Numerical Study Of Gas Mixing Effect On Block 3 & 4 Muara Tawar’s Gas Turbine Combustion Stability

DOI: 10.1007/978-981-19-1581-9_33

Abstract. Gas turbine operation’s disturbances related to combustion that lead to flame instability greatly influenced by the setting of fuel and air which is adjusted according to gas availability during commissioning. Meanwhile, gas turbine must be able to operate using a variety of natural gases or its mixing depending on the system and gas availability. This study presents the numerical simulation to obtain the combustor’s characteristics by analysis the flame stabilization, temperature distribution, and Nox emission by varying the fuel gas sourced and air mass flow. The numerical analysis has shown that fuel with higher CH4 contains will tend the ombustion become more unstableand and stabilized by the inner recirculation zone. The more excess air also provide more stable combustion as flame lenght decrease, but too much excess air will decrease the total temperature. NOx emission produced from the combustion which produce higher temperature from methane and excess air effect. The recommendation of the research results is to provide a limitation of the composition of the gas mixing and the fuel air ration to obtain the combustion stability. The results of the study simulate that it is possible to use three condition of fuel gas in combustion system.

Keywords: gas composition, stability, temperature distribution, emission NOx, excess
air.

1. Introduction

Power plants managed by PT PJB UP Muara Tawar operates as peak load (peaker) with periodic

start-stop operation. Block 3 and 4 operate V 94.2 Siemens’s type gas turbines. Natural gas as gas turbine

fuel comes from 3 different suppliers, Nusantara Regas (NR), Pertamina EP (PEP) and PGN with their

chemical composition values. Gas turbine operation’s disturbances related to combustion that lead to

flame instability greatly influenced by the setting of fuel and air which is adjusted according to gas

conditions during commissioning. Meanwhile, gas turbine must be able to operate using a variety of

natural gases or its mixing depending on the Java-Bali grid system and gas availability.

There are several studies that have been done to investigate the various gases on combustion system

depend on the gas turbine type For example the author [1] in this study present the simulation on

combustion of methane and biogas mixture within can-type of gas turbine combustion chamber.The

analysis shown that biogas with lower methane contain leads to the lowering flame temperature whose

effect reduce NO emissions. Author [2] aim to examinate different fuels that affect the output character
istics and highlights the benefits of using fuels with higher hydrogen–carbon ratios including higher
power, higher efficiency, and lower carbon emissions. Author [3] the investigation to analyze the V94.2
gas turbine’s fluid flow and heat transfer on burner performance.
Based on these previous studies, the research is carried out by evaluating and optimizing the
combustion characteristics and temperature distribution by varying the fuel gas sourced and provide a
limitation of the composition of the gas mixing and the ratio of air to fuel (air fuel ratio) to obtain the
combustion stability.
2. Methods
Prior to the combustion simulation in the burner, the geometry is made to check whether it is close
to the actual condition based on the parameters in the gas turbine operating parameters. The geometry
(figure 1) test is done by simulating the fuel inlet, air inlet and combustion chamber outlet in one of the
operating conditions based on the composition of the gas used then the parameter results, especially
several points of turbine inlet temperature and turbine inlet pressure, are compared with operating
parameters as seen on table 1.

The result of comparation between operating and simulation condition shown that the temperature

on inlet turbine from four point of calculation gave arround 1% until 5% of error as shown on table 4.2.

This lead that the geometry check has similarity with operating condition.

The simulation’s result the temperature outlet of combustion chamber has similarity compared to
operation condition. Methane is a fuel gas contain that used as the reference of this analysis. Since the
actual composition during operating of the Muara Tawar’s gas turbine couldn’t accurately calculate (no
gas chromatograph to sense mixing gas) there were three gas classification, high composition (CH4
94%), medium (CH4 87%) and low (CH4 71%).

3. Result and discussion
3.1 Flame Stability
The flame length is one of the stability parameters combustion mechanism which closely related to
the mixing of fuel and air. The increasing of flame length indicated the combustion happened away from
the burner tip and tend to flame becomes unstable. The combustor design that applied the air swirl help
to increase the combustion intensity and reduce the flame length. Swirl flow also provided an angular
velocity to the axial incoming flow to produce a central recirculation zone (CRZ) which provides the
main flame stabilization.

