CENTRIFUGAL FLOW COMPRESSORS BASIC INFORMATION AND TUTORIALS

What are centrifugal flow compressors?

Centrifugal compressors are used in small gas turbines and are the driven units in most gas turbine compressor trains. They are an integral part of the petrochemical industry, finding extensive use because of their smooth operation, large tolerance of process fluctuations, and their higher reliability compared to other types of compressors.

Centrifugal compressors range in size from pressure ratios of 1:3:1 per stage to as high as 13:1 on experimental models. Discussions here are limited to the compressors used in small gas turbines.

This means that the compressor pressure ratio must be between 3-7:1 per stage. This is considered a highly loaded centrifugal compressor. With pressure ratios, which exceed 5:1, flows entering the diffuser from the rotor are supersonic in their mach number (M > 1:0). This requires a special design of the diffuser.

In a typical centrifugal compressor, the fluid is forced through the impeller by rapidly rotating impeller blades. The velocity of the fluid is converted to pressure, partially in the impeller and partially in the stationary diffusers.

Most of the velocity leaving the impeller is converted into pressure energy in the diffuser. The diffuser consists essentially of vanes, which are tangential to the impeller. These vane passages diverge to convert the velocity head into pressure energy. The inner edge of the vanes is in line with the direction of the resultant airflow from the impeller.

In the centrifugal or mixed-flow compressor the air enters the compressor in an axial direction and exits in a radial direction into a diffuser. This combination of rotor (or impeller) and diffuser comprises a single stage. The air enters into the centrifugal compressor through an intake duct and can be given a prewhirl by the IGVs as shown in figure below.

The inlet guide vanes give circumferential velocity to the fluid at the inducer inlet. IGVs are installed directly in front of the impeller inducer or, where an axial entry is not possible, located radially in an intake duct. The purpose of installing the IGVs is usually to decrease the relative Mach number at the inducer-tip (impeller eye) inlet because the highest relative velocity at the inducer inlet is at the shroud.

When the relative velocity is close to the sonic velocity or greater than it, a shock wave takes place in the inducer section. A shock wave produces shock loss and chokes the inducer. The air initially enters the centrifugal impeller at the inducer.

The inducer, usually an integral part of the impeller, is very much like an axial-flow compressor rotor. Many earlier designs kept the inducer separate. The air then goes through a 90 degrees turn and exits into a diffuser, which usually consists of a vaneless space followed by a vaned diffuser.

This is especially true if the compressor exit is supersonic as is the case with high-pressure ratio compressors. The vaneless space is used to reduce the velocity leaving the rotor to a value lower than Mach number -- 1 (M < 1).

From the exit of the diffuser, the air enters a scroll or collector. The centrifugal compressor is slightly less efficient than the axial-flow compressor, but it has a higher stability. A higher stability means that its operating range is greater (surge-to-choke margin).

TYPES OF GEAR FAILURES GENERAL PROBLEMS OF WIND GEARS BASIC INFORMATION AND TUTORIALS

GEAR FAILURES TUTORIALS
What are the types of gear failures?

Gearboxes, in general, are subject to a number of problems during their operation. These problems can be enhanced or accelerated if faulty situations happen. Faulty situations are undesirable conditions that may arise for various reasons.

For example, the lubricating oil leaks and the oil level drops, or improper oil is used. As a result of any of these situations insufficient lubrication and cooling takes place and parts become heated or hot spots develop, or other damage can occur. Any damage can lead to tooth breakage and the permanent failure of a gearbox.

There are two particular issues with wind turbine gearboxes. The first one, as mentioned earlier, is that in wind turbines the gearbox is not used as a speed reducer, as is the case in the majority of applications. The second issue is that, because of the nature of wind, the gearbox is subjected to frequent and sudden changes of the power it handles. Such power fluctuations, which in turn translate to load variation on the teeth, is not a desirable situation for the gearbox.

