This publication is a guide only.  Reference to local codes and standards for compliance of system design should always be sought.  No part of this document may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording, for any purpose without the express written permission of Vision Systems.  For more information, please contact your nearest Vision Fire & Security office.

The power generation industry uses different types of fuels to generate electricity. The type of power generation plant is classified by the source of fuel, which includes fossil fuels (coal, gas, oil), nuclear and renewable fuels (hydro-electric, wind and geothermal).

Power plants are normally interconnected by a transmission and distribution system to serve the electric loads in a given region.  The total load on any power system is seldom constant; rather, it varies widely with hourly, weekly, monthly, or annual changes in the requirements of the region served.

The minimum system load for a given period is commonly referred to as the ‘base load’. Maximum loads, resulting usually from temporary conditions, are called peak loads.  The operation of the power plant is closely coordinated with fluctuations in the load; the peaks are usually only a few hours duration.  The output response and capacity factor of a gas turbine power plant improves the base- load efficiency of coal fired and similar generating units.

Natural gas is considered the cleanest, least polluting energy with the lowest carbon content of all fossil fuels.  The combustion of gas releases less carbon dioxide into the atmosphere than coal or oil. The output and low emission of pollutants are particularly suitable for electrical generation and co- generation.  The emergent trend of co-generation uses the extreme heat produced during the generation of electricity to heat water, which is then used for secondary purposes.

An early warning smoke detection system not only reduces the risk of a gas turbine power plant fire, but may also prevent the incidence of nuisance alarms which could result in generation downtime, re- power-up of the site, and loss of revenue.  VESDA can provide reliable warning in a gas power plant by detecting a fire at the incipient (pre-combustion) stage.

This Industry Guide has been developed by a team of VESDA engineers who have extensive design and installation knowledge in power generation environments.

The content herein is to be used as a reference by designers and consultants when specifying a VESDA system.  It discusses relevant design considerations and recommends the correct method of installing an aspirating smoke detection system in a typical gas turbine power plant.

NOTE: This Industry Guide has been produced as a global reference. It should be used in conjunction with regionally specific fire codes and national standards.

The following design aspects should be considered during the specification of an aspirating smoke detection system:

Key fire risks in different operational areas; Airflow aspects in different operational areas;

Gas turbine power plants contain a diverse range of operational areas that have specific equipment, processes and environmental requirements.  Additionally, each area exhibits a varied degree of fire risk.  It is imperative that the specification of the air sampling system be designed to address the requirements of the respective operational areas. The following guidelines are to assist consultants and designers achieve the optimum level of detection required within the identified areas. Local standards and codes of practice should always be taken into consideration.

NOTE: Individual gas turbine power plants can differ in site layout and specification. Certain operational areas listed in this Industry Guide may not feature as part of the actual site configuration.

Alarm levels and appropriate levels of response are determined by individual application environments and are not addressed in this Industry Guide.

The following sections describe design recommendations related to the different detection areas. All pipe work designs should be verified using the VESDA Sampling Pipe Modelling Program – ASPIRE™. This program illustrates the significance of various parameters in an aspirating smoke detection system so that the most appropriate design can be applied.

The choice and level of protection is dependent on the characteristic risks of the power plant and the requirements of local fire codes and standards.  VESDA’s ability to provide performance-based solutions provides the opportunity to address the individual site specification.  This is in addition to the normal method of installing the sampling pipework to replace conventional point or beam detectors.

Performance-based design determines alternate fire protection systems by assessing the environmental risks at the concept design stage. Traditional prescriptive codes and standards have proven to provide an appropriate level of fire protection with a reasonable safety margin.  However, as tools and industry expertise continues to develop, the fire protection strategy in many installations is being designed from a risk and performance-based design approach.  This may include the use of computerised modelling tools and analysis of on-site tests (ie. smoke testing) to determine airflows, fire loading, ventilation, ignition sources and other physical/environmental conditions that may affect the likely development of a fire.

VESDA systems complement performance-based designs by providing an appropriate early warning and response in a gas turbine power plant compared to conventional detection systems. VESDA is also easily incorporated into the overall Fire Response Plan, and verification tests can be administered to confirm that the installed system is providing the specified protection.

