Process Engineering Equipment Handbook Episode 3 Part 7 pot

Temperature and Pressure Sensors see Measurement Thermal Insulation see Commonly Used Specifications, Codes, Standards, and Texts Thin-Film Processors see Chillers Torque Converters, M

independent approval institute Well-known institutes are PTB (Germany), FactoryMutual Research (USA), SAA (Australia), JIS (Japan), and CSA (Canada).

The better tank-gauging instruments do not just meet the safety standards butexceed them by anticipating future safety requirements as well Such requirementsinclude the exclusion of aluminum inside storage tanks (zone 0), the limitation ofthe kinetic energy of moving parts of a gauge to values far less than could causeignition

Lightning and tank gauging. Lightning can cause hazardous situations, andmeasures should be taken to protect the tank installation and tank-gauging systemagainst these hazards Modern tank-gauging systems contain many electroniccircuits Their position on top of storage tanks makes this equipment morevulnerable to lightning damage than any other type of industrial equipment.Today’s communication systems linking all field equipment via one networkincrease the probability of possible damage to the equipment as the networksspread over increasingly larger areas With high reliability and availability one ofthe prime requirements of modern tank-gauging equipment, there is a need for well-designed, field-proven lightning protection methods Figure T-19 shows a tankgauge under high voltage test

In tank farms, lightning causes a direct potential difference between the gauge,grounded to the tank at one end, and the central receiver, at the other end Thisresults in a potential difference between cable and gauge or cable and receiver Thisdifference between equipment and cable tries to equalize itself and searches a low

Tanks T-29

TABLE T-2 Overview of Batch Transfer Uncertainties

Level

Batch transfer uncertainties in (%)

NOTE: For level-based systems (servo/radar) the density is obtained from

the laboratory analysis of a grab sample; the uncertainty is assumed to be

±0.1 percent.

TABLE T-1 Overview of Inventory Uncertainties

Level

Inventory uncertainties in (%)

impedance path between the circuitry connected to the cable and the ground Assoon as the potential difference exceeds the isolation voltage, a breakdown occursbetween the electronics and the ground Additionally, transient currents will beinduced in adjacent components and cabling.

The currents flowing through the electronics cause disastrous effects Everysemiconductor that is not sufficiently fast or capable of handling the currents foreven a short period will be destroyed

Two basic techniques are used for minimizing the damage due to lightning andtransients: suppression and diversion

Suppression. By means of special circuits on all incoming and outgoing instrumentcables it is possible to suppress the magnitude of the transient appearing at theinstrument (Fig T-20) A gas discharge tube forms the kernel Gas discharge tubesare available for voltage protection from 60 V up to more than 1000 V and react inseveral microseconds, after which they form a conducting ionized path Theyprovide no protection until they are fully conducting

A transzorb or varistor, in combination with a resistor and preferably an inductor,can be added to improve the protection These semiconductors react within a couple

of nanoseconds and limit the voltage A major problem is that each time a transientsuppressor reacts, it degrades Reliability is therefore poor, rendering this type ofdevice unsuitable for critical applications such as tank gauges

Diversion. Diversion (Fig T-21) is a much more reliable technique and bettersuited for lightning protection of electronic tank-gauging instruments Modernprotection uses diversion combined with screening and complete galvanic isolation

It is a technique in which the high-voltage spikes are diverted rather thandissipated Specially developed isolation transformers are used for all inputs andoutputs They have two separate internal ground shields between primary and

T-30 Tanks

FIG T-19 Tank gauge under high voltage test (Source: Enraf.)

secondary windings and the transformer core External wiring is physicallyseparated from internal wiring and ground tracks are employed on all circuit boards

to shield electronics Unfortunately this protection method is not suitable with DCsignals In this case a conventional transient protection, enhanced with anadditional galvanic isolation, is used

Grounding and shielding. Proper grounding and shielding will also help protectinstruments and systems connected to field cabling against damage by lightning.The possible discharge path over an instrument flange (e.g., of a level gauge) andthe corresponding mounting flange should have a nearly zero resistance to preventbuildup of potential differences A poor or disconnected ground connection maycause sparking and ignite the surrounding product vapors

Field experience. The diversion method described for internal lightning protectionhas been in use for more than 15 years, with approximately 50,000 installedinstruments Almost 100 percent of this equipment is installed on top of bulkstorage tanks, and interconnected via wide area networking

A large number of installations are situated in known lightning-prone areas Todate, only a few incidents in which lightning may have played a decisive role havebeen experienced The amount of damage was always limited and could be repairedlocally at little expense Before this protection method was applied, more extensivelightning damage had been experienced

Tanks T-31

FIG T-20 Suppression circuit (Source: Enraf.)