In this sense, the chances of damage and failure in wind turbine gearboxes are more than in many other applications. Moreover, changing a gearbox in a wind turbine is extremely expensive, since the operation is not on the ground. In summary, the failure in a wind turbine gearbox can be due to (a)
design and manufacturing, (b) mounting and usage, or (c) operation and maintenance.

Poor or wrong design defi nitely can lead to problems during operation. Similarly, defects in manufacturing create unnecessary stresses, wear, heat, and fatigue. In mounting a gearbox, alignment with the rotating shaft , in both the input side and the output side, is absolutely necessary. Alignment implies that the axes of two mating gears are completely parallel to each other.

Misalignment causes the teeth to go under fluctuating stress, meaning that the stress in parts of a tooth continuously changes from compression to tension and vice versa. Th is is called fatigue and, compared to a part under the same magnitude but constant stress, can tremendously decrease the useful life of a machine part.

Proper mounting of a gearbox implies taking care that the alignment remains within the recommended tolerance. Bad usage implies if a wrong gearbox is selected for an application; for instance, when a gearbox cannot handle the power or cannot transfer the torque it is applied on.

This leads to overload on the gear teeth.

Examples of faults in operation were just mentioned. Th ese are essentially based on the lubrication oil, overheating, and malfunctioning of an associated device such as a pump that must circulate the oil.

Other examples are contamination of oil, say if humidity gets to the oil; and if the oil fi lter clogs and oil pressure drops. Faults in maintenance refers to any mistake concerning prolonged delays in oil change, the wrong oil type, and so on.

Typical gear damage or failures are:
1. Fracture
2. Bending
3. Wear
4. Fatigue
5. Cracking
6. Scuffi ng

Fracture implies that a piece of tooth breaks apart. In such an occurrence, not only does the tooth with the broken part become weaker, the broken part has a grinding action on all other parts, because it can move around with oil into all the other teeth, until it is stopped by the oil filter.


Gear tooth bending is a deformation of teeth in the direction of load. Note that when two objects (here the gear teeth of the two meshing gears) push each other, the force on one is the opposite of the force on the other (see the discussion on force and stress).

This can happen due to overloading and/or high temperatures. High temperatures can weaken a metal part in terms of lessening its strength to loads. Thus, the part can bend or deform more easily.

Wear is normally in the form of abrasion, where two matching parts can grind against each other. In the case of two gear teeth, this happens if there is not suffi cient room for smooth rolling motion between the teeth. Th is can also happen due to contamination and debris in the oil. A physical abrasion action is normally called erosion, and if it happens due to chemical reactions (acid in the oil, for instance) it is called cavitation.

Fatigue, as mentioned earlier, occurs when a machine element is subject to frequent change in the applied force. As a result, the part is forced to compress and then pulled to extend. Consider a gear tooth that is pushed up, then pushed down, on a permanent basis. Th is causes fatigue. Vibration in gears causes such a phenomenon.

Fatigue can occur in the tooth roots due to the bending of teeth (which can be called bending fatigue), or it can happen at the points of contact where two teeth press each other and then are relieved. Note that we also have thermal fatigue, which happens in parts of devices that are subject to temperature fl uctuations (becoming very hot and then very cold, repeatedly). Th is does not normally happen in gears.

Cracking is self-explanatory. It is the starting point for eventual fracture, and it is due to localized stress. Cracks can start under the surface, and eventually expand to the surface at different parts of a tooth body.

Scuffing is the term used when tiny pieces of metal tear from one gear tooth and due to localized heat adhere (weld) to the body of the matching teeth. Th is causes the surfaces to become rough, instead of being smooth, which in turn causes the development of more stress in the teeth and vibration in the gears.

SAFETY ON WIND TURBINE OPERATION BASIC INFORMATION AND TUTORIALS

WIND TURBINE SAFETY OPERATION
How to stay safe in operating wind turbines?

• When working on wind turbines, technicians and workers are subject to a number of hazards. These hazards are mainly due to the height, the confined environment, the electrical equipment, and the possibility of turbine motion.

