1B3 Chapter 6 Rotating Equipment Maintenance

1st Class Power Engineering

What are the typical maintenance problems of a large steam turbine?

Problems primarily relate to the steam path, bearings, and mechanical reliability, affecting performance and efficiency.

What factors determine the maintenance required for a steam turbine?

Quality of steam, operational factors (frequency and nature of starts/stops, load changes), age of equipment, and time in service.

What are common causes of efficiency losses in steam turbines?

Blade deposits, erosion, corrosion, foreign object damage, and damaged seals.

What are the categories of efficiency losses in steam turbines?

Leakage loss, friction loss, aerodynamic losses, and losses due to changes in flow passage areas.

What causes leakage losses in steam turbines?

Steam flow bypassing the regular flow path through interstage seals, blade shrouding, and blade root seals.

What is High Cycle Fatigue (HCF) in steam turbines?

A damaging mechanism of failure caused by fluctuating stresses leading to small cracks that propagate over time.

What is Low Cycle Fatigue (LCF) in steam turbines?

Failure due to high stress levels close to the elastic limit, often caused by thermal cycling during starts and stops.

What is creep in the context of steam turbines?

Time-dependent deformation of blading at high stress and temperature, leading to gradual lengthening and plastic deformation.

How does erosion affect steam turbines?

Solid particles in steam cause gouging and impact damage, particularly in high-pressure and reheat sections.

What is the main cause of corrosion in steam turbines?

Ions carried into the steam path, particularly where moisture forms during the expansion process.

What are the effects of moisture in steam on turbine blades?

Large water droplets can cause impact damage and erosion on turbine blades.

What are blade deposits in steam turbines?

Impurities in the steam that settle on turbine blades, causing unbalance and reduced efficiency.

What methods can be used to remove blade deposits?

Manual removal, cooldown cracking, and water washing with low-pressure saturated steam.

What is the purpose of running a turbine at low speed during washing?

To minimize erosion while allowing moisture to dissolve and flush away deposits.

What is the significance of conductivity meter readings in turbine maintenance?

They monitor the effectiveness of the blade washing process by indicating when impurities are removed.

What happens during washing at full speed in steam turbines?

Feedwater is injected into the steam, and exhaust steam is monitored until contaminants are cleared.

What is the impact of a 10% increase in nozzle throat area on turbine efficiency?

It can cause a 3% decrease in stage efficiency.

What are aerodynamic losses in steam turbines?

Losses that occur when blade angles change to less than optimum angles, affecting performance.

What is the role of operational factors in steam turbine maintenance?

They influence the frequency and nature of starts/stops and load changes, affecting wear and maintenance needs.

How does the age of equipment affect steam turbine maintenance?

Older equipment may require more frequent and intensive maintenance due to wear and degradation.

What is the relationship between steam impurities and turbine performance?

Impurities can lead to deposits that reduce efficiency and cause operational issues.

What is the consequence of high stress concentrations in steam turbines?

They can initiate cracking and rupture, particularly during thermal cycling.

What is the impact of blade deposits on turbine spindle balance?

Uneven deposits can cause the spindle to become unbalanced, indicated by increased shaft vibration.

What is the primary focus of steam turbine maintenance?

To ensure performance and efficiency are retained over the longest life possible.

What is the significance of monitoring steam quality in turbine maintenance?

It helps identify potential issues related to impurities that can affect turbine performance.

What effect does overnight shutdown have on a turbine plant?

It undergoes a blade washing effect during the following morning runup and is generally kept free of deposits.

What causes blading deposits in turbines?

Silica carried over in the boiler steam.

How do silica deposits affect turbine efficiency?

They can affect blade surfaces and reduce the efficiency of the machine.

How are silica deposits removed from turbine blades?

They require special procedures such as acid washing or blasting with an abrasive substance during overhaul.

What is the most common type of mechanical damage in turbines?

Wear to the bearing surfaces.

What can cause wear on blade tips in turbines?

