A case study Zubair Ali (12-bsm-118) Associate prof. Mr. Touqeer Ahmad Khan sb. (B.sc Mechanical engineer) 26-JAN-2015 UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE
A Case Study on Reliability-Centered Maintenance Methodology and Application in EMISAL Abstract
This paper describes the application of reliability-centered maintenance methodology to the development of maintenance plan for a steam-process plant. The main objective of reliability-centered maintenance is the cost-effective maintenance of the plant components inherent reliability value. The process-steam plant consists of fire-tube boiler, steam distribution, dryer, feed-water pump and process heater. Within this context, a maintenance program for the plant is carried out based on this reliability-centered maintenance concept. Applying of the reliability-centered maintenance methodology showed that the main time between failures for the plant equipments and the probability of sudden equipment failures are decreased. The proposed labor program is carried out. The results show that the labor cost decreases from 295200 $/year to 220800 $/year (about 25.8% of the total labor cost) for the proposed preventive maintenance planning. Moreover, the downtime cost of the plant components is investigated. The proposed PM planning results indicate a saving of about 80% of the total downtime cost as compared with that of current maintenance. In addition, the proposed spare parts programs for the plant components are generated. The results show that about 22.17% of the annual spare parts cost are saved when proposed preventive maintenance planning other current maintenance once. Based on these results, the application of the predictive maintenance should be applied.
Introduction Steam system is an important part of many processing. Maintenance, availability, reliability and total maintenance reliability cost are some of the most important factors of steam-process plant. The plant provides heat energy to Egyptian Minerals and Salts Company (EMISAL), EL-Fayoum, Egypt. The main product of the company is sodium sulphate unhydrous and sodium chloride. This work aims to generate a maintenance program that based on the RCM technique for the process- steam plant components. This technique should be able to minimize the downtime (DT) and improve the availability of the plant components. Also, it should benefits to decrease the spare parts consumption system components. RCM is a systematic approach to determine the maintenance requirements of plant and equipment in its operating.it is used to optimize preventive maintenance (PM) strategies. The developed PM programs minimize equipment failures and provide industrial plants with effective equipment . RCM is one of the best known and most used devices to preserve the operational efficiency of the steam system. RCM operates by balancing the high corrective maintenance costs with the cost of programmed (preventive or predictive) polices, taking into account the potential shortening of “useful life” of the item considered. But it is difficult to select suitable maintenance strategy for each piece of equipment and each failure mode, for the great quantity of equipment and uncertain factors of maintenance strategy decision RCM philosophy employs preventive maintenance, predictive maintenance (PdM), real-time monitoring (RTM), run-to-failure (RTF) and proactive maintenance techniques is an integrated manner to increase the probability that a machine or component will function in the required manner over its design life cycle with a minimum of maintenance.
1. Reliability-Centered Maintenance 1.2. System Selection and Data Collection Methodology
Determining the list of the system components is one of Reliability-centered maintenance (RCM) is the optimum the first steps in RCM. The criticality analysis requires mix of reactive, time or interval-based, condition-based, different kind of data of each component that build up the and proactive maintenance practices. These principal system. The effect of failure of the system main maintenance strategies, rather than being applied components may effect system productivity and independently, are integrated to take advantage of their maintenance cost. The factors effecting selection of respective strengths in order to maximize facility and critical system are as follows: equipment reliability while minimizing life-cycle costs. 1) Mean-time between failures (MTBF). Total productive maintenance (TPM), total 2) Total maintenance cost. maintenance assurance, preventive maintenance, 3) Mean time to repair (MTTR). reliability- centered maintenance (RCM), and many other 4) Availability. innovative approaches to maintenance problems all aim at
enhancing the effectiveness of machines to ultimately 1.3. Logic Tree Analysis (LTA) improve productivity
The basic (LTA) uses the decision tree structure shown in 1.1. Reliability-Centered Maintenance Figure 3. From this figure, decision bins: 1) safety- Components 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: Does the The components of RCM program are shown in Figure operator, in the normal course of his or her duties, know 1. This figure showing that RCM program consists of that something of an abnormal or detrimental nature has (reactive maintenance, preventive maintenance, condition occurred in the plant? It is not necessary that the operator based maintenance, and proactive maintenance) and its know exactly what is awry for the answer to be yes. patterns.