FIG T-21 Diversion circuit (Source: Enraf.)

Developments in tank-gauging technology

Servo gauges. Modern servo gauges are already members of the sixth generation(Fig T-22) They use modern embedded microcontrollers, minimizing the totalamount of electronics Advanced software development tools and higher orderprogramming languages provide reliable operation Fuzzy control algorithmsimprove interaction of mechanics and electronics, reducing the number ofmechanical parts

Current advanced servo tank gauges (ATG) have less than five moving parts.The main features of an advanced technology servo gauge are:

 Low operating cost

 Typical MTBF of more than 10 years

 Low installation cost, especially when used to replace existing servo gauges

 A standard accuracy of better than 1 mm (0.04 in)

 Software compensation for hydrostatic tank deformation, making support pipes

no longer a must for accurate measurement

 Full programmability for easy setup and simple maintenance without having toopen the instrument

 Compact and lightweight construction requiring no hoisting equipment

 Possibilities for installation while the tank stays in full operation

 Continuous diagnostics to provide maximum reliability and availability

 Water-product interface measurement for time-scheduled water measurement

 Spot and average product density measurement

 Interfacing to other smart transmitters, e.g., for product and vapor temperature,and pressure via a digital protocol, including average density support

The German legislation currently accepts advanced servo gauges as a single alarmfor overfill protection

T-32 Tanks

FIG T-22 Advanced technology servo gauge (Source: Enraf.)

Radar gauges. Radar gauges play an important role in tank gauging (Figs T-23and T-24) Their nonintrusive solid-state nature makes them very attractive Theaccuracy of the newest generation radar gauge meets all requirements for custodytransfer and legal inventory measurements.

Reliability is high and maintenance will be further reduced The onboard intelligence allows for remote diagnosis of the total instrument performance Thecompact and lightweight construction simplifies installation without the need forhoisting equipment Installation is possible while the tank stays in full operation.Current developments are aimed at more integrated functions Improved antennadesigns, full digital signal generation, and processing offer better performance withless interaction between tank and radar beam

The main features of the new generation radar level gauge are:

 No moving parts

 Very low maintenance cost

 Low operational cost

 Nonintrusive instrument

 Low installation cost

 Typical MTBF of more than 60 years

 Low cost of ownership

 Modular design

 A standard accuracy of ±1 mm (0.04 in)

 Software compensation for the hydrostatic tank deformation, making supportpipes no longer a must for accurate measurement

 Full programmability for easy setup and verification facilities

 The compact and lightweight construction eliminating the need for hoistingequipment

Tanks T-33

FIG T-23 Radar level gauge with Planar antenna technology (Source: Enraf.)

 Installation possibilities while the tank stays in operation

 Continuous diagnostics providing a maximum of reliability

 Water-product interface measurement using digital integrated probe

 Density measurement via system-integrated pressure transmitter (HIMS)

 Interfacing to other transmitters, e.g., for product and vapor temperature, andpressure via digital protocol

Temperature gauging. Accurate temperature measurement is essential for based tank-gauging systems

level-Spot temperature elements are widely accepted for product temperatureassessment on tanks with homogeneous products Installation is simple and thereliability is good The graph of Fig T-25 shows that spot measurements areunsuitable to accurately measure the temperature of products that tend to stratify.The effects of temperature stratification can be neglected only for light products,mixed frequently

In general, average temperature-measuring elements are used in case of temperature stratification The latest development is the multitemperature thermometer (MTT) shown in Fig T-26 that utilizes 16 thermosensors evenlydistributed over the maximum possible liquid height A very accurate class A Pt100element at the bottom is the reference Accuracies of better than 0.05°C (0.08°F)are possible The elements can also be individually measured to obtain temperatureprofiles and vapor temperatures MTTs are available with both nylon and stainlesssteel protection tubes It provides a rugged construction suitable for the harshenvironments of a bulk storage tank

Another type of average temperature measuring element is the multiresistancethermometer (MRT) Its operation is based on a number of copper wire temperaturesensing elements of different lengths Average temperature measurement isachieved by measuring the longest fully immersed resistance thermometer chosen

T-34 Tanks

FIG T-24 Radar level gauge for high-pressure applications (Source: Enraf.)

by a solid-state element selector A drawback of MRTs is the delicate construction

of the elements The very thin copper wire used makes the device susceptible todamage, especially during transport and installation

Hydrostatic tank gauging. Recent developments of smart transmitters opened a newera for HTG The development of smart pressure transmitters with microcomputers

Tanks T-35

FIG T-25 Temperature stratification in a storage tank (Source: Enraf.)