As seen on figure 1 the flame length combustion with fuel with a methane content of 94% produces the
longest flame length arround 4.7 m from burner tip. Amer and Gad [4] studied experimentally the effect  of increasing air to fuel ratio on experimental study of LPG combustion. Increasing the air to fuel mass
ratio (excess air) from 5% to 20%, the flame length decreases by about 6% to 16%. The flame length
that indicated the lift will be stabilized by inner recirculation zone.

Recirculation zone on figure 2 showed the negative axial velocity in the center of the combustion
chamber indicates the presence of an inner recirculation zone due to circulating air flow made by swirl
air inlet which results in a vortex breakdown process and initiates a recirculation zone in the center of
the combustion chamber. The composition of the fuel with a higher methane content results in a wider
circulation zone when compared to fuel with a lower methane content as seen on figure 2. The length of
recirculation about 3.4 m and for low methane reached 2.1 m. The result of analysis clearly explain on
figure 3 that calculate on each gas composition and excess air.

The addition of excess air changes the characteristics of the recirculation zone, the more excess air
resulting in shorter recirculation center distances with a larger recirculation zone area. Hong, et al [5]
made some study on recirculation zone as excess air raised. The higher temperature of the products
reduces the velocity gradient in the shear layer and thus the reattachment length. The addition of 5%
excess air reaching 504 kg/s resulted in a recirculation center distance of 2.38 m from the burner tip.
3.2 Temperature Distribution
Combustion process is a reaction between fuel gas and oxygen in the air. The result of this process
were carbon dioxide (CO2), water (H2O), and a great deal of energy. The higher methane content, the
higher the maximum temperature reached. The length of maximun temperature also increases from the
tip burner. The addition of excess air as showed on figure 4 shows that as the amount of air increases,
the temperature to the outlet will decrease this is due to the combustion losing a certain amount of energy
because too much air enters the combustion chamber.

The addition of excess air from 15% to 20% does not cause a significant increase in maximum
temperature and energy. The combustion efficiency increases with increased excess air until the heat
loss in the excess air is larger than the heat from combustion. Munir et al [6] evaluate the effect of excess
air on combustion concluded that an optimum air fuel ratio should be maintained to ensure complete
combustion as well as to decrease the excessive losses due to surplus air.
3.3 NOx Emission Characteristic
Nox emission is produced by the oxidation of atmospheric nitrogen in high temperature regions of
the combution flame and postflame gases at the outlet. Previously reported sitgnificant effects on NOx
characteristic by Thomson, et al [6] that the nitric oxide formation rate in post flame gases of
hydrocarbon flames (T > 1800°K) and follows the Zeldovich chain mechanism. The combustion process
will lead the creation of nitrogen oxides from nitrogen from air or gas fuel. At higher temperatures both
can react to form NOx in large quantities. The formation of the NOx mass fraction in combustion with
variations in the methane content shows a higher value due to ethane increasesing as shown in figure 5.

The addition ofexcess air in combustion will also affect the reaction of NOx mass fraction as a result
of combustion temperatures that becone lower. This is the point that the excess air become too much.
as excess air.
4. Conclusion
The analyses were carried out for combustion characteristic for gas turbine type V94.2 using several
composition of natural gas that being used as fuel in Muara Tawar power plant. Through the numerical
simulations it was possible to notice that:
• The more flame lenght on Fuel with a greater methane content results the flame lenght increasing
and tend to unstable combustion.
• The CRZ distance is about 3.4 m and with a methane content of 94% and the more excess air will
lead the CRZ becomes shorter but the area of recirculation become wider.
• The addition of excess air causes the temperature to decrease at the outlet area of the combustion
chamber due to combustion losing some energy because too much air enters the combustion chamber.
• Temperature plays an important role in the formation of the mass fraction of NOx, the lower the
temperature the less mass fraction of NOx is produced.
• The addition of excess air of about 5% provides the most optimal combustion stability and emission
factor values.
• The results of the analysis clearly demonstrate that it is possible to use such fuels in combustion
systems with swirl burners.