As shown in Figure 2, the RCM steps are presented. The steps describe the systematic approach used to 1.4. Criticality Analysis implement the preserves the system function, identifies
failure mode, priorities failure used to implement the Criticality analysis is a tool used to evaluate how preserves the system function, identifies failure mode, equipment failures impact organizational performance in priorities failure modes and performs PM tasks. The order to systematically rank plant assets for the purpose RCM steps are as follows : of work prioritization, material classification, PM/PdM • Step1: system selection and data collection. development and reliability improvement initiatives . • Step2: system boundary definition. In general, failure modes, effects and criticality analysis • Step3: system description and functional block. (FMEA/FMECA) requires the identification of the • following basic information in Table 1. In Figure 4,
Step4: system function functional failures. • algorithm for the calculation of equipments criticality is
Step5 : failure mode effect analysis • presented. This figure shows the calculation steps of the
Step6: logic tree diagram. • equipments criticality
Step7: task selection.
Re liability Centered Maintenance Reactive Interval (PM) CBM Proactive Small items Subjected to wear Random failure RCFA Non-critical Consumable patterns FMEA Inconsequential replacement Not subjected to wear Acceptance Unlikely to fail Failure pattern PM induced failures testing
Figure 1. Components of RCM program.
1. 1 S election of critical equipment Step 1 : System selection and data collection . 1 2 Operation and maintenance data collection . 2 1 Boundary overview Step 2 : Sy
stem boundary definition 2 . 2 Boundary details 3. 1 S ystem description Step 3 : Sy
3 . 3 Equipment history 4.1 System function Step 4 : System function functional failures 4.2 Functional failures Step 5 : FMEA . 5 Failure mode and effect analysis (FMEA) Step 6 : LTA . 6 Logic tree analysis (LTA) Step 7 : 7 . Task selection
Figure 2. Main steps of the RCM.
2. Case Study The criticality is assessed based on the effect of er- rors/faults and on the time from the occurrence until the We select the most critical system in the sodium chloride effect occurs on the installation and is quantified with 1, plant which contains the most critical items. The plant 2, 3 in Table 1. provides heat energy to perform the drying process for the sodium chloride anhydrous lead to the aimed degree. EC = (30*P + 30*S +25*A+15*V)/3 (1)
where, EC: is the equipment criticality, % 2.1. System Description P: is the product The structure of the steam-process plant is presented in S: is the safety A: is the equipment stand by V: is the capital cost.
Will the failure have? No a direct and adverse effect on environment, health, No security. safety? Will the failure have a direct and adverse effect on mission (quantity or quality)? yes yes Will the failure result in other yes economic loss (high cost damage to machines or system)? Is there an effective CM technology No or approach? yes Is there an effective No
Interval-Based task? Run-to-Fail? Develop & schedule CM task to monitor condition. y es Redesign system accept Develop and schedule the failure risk, or install Interval-Based task. redundancy. yes Perform condition- Based task.
(3) Without standby Availability of standby A 25% (2) With stand by and medium availability, and
(1) With standby and high availability
(3)High value Equipment value V 15%
(2) normal, and
(1) Low value
other being the water tube boiler. A fire tube boiler can Figure 5. The steam-process plant consists of a fire-tube be either horizontal or vertical. A fire-tube boiler is boiler, feed-water pump, condensate tank, dryers and heat sometimes called a “smoke-tube” or “shell boiler” boiler. exchanger (PH).
Fire-tube boiler components are shown in Figure 6. A 2.2. System Boundary Defination fire-tube boiler is a type of boilers in which hot gases
from the fire pass through one or more tubes within the Some gross system definition and boundaries have been boiler. It is one of the two major types of boilers, the established in the normal course of the plant or facility
design, and these system definitions have already been boundaries that must be identified for the RCM analysis used as the basis of system selection. These same defini- process. tions serve quite well to initially define the precise Start Process = P Safety = S Standby = A
Value = V EC = (0.3*P + 0.3*S + 0.25*A + 0.15*V)/3 If EC < 45 Class D Then EC < 60 Class C Else Class B Ec < 74 Class A Output Ec and Class
Figure 4. Algorithm for the calculation of equipments criticality.