The diverse range of operational areas within a gas turbine power plant varies according to the size of the physical installation and the layout of the site.

The performance of conventional smoke detectors may be reduced in many of these operational areas due to a number of factors including smoke diffusion, stratification, high background smoke levels. Air sampling smoke detection is able to overcome these shortfalls with intuitive system design to match the requirements of a particular site.

For example, the size and volume of a Generator/Turbine Hall exposes the area to the incidence of smoke stratification and thermal layering.  In comparison, the high forced airflow inside a High Voltage Switching or Electrical Control Room can remove smoke to recirculation/air handling systems and bypass conventional ceiling mounted detection.

The key fire risk areas in a typical gas turbine power plant include the electrical control equipment used for turbine and generator control and the support equipment located in the Mechanical Control Room.  Additionally, the electrical transmission equipment and the vast array of both data and electrical cables that link all associated systems are also primary risk areas.

Turbine and Generator Housings: The turbine and generator are enclosed within steel or concrete housings or casings that are approximately 10 x 20m (30 – 65ft) and 5 – 6m high (16 – 20ft).   The purpose of the housing is to control the high temperature and volume of noise that are characteristic of a gas turbine or generator.

The risk of fire is generally attributed to faulty/broken oil, lubrication or fuel lines, or electrical faults on the control/monitoring equipment.

Control Rooms – Mechanical: The Mechanical Control Room is usually located to the side or rear of the turbine housing, and contains the lubrication, fuel and cooling pumps and electrical control equipment.   The incidence of electrical failure and broken or faulty fuel and lubrication lines exposes this area to a significant fire risk.

Control Rooms – Electrical & High Voltage Switching: The Electrical Control Room typically contains a series of cabinet-enclosed PLC and electrical switching equipment that controls the site’s pumps and ancillary machinery. The area is typically a small, enclosed space (3 x 5m/16 x 20ft), with cabinets located along the walls.

The High Voltage Switching Control Room is generally located adjacent to the generator, and contains the control equipment that provides the connection of the generator’s output to the power grid.

Cable Tunnels: Transporting electrical current and data to the control and high voltage switch rooms, the cables are located in trenches or overhead trays.  The large and constant transportation of power along the cables exposes the area to a significant level of fire risk.

Manned Control Rooms: Although there is a low risk of an electrical fire in this area, the consequence of a fire event classifies the risk as critical.  Not only can failures effect the site’s power generation output, but a small cabling or switching failure may completely disable the power plant and cause shut-down.

High Voltage Switching Control Room

Standard room sampling is typically installed in a grid format so that each sampling hole corresponds to the location of a point detector (Refer Figure 2 and 3).  Reference should always be made to local fire codes and standards.

Certain operational areas may have support beams and voids as part of the ceiling configuration.  In areas with ceiling voids exceeding 600mm (1.96ft) it may be necessary to sample within the voids for code compliance. This is most easily achieved by incorporating ‘walking stick’ style capillary sampling, rather than using pipe bends.  This method of sampling allows the shortest possible smoke transport time while allowing a longer pipe run than would otherwise be possible. (Refer to Figure 4)

The occurrence of smoke stratification must be considered when installing sampling pipework. Stratification refers to the incidence of smoke layering that results from heating and cooling of air within an enclosed area, such as Generator/Turbine Halls. Factors such as temperature, ventilation and roof height can affect the degree of stratification and level of smoke rise.

To overcome the effect of stratification, it is recommended vertical sampling pipe be installed in addition to standard room sampling.  The vertical sampling pipe allows air to be drawn from multiple heights or layers, and therefore provides earlier detection of smoke.  (Refer to Figure 5)

Generator/Turbine Halls are generally large volume areas with roof heights in excess of 10m (32.8ft). The primary function of the area is to accommodate the turbine, generator and associated cable network, electrical switching and control equipment (typically enclosed within cabinets).  The basic premise of the turbine and generator is to produce electricity for transmission. An energy source rotates the turbine, which then drives the generator to produce electrical output. Exciter cabinets and the low voltage and high voltage control cabinets assist in the control of this output.