• Falling from a height is the most serious hazard. The number of workrelated fatal occupational injuries in 2008 in the United States due to falls from height was 700.

• Human error or lack of knowledge and ignoring safety rules are among the other causes of accidents.

• It is necessary that technicians learn about safety rules and follow them carefully. Training for safety and rescue is provided to workers by employers.

• It is the responsibility of an employer to provide a safe environment for the workers. Because of the hazardous nature of work on wind turbines, moreover, it is a worker’s responsibility to care for his or her safety as well as the safety of other workers.

• Safety regulations are set and monitored by authorities in each country. In the United States, these regulations are set by the Occupational Safety and Health Administration (OSHA).

• A mindset for a good technician is that “safety is rule number 1, always.”

• For any job that involves hazards, personal protection equipment (PPE) is to be used in order to reduce the risk of having accidents. PPE is not the same for all jobs and depends on the nature of a hazard(s) involved in a job and the work environment.

• For working on wind turbines, PPE comprises climbing gear, a hard hat, and fall arrest equipment. In the simplest version, climbing gear includes a harness to wear, a cable grip, and a locking carabiner. The fall arrest gear is a lanyard with which a worker can hook himself or herself to a secure point during work.

• Long-sleeve shirts and pants, with no jewelry or unnecessary items on the body (or in the pockets) are part of the proper clothing that a wind turbine worker must bear in mind. A pair of gloves is often very useful when climbing a ladder.

• PPE must always be checked before each use to make sure it is in perfect order. A periodic professional inspection, for instance, a yearly inspection, may also become necessary, depending on the frequency of usage.

• A good safe action is to respect tagout and lockout practice at work. Tagout implies putting a note on a device or equipment, notifying others about a hazard or danger. Lockout implies that equipment or a device, or a place containing such a device, be physically locked to prevent it from being accessed.

• In addition to training for safety, a technician working on wind turbines receives training for self-rescue in case it becomes necessary to escape from a turbine without using the ladder inside the tower.

• It is also essential that a worker learns how to rescue a colleague who might get involved in an accident. A fi rst rule to follow is that one must never put his or her safety at risk in such a rescue mission.

ADVANTAGES AND DISADVANTAGE OF DIESEL POWER PLANT BASIC INFORMATION

ADVANTAGE OF DIESEL POWER PLANT
The advantages of diesel power plants are listed below.


1. Very simple design also simple installation.

2. Limited cooling water requirement.

3. Standby losses are less as compared to other Power plants.

4. Low fuel cost.

5. Quickly started and put on load.

6. Smaller storage is needed for the fuel.

7. Layout of power plant is quite simple.

8. There is no problem of ash handling.

9. Less supervision required.

10. For small capacity, diesel power plant is more efficient as compared to steam power plant.

11. They can respond to varying loads without any difficulty.


DISADVANTAGE OF DIESEL POWER PLANT
The disadvantages of diesel power plants are listed below.

1. High Maintenance and operating cost.

2. Fuel cost is more, since in India diesel is costly.

3. The plant cost per kW is comparatively more.

4. The life of diesel power plant is small due to high maintenance.

5. Noise is a serious problem in diesel power plant.

6. Diesel power plant cannot be constructed for large scale.

ASME PTC 22-1997 BASIC INFORMATION AND TUTORIALS

What is ASME, Performance Test Code on Gas Turbines, ASME PTC 22 1997?

The object of the code is to detail the test to determine the power output and thermal efficiency of the gas turbine when operating at the test conditions, and correcting these test results to standard or specified operating and control conditions. Procedures for conducting the test, calculating the results, and making the corrections are defined.

The code provides for the testing of gas turbines supplied with gaseous or liquid fuels (or solid fuels converted to liquid or gas prior to entrance to the gas turbine). Tests of gas turbines with water or steam injection for emission control and/or power augmentation are included. The tests can be applied to gas turbines in combined cycle power plants or with other heat recovery systems.