Air Draft Fan Steam Dryer Steam
distribution Sodium sulphate
plant PH CNT12 Fire-Tube Boiler CP13 Feed Tank Softener FWP6
Figure 5. The structure of steam-process plant.
• Brine entrance to the sodium sulphate plant.
• Steam entrance to the sodium chloride plant. Ac electric power entrance to the sodium chloride plant Remain brine entrance to the sodium sulphate plant. Sodium chloride, as a product, exit from the sodium sulphate plant. • Remain brine exit from the sodium chloride plant.
2.3. Information Collection
Uniformity, by researching some of the necessary system documents and information that will be needed in subsequent steps, the absence of documentation and data records was a huge problem that makes the system analysis process more difficult. Thus, a greater effort must be done to collect the missing data. 1) Some cards that contain few maintenance actions that have been under taken to some equipments. 2) Some of the operating and maintenance manuals for a few number of equipment. All of the other information has been collected through a walk down through EMISAL faculties, and personal
meeting with EMISAL staff. Fortunately, in most Figure 6. Fire-tube boiler. situations, there are plant personnel on site who have
essential elements of this data stored either in their desks 2.2.1. Boundary Overview (see Table 2) or their minds. Also Original Equipment Manufacturer
(OEM) recommendation stands ready to supply some Table 2. Boundary overview. information.
Demineralization Plant. • Sodium Chloride Plant. Central The Functional block diagram for the process is Laboratories. introduced in Figure 7. This figure shows the input
resource and output for the system main components. 2. primary Physical bondries:
• Brine entrance to the concentration ponds. • Brine exit from the concentration ponds.
2.5. System Root Cause Failure Analysis (RCFA) same two equipments we represent its root cause failure
analysis : As shown in Tables 3 and 4, root cause failure analysis 1) Fire-tube boiler. for critical equipments in steam system (fire tube boiler 2) Multi-stage centrifugal pump. and feeding pump) is presented. The cause analysis
(failure mode, reason and root cause) for the most critical 2.7. Criticality Analysis for Plant Components equipments in the steam system which is.
1) Fire tube boiler. Then the safety related effects take weight of 40%, 2) Multi-stage centrifugal pump. Production related effects 40%, and the cost related
effects 20%. The failure mode categories A, B, C, and D 2.6. Failure Mode and Effect Analysis (FMEA) depending on the criticality index are as shown in Table
7. Tables 8 and 9 show the criticality analysis for boiler, Failure mode and effect analysis is a tool that examines pump, respectively. potential product or process failures, evaluates risk
priorities, and helps determine remedial actions to avoid 2.8. Task Selection identified problems. The spreadsheet format allows easy
review of the analysis. Failure mode and effect analysis A great strength of RCM is the way it provides simple, help on identifying and the creation of functional failure. precise and easily understood criteria for deciding which At the following tables (Tables 5 and 6) we will represent the failure Mode and effect analysis for the
Figure 7. The functional block diagram.
