Outage Optimization of Steam Turbine and Auxiliaries

Parichart Suttiprasit#1, Santhiti Chanthauthai#2, Wannadit Umpuch#3 Steam Turbine Department, Mechanical Maintenance Division Electricity Generating Authorityof Thailand (EGAT), Thailand

1parichart.su@egat.co.th 2santhiti.c@egat.co.th 3wannadit.u@egat.co.th

Abstract - There is no standard of outage interval that power plant maintenance should be followed as the guidance, because the core of maintenance is to execute in the least cost. Condition Monitoring becomes the first priority tool to plan maintenance work analytically. Moreover, the nature of each power plant is really specific, so all outage need to be arranged upon the equipment condition. This is called Outage Optimization, which is a necessary system that collects monitoring data to update and analyze the equipment health all the time before planning. This paper presents the optimization of 115 MW Steam Turbine and Auxiliaries in South Bangkok Combined Cycle Power Plant which was first synchronized in 1995. What the monitoring data should be collected, how to collect them and how to analyze them are included. Moreover, the maintenance planning and the results of this condition based maintenance compared with the previous time based maintenance are disclosed. We trusted that this optimization method can be applied with specific equipment of all power plants to process their results.

I. INTRODUCTION The outage maintenance work of Steam Turbine normally

is the critical path of all outage activities. It is obvious that the equipment working needs to be planned thoughtfully. The configuration is series type, so it has no standby component. And Steam Turbine’s condition has to be analyzed cautiously. Therefore, it needs maintenance experiences, conceptual consideration, and economic analysis to achieve this outage optimization completely. The main objective of outage optimization is to maintain the same effectiveness which is plant reliability and performance in the same time. In addition, the Auxiliaries which are overhauled in the outage need to be monitored for their condition. It is very crucial to gather all information to plan the outage.

Ideally, the uptime must be 100 percent, and the downtime needs to be zero. Although, it cannot be maintained in the ideal world, the main objective of maintenance world is to perform preventive maintenance only with the equipment which has abnormal symtoms. The results which we can gain shows a decrease of defect, the most efficient power generation, a decrease of downtime, cost savings which includes spare parts inventory, manpower, maintenance cost, etc. Also, the root cause of equipment problem can be pinpointed instantly.

II. STEAM TURBINE The 115 MW- Steam Turbine of South Bangkok Combined

Cycle Power Plant (SB-C10) which was first synchronized in 1995 had been executed with 4 minor inspection (MI) and 1 major overhaul (MO) outages. During the MO scope planning in 2008, the feasibility to minimize the outage duration has been found by our record and analysis as follows.

A. Condition Monitoring Steam Turbine vibration record was measured by ADRE#7

analyzer and Bentley 3300 transducer and analyzed by EGAT Mechanical Testing Department. The overall shaft vibration was recorded at bearing no.1, 2, 3 and 4. The bearing no. 1 and no. 2 support HP-LP Rotor and the rest support generator rotor. The vibration trend presented as in the following figure tended to be decreased and the latest record was in admissible range, 90 μmp-p, accepted by MHI criteria [1].

In case of physical oil analysis, the lubricaiton oil particle count record has been in normal range since 1998. As regards to thermal technique, the bearing metal temperature which is represented the bearing load condition was started collecting in 1997. All of the temperature record was not exceeded 91oC which was much less than the alarm setting at 107oC. It can be described that the bearing condition is in good condition and the oil is confined in sufficient amount. So, the possible problem of oil leakage, babbit melting, bearing damage hardly occurs. Another measurement of thermal concern was implemented by an infrared camera, ThermaCAM PA695, earlier before the outage to point out the temperature on high-temperature zone which is shielded by insulation such as high pressure (HP) turbine, HP bolt, the main steam pipeline, and the main steam stop and control valves. According to the sample of temperature record in the appendix 1, the measurement of this unit does not reveal the overheated or leakage point of insulation.

