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Interaction | December 2014

The best of O&M practices

The author outlines best O&M (operation & maintenance) practices that enhance asset management and can protect the life of power plant equipment to the desired life-cycle of 30 years.
In India, the current installed capacity is 253,389 MW from all sources of energy and coal-fired alone contributes around 60 per cent. With the introduction of the Electricity Act-2003, the private companies in the country have shown tremendous interest in the power industry and as a result, various independent power projects (IPPs) have come up in India. The capacity addition for the 12th Plan target is to add 88,537 MW of power generation.

The purpose of this article is to outline O&M best practices that enhance asset management and can protect the life of power plant equipment to the desired life-cycle of 30 years. The best O&M practices include following well-established operating procedures, minimising frequent cold start-ups, reducing prolonged period of operating the unit with low load and operating the unit beyond the design limit, cycling the unit per OEM´s recommendation, following condition-based monitoring, introducing online monitoring system, periodic condition assessment - NDE, maintaining proper water chemistry, utilising proper spares, operating controllable parameters per OEM´s guidelines and staying well within the O&M budgeted cost. O&M should implement proper cycle chemistry to prevent boiler tube failures without causing significant loss to the company, annual plant audit, implement preventive maintenance program, and reduce forced outages, RLA assessment and comprehensive cost analysis study performed including damaging cost on annual basis.

Introduction
During day-to-day operations, O&M managers need to be vigilant about the impact of frequent outage-related cold startup, prolonged period of low load and beyond the design limit operations, frequently load cycling to meet load fluctuation that significantly varies the water chemistry and not being able to maintain the critical plant parameters, which impacts plant efficiency.

It is clear that IPPs should meet the power purchase agreement obligations that are linked with many challenges such as coal and water availability, low power selling price, low load operation, frequently planned shutdown and load cycling. Some of these items may further aggravate by increasing the renewable power supply from wind, solar and hydro that are extremely variable. The O&M operating team is caught in the complex environment between the unit cycling (cold startup, warm startup, hot startup, load ramp rate, low load operation and load cycling) and generating power. The author´s intent is to establish a road map for the O&M team and they can adopt best practices for operating a thermal power plant to manage the plant asset & improve reliability and operate the plant to the desired period of life-cycle of 30 years.

Frequent unit outages
The present power plant scenario is, availability of coal with acceptable price, continuously operating the power plant with desired load, seasonal factors associated with renewable energy and priority dispatch, which pose challenges to the thermal power plant. These factors are specifically influenced by larger size units such as 600 MW, 660 MW and 800 MW for frequent shutdown/cold startup. EPRI and other research organisations have investigated and performed analysis that states that frequent shutdown and cold startup due to forced outages or planned outages may impact the power plant life including extensive maintenance activities associated with capital replacements. To start up the unit, use of start-up fuel such as oil or gas is required and also to support the unit to bring the desired load with proper water chemistry.

Potential risk for cold startup
Frequent shutdown and cold startup may impact rotating equipment due to vibration & balancing, anticipate short-term and long-term overheating phenomena including SH/RH metal temperature excursions, stub to headers ligament cracking, potential to avert proper preservation methods, boiler back-end corrosion due to the acid dew point, chances of mills puff and explosions.

Electric Power Research Institute reports indicate that steam turbine blades erosion, thermal stress, pitting and low cycle fatigues, condenser wear and tear, gland seal erosion and FW heaters, valves and fittings thermal fatigue cracking are anticipated due to frequent cold startup. During startup phase, various challenges are experienced such as maintaining proper water chemistry & additional burden on WTP because of high water makeup, concern about the integrity of the magnetite layer inside the boiler tube, proper load ramp rate, vigilance on the metal temperature rise (SH/RH), process control stability, oil/gas consumption, control of spray valves, introduction of coal through mills, auxiliary power consumption, plant performance & efficiency. Load cycling damage severity is in this magnitude: Cold startup (Highest), warm startup (High), hot startup (Low) and Load Cycling (Lowest). A preventive approach is to reduce the long-term wear and tear on the equipment.