FIG T-26 Average temperature sensor with selector/interface unit (Source: Enraf.)

made HTG feasible Only a couple of years ago, high-accuracy pressure transmitterswere still rare and quite expensive Several manufacturers now offer 0.02 percentaccuracy transmitters Digital communication by means of de facto standards, asthe HARTTM

-protocol, permits simple interfacing to almost any transmitter Thiswide choice simplifies selection for specific applications and allows the user tochoose his own preferred transmitter The inherent standardization for the end userreduces the cost of maintenance

Hybrid inventory measurement system. HIMS are also based on the integration ofsmart pressure transmitters Modern level gauges, either servo or radar, providethe possibility for direct interfacing to smart pressure transmitters HIMS opensthe ideal route to total tank inventory systems, measuring all tank parameters viaone system

Central inventory management system. The interface to the operators and/or thesupervisory control and management system is the tank-gauging inventorymanagement system (Fig T-27) These high-speed systems collect the measurementdata from all tank-gauging instruments, continuously check the status of alarmsand functional parameters, and compute real-time inventory data such as volumeand mass The hardware used is generally off-the-shelf personal or industrialcomputers loaded with dedicated inventory management software It is thissoftware, together with the reliability and integrity of the field instrumentation thatdetermines the performance and accuracy of the inventory management system Allfield instruments, regardless of age or type, should communicate via the sametransmission bus

Product volumes and mass should be calculated the same way as do the appointed authorities and surveyors The system software should store the tanktable parameters, calculate observed and standard volumes, correct for free waterand, if applicable, correct for the floating roof immersion The GSV calculationsmust be in accordance with API, ASTM, and ISO recommendations implementingtables 6A, 6B, 6C, 53, 54A, 54B, 54C, and 5

owner-T-36 Tanks

FIG T-27 Central inventory management system (Source: Enraf.)

The quality of the inventory management system can be evidenced from theavailability of Weights & Measures or Customs & Excise approvals Inventorymanagement systems can have their own display consoles or can make all dataavailable for a supervisory system.

Networked systems are available when required Apart from a large number ofinventory management functions, the system can also control inlet and outlet valves

of the tanks, start and stop pumps, display data from other transmitters, provideshipping documents, provide trend curves, show bar graph displays, performsensitive leak detection, calculate flow rates, control alarm annunciation relays,perform numerous diagnostic tasks, and much more For examples of displayformats of an inventory management system see Fig T-28 for tabular displays andFig T-29 for graphical displays

The operator friendliness of the system is of paramount importance The betterand more advanced systems have context-sensitive help keys that make the properhelp instructions immediately available to the operator

Interfacing to host systems. The receiving systems can also be equipped with hostcommunication interfaces for connection to plant management systems, e.g., Dis-tributive Control Systems (DCS), Integrated Control Systems (ICS), oil accountingsystems, etc (Fig T-30) Protocols have been developed in close cooperation withthe well-known control system suppliers

These are needed in order to transmit and receive the typical tank-gauging measuring data Standard protocols as Extended MODBUS, Standard MODBUS,and others are also available for smooth communication between tank inventorysystems and third-party control systems Modern DCS or other systems have sufficient power to handle inventory calculations, but often lack the dedicatedprogramming required for a capable inventory management

Tanks T-37

FIG T-28 Tabular screens of an inventory management system (Source: Enraf.)

Tank inventory management systems, specially developed for tank farms andequipped with suitable host links, will have distinct advantages.

 It frees the host system supplier from needing detailed knowledge of transmitterand gauge specific data handling

 Maintaining a unique database, with all tank-related parameters in one computeronly, is simple and unambiguous

T-38 Tanks

FIG T-29 Graphical screens of an inventory management system (Source: Enraf.)

FIG T-30 Interfacing to distributed control system and management information system (Source: Enraf.)

 Inventory and transfer calculation procedures outside the host system are easierfor Weight and Measures authorities.