Carbon Capture & Storage

Carbondioxide Capture and Storane (CCS) merupakan salah satu mitigasi adanya pemanasan global dengan cara mengurangi emisi CO2. Teknologi CCS merupakan rangkaian pelaksanaan proses yang terkait satu sama lain, mulai dari pemisahan dan penangkapan (capture) CO2 dari sumber emisi gas buang (flue gas), pengangkutan CO2 yang tertangkap ke tempat penyimpanan (transportation), dan penyimpanan ke tempat yang aman (storage). 

CSS memiliki potensi untuk mengurangi keseluruhan biaya mitigasi dan meningkatkan upaya pengurangan emisi gas penyebab efek rumah kaca. Penerapan teknologi CSS akan sangat tergantung pengembangan teknologi yang semakin feasible, potensi penerapan dan pengaplikasian, biaya, transfer teknologi antar negara, aspek peraturan, isu lingkungan dan adanya persepsi atau cara pandang masyarakat. Emisi gas rumah kaca dihasilkan paling besar akibat aktivitas manusia berupa pembakaran bahan bakar fosil, biomass, pembakaran lahan dan aktivitas industry lain.

Emisi dari gas rumah kaca sendiri adalah gas-gas di atmosfer yang dapat menimbulkan perubahan dalam kesetimbangan radiasi (daya pantul maupun daya serap pada atmosfer) sehingga mempengaruhi suhu atmosfer dan juga permukaan bumi. Gas-gas tersebut dinamakan gas rumah kaca karena kemampuannya dalam menyerap dan memantulkan kembali radiasi gelombang panjang yang bersifat panas seperti yang dilakukan oleh kaca, sehingga menimbulkan efek pemanasan global yang disebut efek rumah kaca.

Karbondioksida berkontribusi sebesar 76,7% dari total emisi gas rumah kaca dan di tahun 2000 emisi CO2 akibat pembakaran bahan bakar fosil mencapai 23,5 GtCO2 terutama dihasilkan oleh industry energi seperti dalam table 1.

Peningkatan emisi CO2 dari tahun 1971 hingga 2001 menunjukkan peningkatan seiring dengan peningkatan pemakaian sumber energi dari fosil yang semakin dominan dan mencapai angka 86% dari total pemakaian di seluruh dunia.

Tabel 1. Tingkat Emisi dari Pembakaran Bahan Bakar Fosil

Emisi terbesar dari sector pembangkitan energi berasal dari batu bara yang mencapai 59% atau setara dengan total emisi sebesar 7.984 MtCO2 pertahun (data tahun 2002) secara rinci emisi CO2 yang dihasilkan dari sector energi terdapat dalam table 2 berikut;

 Tabel 2. Emisi dari sector pembangkit energi

Pemanfaatan teknologi untuk mengurangi emisi CO2 ke atmosfer terdiri dari beberapa pendekatan yang meliputi :

• Mengurangi konsumsi energi bahan bakar, contohnya dengan meningkatkan efisiensi dari konversi energi.
• Beralih penggunaan bahan bakar dengan kandungan karbon yang lebih rendah, seperti penggunaan gas dibanding batu bara.
• Peningkatan penggunaan energi terbarukan atau energi nuklir yang menghasilkan emisi CO2 yang rendah atau tanpa emisi sekali.
• Penangkapan gas CO2 secara biologi atau secara alami dengan meningkatkan kemampuan hutan dalam penyerapan CO2.
• Teknologi menangkap dan menyimpan CO2 secara kimiawi dan metode fisis.