The impact and consequence of a critical loss and the cost of turbine and generator replacement, equipment repairs and revenue loss caused by a fire event make it imperative to install an early warning smoke detection system throughout the area.

The combination of high voltages, electrical currents, elevated temperatures and rotating/vibrating machinery increases the possibility of overheating cabling and equipment.  The switching gear and control equipment housed within cabinets (and typically located around the exterior wall of the area) requires monitoring for signs of incipient smoke in the event of overheating or sparking.

The stratifying air layers and variable airflows may create a challenge to conventional smoke detection. Smoke stratification results from heating and cooling of air within an enclosed area.  The low levels and/or small increases in smouldering smoke become increasingly difficult to detect by ‘passive’ conventional detectors, which only activate an alarm when a significant amount of smoke is generated.

External detection of smoke during the incipient (pre-combustion) stage of an in-cabinet fire may also be affected due to low thermal energy within the cabinets, poor room ventilation, or smoke dilution due to forced air ventilation in cabinets.

It is recommended that room sampling (ceiling and multiple level vertical sampling pipes, and in- cabinet sampling is installed in the area.  VESDA’s cumulative early warning smoke detection allows positioning of sampling points at various heights to compensate for stratification; actively sampling the area for smoke.

The Switch Room is a dedicated space that accommodates a high concentration of electronic cabinets and automated switchgear.  The in-cabinet equipment maintains the primary functions of the power plant, and is the switching interface between the Control Room/s and the field equipment.

Although the fire risk may be considered low, it is imperative to install early warning fire detection due to the critical nature of the equipment and demand on the continuous operation of the Switch Room.

There are several challenges to the detection of a Switch Room fire.  The electrical demand of the in- cabinet equipment is a major risk and is the primary cause of a fire incident.

In the occurrence of an in-cabinet fire, the initial low thermal energy inside the cabinet may affect the external detection of smoke during the incipient (pre-combustion) stage.  The poor ventilation of cabinets can also affect the detection of smoke by external devices during the crucial incipient stage.

A further consideration is that the nature of the in-cabinet equipment may require a high level of airflow to maintain a suitable operational temperature.  Forced air ventilation within the cabinet can increase the risk of smoke dilution and therefore limit the ability to detect within the cabinet when using conventional detection systems.

It is recommended that both ceiling mounted and cabinet sampling be located in the Switch Room. Cabinet sampling eliminates the risk of delayed smoke detection, which can result from low thermal energy and/or smoke dilution.

The positioning of ceiling mounted pipework and sampling holes is dependant on the layout and configuration of the Switch Room.  Reference should be made to local standards regarding the appropriate spacing and density of the sampling holes (detection points).

The cabinet configuration is the main consideration when determining the type of pipework sampling method for cabinet protection.  Depending on the cabinet construction, either in-cabinet (fully- enclosed) or above-cabinet (top-vented) sampling is recommended.

Above cabinet sampling is achieved by locating the sampling pipe directly over the ventilation grille of the cabinet.  It is recommended that each cabinet must have a minimum of one dedicated sampling hole that faces the direct airflow out of the cabinet.

Refer to Section 10: Cabinet Protection for further explanation on the above air sampling methods.

The control room is the main command centre of the power plant. The entire site operation is monitored, maintained and adjusted from this central location.  Control rooms are a key consideration due to the high concentration of computers, control equipment, electronic switching devices and underfloor cabling.  While the actual risk of fire is low, the consequential impact of a control room fire necessitates earliest warning.

Control rooms can range from a small, non-ventilated space, to a massive high-tech air conditioned command and control station with multiple consoles and operators controlling every function of the facility.

Control rooms provide a challenge to traditional detection methodologies due to the fact that most computer and control equipment is housed within cabinets, which can impede the detection of smoke at ceiling level.   The ability of conventional systems to detect a fire can also be affected by smoke dilution caused by the control room’s HVAC system.

VESDA system design provides several methods to overcome the issue of compartmentalised and/or diluted smoke.