High vibration due to imbalance or misalignment, and differential temperatures during starts and stops.

What are the common methods of turbine monitoring?

Performance analysis and vibration monitoring.

What factors determine the frequency of turbine overhauls?

Operating hours, operating history, and running efficiency checks.

How often do some engineers recommend a major overhaul for turbines?

Every four years.

What does a minor overhaul of a turbine typically include?

General external inspection and servicing of auxiliary machinery.

What is examined during a minor overhaul?

Bearings, thrusts, glands, valve gear, main steam strainers, and running clearances.

What is involved in a major overhaul of a turbine?

Lifting all turbine cylinder covers and spindles, examining blading, bearings, steam valves, and governors.

What is the purpose of advanced planning for a turbine overhaul?

To estimate the time required and provisions for loss of machine output.

How is bearing wear measured during inspection?

Using a bridge gauge placed over the shaft journal.

What is the general rule for allowable oil clearances in bearings?

0.025 mm per 25 mm of shaft diameter.

What is checked during thrust bearing examination?

Axial float and wear on thrust pads and shaft thrust collar.

What is the primary purpose of flexible couplings in turbines?

To allow for relative axial movement between the shafts they connect.

What should be done before lifting turbine cylinder covers?

Ensure all shaft coupling alignment readings are taken and cylinder glands are inspected.

What is done with cylinder joint bolts during reassembly?

They are marked or recorded to ensure they are replaced in the same position.

What is measured during the blading examination after removing the top half casing?

Axial clearances on each blade wheel and preceding diaphragm.

What does the examination of the white metal in bearings involve?

Checking for cracks or hard spots.

What is the role of graduated steel columns during the lifting of cylinder covers?

To guide the relative positions of each end of the cover during lifting.

What should be done with the turbine spindle before lifting the top half?

Ensure it is set right back on the thrust pads for maximum blade clearances.

What is the significance of recording measurements during turbine inspections?

To compare with previous records for detecting displacement or distortion.

What is the typical procedure for removing the bottom half of the bearing shell?

Support the turbine shaft with shaft raising gear and turn out the bottom half shell.

What is the purpose of examining oil passages in the bearing housing?

To inspect and clean out any deposits.

What can excessive wear on flexible couplings indicate?

Faulty shaft alignment.

What is the recommended action if the axial float is incorrect during thrust bearing inspection?

Adjustment of the thrust bearing liners is required.

What is the major cause of short-term deterioration in gas turbine performance?

Fouling of the compressor blading.

What factors influence the maintenance required for gas turbines?

Type and quality of fuel, frequency and nature of starts and stops, how close it operates to maximum temperature limits, contaminants in the air stream, age of equipment.

What is the impact of compressor fouling on gas turbine performance?

It decreases compressor efficiency, reduces overall thermal efficiency, and can cause compressor surging and acceleration problems.

What contaminants can cause compressor fouling?

Dust, salt, and other airborne particles not trapped by intake filters.

What is the consequence of erosion in gas turbines?

It primarily affects the first few stages of the compressor due to ingestion of dust particles.

How can corrosion occur in gas turbines?

Through airborne contaminants like salt or due to contaminants in the fuel, especially liquid fuels containing sulphur products and metals.

What is high cycle fatigue in gas turbines?

A phenomenon affecting rotating and stationary components due to high levels of pressure fluctuation in the combustion section.

What is low cycle fatigue in gas turbines?

A phenomenon similar to high cycle fatigue, affecting components due to repeated loading and unloading cycles.

What maintenance is required for gas turbine intake filters?

They must be periodically maintained to prevent erosion.

What is the importance of checking cylinder drainage holes in turbines?

Clear drainage holes are crucial for the life of the blading.

What should be done before replacing the turbine spindle?

A careful check for extraneous matter, including tools and debris, should be performed.

What is the recommended thickness of insulation for a high-pressure turbine cylinder?

About 150 mm for a turbine with stop valve steam conditions of 6200 kPa and 480°C.

