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Reliability Centered Maintenance in Power Plants

Team members: Seenuvasan Arun Ravindran, Prem Sagar, Sindhu Nair ------------------------------------------------------------------------------------------------------------------------ Abstract The power industry worldwide has been the subject of major reviews and reforms in recent years, which have resulted in changing demands in respect of enhanced safety, reliability, environmental safeguards and commercial competition. In such an environment it is essential that the personnel and the plant and equipment involved, perform to their optimum levels of capability. Reliability Centred Maintenance is a maintenance Optimization tool which has a role in providing an effective response to such demands on the industry, by enhancing the effectiveness of operations and maintenance programmes This paper describes the application of reliability-centered maintenance methodology to the development of maintenance plan for a thermal power plant. The main objective of reliability-centered maintenance is the cost-effective maintenance of the plant components inherent reliability value. Application of the reliability-centered maintenance methodology will reduce the mean time between failures for the plant equipments, probability of sudden equipment failures an also it will reduce the maintenance labor cost and spare parts cost. Introduction: Reliability centered maintenance (RCM) is a technique initially developed in late 1960’s by the airline industry that focuses on preventing failures whose consequences are most likely to be serious. Preventive maintenance is time based but RCM is condition-based, with maintenance intervals based on actual equipment criticality and performance data. Preventive maintenance programmes at thermal power plants were based on Original Equipment Manufacturers (OEM) recommendations, without sufficient consideration of overall system functions. In other cases, too little preventive maintenance was performed on key components that had not been identified as critical, leading to failures that increased corrective maintenance costs and reduced plant availability. Reliability centered maintenance (RCM) analysis is a systematic evaluation approach for developing or optimizing a maintenance programme. RCM utilizes a decision logic tree to identify the maintenance requirements of equipment according to the safety and operational consequences of each failure and the degradation mechanism responsible for the failures. Reliability-centered maintenance (RCM) is the optimum mix of reactive, time or interval-based, condition-based, and proactive maintenance practices. RCM is an ongoing process that gathers data from operating systems performance and uses this data to improve design and future maintenance. These maintenance strategies, rather than being applied independently, are integrated to take advantage of their respective strengths in order to optimize facility and equipment operability and efficiency while minimizing life-cycle costs.

Figure 1: Components of an RCM programme

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Steps of RCM The seven steps of RCM is shown in Table 1

Table 1: Steps in RCM

The main steps of the RCM process are illustrated through a case study. Case Study System selection: Naphtha Fuel system of gas turbine. System description: Fuel system for gas turbine consists of Naphtha storage tanks, fuel filters, forwarding pumps, Naphtha booster pumps. The most critical equipment in the fuel system is naphtha forwarding pumps and naphtha booster pumps. In this paper we have selected one critical equipment,ie naphtha forwarding pump for analysis. System Functions: Naphtha received from IOCL is centrifuged and stored in clean naphtha storage tanks.Naphtha forwarding pump forwards naphtha from tank to booster pump suction.Booster pump boosts the naphtha pressure and supply to gas turbine. The process is shown in the block diagram (refer figure 2)

Figure: 2 : Naphtha fuel system

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System Functional Failure: For each function described, there must be at least one functional failure mode/mechanism. The functional failure statement documents the mechanism of failure for the function and its consequences. System functional failure for naphtha forwarding pump is shown in Table 2

Failure Mode Reason Root cause

Pump shutdown

Low flow rate

Suction filter choked Impeller damaged Impeller loose on shaft Insufficient suction level in tank

High vibration

Mis alignment Worn Bearing Loose foundation bolts Bent shaft

Coupling failure Worn out spiders

Fuel leak Mechanical seal failure Pipe line leaks/gasket damage

Table 2: System functional failure for naphtha forwarding pump.

Failure Mode Effect Analysis (FMEA): Failure mode and effect analysis is a tool that examines potential product or process failures, evaluates risk priorities, and helps determine remedial actions to avoid identified problems. FMEA is often the first step of a system reliability study. It involves reviewing as many components, assemblies, and subsystems as possible to identify failure modes, and their causes and effects. For each component, the failure modes and their resulting effects on the rest of the system are recorded.The FMEA for naphtha forwarding pump is shown in Table 3

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Item Failure Mode Effect-Local Effect on Gas Turbine

Impeller Worn Impeller

Pump low efficiency - Vibration - Reduce in suction power

Gas turbine trip

Bearing Faulty bearing

Excessive pump vibration - Motor may be overload - Increased in shaft radial movement - Eventual pump shutdown

Gas turbine trip

Shaft Shaft deforming

Pump low efficiency - Vibration - Increase in shaft radial movement - Possible bearing damage - Eventual coupling failure

Gas turbine trip

Coupling Faulty shaft coupling

Losses of pumping efficiency - Noise and vibration on the pump - Possible seals damage - Eventually pump shutdown

Gas turbine trip

Table 3: FMEA for naphtha forwarding pump Criticality Analysis for Pump Components The criticality is assessed based on the effect of errors/faults and on the time from the occurrence until the effect occurs on the installation and is quantified with 1, 2, 3 as shown in the Table 4.