Meetings should be held with all parties concerned as to how the test will be conducted and an uncertainty analysis should be performed prior to the test. The overall test uncertainty will vary because of the differences in the scope of supply, fuel(s) used, and driven equipment characteristics.

The code establishes a limit for the uncertainty of each measurement required; the overall uncertainty is then calculated in accordance with the procedures defined in the code and by ASME PTC 19.1.

Mechanical Parameters
Some of the best standards from a mechanical point of view have been written by the American Petroleum Institute (API) and the American Society of Mechanical Engineers, as part of their mechanical equipment standards.

The ASME and the API mechanical equipment standards are an aid in specifying and selecting equipment for general petrochemical use. The intent of these specifications is to facilitate the development of high-quality equipment with a high degree of safety and standardization.

The user's problems and experience in the field are considered in writing these specifications. The task force, which writes the specifications, consists of members from the user, the contractor, and the manufacturers. Thus, the task-force team brings together both experience and know-how.

The petroleum industry is one of the largest users of gas turbines as prime movers for drives of mechanical equipment and also for power generation equipment. Thus the specifications written are well suited for this industry, and the tips of operation and maintenance apply for all industries. This section deals with some of the applicable API and ASME standards for the gas turbine and other various associated pieces.

It is not the intent here to detail the API or ASME standards, but to discuss some of the pertinent points of these standards and other available options. It is strongly recommended that the reader obtain from ASME and API all mechanical equipment standards.

ASME PTC 46 1996 BASIC INFORMATION AND TUTORIALS

What is ASME, Performance Test Code on Overall Plant Performance, ASME PTC 46 1996?

This code is written to establish the overall plant performance. Power plants, which produce secondary energy output such as cogeneration facilities, are included within the scope of this code.

For cogeneration facilities, there is no requirement for a minimum percentage of the facility output to be in the form of electricity; however, the guiding principles, measurement methods, and calculation procedures are predicated on electricity being the primary output.

As a result, a test of a facility with a low proportion of electric output may not be capable of meeting the expected test uncertainties of this code. This code provides explicit procedures for the determination of power plant thermal performance and electrical output.

Test results provide a measure of the performance of a power plant or thermal island at a specified cycle configuration, operating disposition and/or fixed power level, and at a unique set of base reference conditions.

Test results can then be used as defined by a contract for the basis of determination of fulfillment of contract guarantees. Test results can also be used by a plant owner, for either comparison to a design number, or to trend performance changes over time of the overall plant.

The results of a test conducted in accordance with this code will not provide a basis for comparing the thermoeconomic effectiveness of different plant design.

Power plants are comprised of many equipment components. Test data required by this code may also provide limited performance information for some of this equipment; however, this code was not designed to facilitate simultaneous code level testing of individual equipment.

ASME PTCs, which address testing of major power plant equipment, provide a determination of the individual equipment isolated from the rest of the system. PTC 46 has been designed to determine the performance of the entire heat cycle as an integrated system.

Where the performance of individual equipment operating within the constraints of their design-specified conditions are of interest, ASME PTCs developed for the testing of specific components should be used.

Likewise, determining overall thermal performance by combining the results of ASME code tests conducted on each plant component is not an acceptable alternative to a PTC 46 test.

GAS TURBINES CATEGORIES BASIC INFORMATION AND TUTORIALS

Categories of Gas Turbines defined.
What are the different types of gas turbines?

The simple-cycle gas turbine is classified into five broad groups:

1. Frame Type Heavy-Duty Gas Turbines. 

The frame units are the large power generation units ranging from 3 MW to 480 MW in a simple cycle configuration, with efficiencies ranging from 30-46%.

2. Aircraft-Derivative Gas Turbines Aero-derivative. 

As the name indicates, these are power generation units, which originated in the aerospace industry as the prime mover of aircraft. These units have been adapted to the electrical generation industry by removing the bypass fans, and adding a power turbine at their exhaust.