Cold startup & cost implications
Every cold startup cycle involves additional O&M cost that includes maintenance & capital, anticipated further forced outages, startup fuel, auxiliary power consumption during unit shutdown, efficiency loss from low load to the desired load operation with oil & coal, water chemistry and maintenance supports. Reports suggest that the power plant life will be significantly reduced from 100-120 hours for every shutdown & cold startup. Allowable limit for cold startup per annum should be less than four as per best practices. Even though thermal power plants are designed for economic load operation of 70-100 per cent MCR or optimised load in and without mills, there are many IPP plants operating at minimum or low load of 45-50 per cent MCR for prolonged period without oil support due to various reasons. Some of them are inadequate infrastructures to evacuate power, merchant power sale price is not commercially viable, coal availability, ash disposal facility limitations and generating cost is higher than the power purchase agreement cost.

It is imperative that operating the power plant at prolonged low load will not provide the fuel economics (high heat rate with high auxiliary power consumption) at the plant. Table-2 indicates heat rate impact at various loads. At 50 per cent load, the heat rate is around 2,550 Kcal/kWh which is not an economic and efficient operation of the power plant (plant efficiency is 33.7 per cent). It may be acceptable to operate the plant at low load for short duration, say 2- 4 hours per day.

Equipment impact on low load operation
Prolonged low load operation is not a desirable practice for coal-fired thermal power plants due to thermally induced bending at water wall tubes, top to bottom and introduced thermal & creep fatigues of water wall at heat flux zones, increase in dew point corrosion of flue gas ducts and dust buildup in electrostatic precipitator as a result of condensation.

In steam turbines, LP rotor is exposed
to stress corrosion cracking of rotor disks, keyways and fatigue and corrosion failure of final row of blades. The plant operating team needs to operate the unit without impacting the combustion stability associated with drum level fluctuations and high SH/RH metal temperatures (per Larson Method, 5 Deg. C above the design limit operation, 20 per cent life reduction is expected). Performance on low load operation
One of the main concerns at low load operation is, the combustion stability, maintaining air/fuel ratio and fuel firing equipment controls, in particular in the mills. The aim should be to optimise the SH/RH and airflow that would maximise the overall unit efficiency. The power plant is gauged by the plant efficiency and unutilised available station capacity. The station heat rate (SHR) and auxiliary power consumption (APC) decides the cost of generation. A typical 600 MW unit should be operational with SHR of 2220-2300 Kcal/kWh (Subcritical units efficiency ~ 36-38 per cent and Supercritical units efficiency ~ 39- 42 per cent) and APC of 6 - 6.5 per cent of the MW generation.

Frequent load cycling
Thermal power plant cycling consists of various aspects including startups (cold, warm and hot), ramp rate MW/min, low load & peak load operation and typical load following or load cycling. Cold startup and low load operation and its impact has already been reviewed in this article.

The generating station is originally designed for base load operation and operating in cyclic mode from minimum advertised load to the maximum load (60 per cent to 100 per cent MCR -> 4- 6 times per day is manageable) with one or two mills in and out. The need for load cycling is mainly due to the load flexibi¡lity, renewable energy dispatch, generating units are offline, load demand is swinging, and transmission system undergoes repair, seasonal factors and merit order dispatch by load dispatch centre.

Load cycling operations
The drum type units are more challenging than the supercritical units. EPRI reports suggest the need for improved controls, online monitoring and diagnostic systems for cycling. Necessary controls are needed to provide the desired level of flexibility, alarms and protection systems to avoid component damage.

The cycling operation also leads to increase in O&M and coal cost, high incremental or dynamic heat rate, plant cycle efficiency impact, auxiliary power consumption increase, potential for high ramp rate, frequently tuning operating parameters, in and out of mills operation that leads to operator error.