1996 (MI) 1999 (MI) 2001(MO) 2003 (MI) 2006 (MI)

1X

B. Performance The Steam Turbine performance test was reported in 2007

by EGAT production efficiency division following ASME PTC6 testing standard [2] , which showed that the heat rate from 1998 to 2006 increased from 10216.37 to 10285.72 kJ/kWh respectively. Therefore, this result can be diagnosed in 2 sections as follows.

1) Loss of fuel: The Efficiency of Steam Turbine decreased only 0.24% in an 8-year operation. The assumption had been identified that heat rate since 1995 will be increased in the constant rate of 69.35 kJ/kwh per 8 years in case the Turbine is not performed with any maintenance activities. The power plant will lose in fuel cost as shown in the calculation below.

Fuel Cost of SB-C10 (Natural Gas) = 1.271 Baht/kwh Loss of Fuel = (1.27 Baht/kwh)(115 Mw)(24 hr/day)(1/2)(120.35(15) -

104.31(13) year.kJ/kwh) = 79.82 million Baht Therefore, this Steam Turbine which is operated continuously in this condition in the next 2 years must be calculated for the loss of fuel to be 79.82 million Baht. Indeed, the fuel price tendency is increased year after year, so the loss of fuel as shown above is the minimum loss which can possibly occur. Because we may lose the fuel cost upon the performance, we have to conduct economic analysis to decide whether we should shift the MO outage onto another 2 years.

2) Economic Analysis: According to the entire organizational synergy, the replacement energy concept is applied. All EGAT power plants which are regularly base load operation type comprise coal, natural gas, and petroleum fired plants. They are classified by generating fuel cost as the first, second, and third rank respectively. The conceptual emphasis on energy preservation and reduction of the overall fuel cost is to manage electricity generation and distribution in Thailand

efficiently. And operation and power loss are neglect in this concept. So, this combined cycle plant which is gas-fired type of HRSG, SB-C10, should be operated rather than other thermal power plants where petroleum-fired type of boiler functioned in. Obviously, Bangpakong Thermal Power Plant, BPK-T3, which is oil-fired plant, was selected to be an operating replacement by SB-C10 because fuel oil of BPK-T3 is in highest cost rank. The fuel cost difference between BPK-T3 and SB-C10 is 1.78. It is summarized that the current MO outage duration would reduce to 8 days as a result of avoidance to overhaul the Steam Turbine. The result was calculated as shown below.

Given, Standard MO duration = 45 days MO – overhaul Steam Turbine duration = 37 days If BPK-T3 was operated instead of SB-C10 which is

comprised of 2 x 110 Gas Turbine and 1x115 Steam Turbine as a full block, the fuel cost would be increased:

= (1.78 Baht/kwh)(335 Mw)(24 hr/day) = 14.31 million Baht per day

So, the fuel cost savings for 8 days operation would be:

= 114.49 million Baht Comparatively, the fuel cost savings was approximately

twice more than the fuel loss by lower efficiency; therefore, this outage plan had to be optimized to 37-day duration without Turbine overhaul, so the savings is 34.67 million Baht by deducting 79.82 from 114.49 million Baht (1.06 million US$; 32.58 Baht = 1 US$ [4]). Additionally, the operation and maintenance cost which was not included in this calculation would be considerably reduced.

C. Risk Assessment In addition, we need to identify the risk by multiplying the

failure effects and failure mode likelihood. Referring to Table I and II below, we can indicate risk in this case as 1 of “Very unlikely” mode and multiply by 10 of failure effect weight

Fig. 1 Vibration Trend of Steam Turbine from the year of 1996 to 2006

Admissible Range 90 μm p-p

that can be affected in unit shutdown. However, the result is equal to 10% as the moderate probability, we can mitigate by our Steam Turbine life assessment record. Rarely the critical failure, for example, stress corrosion cracking, corrosion fatigue, thermal fatigue cracking, can be usually found earlier than 20 years of its lifetime. Some failure damages such as erosion at turbine nozzle or a crack on high-pressure casing had been repaired by the mean of welding and/or grinding simply.

