spreadsheet of the ejectors it is seen that the maximum number of times a part has to be maintained is six, which is for water-ring pumps. So, if the maintenance of ejectors as a whole is considered, it can be said that it should be done six times in a year, i.e. every two months.
5. Conclusion The maintenance tasks for all the subsystems were decided and a schedule was prepared. This was then compared with the company’s existing preventive maintenance schedule. The comparison is shown clearly in Table 4.
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Table 4 Maintenance task comparison Subsystems
Existing schedule
RCM-based schedule
Ejectors Condensers Water-ring pumps Cooling pumps Oxygen lance RLC/ladle car/furnace
Monthly N.A. Monthly Monthly Yearly Monthly
Bi-monthly Bi-monthly Quarterly Monthly Quarterly Bi-monthly
For subsystems such as ejectors, water-ring pumps and the RLC/ladle car/furnace, existing maintenance schedule is conservative. For the ejectors, water-ring pumps and the RLC/ladle car/furnace, the frequency of the maintenance need not be monthly. For oxygen lance and cooling pumps maintenance frequency is to be increased to quarterly from yearly in the present schedule. For condensers bimonthly schedule is recommended as compared to none, whereas for cooling pumps the schedule remains unchanged. This reveals that RCM based tasks need not necessarily increase the frequency but can, retain or even decrease the frequency of maintenance based on functional priorities. RCM can also recommend the additional maintenance tasks. The correctness of the frequencies could be argued. The company has been following its schedule for several years and in the present schedule certain changes are suggested. Application of a totally new schedule would have to face resistance from a lot of people in the company. To find a solution to this situation, another technique known as age exploration (AE) technique can be used. AE is a proven technique that can be employed to predict more accurately the correct task interval. In AE method during the initial PM task/overhaul, the entire condition of the system along with its components and parts is meticulously recorded where aging and wear out are thought to be possible. If the inspection does not reveal any wear or aging sign the initial interval is increased by 10% (or more) and continued until on one of the overhauls the incipient signs of aging or wear out are observed. At this point AE is to be stopped and backed of by 10% and this can be defined as correct task interval. It can be best explained by an example. For the
ejectors, as the company cannot shift from monthly to bimonthly directly, it can use this technique to arrive at the right interval. According to this technique, the company should first increase the interval by one week. If there are no breakdowns or problems arising, it can further increase it by another week. This process can be continued till a stage is reached where increasing the interval further would result in major breakdowns or damages to the system. At this point it can stop or perhaps back off by one week. This interval obtained would be more accurate than any theoretical value and can be applied to the other subsystems to obtain accurate tasks intervals for all. Very often it has been observed and experienced that the checklist as per the existing PM schedule (daily checklist) is not attended resulting in corrective maintenance forced outage. This situation again needs to be altered. RCM methodology can be applied to manufacturing facility for establishing the preventive maintenance schedule based on safety as well as economical consequences.
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