Process Engineering Equipment Handbook Episode 3 Part 7 pot

independent approval institute. Well-known institutes are PTB (Germany), Factory
Mutual Research (USA), SAA (Australia), JIS (Japan), and CSA (Canada).
The better tank-gauging instruments do not just meet the safety standards but
exceed them by anticipating future safety requirements as well. Such requirements
include the exclusion of aluminum inside storage tanks (zone 0), the limitation of
the kinetic energy of moving parts of a gauge to values far less than could cause
ignition.
Lightning and tank gauging. Lightning can cause hazardous situations, and
measures should be taken to protect the tank installation and tank-gauging system
against these hazards. Modern tank-gauging systems contain many electronic
circuits. Their position on top of storage tanks makes this equipment more
vulnerable to lightning damage than any other type of industrial equipment.
Today’s communication systems linking all field equipment via one network
increase the probability of possible damage to the equipment as the networks
spread over increasingly larger areas. With high reliability and availability one of
the prime requirements of modern tank-gauging equipment, there is a need for well-
designed, field-proven lightning protection methods. Figure T-19 shows a tank
gauge 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. This
results in a potential difference between cable and gauge or cable and receiver. This
difference between equipment and cable tries to equalize itself and searches a low
Tanks T-29
TABLE
T-2 Overview of Batch Transfer Uncertainties
Level
Transfer Servo/Radar HIMS HTG
(m) (ft) Mass GSV GSV Mass Mass GSV
20–18 66–60 0.31 0.30 0.30 0.28 0.28 3.09
4–2 13–6.5 0.14 0.10 0.10 0.28 0.28 0.61

20–26 66–6.5 0.11 0.04 0.04 0.03 0.03 0.47
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 Servo/Radar HIMS HTG
(m) (ft) Mass G.S.V. G.S.V. Mass Mass G.S.V.
20 66 0.12 0.06 0.06 0.04 0.04 0.43
10 33 0.12 0.07 0.07 0.08 0.08 0.41
2 6.5 0.13 0.08 0.08 0.40 0.40 0.34
Inventory uncertainties in (%)
impedance path between the circuitry connected to the cable and the ground. As
soon as the potential difference exceeds the isolation voltage, a breakdown occurs
between the electronics and the ground. Additionally, transient currents will be
induced in adjacent components and cabling.
The currents flowing through the electronics cause disastrous effects. Every
semiconductor that is not sufficiently fast or capable of handling the currents for
even a short period will be destroyed.
Two basic techniques are used for minimizing the damage due to lightning and
transients: suppression and diversion.
Suppression. By means of special circuits on all incoming and outgoing instrument
cables it is possible to suppress the magnitude of the transient appearing at the
instrument (Fig. T-20). A gas discharge tube forms the kernel. Gas discharge tubes
are available for voltage protection from 60 V up to more than 1000 V and react in
several microseconds, after which they form a conducting ionized path. They
provide 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 transient
suppressor reacts, it degrades. Reliability is therefore poor, rendering this type of
device unsuitable for critical applications such as tank gauges.
Diversion. Diversion (Fig. T-21) is a much more reliable technique and better
suited for lightning protection of electronic tank-gauging instruments. Modern
protection uses diversion combined with screening and complete galvanic isolation.
It is a technique in which the high-voltage spikes are diverted rather than
dissipated. Specially developed isolation transformers are used for all inputs and
outputs. 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 physically
separated from internal wiring and ground tracks are employed on all circuit boards
to shield electronics. Unfortunately this protection method is not suitable with DC
signals. In this case a conventional transient protection, enhanced with an
additional galvanic isolation, is used.
Grounding and shielding. Proper grounding and shielding will also help protect
instruments and systems connected to field cabling against damage by lightning.
The possible discharge path over an instrument flange (e.g., of a level gauge) and
the corresponding mounting flange should have a nearly zero resistance to prevent
buildup of potential differences. A poor or disconnected ground connection may
cause sparking and ignite the surrounding product vapors.
Field experience. The diversion method described for internal lightning protection
has been in use for more than 15 years, with approximately 50,000 installed
instruments. Almost 100 percent of this equipment is installed on top of bulk
storage tanks, and interconnected via wide area networking.
A large number of installations are situated in known lightning-prone areas. To
date, only a few incidents in which lightning may have played a decisive role have
been experienced. The amount of damage was always limited and could be repaired