Table 4: Criticality Analysis for Pump Components

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Safety(S) related effects take weight of 40%, Production (P) related effects 40%, and the cost(V) related effects 20%. Equipment criticality is calculated using the following formula. EC = (40*P + 40*S +20*V)/3 The failure mode categories A, B, C, and D depending on the equipment criticality is calculated as per the algorithm mentioned in Figure 3 and tabulated in Table 5

Figure3: Algorithm for calculating equipment component criticality

Table 5: Naphtha forwarding pump component criticality analysis

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Logic tree Analysis The basic (LTA) uses the decision tree structure shown in Figure 4. From this figure, decision bins: 1) safety related, 2) outage-related, or 3) economic-related were noticed. Each failure mode is entered into the top box of the tree, where the first question is posed.

Figure 4: Logic Tree Analysis

Task selection A great strength of RCM is the way it provides simple, precise and easily understood criteria for deciding which(if any) of the proactive tasks is technically feasible in any context, and if so for deciding how often they should be done and who should do them. Whether or not a proactive task is technically feasible is governed by the technical characteristics of the task and of the failure which it is meant to prevent. If a proactive task cannot be found which is both technically feasible and worth doing, then suitable default action must be taken. Maintenance tasks are consisting of run-to-failure (RTF), time-directed maintenance, condition-directed maintenance (CD). and failure-finding (FF). Based on the logic tree analysis and task selection, maintenance tasks are arrived for naphtha forwarding pump and is shown in Table 6

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S= Once in six months, M=Monthly, A=Annually

Table 6:Maintenance Tasks for naphtha forwarding pump Final Phase of Analysis Task Selection has been completed and reviewed. Recommended tasks developed in the analysis is compared with the existing planned maintenance program. The purpose of this comparison is to identify needed changes in the existing program, and thereby optimise the facility’s Preventive Maintenance program. The comparison also provides another check of the analysis to assure validity of assumptions and completeness and gaps derived between RCM analysis and existing Preventive Maintenance program. Perform Task Comparison RETAIN: Existing tasks for the components that have RCM recommended tasks that exactly match the existing PM tasks (content and frequency). MODIFY: Existing tasks for the components that have RCM recommended tasks that differ slightly in context or frequency from the existing PM tasks and will make these tasks more applicable and effective. DELETE: Existing tasks for the components that may be replaced by more applicable and effective RCM recommended tasks. DELETE may apply to existing tasks that are redundant. DELETE may also apply to existing tasks where RCM recommended the component be run-to-failure. ADD: New PM tasks intended to prevent or mitigate identified failures for the components whose existing tasks do not provide this appropriately. Add new tasks would also apply to all of those components for which there are no existing PM tasks but RCM has identified applicable and effective tasks. Note: Most new, or add, tasks are usually condition monitoring which should replace existing time-directed overhauls and inspections. Conclusion: Based on the above case study the results of the RCM technique applied to naphtha forwarding pump show that the PM proposed tasks and planning are generated. Moreover, PM is consisted of on-condition and scheduled maintenance. It is recommended to perform these tasks as per the frequency. The outcomes of an RCM analysis can result in changes to existing preventive maintenance tasks, the use of condition monitoring, inspections and functional testing, or the addition or elimination of such tasks. The overall aim of the RCM process is not necessarily to reduce the cost of the maintenance programmes but to improve the functional performance of the plant equipment. Enhanced reliability and efficiency will in turn contribute to improved economic and safety performance of the plant equipment. However by optimizing the PM tasks and condition monitoring techniques will lead to reduction in maintenance labor and spare parts costs. .

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References:

• The National Aeronautics and Space Administration, “Reliability-Centered Maintenance Guide for Facilities and Collateral Equipment,” NASA, Washington D.C.,February 2000.

• A. M. Smith, “Reliability-Centered Maintenance,” McGraw-Hill, New York, 1993. • M. Rausand, “Reliability-Centered Maintenance,” Reliability Engineering and System Safety, Vol. 60,

No. 2, 1998, pp. 121-132.