Steam turbine oil analysis practices
The market studies done by the various analyst on “Global market analysis on Turbine oil “forecast that the global turbine oil market to grow at a CAGR of 3.498% during the period 2017-2021 and the market size has a potential to grow by 1500.05 K MT during 2020-2024, and its momentum will accelerate during the forecast period. Turbine oil manufacturers are continuously working with OEMs to provide high-performance turbine oils to meets the rising demands globally. Nowadays the industries are more focused on extending the life of both lubricants and the equipment by implementing a robust Reliability lubrication Program.
In this article, we have tried to cover the maximum information about turbine oil which could help the power generation & manufacturing industries to move further with a cost-effective sustainable solution.
Turbine & it's lubrication system:
The turbine which is the heart of the power plant extracts thermal energy from high-pressure steam and converts it into mechanical energy on a rotating output shaft. This mechanical energy is used to drive rotating equipment like pumps, compressors, fan, blower, and ultimately to drive a generator for the production of electricity.
Let’s understand a brief of oil flow along with major component and its function in the lubrication system of the turbine with the following steps:
- Oil system which is composed of an oil tank (with a heater for a start-up)
- Oil pumps to provide the necessary level of oil pressure
- Oil coolers to dissipate the heat from the oil during operation
- Oil filters to remove erosional debris from the circulating oil
- Pressure control valve to regulate an exact and constant supply oil pressure to the bearings
- In addition, one pressure relief valves to protect the pumps from overpressure (usually rotary type)
- An accumulator for minimizing pressure surge when any operational change occurs, like switching-over between main and stand-by equipment.
- A Temperature Control Valve (TCV) is provided to by-pass the coolers when the oil temperature is low, to ensure correct oil viscosity and efficient lubrication. Sometimes it is done by controlling water flow through oil coolers.
Importance of turbine oil:
In this growing modern world where electricity has become the most important part of our life and spending a day without it could slow down our lifestyle, productivity and overall impacting the global economy; then the generation of electricity indeed became fundamental to us. To meet this ever-increasing demand, power plants are delicately performing to improve their output and reliability. In the Power plants, Turbine is the most crucial machine which is the prime movers for other rotating equipment and hence the generation of electricity. Its reliability and availability critically depend on the turbine oil, which makes the lubricant selection very important part in the reliability program. However, the OEM of the turbine carefully specifies the characteristic of the lubricants, laboratory test for oil analysis and standard references for the best performance of the turbine.
Turbine oil has to undergo through an adverse condition like large temperature fluctuation, heavy load, ingress of contamination, moisture, entrained air, heat and many physical & chemical changes. Important roles of oil are cooling of bearings & journals, flushing contaminants away from rotating parts, preventing in leakage of gases, providing hydrostatic lift for shafts, actuating valves in the hydraulic circuit, and protecting lube-system internals.
Modern technologies in lubrication science show clearly a great advancement in the formulation of the turbine oil. Almost 90-95% of turbine oil is a base oil with additives making up the remaining per cent which includes antioxidants, rust inhibitors, metal deactivators, antifoam agents, demulsifier, pour depressants and antiwear additives.
Turbine oil properties, type and brand:
Integral properties of the steam turbine oil are to allow uniform lubricating performance over a wide range of ambient and operating temperatures, to reduce hydrolysis, to reduce the formation of varnish, sludge, emulsion & entrained gas/foam, to prevent rust, minimize acid attack on copper alloy parts, improves lubricity, minimize oxidation, good oil degradation solubility improves filtration, etc. The oil which meets all these mentioned properties is of ISO VG 46.
The below chart is summing up all properties along with the recommended range and testing methods for its analysis:
Though turbine oil is designed to have a long service life working in adverse conditions changes the properties of the oil. Here we will discuss the causes of changes in a few important properties and suggested action to be taken:
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Lubricant supplier selection and brands:
Selecting a suitable supplier who can meet the performance requirement for the turbine is a vital part of the reliability lubrication program. While finalizing the vendors, many points should be considered like lubricant quality & services, technological advancement, troubleshooting support, emergency preparedness, technical support, laboratory support, price, and delivery capacity.
Top suppliers of turbine oil in India and international are IOCL, BPCL, HPCL, Gulf Oil, Mobil, Castrol, Shell, Chevron, Valvoline, and many more. Below chart is a brief of turbine oil properties considered by the manufacturer:
Turbine oil maintenance (sampling points, testing standards, testing schedule)
- Maximize data density of oil information
- Minimize the distortion of information
- Designated sampling location of primary points (return line or drain valve, elbow, turbulent area) and secondary points (anywhere on the system to isolate upstream component)
- Proper frequency
- Proper sampling hardware
- Sample when the machine is running at normal operating temperature
- Sample after flushing a small quantity of oil (0.5 - 1.0l) through the sampling point
- Use only dedicated clean and dry sampling equipment intended for the sampling of used oils.
- Samples should be sent to the lab or processed for on-site analysis before 24 hours from the time the sample was extracted.
- To avoid leakage, fill the sample bottle to 90% capacity and ensure it is properly sealed before despatch to the laboratory.
Below images are illustrating both primary sampling points and secondary sampling points of turbine lube oil system:
Primary Sampling Point: Location where regular routine oil samples are taken for monitoring and trending wear metal debris, oil condition, and oil contamination.
Secondary Sampling Point: For diagnostic and troubleshooting and for measuring wear metal debris and oil contamination by individual components.