locally at little expense. Before this protection method was applied, more extensive
lightning 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 total
amount of electronics. Advanced software development tools and higher order
programming languages provide reliable operation. Fuzzy control algorithms
improve interaction of mechanics and electronics, reducing the number of
mechanical 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 to
open 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 alarm
for 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-23
and T-24). Their nonintrusive solid-state nature makes them very attractive. The
accuracy of the newest generation radar gauge meets all requirements for custody
transfer 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. The
compact and lightweight construction simplifies installation without the need for
hoisting equipment. Installation is possible while the tank stays in full operation.
Current developments are aimed at more integrated functions. Improved antenna
designs, full digital signal generation, and processing offer better performance with
less 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 support
pipes no longer a must for accurate measurement
᭿
Full programmability for easy setup and verification facilities
᭿
The compact and lightweight construction eliminating the need for hoisting
equipment
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, and
pressure via digital protocol
Temperature gauging. Accurate temperature measurement is essential for level-
based tank-gauging systems.
Spot temperature elements are widely accepted for product temperature
assessment on tanks with homogeneous products. Installation is simple and the
reliability is good. The graph of Fig. T-25 shows that spot measurements are
unsuitable 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 evenly
distributed over the maximum possible liquid height. A very accurate class A Pt100
element 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 temperature
profiles and vapor temperatures. MTTs are available with both nylon and stainless
steel protection tubes. It provides a rugged construction suitable for the harsh
environments of a bulk storage tank.
Another type of average temperature measuring element is the multiresistance
thermometer (MRT). Its operation is based on a number of copper wire temperature
sensing elements of different lengths. Average temperature measurement is
achieved 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 to
damage, especially during transport and installation.
Hydrostatic tank gauging. Recent developments of smart transmitters opened a new
era 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 transmitters
were still rare and quite expensive. Several manufacturers now offer 0.02 percent
accuracy transmitters. Digital communication by means of de facto standards, as
the HART
TM
-protocol, permits simple interfacing to almost any transmitter. This
wide choice simplifies selection for specific applications and allows the user to
choose his own preferred transmitter. The inherent standardization for the end user
reduces the cost of maintenance.
Hybrid inventory measurement system. HIMS are also based on the integration of
smart pressure transmitters. Modern level gauges, either servo or radar, provide
the possibility for direct interfacing to smart pressure transmitters. HIMS opens
the ideal route to total tank inventory systems, measuring all tank parameters via
one system.
Central inventory management system. The interface to the operators and/or the
supervisory control and management system is the tank-gauging inventory
management system (Fig. T-27). These high-speed systems collect the measurement
data from all tank-gauging instruments, continuously check the status of alarms
and functional parameters, and compute real-time inventory data such as volume
and mass. The hardware used is generally off-the-shelf personal or industrial
computers loaded with dedicated inventory management software. It is this
software, together with the reliability and integrity of the field instrumentation that

determines the performance and accuracy of the inventory management system. All
field instruments, regardless of age or type, should communicate via the same
transmission bus.
Product volumes and mass should be calculated the same way as do the owner-
appointed authorities and surveyors. The system software should store the tank
table parameters, calculate observed and standard volumes, correct for free water
and, if applicable, correct for the floating roof immersion. The GSV calculations
must be in accordance with API, ASTM, and ISO recommendations implementing
tables 6A, 6B, 6C, 53, 54A, 54B, 54C, and 5.
T-36 Tanks
FIG.
T-27 Central inventory management system. (Source: Enraf.)
The quality of the inventory management system can be evidenced from the
availability of Weights & Measures or Customs & Excise approvals. Inventory
management systems can have their own display consoles or can make all data
available for a supervisory system.
Networked systems are available when required. Apart from a large number of
inventory management functions, the system can also control inlet and outlet valves
of the tanks, start and stop pumps, display data from other transmitters, provide
shipping documents, provide trend curves, show bar graph displays, perform
sensitive leak detection, calculate flow rates, control alarm annunciation relays,
perform numerous diagnostic tasks, and much more. For examples of display
formats of an inventory management system see Fig. T-28 for tabular displays and
Fig. T-29 for graphical displays.
The operator friendliness of the system is of paramount importance. The better
and more advanced systems have context-sensitive help keys that make the proper
help instructions immediately available to the operator.
Interfacing to host systems. The receiving systems can also be equipped with host
communication interfaces for connection to plant management systems, e.g., Dis-
tributive Control Systems (DCS), Integrated Control Systems (ICS), oil accounting