 Implementation of software required for handling of new or more tank gaugescan be restricted to the tank-gauging system This will improve the reliability andavailability of the host system

Connecting all field instruments via one fieldbus to the supervisory system, DCS

or tank-gauging system is advantageous for operations It simplifies maintenanceand service, and allows fast replacement of equipment in case of failure

Future trends in tank-gauging technology

Combining static and dynamic measuring techniques provides a possibility for continuously monitoring physical stock levels on a real-time basis By reconcilingrecorded changes in stock levels against actually metered movements, the systemcan detect and immediately identify any product losses

Unexpected product movements can then be signaled to the operator by an alarm.Statistical analysis of static data from the tank-gauging system and dynamic data from flow meters could also be used to improve the accuracy of the tank capacity table Cross-correlation of gauges versus flow meters could further reduce measurement uncertainties With high-accuracy tank-gauging instruments combined with powerful computing platforms, automatic reconciliation becomesrealistic

Interfaces to multiple supplier systems, ranging from tank gauging to loading andvalve control systems, will be feasible via internationally accepted communicationstandards

In summary, a wide range of different tank-gauging instruments is available Theemployed techniques are more complementary than competitive as each measuringprinciple has its own advantages See Table T-3 Modern servo and radar gaugeshave improved considerably They hardly need any maintenance and can providetrouble-free operation if applied correctly The possibility of mixed installations withservo, radar, HTG, and HIMS provides optimal flexibility and utilizes the capability

of each gauging technique

HTG is to be preferred if mass is the desired measurement for inventory andcustody transfer

The costs of any tank-gauging system are mainly determined by the cost of installation including field cabling In upgrading projects, costs depend very much

on the possibility of retrofitting existing facilities

Because of worldwide commercial practice, volume measurement will continue toplay an important role

The combination of volume and mass offers great advantages A globally accepted

measurement standard will probably not be published for several years.Implementation of volume and mass calculations outside the managementinformation, DCS, or host systems remains preferable Integrity requirements forvolume and mass calculations imposed by the Weight and Measurement authoritiesare easier to fulfill externally and justify the additional hardware.

Standard field buses may play a decisive role in the direct interface betweendedicated tank-gauging systems and other systems However, the quality of themeasurements should never be sacrificed for the sake of bus standardization

Temperature and Pressure Sensors ( see Measurement)

Thermal Insulation (see Commonly Used Specifications, Codes, Standards,

and Texts)

Thin-Film Processors (see Chillers)

Torque Converters, Measurements, and Meters (see Power Transmission)

Towers and Columns

Towers and columns are heat- and mass-transfer devices in which reactions mayoccur Reactors frequently are large enough to require the structural designtechniques used with towers The term “reactor tower” might be used to describe atower that does reactor functions

The tower may be dealing with fractional distillation or component contentchange(s) (two substances mutually insoluble, but where one contains a dissolved

substance that needs to be transferred to solution in the other) A scrubber is the

term given to a tower where the solute is transferred from a gas to a liquid phase

In a stripper (or regenerator) the reverse occurs.

When towers are tall, wind loading factors become severe Towers a few hundredfeet high are not that uncommon now

Tray-type reactors. Internally, a variety of different tray types may be used Thedescriptive terms for these trays include: bubble cap trays, “flexitrays,” ballasttrays, float tray, sieve trays, “turbogrid,” and “kittel” trays They use a variety oftechniques, including sieve slots and holes, as well as caps or fitted mini “skirts,”

to alter the residence time of the fluid that passes over them, thereby enabling amore complete reaction

Packed reactors. A packed reactor is more popular with very corrosiveapplications A designer needs to allow for good distribution and avoid overly largepacking and bed depth Packing types include various kinds of rings and saddles

Toxic Substances (see Pollutants, Chemical)

Transportation, of Bulk Chemicals, of Large Process Equipment

For regulations and guidelines covered for these items as well as spills during transportation of same, consult the appropriate government protection agency Inthe United States, this would be the EPA; in Canada, Environment Canada; in theUnited Kingdom, DOE If traveling across borders, one needs to look at all thespecifications from different countries and pick the most stringent one to worktoward

T-40 Temperature and Pressure Sensors

Reference and Additional Reading

1 Soares, C M., Environmental Technology and Economics: Sustainable Development in Industry,

Butterworth-Heinemann, 1999.

Triple Redundancy

Triple redundancy is a term generally associated with aircraft engine control.However, as with other aspects of gas turbine system design—-such as metallurgy,where Rolls Royce RB211 and Trent flight engines lend their metallurgicalselections to their land-based counterparts in power generation and mechanicaldrive service—-the technology is starting to move to “ground level.” The governingfactor, as always, is economics