Table 3. Boiler Root Cause Failure Analysis. Failure Mode Mechanism Reason Root Cause
High temperature of stack - Too much excess air Smoke stack gas - Dirty firesides
- Low water Boiler low efficiency Steam pressure Low steam pressure - Excessive steam demands - Poor combustion Combustion Combustion gases entering - Leakage through soot blower casing fire room seal
- Incorrect viscosity, temperature, or pressure of Boiler tubes Fuel Fuel impingement on fuel corrosion furnace walls and tubes - Improperly made up atomize assemblies - water in fuel - Sudden change in steam demand High fuel combustion - Too much or too little excess air
Table 4. Pump root cause failure analysis. Failure Mode Mechanism Reason Root Cause Pump low Discharge pressure Low discharge pressure - Water excessively hot efficiency Low flow rate & - Impeller damaged Impeller Low delivery - Impeller loss on pressure shaft
- Flooding of oil reservoir Pump shutdown oil Loss or oil contamination - Over filling of oil reservoir - Mechanical seal failure - Improper installation of bearing - Insufficient NPSH - Water excessively hot Low flow Operation condition - Impeller damaged or loose on shaft Bearing High bearing temperature - Bent shaft
Fails to open Low effect Low effect No effect Y Check valve Remain open Low effect Low effect No effect Y
Crack valve Low effect Low effect No effect Y
Incorrect burner sequence Boiler trip Steam system trip Production stooping
Too much fuel being fired Boiler trip Steam system trip Production stooping Y Combustion Too much excess air Boiler low Steam system trip Lower production Y room performance Faulty flam detector Y Boiler trip Steam system trip Production stooping
Combustion air very Low flow Boiler trip Steam system trip Lower production Y
Combustion gas pass failure Boiler trip Steam system trip Lower production Y Forced draft fan Noise in motor Boiler trip Steam system trip Low production Y
Air filter Dirt on surface Low effect Low effect No effect Y on boiler
Furnace Hole in tube Boiler low Low output Low effect Y efficiency Fuel system Relieve valve damage Boiler trip Steam system trip Production stooping Y Faulty of the trip valve Boiler trip Steam system trip Production stooping Y Piping system Corrosion Boiler shutdown Steam system trip Production stooping Y Safety valve Fail to open Boiler shutdown Steam system trip Low productivity Y Water softener Water contamination Boiler trip Low output Low productivity Y Feed water Pump trip Boiler trip Steam system trip Production stooping Y system No softening Boiler low Lower production No effect Y Too much efficiency Water softener softening Boiler low Lower production No effect Y efficiency
Table 6. Pump failure mode effect analysis.
Mode Local Boiler Steam System - Pump low efficiency Impeller Worn impeller - Vibration Boiler trip System trip Y - Reduce in suction power - Excessive pump vibration - Motor may be overload Faulty thrust Bearing - Increased in shaft radial Boiler shutdown System shutdown Y bearing movement - Eventual pump shutdown
- Pump low efficiency - Boiler low - System low - Vibration efficiency efficiency Shaft - Increase in shaft radial - low effect - low effect Shaft N deforming movement - low effect - low effect - Possible bearing damage - low effect - low effect - Eventual coupling failure - Reduce pumping rate - Boiler low - System low Casing Leaking casing - Possible corrosion on all efficiency efficiency N pump components - low effect - low effect - Losses of pumping efficiency - Boiler low - System trip - Noise and vibration on the efficiency - System trip Faulty shaft pump - boiler - System trip Coupling Y coupling - Possible seals damage shutdown - System trip - Eventually pump shutdown - Boiler trip - Boiler trip - Internal liquid leakage - Boiler low - System low - Eminent impeller wear efficiency efficiency - Faulty - Potential corrosion effect - Boiler trip - System trip Ring impeller wear Y on pump internals - Boiler trip - System trip ring - Pump capacity greatly - Boiler low - System low reduce. efficiency efficiency Table 7. Criticality group.
Group Criticality Index A 3 – 2.5 B 2.5 – 2 C 2 – 1.5 D 1.5 – 1
Table 8. Criticality analysis for boiler.
Failure Failure Criticality Analysis Criticality
Mode Cause Safety Production Cost Index
Too much excess air 2 1 3 1.4 D
Incorrect burner 3 2 1 2.2 B Excessive sequence high steam Too much fuel being 3 3 3 3 A outlet fired temperature Dirty generating 2 2 3 2.2 B surface Boiler
Dirty economizer 3 3 3 3 A Low discharge Water excessively hot 1 3 1 1.8 C pressure High Too much excess air 2 1 3 1.8 C A temperature Dirty firesides 3 2 3 2.6 of stack gas Leakage through soot 3 3 3 3 A Combustion blower casing seal
gases entering Leakage through fire room economizer drain line 3 3 3 3 A
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Table 9. Criticality analysis for the pump.
Failure Failure Criticality Analysis Criticality
Mode Cause Safety Production Cost Index Low Water excessively 2 3 1 2.2 B discharge hot
Bent shaft 3 3 3 3 A High Worm bearing 3 3 2 2.8 A
bearing Lack of lubrication 3 3 2 2.8 A temperature Pump Improper 3 3 2 2.8 A
installation of bearing Pump Misalignment of pump 3 3 3 3 A casing drive motor 3 3 3 3 A overheats Shaft sleeve worn Low flow Impeller damaged 3 3 3 3 A or loose on shaft
(if any) of the proactive tasks is technically feasible in and failure-finding (FF). The maintenance task for the any context, and if so for deciding how often they should boiler is illustrated in Table 10. be done and who should do them.