Teknologi penangkapan CO2 dan penyimpanan yang akan dibahas lebih lanjut bertujuan untuk menghasilkan gas yang terkonsentrasi sehingga memudahkan proses pemindahan CO2 ke tempat penyimpanan. Penangkapan CO2 akan optimal apabila dilakukan pada lokasi industry dan pembangkitan energi dimana dihasilkan emisi CO2 dalam jumlah yang banyak. Terdapat 4 teknologi dalam upaya menangkap emisi CO2 yang meliputi seperti dalam gambar 3 berikut

Berdasarkan gambar 3. di atas terdapat beberapa proses penangkapan gas buang CO2 yang dilakukan oleh industry di seluruh dunia, diantaranya adalah :

• Menangkap CO2 dari proses industry, salah satunya dikenal dengan metode sweetening yaitu mengurangi kandungan CO2 dalam gas alam.
• Teknik post-combustion, menangkap CO2 dari gas buang pembangkit listrik setelah   bahan bakar fosil dibakar. Gas buang akan melewati absorber tower yang mempunyai bahan kimia khusus (biasanya amina). Amina berfungsi untuk menyerap CO2 dari gas buang
• Teknik pre-combustion biasanya diterapkan pada Integrated Gasification Combine Cycle (IGCC) yaitu pembangkit listrik tenaga batu bara dan penangkapan CO2 dilakukan sebelum batu bara benar- benar membara. Batu bara dipanaskan secara perlahan untuk mengeluarkan synthetic gas yang terdiri dari karbon monoksida dan hydrogen.
• Teknik oxyfuel combustion yaitu membakar bahan bakar fosil dengan oksigen murni alih-alih dengan udara. Gas buang yang dihasilkan hampir seluruhnya terdiri dari CO2 dan air. Air dikeluarkan melalui kondensasi sedangkan CO2 dikompresi agar dapat dipindahkan. Tehnik ini dapat menghasilkan tingkat penangkapan CO2 yang sangat tinggi, kekurangannya metode ini membutuhkan banyak energy untuk menghasilkan oksigen murni sehingga relative tidak efisien.

Dari penjelasan di atas kemudian dalam tulisan ini akan dirangkum dua jenis teknologi yang secara umum digunakan oleh industry diantaranya adalah teknologi pre combustion dan post combustion.

Optimization of Prefill Mode to Increase Reliability of Gas Turbine 13E2 PT. PJB UP Muara Tawar

IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE). e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 14, Issue 3 Ver. VII. (May - June 2017), PP 13-18. www.iosrjournals.org

Abstract: Muara Tawar block V is constructed by PT. PLN (Persero) Power Plant Master Project ofJava, Bali and Nusa Tenggara Networks based on contract No. 261. PJ / 041 / DIR / 2007 (Concerning Gas-Fired Power Plant Extension Project). The plant uses gas turbine type 13E2. The operational concept of gas turbine type 13E2 is regulating the amount of fuel that is divided from three fuel control valves and the operating pattern is in an area known as the pilot valve and premix valve which must be maintained the stability of combustion shown by pulsation parameters. The prefill concept is used to avoid flame off when gas turbine loading often passes over point switch or back point switch. Prefill itself will momentary activate a pilot valve to fill the Fuel Distribution System (FDS) line from MBP43. However, prefill gives the effect of high pulsation that triggers Gas Turbine experience PLS or derating. This effect can be overcome by improving the prefill concept and modifying the prefill system through Logic Advant, resulting in a more stable burning in the gas turbine burner.

Keywords: Gas-Fired Power Plant, FDS, pilot valve, pulsation, prefill.

I. Introduction
The utilization of gas turbines in power generation has increased in recent years and it will be increased up to a medium level of application [1]. Muara Tawar Block V gas turbine system was designed to operate at peak or under load continuously in both open and combined cycles as per network requirement. Muara Tawar Block V power generation can generate 242.5 MW electricity (gross output). It comes from the gas turbine generator alone about 161.5 MW and from the steam turbine is about 81 MW [2]. The reliability of a power plant to
provide electricity to the electricity network is the most important task to ensure energy availability [3]. Gas Turbine type 13E2 consist of three fuel control valves to maintain mass and energy balances in the burner system under the pilot and premix valve. To maintain stable combustion, changes in the operating condition between the pilot valve and the premix valve are required and it's known as switch over and switch back point. During these conditions, the pulsation parameter should be monitored. If the gas turbine operates in high load or above switch over point continuously, it tends pilot valve MBP43 cannot provide sufficient gas fuel to the combustion system. Moreover, during the load decrease, the operating condition will change to a switchback point. It condition leads to mass unbalance and it can be very dangerous because the pilot valve must close immediately. Moreover, in this condition, the pilot valve status is closed due to operation in high load. This condition triggers flame off resulting in the unit trip. The prefill concept is used to overcome this hazard and unscheduled shut down by the pilot valve in the MBP43 area pulse opening. However, prefill can also affect the emergence of high pulsation that resulted in gas turbines experiencing PLS or derating.