These units range in power from 2.5 MW to about 50 MW. The efficiencies of these units can range from 35-45%.

3. Industrial Type-Gas Turbines.

These vary in range from about 2.5 MW- 15 MW. This type of turbine is used extensively in many petrochemical plants for compressor drive trains. The efficiencies of these units are in the low 30s.

4. Small Gas Turbines. 

These gas turbines are in the range from about 0.5 MW-2.5 MW. They often have centrifugal compressors and radial inflow turbines. Efficiencies in the simple cycle applications vary from 15-25%.

5. Micro-Turbines. 

These turbines are in the range from 20 kW-350 kW. The growth of these turbines has been dramatic from the late 1990s, as there is an upsurge in the distributed generation market.

ADVANTAGES OF STEAM TURBINE OVER STEAM ENGINE BASIC INFORMATION AND TUTORIALS

Comparing steam turbine and steam engine, which is better?


The various advantages of steam turbine are as follows :

(i) It requires less space.

(ii) Absence of various links such as piston, piston rod, cross head etc. make the mechanism simple. It is quiet and smooth in operation,

(iii) Its over-load capacity is large.

(iv) It can be designed for much greater capacities as compared to steam engine. Steam turbines can be built in sizes ranging from a few horse power to over 200,000 horse power in single units.

(v) The internal lubrication is not required in steam turbine. This reduces to the cost of lubrication.

(vi) In steam turbine the steam consumption does not increase with increase in years of service.

(vii) In steam turbine power is generated at uniform rate, therefore, flywheel is not needed.

(viii) It can be designed for much higher speed and greater range of speed.

(ix) The thermodynamic efficiency of steam turbine is higher.

SIMPLE IMPULSE TURBINE BASIC INFORMATION AND TUTORIALS

This type of turbine works on the principle of impulse and is shown diagrammatically. It mainly consists of a nozzle or a set of nozzles, a rotor mounted on a shaft, one set of moving blades attached to the rotor and a casing.

The uppermost portion of the diagram shows a longitudinal section through the upper half of the turbine, the middle portion shows the development of the nozzles and blading i.e. the actual shape of the nozzle and blading, and the bottom portion shows the variation of absolute velocity and absolute pressure during flow of steam through passage of nozzles and blades.

The example of this type of turbine is the de-Laval Turbine.

It is obvious from the figure that the complete expansion of steam from the steam chest pressure to the exhaust pressure or condenser pressure takes place only in one set of nozzles i.e. the pressure drop takes place only in nozzles.

It is assumed that the pressure in the recess between nozzles and blades remains the same. The steam at condenser pressure or exhaust pressure enters the blade and comes out at the same pressure i.e. the pressure of steam in the blade passages remains approximately constant and equal to the condenser pressure.

Generally, converging-diverging nozzles are used. Due to the relatively large ratio of expansion of steam in the nozzles, the steam leaves the nozzles at a very high velocity (supersonic), of about 1100 m/s.

It is assumed that the velocity remains constant in the recess between the nozzles and the blades. The steam at such a high velocity enters the blades and reduces along the passage of blades and comes out with an appreciable amount of velocity.

As it has been already shown, that for the good economy or maximum work, the blade speeded should be one half of the steam speed so blade velocity is of about 500 m/s which is very en high. This results in a very high rotational speed, reaching 30,000 r.p.m. Such high rotational speeds can only be utilised to drive generators or machines with large reduction gearing arrangements.

In this turbine, the leaving velocity of steam is also quite appreciable resulting in an energy loss, called “carry over loss” or “leaving velocity loss”. This leaving loss is so high that it may amount to about 11 percent of the initial kinetic energy.

This type of turbine is generally employed where relatively small power is needed and where the rotor diameter is kept fairly small.

INDUSTRIAL TYPE GAS TURBINES BASIC INFORMATION AND TUTORIALS

What are industrial type gas turbines?