Load cycling cost impact
Among cold startup, hot startup and warm startup, load cycling to low load cost is relatively lower. To optimise operations and determine the true cost of each operation, a thorough analysis should be performed for load cycling. The areas involved for cost estimate are forced outage effects - repair time and energy lost, efficiency loss and mills on/off (cycle). Typical cost generating cost components are fuel, O&M, capital, depreciation and others. Cost components impacted by load cycling are O&M and other costs. Most of the cycling related O&M costs consists of boiler (50-60 per cent), steam turbine (20-30 per cent), fuel handling (5-10 per cent) and BOP (10-15 per cent).

Water chemistry effects on plant assets
One of the biggest challenges for O&M personnel is maintaining good water chemistry if the water supply comes from multiple sources and makeup water consumption increases for cycling the unit. Maintaining & monitoring water chemistry parameters such as sodium, silica, pH, dissolved oxygen and cation conductivity during operation is a major requirement. If there are any deviations of water chemistry parameters, it may cause metal to overheat, corrosive wall thinning, and localised pitting, any or all of which can lead to premature boiler tube failures and leads to forced outages. On the water/steam side, there is high potential for oxidation of boiler tubes due to high temperature steam and the corrosive action of chemicals in the water supply. Typical issues anticipated for cycling are corrosion fatigue, fireside corrosion, oxygen pitting, phosphate hideout leading to acid and caustic attack, silica and iron deposits, air in-leakages and acid and caustic attack.

Key O&M best practices
The best O&M practices are identified in the previous sections where those actions directly influence plant reliability as well as the asset protection. In addition, the following practices should be followed for the process & performance improvement and asset management.

Conclusion
The Rankine cycle based power plants cycling impact of frequent cold startup, prolonged period of low load and beyond the design limit operation, too frequently load cycling the units from 50 per cent to 100 per cent MCR load and high ramp rate MW/min are discussed. Low load (50 per cent MCR) to 100 per cent MCR load cycling should be minimised. The minimum operating load (60 per cent MCR) criteria as well as allowable load cycling (4-6 times is an acceptable practice per day) operation including ramp rates for controlled circulation units are better than the natural circulation units.

Every cold startup that is being performed that leads to the highest damage cost. The life of the unit is reduced by around 120 hours and the typical cost of damage estimated including maintenance with startup oil is in the range of Rs. 1.1 crore to Rs 1.9 crore per cold startup. This cost data may vary from plant to plant and is site-specific. In most cases, this cost is hidden and is not covered in the power purchase agreement for IPPs. The O&M team should understand, follow, plan and react to this complex nature and ensure meeting asset obligations.

Asset management is of paramount importance to everybody associated with thermal power plants, specifically larger size units and those who are managing the technical, cost, management and plant O&M. Unit cycling should be minimised and not compromised for short-term cost benefits.

This article´s primary author is Sundara Kavidass, Sr. Vice President, Operation Services, Essar Power, and the co-author is Nilesh Narain, GM (Tech), Essar Power Gujarat Limited, Jamnagar and Gujarat.

This article´s primary author is Sundara Kavidass,
Sr. Vice President, Operation Services,
Essar Power, and the co-author is Nilesh Narain,
GM (Tech), Essar Power Gujarat Limited,
Jamnagar and Gujarat.



Typical cold startup cycling cost for 600 MW
Item Category Estimated Cost (Rs.1000)
  Low High
1 Maintenance & Capital 2,000 4,000
2 Forced outages due to cold start 1,500 3,000
3 Startup fuel 5,500 7,500
4 Auxiliary Power 2,000 3,000
5 Efficiency loss 850 1,300
6 Water chemistry& support 150 250
Total Cost 11,000 19,000


A Typical Load Vs Heat Rate & Aux. Power Consumption
Load
(MW)
Heat Rate
(Kcal/
kWh)
Aux. Power
Consumption
per cent
Assumed GCV (12 per
cent Moisture) - India
Coal (Kcal/kg)
300 2550 8.5 3300
360 2420 8.0 3300
420 2385 7.5 3300
600 2300 6.5 3300
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