Power Plant lubrication monitoring is broadly specified under ASTM D4378 and ASTM D6224, and these define almost every test used to qualify lubricants for new and in-service monitoring for power plants. Following are major standards and OEM approval standards
- ASTM D4378, "Standard Practice for In-Service Monitoring of Mineral Turbine Oils for Steam, Gas, and Combined Cycle Turbines"
- ASTM D6224, "Standard Practice for In-Service Monitoring of Lubricating Oil for Auxiliary Power Plant Equipment", focuses on auxiliary equipment that supports power-generating turbines.
- FED-STD-791D (testing method of lubricants, liquid fuels, and related products)
- Ahlstom - Gas and Steam - HTGD901117
- GE - Gas - GEK 32568f
- GE - Steam - GEK 46506D
- Solar - ES9-224
- MHI - Steam & Gas - MS04-MA- CL001 and CL002
- Siemens/ Westinghouse - K-8962-11
Testing schedule with standard reference:
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Steam turbine breakdown problems:
1. Bearing Failure
Bearing & Journal related issues are one of the main causes of turbine generator outages. While faulty design or manufacturing can be occasionally responsible, the most frequent cause of these problems is improper operating conditions. A turbine trip is likely to occur when any bearing/shaft vibration reaches the trip limit. Major operational causes of accelerated wear and failure of turbine generator bearings are:
- Improper lubrication (inadequate flow, dirty oil, improper oil temperature)
- Excessive load (ie. the force pressing the shaft against the bearing)
- Prolonged operation at high vibration levels which can result in fatigue cracking of the bearing lining.
- Insufficient oil flow and pressure causes wiping off of babbitt material
Such operating conditions can result in bearing overheating, scoring, or erosion of bearing surfaces by oil contaminants, or fatigue damage.
2. Blade Failure
Turbine blade failure is a nightmare for any power player. It is another major problem and typical operational issues responsible for it are deposits, erosion, corrosion and vibration, fatigue, and manufacturing defect. The most common causes of blade failures are stress corrosion cracking and fatigue cracking due to excessive blade vibration.
Blade failure can cause large scale damage to the turbine. In the worst case, one or more of the long moving blades in an LP turbine may pierce the casing and become turbine missiles. In a less severe case, the broken blade (interfering with the motion of other blades, can shear off or bend some of them. The resultant very high vibration can destroy turbine generator bearings, seals, oil lines, etc. This type of case happened in 2014 at one of the large power generation stations in India where 600 MW LP turbine last stage blade broken at lacing wire area and subsequently damaged 5 more blades. It caused a unit outage of more than 90 days!
The serious consequences of such breakdowns, determination of the causes of bearing failure, and methods of effective repair are of paramount importance. Sublevel maintenance practices or overextending the life of the oil are also important factors for the breakdown of the turbine *A very common myth with lubrication system is that “keeping filters in line with oil circuits will prevent failures due to contamination”. This is the paradox of the situation where emergency oil supply systems bypass all the filters and obstacles in the circuit to fulfill the demand, but contaminants use it as an opportunity to fail the emergency system itself. (Bases on the facts when DC pumps started and failed due to oil contamination).
*Another myth with filters is that “all the filters are the same”. The filter is a vast subject and it’s not just putting some filter media in a can. Starting from the media selection it has a long journey which includes filter sizing, pressure drop, dirt holding capacity, rupture pressure, fugitive particles, etc. So, it is always suggested to go with a proven supplier.
Turbine bearing failure is not only limited to bearing replacement cost and time but results in rotor damage as well. In 2017, in India, one 300 MW Chinese supplied machine encountered oil starvation during a blackout and resulted in rotor damage which further cost billions of rupee revenue loss to the company.
Consolidated turbine oil care and maintenance points:
Apart from the oil analysis, below lubrication practices should be followed for best performance of Steam Turbine:
- Keep oil clean, cool, and dry
- Inspect bulk delivery of new oil before acceptance
- Maintain proper alignment
- Perform regular oil analysis
- Install high-capture-efficiency breathers
- Inspect for signs of foaming and/or air entrainment
- Install primary and secondary sampling points
- Routinely inspect sight glasses for level, foam, and water
- Keep tank headspace dry using blowers, condensers, or instrument air
- Use off-line fluid conditioning for precision contamination control
- Regularly inspect for varnish, especially in gas turbines
- Analyze the oil periodically
- Keep temperature and operation records
- Install filter and strainers to retain solid contaminants
- Install a multipurpose oil purification unit to prevent solid and liquid contaminants
- Check oil filter cartridges or elements periodically
- During inspection and overhauls, the turbine oil must be drained from the main oil tank, oil cooler, and the bearing pedestals
- Reuse the oil only after ensuring all the parameters within the prescribed limits
- Any small change in vibration must be considered on priority
- Keep a close view on condition monitoring results and repeat the test in case of any deviation
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About the Author: Ms. Preeti Prasad associated as Technical Consultant and Business Development Manager with Minimac system Pvt Ltd. She is a chemical engineer with work experience in Oil Refinery and also providing lubrication consultancy services to many companies/sectors. She holds Level I Machine Lubrication Technician(MLT) certification through the International Council for Machinery Lubrication (ICML).
- Machinery Lubrication India- Noria Corporation Ltd
- Monitoring and Optimizing life of Turbine Oil by Analyst, Inc
- Troubleshooting bearing and lube oil system problems by Thomas H.McCloseky
- BHEL documents on Stem Turbine Maintenance and Turbine oil care maintenance
- Turbine Operational Problem (module 234-14)
- PDS of mentioned oil manufacturer