TABLE I FAILURE MODE LIKELIHOOD

1 Very unlikely has not happened to equipment or similar equipment

2 Unlikely >2 times in 5-10 years to equipment or similar equipment

3 Likely >3 times in last 5 years

6 Likely >2 times in last year

10 Very likely many times to this equipment

TABLE II

WEIGHT OF CONSEQUENCE/FAILURE EFFECT

Failure Effect Weight

Unit Shut down 10

Unit Through put Loss 8

Minor Through put Loss 4

Start up Delay 4

Personnel Hazard 10

Personnel Hazard – Low Probability of Injury 5

Environment Regulations Event – Major Incident 10

Environment Regulations Event – Incident 7

Environment Regulations Event – Near Miss 3

III. AUXILIARIES Biannually, managing the outage plan interval along with

Time Based Maintenance (TBM), we determine the number of the Auxiliaries to overhaul. Most of Auxiliaries to be considered in Condition Based Maintenance (CBM) are pumps as shown in quantity (EA.) in Table IV. The optimization of the work scope to overhaul the really needed equipment by its health is our main concept. The primary reason why there should be no standard of outage interval like TBM is to execute at the least cost of maintenance. The tool that we applied in CBM plan is “Condition Monitoring” which was started roughly 3 months before the outage. We selected the auxiliary equipment to be inspected in the MO scope from January 29th to March 3rd, 2008 as summarized in table IV.

A. Condition Monitoring In terms of human senses, we inspected various items such

as looseness, leakage, cleanliness, as well as tightness at base plate and bolts. We also measured noise level, cavitation formation by stethoscope also noise condition at bearing as in the following figure. Our professional team had diagnosed noise pattern and pinpointed what can possibly be the source of the abnormal noise. For thermal measurement, we applied the infrared thermometer to indicate the bearing temperature.

Fig. 2 Stethoscope Measurement of Bearing Noise Condition

As for vibration technique, the vibrometer told us the overall vibration which was initially analyzed. The acceptance criterion is ISO standard 10816-3[5] in velocity range and mm/s rms unit of measurement.

Our teamwork which comprised engineering team, operators and the owner had the meeting together to optimize the outage work scope by all of collected information and data. The one of our report on condensate pump in appendix 2 shown as the example of condition monitoring inspection sheet. Referring to their condition, we finally summarized which Auxiliaries need to be overhauled and what the specific parts need to be inspected in the overhaul. The result is presented in table IV.

B. Performance Not only was the Auxiliaries’ health, but also power

consumption which is derived from efficiency consideration. We identified power consumption at shop test and current condition as shown in appendix 3, and then the optimum time to overhaul was found out from the concept when the cumulative cost of increased power consumption equals the overhaul cost.

When; Pi = Intial Power Consumption, kW Po = Current Power Consumption, kW M = Motor Efficiency, % E = Electricity Cost per unit = 2.37 Baht/kwh (Updated June 2007) C = Capacity of Operation, % Therefore, the deterioration cost, d, is equal to;

d = (Po – Pi)(E)(C) Baht/hr M

The deterioration cost in 1 month, D, must be multiplied by 720 hr.

TABLE III OPTIMUM TIME TO OVERHAUL AUXILIARIES, SB-C10

TABLE IV LIST OF AUXILIARIES AND OVERHAUL COST IN THE MO SCOPE, SB-C10

Since commissioning in 1995, the operating duration is 132

months, so the average deterioration cost rate is given from dividing by 132. The optimum time to overhaul (T), month, can be calculated from [6]:

T = 2(Overhaul Cost per EA.) m The Avg. Deterioration Cost Rate The sample result of closed cycle cooling water pumps and

condensate pumps is presented in table III that the optimum time to overhaul these was not earlier than 6 years. It supports our analysis that the Auxiliaries were not needed to overhaul this time.