systems, etc. (Fig. T-30). Protocols have been developed in close cooperation with
the 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 inventory
systems and third-party control systems. Modern DCS or other systems have
sufficient power to handle inventory calculations, but often lack the dedicated
programming 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 and
equipped with suitable host links, will have distinct advantages.
᭿
It frees the host system supplier from needing detailed knowledge of transmitter
and gauge specific data handling.
᭿
Maintaining a unique database, with all tank-related parameters in one computer
only, 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 easier
for Weight and Measures authorities.
᭿
Implementation of software required for handling of new or more tank gauges
can be restricted to the tank-gauging system. This will improve the reliability and
availability 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 maintenance
and 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 reconciling
recorded changes in stock levels against actually metered movements, the system
can 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 becomes
realistic.
Interfaces to multiple supplier systems, ranging from tank gauging to loading and
valve control systems, will be feasible via internationally accepted communication
standards.
In summary, a wide range of different tank-gauging instruments is available. The
employed techniques are more complementary than competitive as each measuring
principle has its own advantages. See Table T-3. Modern servo and radar gauges
have improved considerably. They hardly need any maintenance and can provide
trouble-free operation if applied correctly. The possibility of mixed installations with
servo, 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 and
custody 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 to

play an important role.
The combination of volume and mass offers great advantages. A globally accepted
Tanks T-39
TABLE
T-3 Suitability of the Different Gauging Techniques
Servo Radar HTG
Asphalt -++-
Fuel oil, crude ++++
Black products +/-++/-
White products ++ + +
LPG/LNG ++ +/
measurement standard will probably not be published for several years.
Implementation of volume and mass calculations outside the management
information, DCS, or host systems remains preferable. Integrity requirements for
volume and mass calculations imposed by the Weight and Measurement authorities
are easier to fulfill externally and justify the additional hardware.
Standard field buses may play a decisive role in the direct interface between
dedicated tank-gauging systems and other systems. However, the quality of the
measurements 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 may
occur. Reactors frequently are large enough to require the structural design
techniques used with towers. The term “reactor tower” might be used to describe a
tower that does reactor functions.
The tower may be dealing with fractional distillation or component content

change(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 hundred
feet high are not that uncommon now.
Tray-type reactors. Internally, a variety of different tray types may be used. The
descriptive terms for these trays include: bubble cap trays, “flexitrays,” ballast
trays, float tray, sieve trays, “turbogrid,” and “kittel” trays. They use a variety of
techniques, 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 a
more complete reaction.
Packed reactors. A packed reactor is more popular with very corrosive
applications. A designer needs to allow for good distribution and avoid overly large
packing 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. In
the United States, this would be the EPA; in Canada, Environment Canada; in the
United Kingdom, DOE. If traveling across borders, one needs to look at all the
specifications from different countries and pick the most stringent one to work
toward.
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 metallurgical
selections to their land-based counterparts in power generation and mechanical
drive service—-the technology is starting to move to “ground level.” The governing
factor, as always, is economics.
Triple redundancy means a more reliable, available system that is prone to fewer
failures. When that translates into money, the more sophisticated technology is
adopted. At this point, many power plants in Asia have unused capacity and are
not always hurt financially if there is an interruption in availability in one of their
power 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 power
plants 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 critical
compressor, pump, or blower could mean $250,000 to $500,000 in lost income.) At
this point, triple redundancy technology is more popular with these users, but
power operators need to take notice. As other more elementary problems, such as
transmission-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 guards
against 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). The
processors vote about:
᭿
Input values
᭿
Output values
᭿
Data results

᭿
Condition codes
᭿
Condition interrupts
᭿
Memory locations
᭿
Diagnostics
The processors communicate during the vote so that they have high test coverage.
Communication occurs through read-only isolated links.
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 economics
and cost per running hour.
Triple Redundancy T-41
3. Shutdown/malfunction/loss of availability might violate contracts (such as
PPAs)/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/unwanted
shutdowns put operational staff under less pressure and make them more
inclined 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 and
started. The system can be run, albeit imperfectly, until optimized timing for
taking the turbomachinery units and/or accessories can be arranged. This cuts

down 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 (duplex
systems), these types of failure are likely to result in shutdown. TMR eliminates
control system-caused shutdowns. It cuts down on the number of operator-caused
shutdowns, 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 generators
reduces wear factors on generators, couplings, and turbines. Smooth transfer of
fuel types in a dual (gas/liquid) or tri (gas/liquid/gas and liquid mixed) fuel system
greatly 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 NO
x