Triple redundancy means a more reliable, available system that is prone to fewerfailures When that translates into money, the more sophisticated technology isadopted At this point, many power plants in Asia have unused capacity and arenot always hurt financially if there is an interruption in availability in one of theirpower modules On the other hand, some of them have power purchase agreements(PPAs) that guarantee them income if they run (the YTL and Genting IPP powerplants in Malaysia are in this category) They may not be in as bad an income-lost-in-the-event-of-failure situation as some of their mechanical drive counterparts,however (On critical mechanical drive applications, 24 h of downtime on a criticalcompressor, pump, or blower could mean $250,000 to $500,000 in lost income.) Atthis point, triple redundancy technology is more popular with these users, butpower operators need to take notice As other more elementary problems, such astransmission-line losses, are brought under control, they have to look elsewhere for

further optimization Triple redundancy or triple modular redundancy (TMR) is an

expensive option, so foreknowledge is important

Software-implemented fault tolerance is the most common TMR technique in use.This method involves three processors that run asynchronously This guardsagainst transient errors Each processor waits for the other two to “cast their vote”

at certain points in the program cycle (at least once per input/output scan) Theprocessors vote about:

Choice of TMR Control

TMR is generally selected if:

1 Shutdown/malfunction/loss of availability might endanger operators

2 Shutdown/malfunction/loss of availability might hurt overall plant economicsand cost per running hour

Triple Redundancy T-41

3 Shutdown/malfunction/loss of availability might violate contracts (such asPPAs)/legislation (such as environmental laws).

4 There are remote control situations

5 Damage to the overall system may result

The Effect on Life-Cycle Costs

 Improved safety: A safer operation is a less costly one Fewer faults/unwantedshutdowns put operational staff under less pressure and make them moreinclined to contribute to safety

 A TMR can keep a system operating even if there are one or more system faults(electronic or field equipment) Backup machinery can be accurately cued andstarted The system can be run, albeit imperfectly, until optimized timing fortaking the turbomachinery units and/or accessories can be arranged This cutsdown on lost production losses

 Unscheduled outages are costly They can occur due to

 Machinery and hardware failures

 Control system failures

 Operator or maintenance personnel errors

If there is no redundancy (simple or simplex system) or dual redundancy (duplexsystems), these types of failure are likely to result in shutdown TMR eliminatescontrol system-caused shutdowns It cuts down on the number of operator-causedshutdowns, as much because of the speed of TMR’s diagnostics as anything else

 Maintenance costs per fired hour: With optimized diagnostics, the system is likely

to run “unknowingly” with faulty components that would have gone undetected

in a simplex or duplex system

 System component longevity: Smooth (bumpless) synchronization of generatorsreduces wear factors on generators, couplings, and turbines Smooth transfer offuel types in a dual (gas/liquid) or tri (gas/liquid/gas and liquid mixed) fuel systemgreatly compensates for the temperature bursts that take a severe toll on hot-section component lives

 Efficiency: Total system thermal efficiency is influenced by many factors including

 Improved NOxcontrol (combustion stability)

 Automatic operation, starting, loading, or synchronizing

 Integration of control systems of multiple plant units This is additionallysignificant in many power development projects where modules or cogeneration

or waste-heat recovery schemes are commissioned after the main unit has runfor a while (add-ons)

Operation of the TMR

The TMR operates according to a two-out-of-three algorithm that is generallyconfigured for fail-safe operation If there is one input/output failure, the systemcontinues to operate If there is another component failure, the system can beconfigured to continue running (3:2:1 mode) or shut down safely This meets all thesafety codes that are required for plant operation as well as international standardsfor system management It gives full fault tolerance between input and output

T-42 Triple Redundancy

terminals PC controllers with Windows software can be used to monitor the overallsystem, so this high degree of sophistication is quite user-friendly.

 Load and load-sharing controls (using temperature or speed variables)

 Alarms and shutdowns

 Dual/triple fuel changeovers

 Turning gear controls

 Automatic compressor module washes

 Safety interlock control

 Monitor and diagnostics of condition monitoring systems (CMS)

 Antisurge control systemsFor generators, the TMR provides:

 Synchronization to local transmission bus systems, controllers, and exciters

 Control of the main breaker

 Control of safety interlocks

 Monitoring diagnostics of CMS

In summary, TMRs are worth the investment when a system must remainrunning They provide availabilities of 99.999 percent These applications in thepower industry will grow in number as Asia’s demand growth accelerates Whilesimplex control may be acceptable as long as a turbine and generator are essentiallyall the system consists of, the justification for TMR rises with the addition of othermodules, other system complexities, or increased demands on availabilities

Turbines, Gas

Gas Turbine: Basic Description*

The gas turbine is a heat engine, i.e., an engine that converts heat energy intomechanical energy The heat energy is usually produced by burning a fuel with theoxygen of the air In that way the engine converts the potential chemical energy ofthe fuel first to heat energy and then to mechanical energy However, in a gasturbine, as well as in other types of heat engines, only a part of the released heatenergy can be converted into mechanical energy The remaining heat energy will

be transferred to the atmosphere See Fig T-31

The efficiency of the energy conversion tells the portion of the input energyconverted into useful energy and is generally designated h In a gas turbine 25–40

Turbines, Gas T-43

* Source: Alstom.

terminals PC controllers with Windows software can be used to monitor the overallsystem, so this high degree of sophistication is quite user-friendly.