II. Theory
Prefill is used to reduce the disturbance that may arise due to the changing of electricity load from high to low load and vice versa. Prefill is used to fill up the volume between Fuel Distribution System (FDS) to the burner [4]. This prefill is performed by opening the pilot valve or MBP43 at a certain time to ensure the volume of the FDS until the burner fills up with approximately 20% of additional fuel. Mass unbalance occurs when the load is over the SwitchBack Point (SBP) area. Therefore, the prefill on the FDS pilot will not cause the flame off when SBP is present.

The prefill will be activated when the decrease in electricity load occurs. The prefill system will open the MBP43 control valve from the minimum mass flow rate and increase up to the maximum mass flow rate gradually to fill up the volume between FDS to the burner. The gas turbine at Muara Tawar Block V mass flow rate was designed at 0.285 kg/s in maximum load and 0.250 kg/s in minimum load [5]. The prefill process will stop if any of the criteria are reached i.e. Delta TAT (Temperature After Turbine difference) reaches 5 deg C or high pulsation reaches 35 mbar, or maximum prefill time has reaches 10 seconds.

III. Method
3.1 Analyze DEPP Prefill System of Gas Turbine
Logic prefill system is simulated under Advant Egatrol 8 software. Simulation on the logic includes the operation of pilot valve MBP43 and the operating pattern of the prefill system.

Fig. 2. describes prefill system, where prefill will be enabled or start (PltPrfSqPrel_START) with 2

modes i.e. periodically time (PltPrfSeqCyc_START) or decrease in load. Every 4 hours, the gas turbine will experience periodic prefill at any load that causes interruption to Protective Load Shedding (PLS). Interruption to PLS by high pulsation is caused by an over mass flow rate from MBP43 or pilot valve.

Fig. 3. shows the high pulsation which resulted in PLS 148 mbar. High pulsation occur because MBP43 

(number 4) is activated and mass flow rate or gas fuel flow rate of 0.250 kg/s up to 0.285 kg/s during 10 seconds. This logic set can be observed in Fig. 4. Periodic prefill is set up for every 4 hours which often results in high pulsation. Total mass flow rate can be obtained in two ways i.e. the mass flow rate on the scale is too large or the setting time is too long for each prefill process.

3.2. Fix the Prefill Function on the Operational Concept from the Advant logic side
Based on the investigation of the logic prefill compared to the DEPP data trending it is necessary to make some parameters modification that will affect the prefill process as well as Prefill operation pattern. Improvement of the prefill logic is done in several stages. The first stage is to modify the maximum time for each prefill (TIME_MAX_PRF_C1). The maximum time for prefill is gradually lowered and applied directly when the gas turbine operates by monitoring DEPP trending data continuously.

Based on Table 1. It can be seen that a decrease in setting time will reduce the pulsation that occurs in the burner. However, the maximum limit of changes is shown by the absence of diff TAT. Diff TAT indicates that the prefill has perfectly filled the FDS. The maximum prefill time decrease gradually and reaches the optimum value of 7 seconds. However, a change in time setting is not the best choice because the gas turbine is still experiencing PLS due to prefill for a while. The next step is decreasing the pilot valve mass flow rate value in logic prefill shown by Fig. 6. The mass flow rate value decreases gradually while pulsation pattern and changes in TAT are continuously observed.
Changing of pilot valve mass flow rate parameters compared to the pulsation values can be observed in Table 2.

Any decrease in mass flow rate will result in a decrease of high pulsation value. However, the maximum limit of changes is indicated by the absence of diff TAT. Change in mass flow rate value obtained minimum high pulsation at the point of 0.2 kg/s for both minimum and maximum mass flow rate.