Why is it important to dry each layer of insulation before applying the next?

To prevent serious distortion of the cylinder.

What should be checked when a turbine is put back into service after an overhaul?

Lubricating system operation, bearing temperatures, gland rubbing, and vibration levels.

What is the role of thrust bearings in turbine maintenance?

They are replaced and blade clearances are checked during spindle replacement.

What can cause a loss in maximum power output in gas turbines?

Blade deposits, erosion, corrosion, foreign object damage, and damaged seals.

What is the significance of following the manufacturer's instructions during turbine maintenance?

To ensure proper operation and prevent damage during startup after an overhaul.

What can happen if insulation is not properly replaced after maintenance?

It can lead to distortion of the cylinder and operational issues.

What is the function of the cylinder glands in turbine maintenance?

They are replaced and joints are remade to ensure proper sealing.

What types of contaminants can affect the combustion section of gas turbines?

Contaminants in the fuel, particularly those containing sulphur products.

What is the purpose of using compressed air during turbine maintenance?

To remove dust and small particles of dirt from the cylinder.

What is the procedure for tightening cylinder bolts?

Bolts are replaced in their correct positions and tightened in the correct sequence according to manufacturer's instructions.

What is the effect of operating a gas turbine close to maximum temperature limits?

It can increase the likelihood of maintenance issues and deterioration.

What maintenance action is taken to prevent future seizing between diaphragms and cylinders?

Diaphragms are sometimes removed at each overhaul.

What should be done with the turbine rotor after it is lifted out?

It should be placed on trestles for a careful examination of the blading.

What is a common maintenance problem related to gas turbine operation?

Fouling, which can occur over about 1000 operating hours.

What is the importance of checking steam pipe drain valves during maintenance?

To ensure they are operating correctly, which affects overall turbine performance.

What contributes to high cycle fatigue in gas turbines?

High level of pressure fluctuation in the combustion section.

What type of fatigue is especially relevant for peaking gas turbines?

Low cycle fatigue.

How much does maintenance increase for peaking gas turbines that start and stop frequently?

By a factor of 2-3 times.

What is creep in the context of gas turbines?

The fundamental life-limiting factor for first and second stage turbine blades due to operating close to metallurgical limits.

What is a standard creep limit for turbine blades?

Around 100,000 operating hours.

What technique can restore the creep life of turbine blades?

Hot isostatic pressing.

What causes thermal damage in gas turbines?

Uneven temperature distribution due to problems with fuel nozzles or air flow.

What components are most affected by mechanical damage in gas turbines?

Bearings, blade tips, guide vane, and bleed valve linkages.

What factors influence the maintenance of gas turbines?

Type of gas turbine, design, and manufacturer's recommendations.

What are the two common types of gas turbines?

Aeroderivative and heavy-duty (frame type) gas turbines.

What is the typical frequency for routine inspections of gas turbines?

Yearly or every 8,000 running hours.

What is the most effective method for cleaning gas turbine compressors?

Off-line water-wash method.

What instrument is used for inspecting the internal gas path of a gas turbine?

Borescope.

What maintenance activities are performed every 3-5 years or 24,000 to 30,000 running hours?

Intermediate level maintenance.

What is the purpose of major maintenance or overhaul in gas turbines?

To restore the engine to almost new condition, typically every 48,000 to 60,000 operating hours.

What does a major overhaul typically include?

Complete disassembly of the engine, replacement or refurbishment of components, and performance testing.

What is the cost implication of major overhauls in gas turbines?

A major part of the cost is the replacement or refurbishment of hot gas path components.

What is the purpose of condition monitoring in gas turbine maintenance?

To potentially extend the intervals between intermediate and major overhauls.

What is the crank-soak method in compressor cleaning?

Injecting water wash fluids into the intake while the unit is stopped, then restarting the compressor.

What is the typical interval for major maintenance or overhaul?

Every 7-8 years or 48,000 to 60,000 running hours.