Whether or not a proactive task is technically feasible 3.9. Maintenance Labor Force is governed by the technical characteristics of the task
and of the failure which it is meant to prevent. Whether it The maintenance labor force is presented in Table 11. is worth doing is governed by how well it deals with the This table shows the size of maintenance labor force consequences of the failure. If a proactive task cannot be calculations for the PM levels (six monthly, monthly and found which is both technically feasible and worth doing, weekly). In addition, the labor saving cost is introduced then suitable default action must be taken. Maintenance in Table 12. Not that the proposed labor cost (295200 tasks are consisting of run-to-failure (RTF), time-di- $/year) decreased with respect to the current values rected maintenance, condition-directed maintenance (CD) (220800 $/year). Table 10. Maintenance task. Equipment Failure Failure Group Task Description Frequency Mode Cause
Too much excess D RTF …………. ………. air
Measure the diameter of
CD the fuel opening holes M inside burner Boiler Excessive Incorrect burner B Replacement of fuel hose TD S high steam sequence and gaskets outlet Check the deflector
FF temperature position W Too much fuel Measure the diameter of being fired A CD the fuel opening holes M inside burner Dirty generating B CD Cleaning generating S surface surface Dirty economizer A CD Measure the M temperature of the stack gases
Table 11. The size of maintenance labor force.
Duration PM Level Frequency (Hours) No. of Workers Man-hour per PM level Six Monthly 2
21 4 168 Monthly 10
5 2 100 Weakly 50
6.15 1 325 Maintenance labor force = 1 labor.
Table 12. Labor savings Cost. Labor type Number of labors Number of labors Item Per day Per day (current maintenance) (proposed ) Mechanical 5 5 4 4 Engineers Electrical Control 5 4 (1000$/month) Technicians Mechanical 6 4 (800$/month) Electrical 6 4 Total cost
295200 220800 ($)/year)
Saving cost (%) = 25.2
Downtime Cost indicate a saving of about 22.17% of the spare parts total • Average CM downtime of the feed water pump cost as compared with that of the current maintenance = 20 hr/year. (CM). • Average CM downtime of the fire tube boiler =
30 hr/year. 3. Conclusions • Average CM downtime of the system auxiliaries
= 10 hr/year. The results of the RCM technique applied of the plant • Average down cost rate = 10000 $/hr show that the PM proposed tasks and planning are • Total downtime cost rate = 600,000 $/year generated. Moreover, PM is consisted of on-condition • Proposed saving downtime cost = 480,000 and scheduled maintenance. The RCM had great impact $/year Spare Parts Program on the PM tasks. The Run-To-Failure (RTF) frequency The proposed spare parts program is shown in Table has been decreased. It is recommended to perform these 13. This table shows that the spare parts for the plant tasks (CD, TD and FF) every yearly, six monthly and main components (feed-water pump, water tube boiler ant monthly. steam turbine). Proposed spare parts program results
Table 13. Proposed spare parts program (yearly). Quantity Cost (current) Quantity Cost (Proposed) Equipment Spare part (Current) $/year (Proposed) $/year
Coupling 2 3000 1 1500 Feed Mechanical seal 6 12000 4 8000 water pump Motor bearing 6 24000 4 16000
Pump bearing 6 12000 4 8000
Gasket 12 12000 10 10000
Fuel house 2 40000 2 40000
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Fire tube boiler
Fan bearing 4 12000 2 6000 Total cost
Saving cost %
The proposed labor program is carried out. The results show that the labor cost decreases from 295200 $/year to 220800 $/year (about 25.2% of the total labor cost) for the proposed PM planning. Moreover, the downtime cost (DTC) of the co-generation plant components is investigated. The proposed PM planning results indicate a saving of about 80% of the total downtime cost as compared with that of current maintenance (RTF). The system reliability increase with decreasing the labor cost. The proposed spare parts program for the co-genera- tion plant components (feed water pump, boiler and turbo-generator) are generated. The results show that about 22.17% of the annual spare parts cost are saved when proposed PM planning other current maintenance (RTF) once.