 Load and load-sharing controls (using temperature or speed variables)

 Alarms and shutdowns

 Dual/triple fuel changeovers

 Turning gear controls

 Automatic compressor module washes

 Safety interlock control

 Monitor and diagnostics of condition monitoring systems (CMS)

 Antisurge control systemsFor generators, the TMR provides:

 Synchronization to local transmission bus systems, controllers, and exciters

 Control of the main breaker

 Control of safety interlocks

 Monitoring diagnostics of CMS

In summary, TMRs are worth the investment when a system must remainrunning They provide availabilities of 99.999 percent These applications in thepower industry will grow in number as Asia’s demand growth accelerates Whilesimplex control may be acceptable as long as a turbine and generator are essentiallyall the system consists of, the justification for TMR rises with the addition of othermodules, other system complexities, or increased demands on availabilities

Turbines, Gas

Gas Turbine: Basic Description*

The gas turbine is a heat engine, i.e., an engine that converts heat energy intomechanical energy The heat energy is usually produced by burning a fuel with theoxygen of the air In that way the engine converts the potential chemical energy ofthe fuel first to heat energy and then to mechanical energy However, in a gasturbine, as well as in other types of heat engines, only a part of the released heatenergy can be converted into mechanical energy The remaining heat energy will

be transferred to the atmosphere See Fig T-31

The efficiency of the energy conversion tells the portion of the input energyconverted into useful energy and is generally designated h In a gas turbine 25–40

Turbines, Gas T-43

* Source: Alstom.

percent of the input energy is transformed into mechanical energy The remaining60–75 percent will be transferred to the atmosphere in the form of waste heat(exhaust losses) The efficiency is consequently 15–40 percent Where a part of thewaste heat can be recovered, e.g., in a waste heat recovery boiler, the efficiencyincreases correspondingly.

Operating cycle main parts

In a gas turbine the operating medium is air and gas, and the flow runs throughthe cycle COMPRESSION—HEATING—EXPANSION

In an open gas turbine cycle, ambient air is sucked in, compressed in a

COMPRESSOR, heated in a COMBUSTION CHAMBERby injection and burning of a fueland then expanded through a TURBINE back to the atmosphere The operatingmedium of an open gas turbine cycle consequently is air and a mixture of air andcombustion gases

In a closed gas turbine cycle an enclosed gas, which cannot be air, runs through

the same phases as in the open cycle, but the heating takes place in a heatexchanger and the gas expanded through the turbine must be cooled before it is ledback to the compressor See Fig T-32

In practice the open gas turbine cycle is completely dominating and the furtherdescription is fully concentrated on the open gas turbine cycle

Function principle

As mentioned in the previous part, the gas turbine consists of three main parts:compressor, combustion chamber, and turbine How heat energy, by the operatingmedium flowing through these main parts, is converted into mechanical energy can

be explained by means of the simple model shown in Fig T-33

A tube is in either end equipped with a simple fan One of the fans is namedcompressor and the other fan is named turbine An external power source, or starter,

is through a coupling connected to the compressor

Through the tube an airflow is created that will speed up the turbine Energy issupplied to the compressor and is transferred to the airflow From the airflow energyflows to the turbine, which through its rotation gives off a mechanical output The

Efficiency output energy

energy flow can be noticed as compressor speed, increase of airflow velocity andpressure (by virtue of pressure increase as well as temperature increase), andturbine speed If the process goes on without losses (which in practice is impossible but temporarily accepted to simplify the understanding), the turbineenergy output is equal to the energy sacrificed to drive the compressor.

The airflow is heated

The heating means that the air temperature increases Since the air pressure insidethe tube is created by the compressor, the heating of the air does not result infurther increased air pressure Instead the air volume is increased Increased airvolume results in increased air velocity through the turbine A larger amount ofenergy is transferred to the turbine, which then can give off a larger mechanicaloutput If the process goes on without losses, the turbine mechanical energy output

is equal to the sum of the mechanical energy supplied to the compressor and theheat energy supplied to the airflow See Fig T-34

Turbines, Gas T-45

FIG T-32 Open and closed cycles for a gas turbine CC = combustion chamber, C = compressor,

T = turbine, GC = gas cooler (Source: Alstom.)