control (combustion stability)
᭿
Automatic operation, starting, loading, or synchronizing
᭿
Integration of control systems of multiple plant units. This is additionally
significant in many power development projects where modules or cogeneration
or waste-heat recovery schemes are commissioned after the main unit has run
for a while (add-ons).
Operation of the TMR
The TMR operates according to a two-out-of-three algorithm that is generally
configured for fail-safe operation. If there is one input/output failure, the system
continues to operate. If there is another component failure, the system can be
configured to continue running (3:2:1 mode) or shut down safely. This meets all the
safety codes that are required for plant operation as well as international standards
for 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 overall
system, so this high degree of sophistication is quite user-friendly.
Control Features of TMR
For turbines (power generation and mechanical drive applications) and driven
equipment, the TMR system provides:
᭿
Automatic startup sequencing
᭿
Starter control
᭿
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 systems
For 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 remain
running. They provide availabilities of 99.999 percent. These applications in the
power industry will grow in number as Asia’s demand growth accelerates. While
simplex control may be acceptable as long as a turbine and generator are essentially
all the system consists of, the justification for TMR rises with the addition of other
modules, 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 into
mechanical energy. The heat energy is usually produced by burning a fuel with the

oxygen of the air. In that way the engine converts the potential chemical energy of
the fuel first to heat energy and then to mechanical energy. However, in a gas
turbine, as well as in other types of heat engines, only a part of the released heat
energy 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 energy
converted 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 overall
system, so this high degree of sophistication is quite user-friendly.
Control Features of TMR
For turbines (power generation and mechanical drive applications) and driven
equipment, the TMR system provides:
᭿
Automatic startup sequencing
᭿
Starter control
᭿
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 systems
For 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 remain
running. They provide availabilities of 99.999 percent. These applications in the
power industry will grow in number as Asia’s demand growth accelerates. While
simplex control may be acceptable as long as a turbine and generator are essentially
all the system consists of, the justification for TMR rises with the addition of other
modules, 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 into
mechanical energy. The heat energy is usually produced by burning a fuel with the
oxygen of the air. In that way the engine converts the potential chemical energy of
the fuel first to heat energy and then to mechanical energy. However, in a gas
turbine, as well as in other types of heat engines, only a part of the released heat
energy 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 energy
converted 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 remaining
60–75 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 the
waste heat can be recovered, e.g., in a waste heat recovery boiler, the efficiency
increases correspondingly.
Operating cycle main parts
In a gas turbine the operating medium is air and gas, and the flow runs through
the cycle COMPRESSION—HEATING—EXPANSION.
In an open gas turbine cycle, ambient air is sucked in, compressed in a
COMPRESSOR, heated in a COMBUSTION CHAMBER by injection and burning of a fuel
and then expanded through a TURBINE back to the atmosphere. The operating
medium of an open gas turbine cycle consequently is air and a mixture of air and
combustion 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 heat
exchanger and the gas expanded through the turbine must be cooled before it is led
back to the compressor. See Fig. T-32.
In practice the open gas turbine cycle is completely dominating and the further
description 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 operating
medium 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 named
compressor 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 is

supplied to the compressor and is transferred to the airflow. From the airflow energy
flows to the turbine, which through its rotation gives off a mechanical output. The

Efficiency
output energy
input energy
==h
T-44 Turbines, Gas
FIG. T-31 Energy exchange for a gas turbine. (Source: Alstom.)
energy flow can be noticed as compressor speed, increase of airflow velocity and
pressure (by virtue of pressure increase as well as temperature increase), and
turbine speed. If the process goes on without losses (which in practice is
impossible but temporarily accepted to simplify the understanding), the turbine
energy 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 inside
the tube is created by the compressor, the heating of the air does not result in
further increased air pressure. Instead the air volume is increased. Increased air
volume results in increased air velocity through the turbine. A larger amount of
energy is transferred to the turbine, which then can give off a larger mechanical
output. 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 the
heat 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 to
drive 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 starter
has been necessary to create the airflow through the tube. The airflow forces the
process to continue by virtue of its momentum. Heating stationary air inside the
tube would only have meant temperature increase and air expansion backward
through the compressor as well as forward through the turbine. See Fig. T-35.
At self-sustaining condition the mechanical output extracted from the turbine is
just enough to drive the compressor. The whole amount of energy supplied by
heating is waste energy. In reality these losses consist of exhaust losses,
losses due to turbulence, and radiation losses. For thermodynamic reasons the
temperature 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 certain
velocity.
Power generation
To get a useful mechanical output from the turbine the heat supply must be further
increased to speed up the engine above self-sustaining speed. See Fig. T-36.
To make the gas turbine as efficient as possible, by converting heat energy into
mechanical energy, its design must be much more complicated than that of the
described simple model. However, the main features with cylindrical casings
containing compressor, combustion chamber, and turbine, so that the air/gas flow
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 sophisticated
construction to continuously compress an airflow to desired pressure. One of two
basic types of compressors, one giving a radial flow and the other an axial flow, is
normally used in a gas turbine. The axial flow compressor is easier to design for