Industrial Type Gas Turbines are medium-range gas turbines and usually rated between 5-15 MW. These units are similar in design to the large heavy-duty gas turbines; their casing is thicker than the aero-derivative casing but thinner than the industrial gas turbines.

They usually are split-shaft designs that are efficient in part load operations. Efficiency is achieved by letting the gasifier section (the section which produces the hot gas) operate at maximum efficiency while the power turbine operates over a great range of speeds.

The compressor is usually a 10-16 stage subsonic axial compressor, which produces a pressure ratio from about 5:1-15:1. Most American designs use can-annular (about 5-10 combustor cans mounted in a circular ring) or annular-type combustors.

Most European designs use side combustors and have lower turbine inlet temperatures compared to their American counterparts. Figure below shows an Industrial Type Gas Turbine.

The gasifier turbine is usually a 2-3 stage axial turbine with an air-cooled first-stage nozzle and blade. The power turbine is usually a single- or two-stage axial-flow turbine. The medium-range turbines are used on offshore platforms and are finding increasing use in petrochemical plants.

The straight simple-cycle turbine is low in efficiency, but by using regenerators to consume exhaust gases, these efficiencies can be greatly improved. In process plants this exhaust gas is used to produce steam. The combined cycle (air-steam) cogeneration plant has very high efficiencies and is the trend of the future.

These gas turbines have in many cases regenerators or recuperators to enhance the efficiency of these turbines. Figure below shows such a new recuperated gas turbine design, which has an efficiency of 38%.

The term "regenerative heat exchanger" is used for this system in which the heat transfer between two streams is affected by the exposure of a third medium alternately to the two flows. (The heat flows successively into and out of the third medium, which undergoes a cyclic temperature.)

In a recuperative heat exchanger each element of heat-transferring surface has a constant temperature and, by arranging the gas paths in contraflow, the temperature distribution in the matrix in the direction of flow is that giving optimum performance for the given heat-transfer conditions.

This optimum temperature distribution can be achieved ideally in a contraflow regenerator and approached very closely in a cross-flow regenerator.

GAS TURBINE DESIGN CONSIDERATIONS

The gas turbine is the best suited prime mover when the needs at hand such as capital cost, time from planning to completion, maintenance costs, and fuel costs are considered. The gas turbine has the lowest maintenance and capital cost of any major prime mover.

It also has the fastest completion time to full operation of any plant. Its disadvantage was its high heat rate but this has been addressed and the new turbines are among the most efficient types of prime movers. The combination of plant cycles further increases the efficiencies to the low 60s.

The design of any gas turbine must meet essential criteria based on operational considerations. Chief among these criteria are"

1. High efficiency
2. High reliability and thus high availability
3. Ease of service
4. Ease of installation and commission
5. Conformance with environmental standards
6. Incorporation of auxiliary and control systems, which have a high degree of reliability
7. Flexibility to meet various service and fuel needs

A look at each of these criteria will enable the user to get a better understanding of the requirements.

The two factors, which most affect high turbine efficiencies, are pressure ratios and temperature. The axial-flow compressor, which produces the high-pressure gas in the turbine, has seen dramatic change as the gas turbine pressure ratio has increased from 7:1 to 40:1.

The increase in pressure ratio increases the gas turbine thermal efficiency when accompanied with the increase in turbine firing temperature. The increase in the pressure ratio increases the overall efficiency at a given temperature, however increasing the pressure ratio beyond a certain value at any given firing temperature can actually result in lowering the overall cycle efficiency.

It should also be noted that the very high-pressure ratios tend to reduce the operating range of the turbine compressor.

This causes the turbine compressor to be much more intolerant to dirt build-up in the inlet air filter and on the compressor blades and creates large drops in cycle efficiency and performance. In some cases, it can lead to compressor surge, which in turn can lead to a flameout, or even serious damage and failure of the compressor blades and the radial and thrust bearings of the gas turbine.

Got any info about turbines? Feel free to leave your comments in the comment sections.