FIG T-33 The compressor is “speeded up” by the starter (Source: Alstom.)

Self-sustaining speed

Increased heat supply means that the turbine gives sufficient mechanical output todrive the compressor If the compressor and turbine are mounted to a common shaft,the starter can be disconnected and self-sustaining condition is reached The starterhas been necessary to create the airflow through the tube The airflow forces theprocess to continue by virtue of its momentum Heating stationary air inside thetube would only have meant temperature increase and air expansion backwardthrough the compressor as well as forward through the turbine See Fig T-35

At self-sustaining condition the mechanical output extracted from the turbine isjust enough to drive the compressor The whole amount of energy supplied byheating is waste energy In reality these losses consist of exhaust losses, losses due to turbulence, and radiation losses For thermodynamic reasons thetemperature of the exhaust gas must be higher than that of the sucked-in air and

T-46 Turbines, Gas

FIG T-34 Airflow is heated from fuel combustion (Source: Alstom.)

FIG T-35 Starter is disconnected when gas turbine reaches self-sustaining speed (Source:

Alstom.)

that means losses Further the exhaust gas must leave the turbine at a certainvelocity.

is moving straight through the engine, are typical for many gas turbines See Fig.T-37

The compressor

The compressor is in practice not a simple fan, but a far more sophisticatedconstruction to continuously compress an airflow to desired pressure One of twobasic types of compressors, one giving a radial flow and the other an axial flow, isnormally used in a gas turbine The axial flow compressor is easier to design forhigh-pressure ratios, is more efficient, and is thus common in high-performanceunits Only the axial flow compressor is dealt with in this primer

Axial flow compressor design

An axial flow compressor consists of one or more rotor assemblies that carry blades

of airfoil section and are mounted between bearings in the casing In the casing aremounted the stator vanes, which also are of airfoil section The compressor is amultistage unit as the pressure increase by each stage is small (pressure ratio1.15–1.25/compressor stage consists of a row of rotating blades followed by a row

of stator vanes When needed, an additional row of stator vanes, known as inletguide vanes, is used to guide the air on to the first row of rotor blades From thefront to the rear of the compressor, i.e., from the low to the high pressure end, there

is a gradual reduction of the airflow annular area This is necessary to maintain

Turbines, Gas T-47

FIG T-36 For useful work output, gas turbine is driven past self-sustaining speed (Source: Alstom.)

the axial velocity of the air constant as the volume decreases during thecompression To prevent air leakage there are sealings between the stages and atthe inlet and outlet ends of the compressor See Fig T-38.

Function principle

During operation the compressor is turned at high speed by the turbine Air iscontinuously induced into the compressor, where it is accelerated by the rotatingblades and swept rearward In the subsequent stator vane passages, shaped asdiffusers, the air velocity is decreased and thus the air pressure is increased Asimilar process takes place in the rotor blade passages The stator vanes also serve

to correct the deflection given to the air by the rotor blades and to present the air

at a correct angle to the next stage of rotor blades The last stator vane row usuallyacts as “air straightener” so that the air enters the combustion chambers at a fairlyuniform axial velocity

Compressor stall and surging

The airfoil sections, the blade angles, and the reduction of the annular area aredesigned to give best performance at full load (full speed), i.e., for a certainrelationship between airflow and blade velocity and for a certain compression ratio

If the airflow velocity is too low in relation to the blade velocity, which occurs if thecompressor rotor accelerates too quickly or if the air intake filter is clogged, theairflow will break away from the blades That phenomenon is known as stall whenonly a few stages are concerned and is known as surging when the complete airflowthrough the compressor is broken down Stall or surging is a serious problembecause the blading then is exposed to oscillating forces creating unwanted stresses.The compressor is designed to operate below its surge limit, but if the airfoil sectionsare spoiled by excessive fouling the surge limit is lowered so that stall or surging

T-48 Turbines, Gas

FIG T-37 Section through a gas turbine (Source: Alstom.)