high-pressure ratios, is more efficient, and is thus common in high-performance
units. 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 are
mounted the stator vanes, which also are of airfoil section. The compressor is a
multistage unit as the pressure increase by each stage is small (pressure ratio
1.15–1.25/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 inlet
guide vanes, is used to guide the air on to the first row of rotor blades. From the
front 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 the
compression. To prevent air leakage there are sealings between the stages and at
the 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 is
continuously induced into the compressor, where it is accelerated by the rotating
blades and swept rearward. In the subsequent stator vane passages, shaped as
diffusers, the air velocity is decreased and thus the air pressure is increased. A
similar 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 usually
acts as “air straightener” so that the air enters the combustion chambers at a fairly
uniform axial velocity.
Compressor stall and surging

The airfoil sections, the blade angles, and the reduction of the annular area are
designed to give best performance at full load (full speed), i.e., for a certain
relationship 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 the
compressor rotor accelerates too quickly or if the air intake filter is clogged, the
airflow will break away from the blades. That phenomenon is known as stall when
only a few stages are concerned and is known as surging when the complete airflow
through the compressor is broken down. Stall or surging is a serious problem
because 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 sections
are 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 lower
compression 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 be
stalled and the rear stages tend to be choked. This problem increases with the
number of stages and the pressure ratio but can be managed by using bleed-off
valves, variable guide vanes, or twin-spool compressors (each of the two compressor
parts 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 an
intermediate stage, the variable guide vanes decrease the airflow to the rear stages
by throttling the first stage(s), and the twin-spool compressor allows the
relationship 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 manner
that the gas is expanded and accelerated to give a smooth stream of uniformly
heated gas at all conditions required by the turbine. This must be accomplished
with 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 thermal
efficiency and to minimize the exhaust gas emission.
Design
The combustion chambers can be designed in different ways. The following
description refers to the canannular type. A number of flame tubes are fitted inside
a common annular casing. The burners are fitted in the center of the forward end,
known as the flame head, of the flame tubes. The compressor outlet is via a diffuser,
in which the airstream is decelerated and the static pressure raised, connected to
the forward end of the annular casing and each flame tube is via a gas collector
connected to a section of the turbine inlet. During start the combustion is initiated
by means of electrical igniter plugs fitted to one or more of the flame tubes. The
flame 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 the
speed 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 low
axial velocity has to be created inside the flame tube so that the flame will remain
burning throughout the engine operating conditions. To obtain efficient combustion
the flame temperature must be about 1400–2000°C. Since no material known today
can stand such a temperature, excess air must be supplied to cool the flame tube
walls and to dilute the hot gases to a temperature that the material of the turbine
parts can stand. The combustion takes place in the combustion zone inside the flame
tube. 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 radial
holes in the flame tube wall.
The air supplied creates a region of recirculating gas that takes the form of a
torodial vortex, similar to a smoke ring, to stabilize and anchor the flame in the
center of the combustion zone. The recirculating hot gases also greatly assist in
atomizing up the fuel and mixing it with the incoming air. At full load only about
1/4 of the total airflow from the compressor is supplied to the combustion zone. That
part is sufficient to obtain complete combustion. The remaining airflow, the excess
air, 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 nearest
the 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 gases
released from the combustion chambers and expanding them to a lower pressure
and temperature. High stresses are involved in this process. Since the turbine
operates at high speed it is exposed to large centrifugal forces and the operating
medium. 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 flow
turbine. In the radial flow turbine the gas enters the turbine in the radial direction
and 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 the
following description is completely concentrated on that type.
Axial flow
The turbine normally consists of several stages, each stage combined with a row of
stationary 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 to

turbine 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. To
prevent gas leakage there are sealings between the stages and there are also
sealings to prevent leakage of hot gases toward the shafts and bearings.
Those sealings are often supplied with sealing air, bled off from suitable
compressor stages, and this air is led off along the turbine discs to cool them and
prevent 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 hot
gas is expanded. Pressure energy is converted into kinetic energy and the gas is
accelerated. At the same time the gas is given a spin or swirl in the direction of
rotation of the turbine buckets. By the buckets the gas is forced to deflect and, since
the passages are convergent, the gas is further expanded. On impact with the
buckets and during the subsequent reaction through the passages, energy is
absorbed, causing the turbine to rotate and provide the power for driving the
turbine shaft. By the guide vanes of the next stages the gas then is further expanded
and 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