IV. Results And Discussions
Prefill system on turbine gas will help avoid flame off when there is a change in the area of switch over or switchback point. However, the active prefill will cause high pulsation so PLS or Protective Load Shedding will be active. PLS will cause turbine gas load to drop until a normal condition is reached and PLS is reset.

In Fig. 7. There is an active PLS signal from signal number 7 that is 51MBX41EA000_XU03 PLS 2oo3 or PLS 2 out of 3 active signals. PLS is active because of signal high pulsation number 9, 51MBM30AX010Breach 148 mbar. High pulsation reaching 148 mbar would cause PLS. PLS always happens when prefill process takes place. The total event for 2012 is 52 times starting from February 10, 2012 and the last event is November 7, 2012. High pulsation results in PLS because Prefill occurs in an area of about 60% to 65% Relative Power or approximately 112 MW. The operating load in 112 MW is the demand from the network at certain times. However, if prefill operates at a relatively high load (above 120 MW), then the prefill process will not cause high pulsation because the proportion of premix is so large that the effect of the prefill is relatively small. Using this information, logic improvements are needed by modifying the enabled function
PLT_PREFIL_CYC_EN, so when turbine gas operates at a load the cyclic prefill will not be active. This function can be activated when a gas turbine operates with a relatively high load. When under load, the prefill will be active when the gas turbine experiences a minimum load loss of 0.35 MW/s for 10 seconds. Fig. 10. show prefill process after modifying function has been done. Prefill occurs when the Gas Turbine experiences a decrease in load from 143 MW to 113 MW within 2 seconds or a decrease of 15 MW per second. This prefill occurs optimally by being marked by a small power swing and no increase in pulsation value.

V. Conclusion
Fixing the prefill function from gas turbines can reduce and eliminate PLS due to High Pulsation. Prefill function improvement is done by disabling cyclic prefill function under load. With the improvement of prefill function, there is no more derating because of prefilling. Despite the improvement of the prefill system to the operational concept of gas turbine, monitoring of combustion parameters must continue.

Endrik Purbo Yunastyo1, Danan Tri Yulianto1, Asnawadi Hidayat1,

Kevin Sanjoyo Gunawan2, Totok R. Biyanto2*

1PT. PJB UP Muara Tawar, Muara Tawar, Indonesia

2

Engineering Physics Department, Institut Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia.

Implementing Risk-Based Maintenance Strategies for Distributed Control System as Power Plant Asset Management

doi:10.1088/1757-899X/1096/1/012108 

Abstract. The electricity generated from the power plant is subject to several requirements for active power, voltage, and frequency according to the grid system, so the machine must be controlled to achieve requirement by a power plant control system known as Distributed Control System (DCS). DCS system in block 1 Muara Tawar power plant using Procontrol P13 for gas turbine and Procontrol P-14 to control steam turbine. These systems had operated since 1997 and had been nearly operating for 23 years until now, and several failures tend to increase from time to time. The failures of the DCS and the lack of control cards will result in the loss of production. DCS system assets must handle properly to maintain the overall reliability of the power plant system. A method and strategy to maintain DCS must be carried out and ensure reliability and risk always under controlled conditions. Implementing Risk-based maintenance by carrying out quantitative calculations through the reliability approach and the level of the consequence of failure to calculate equipment risk is one of the methods of the DCS system. The result of the Risk-based maintenance method shows the highest risk on the risk map of the DCS system was in cubicles 14CBA02, 14CBA03 with a high-risk level and gas turbine and HRSG cubicle in medium-high level. The interval preventive maintenance time calculated by reliability within a year showed that cubicles 14CBA02 and 14CBA03 suggested being maintained every 29 days to reduce possibilities of failure.

        The number of DCS failures both cards control and monitoring system (human-machine interface) based on record during 2016 to mid-2020 shows the Procontrol DCS system contribute failures and result in operations and production decreasing. Based on data, DCS Procontrol P13 and P14 manufacturers in 2015, show that the card control and the whole system are in the classic, limited, and absolute phases. The classic phase to absolute has a span of 10 years. This situation brought a major issue on the availability of spare cards, especially how to keep and maintain the reliability of the overall DCS system. 