can occur even at normal operating conditions Thus, regular compressor cleaning

is a necessity

Airflow control

At low compressor speeds, i.e., during start or low load, the compressor gives a lowercompression ratio and that calls for a smaller degree of annular duct convergence.That means that at lower speeds the front stages of the compressor tend to bestalled and the rear stages tend to be choked This problem increases with thenumber of stages and the pressure ratio but can be managed by using bleed-offvalves, variable guide vanes, or twin-spool compressors (each of the two compressorparts driven by its own turbine) All three means are used when needed Simplified,the bleed-off valves cut off a part of the front stages by bleeding air from anintermediate stage, the variable guide vanes decrease the airflow to the rear stages

by throttling the first stage(s), and the twin-spool compressor allows therelationship between the speed, and thus the capacity, of the two compressor parts

to alter

The combustion chamber

In the combustion chamber, the fuel, continuously injected through the fuel burners,

is burnt with air, supplied by the compressor, and heat is released in such a mannerthat the gas is expanded and accelerated to give a smooth stream of uniformlyheated gas at all conditions required by the turbine This must be accomplishedwith the minimum loss in pressure and with the maximum heat release for the

Turbines, Gas T-49

FIG T-38 Gas turbine compressor (Source: Alstom.)

limited space available Efficient combustion is necessary to obtain high thermalefficiency and to minimize the exhaust gas emission.

by means of electrical igniter plugs fitted to one or more of the flame tubes Theflame is then spread to the other flame tubes through crossover tubes See Fig T-39

Function principle

The air leaves the compressor outlet at a velocity in the region of 100 m/s, but thespeed of burning fuel at normal mixture ratios is only a few meters per second.Thus, not to blow out the flame, the airflow must be decelerated A region of lowaxial velocity has to be created inside the flame tube so that the flame will remainburning throughout the engine operating conditions To obtain efficient combustionthe flame temperature must be about 1400–2000°C Since no material known todaycan stand such a temperature, excess air must be supplied to cool the flame tubewalls and to dilute the hot gases to a temperature that the material of the turbineparts can stand The combustion takes place in the combustion zone inside the flametube To that zone fuel is injected through a nozzle and air is induced through a

T-50 Turbines, Gas

FIG T-39 Combustion section, gas turbine (Source: Alstom.)

swirl surrounding the fuel nozzle, through the flame head slots, and through radialholes in the flame tube wall.

The air supplied creates a region of recirculating gas that takes the form of atorodial vortex, similar to a smoke ring, to stabilize and anchor the flame in thecenter of the combustion zone The recirculating hot gases also greatly assist inatomizing up the fuel and mixing it with the incoming air At full load only about1/4 of the total airflow from the compressor is supplied to the combustion zone Thatpart is sufficient to obtain complete combustion The remaining airflow, the excessair, is used to cool the flame tube walls and to dilute the hot gases The cooling air

is supplied in such a manner that a comparatively cool airstream is created nearestthe flame tube wall The dilution air is supplied through large holes downstream

of the flame tube

The turbine

The turbine provides the power for driving the compressor(s) and the power to give

a useful mechanical output That is done by extracting energy from the hot gasesreleased from the combustion chambers and expanding them to a lower pressureand temperature High stresses are involved in this process Since the turbineoperates at high speed it is exposed to large centrifugal forces and the operatingmedium The gas enters the turbine at a very high temperature

Two basic types of turbines can be used, the radial flow turbine and the axial flowturbine In the radial flow turbine the gas enters the turbine in the radial directionand in the axial flow turbine the gas flow passes the turbine in the axial direction.Except from very small units the axial flow turbine is totally dominating and thefollowing description is completely concentrated on that type

Axial flow

The turbine normally consists of several stages, each stage combined with a row ofstationary guide vanes or nozzles followed by a row of moving blades or buckets.The guide vanes are mounted to the turbine casing and the buckets are fitted toturbine discs, mostly by means of fir-tree roots See Fig T-40

The discs are mounted to one or more shafts depending on the configuration Toprevent gas leakage there are sealings between the stages and there are alsosealings to prevent leakage of hot gases toward the shafts and bearings

Those sealings are often supplied with sealing air, bled off from suitablecompressor stages, and this air is led off along the turbine discs to cool them andprevent heat transfer to shaft and bearings See Fig T-41

Function principle

In the convergent passages between the guide vanes of the airfoil section, the hotgas is expanded Pressure energy is converted into kinetic energy and the gas isaccelerated At the same time the gas is given a spin or swirl in the direction ofrotation of the turbine buckets By the buckets the gas is forced to deflect and, sincethe passages are convergent, the gas is further expanded On impact with thebuckets and during the subsequent reaction through the passages, energy isabsorbed, causing the turbine to rotate and provide the power for driving theturbine shaft By the guide vanes of the next stages the gas then is further expandedand directed to the following row of buckets See Fig T-42

The number of stages depends on the number of shafts and on the pressure ratio.Several stages were required to compress the air, but since the gas expansion is

Turbines, Gas T-51