C. Economic Analysis We not only recorded all conditions of Auxiliaries, but also

considered the cost savings that we gained from CBM on Auxiliaries. Although the Auxiliaries work scope management cannot reduce the downtime duration in this MO like Steam Turbine, we approached this case by comparing the overhaul cost-savings between TBM and CBM in the followings. Only once MO concerning did we get benefits of 2.5 million Baht (0.08 million US$) from.

IV. CONCLUSIONS In summary, this condition monitoring can help us to

optimize the outage of Steam Turbine and Auxiliaries which was executed in 2008. It means what the maintenance work has to be performed relies on specific condition of equipment. To minimize the risk, the outage plan needs to be diagnosed with the history data, operation and maintenance data, and experiences of maintenance team. In addition, this dynamically planned outage resulted in the maintenance cost savings of Steam Turbine around 34.67 million Baht from 8-day downtime reduction by assuming that heat rate will be constant in 2 years. Also according to the auxiliaries’ optimized scope, we can get the cost savings of 2.5 million Baht. In the outage compared with the previous one, it totally results in the cost savings of 37.17 million Baht (1.14 million US$).

However, outage optimization needs to preserve overall equipment effectiveness, when the outage had been executed; it needs to be assessed plant availability, performance, and quality rate as well.

Moreover, the future plan is to monitor all of the equipment conditions to arrange maintenance plan continuously. The maintenance management such as turnaround management,

reliability centered maintenance, total productive maintenance, must be the target to be accomplished in asset management eternally.

Appendix 1

Appendix 2

ACKNOWLEDGMENT As in the previous section, I have relied on comments and

information from a number of engineers with whom I have worked and cooperated on a number of projects over the past years. Their comments and observations have been of considerable assistance in identifying areas where more detailed information is required and in bringing to my attention subjects that I may not have been, at that time, addressing in sufficient detail, or even at all. For these reasons, their assistance and advice has been invaluable.

As in this paper, my special thanks go to my colleagues Wannadit Umpuch and Nattawut Piriyachita for allowing me to interrupt his work. Their review and input from their experience have added considerably to any value other plant engineers may find in this paper. Their concept and the time taken to read and comment on the text and the way it has been arranged have been invaluable.

I would also like to acknowledge the input from the following engineers. Each has made information available to me or has been prepared to discuss certain aspects of units and our joint experiences.

Santhiti Chanthauthai, head of Engineering Section in Steam Turbine Department, Supaniti Songsawas, Manassavee Thuphom, Sayan Pansang, Thaweesak Thaviroon, Nissara Siddhichai, Bothkanin Lumlertprasert of other departments in Mechanical Maintenance Division, EGAT, R.N. Pal, head of Training and Administration, and assisting lecturers at Indian Institute for Production Management, India.

To make the text clearer, I have used several of figures and tables. Some of these were presented at the power plant site, conferences, and seminars I attended.

Special thanks go to my office staff in Steam Turbine Department, for their assistance, especially Kobchai Wasuthalainan and Tawatchai Chairerk. Their help in particular was invaluable.

REFERENCES [1] Mitsubishi Heavy Industry, Allowable Shaft Vibration, Japan, 1997. [2] American Society of Mechanical Engineers (ASME) PTC6 Testing

Standard, Steam Turbine, 1996. [3] Production Efficiency Division, Electricity Generating Authority of

Thailand, Marginal Cost/Price of Electricity in Thailand, 2006 [4] Oanda Corporation (2008) homepage [Online]. Available: ,

http://www.oanda.com/convert/fxhistory [5] International Organization for Standardization, Mechanical Vibration

–Evaluation of Machine Vibration by Measurements on Non-Rotating Parts, ISO 10816-3, 1st edition, French, 1998.

[6] H.P. Bloch, and A.R. Budris, Pump User’s Handbook: Life extension, New York, 2004.