        According to card assessment, the majority of cards are in the classic and limited phases. This happened in all GT blocks 1-2 and ST 14 that used P13 and P14 Procontrol systems. DCS card failures data according to the type of card control can be shown in figure 3 the Pareto diagram according to failure of each type of card. Maintenance of DCS systems is an asset management challenge, especially on how "card control" management takes part in the situation where the availability of spare cards is starting to be limited and it will greatly affect the operation, production and at the same time the safety factor of the plant. A maintenance pattern is needed to accommodate and maintain the reliability of the DCS system. The ability to maximize the capabilities and reliability of DCS is an important asset to improve the power plant performance and profitability. 

        This research seeks to formulate a method of maintenance based on DCS’s Risk-based Maintenance and is expected to answer some maintenance problems in the DCS system then classify the possible causes of the failure of the DCS system. This research also shows how much the consequences arising from interference are, how likely the occurrence of the interference is, how big is the risk of DCS failure, and how to plan a DCS system maintenance strategy.

        Data analysis is carried out following the steps in the preparation of equipment maintenance strategies related to the DCS in its cubicle system that uses Risk-based maintenance. The following steps are: 

Classification of DCS generating systems based on cubicle according to Pareto chart that reflected the number of failures. 

• Determine the scenario of failure by analyzing the possible causes of disruption of the DCS system.     
• Determine the evaluation of consequences in a semi-qualitative way by determining the risk criteria for ranking loss of performance of units, financial, ecological, and health and safety so that the total consequences result from a system failure. The four factors are then combined to produce a                 total assessment of the consequences of equipment failure on the system and formulated with Consequence = [0.25A2 + 0.25B2 + 0.25C2 + 0.25D2 ] ^0.5 
• Conduct probabilistic analysis with precise distribution in accordance with the characteristics of the failure interval data so that the reliability value can be found as a basis for calculating the failure probability of the DCS system. The reliability value can be calculated using the equation.
• A risk assessment by combining the consequences of risk and analyzing the probability of failure that produces an acceptable or rejected value. At this stage, a matrix of risk is drawn to describe the position of DCS system risk.
• Based on the risk level and duration of operation of the DCS system, the most appropriate type of maintenance interval can be determined to ensure the reliability of DCS system. Evaluation of maintenance by making comparisons the maintenance process before risk-based maintenance is carried out and after this method is implemented. The reference of reliability value must not be less than 80% or 0.80 [8]. The results of the comparison were then plotted into a graph to find out the relationship between DCS reliability values and operational time. 

       The risk mapping of DCS system based on each cubicle in Muara Tawar Block 1 power plant from likelihood risk identification and consequences based on the failure report then visualized in a risk matrix (figure 12) as part of DCS system risk mitigation process in case of failure prevention that has an uncontrollable impact. The DCS system that is classified as high risk is 14CBA02 and 14CBA03, which are the system that handles the control of the steam turbine system. Meanwhile, the medium-risk were 11CRC30, 12CRC30 cubicle that handles the control of gas turbine and 12CBA11 cubicle handles control of HRSG 12. Risk control is carried out by making recommendations for maintenance and modification based on the level of risk and the likelihood of damage to the DCS system in a cubicle.

    

    Maintenance planning is based on the level of risk and the time interval for DCS system failure so that the most appropriate type and maintenance intervals can be seen and the DCS can be maintained. Based on the calculation of the decline in reliability values obtained maintenance time intervals that have been recapitulated in Table

    Determination of the maintenance interval of the DCS system on the cubicle with a critical level is crucial if it were done in a period based on the failure rate from time to time. Spare part readiness can help maintain DCS assets as an important part of a power plant.

Implementing Risk-Based Maintenance Strategies for Distributed Control System as 

Power Plant Asset Management

D T Yulianto, R M Isman, S N Ihsan and H G Susanto 

Pembangkitan Jawa Bali UP Muara Tawar, PLTGU Muara Tawar 1 street, West Java, Indonesia Corresponding email: *danantriyulianto@ptpjb.com