Locating sampling pipework at ceiling level, under the floor void and across the return air grille of the air conditioning units provides the earliest possible detection at the incipient stage of a control room fire event.  (Refer to Figure 6)

In addition, the presence of equipment cabinets and consoles may require direct in-cabinet sampling. VESDA’s high sensitivity and cumulative sampling allows very small levels of smoke to be identified at the earliest stage.

Refer to Section 10: Cabinet Protection for further explanation on the above air sampling methods.

Electrical, relay and communication equipment is typically mounted on equipment racks and housed within enclosed cabinets.  These cabinets may be fully enclosed or have a ventilation system.  The ventilation system may either be located as a grille at the top of the cabinet, or as forced ventilation with fans and grilles located at various points within the cabinet.

The equipment or wiring within a cabinet may overheat presenting a fire risk.  Smoke or fire within a cabinet may not be detected outside the cabinet until either a fire is already in progress or significant damage has occurred.  Cabinet protection can be maximised by using a VESDA Laser System designed to detect overheating wiring and equipment in the pre-combustion (incipient) stage of a fire.

In the occurrence of an in-cabinet fire, the low thermal energy inherent in the cabinet may affect the detection and response of an external smoke detector during the incipient (pre-combustion) stage.

In addition, the poor ventilation (convectional cooling only) of cabinets can also affect the external detection of smoke during this crucial incipient stage.

A further consideration is that the nature of the in-cabinet equipment may require a high level of airflow to maintain a suitable operational environment.  The forced air ventilation can increase the risk of dilution and therefore limit the ability to detect smoke.

The VESDA Laser System can protect in-cabinet equipment by continuously sampling the internal cabinet environment.  This is achieved by In-Cabinet or Above Cabinet Sampling.  (Refer to Figure 7)

Capillary tubes or drop pipes are inserted into a cabinet from the top or from the under floor void to sample the air within a cabinet.

Sampling pipe is positioned over the cabinet and capillaries or drop pipes are attached to the sampling pipe at appropriate intervals.  It is recommended, unless specified otherwise, that the capillary tube or drop pipe should enter the interior of the cabinet to a depth of 25 - 50mm (1 – 2”).

Sampling pipe is installed in the under floor void.  Capillary tubes or riser pipes are attached at appropriate intervals.  Holes are then drilled in the floor and the base of the cabinet.  Capillary tubes or riser pipes are run through the bottom of the cabinet to the top and supported at the cabinet’s roof by a mounting clip.  It is recommended that the sampling hole is faced downwards and, unless specified otherwise, should be located 25 - 50mm (1 – 2”) below the interior of the cabinet top.

Above cabinet sampling is achieved by locating the sampling pipe directly over the ventilation grille of the cabinet.  Stand-off posts are then used to mount the sampling pipe 25 - 200mm (1 - 8”) above the grille (depending upon air movement). Each cabinet must have at least one dedicated sampling hole that faces the direct airflow out of the cabinet.

Cable Tunnels are long passageways that accommodate lengths of racking that support the communication and control cabling, and the various power cabling to and from specific operational areas.  The information distributed by the cables is fundamental to the uninterrupted operation of the facility. In a typical layout, numerous lengths of Cable Tunnels link the Control Room, Switch Room and High/Low Voltage Annex areas.

Cable Tunnels are a significant fire risk due to the large and constant amount of power being carried on the cables.  The entire operational function of the power plant is dependent on the uninterrupted transportation of information.

Generally classified as confined spaces and located underground, Cable Tunnels are classified as a typically hostile environment, with an increased level of ambient background pollution. Additionally, the configuration and location of the tunnels not only impede general access, but can also affect the response to, and containment of a fire.

The most efficient way to protect the Cable Tunnel area is to install ceiling mounted pipework with long single pipe runs extending in two directions from a central detector.  (Refer to Figure 8)

Cable Chambers are an integral link to the control and coordination of a facility.  The cables located along the Cable Tunnels are grouped together in Cable Chambers for further distribution into various cabinets and control equipment.

Cable Chambers can range in size from 5-30m (16 – 98ft) in width and length and 2-3m (6.5 – 9.8ft) in height.

Cable Chambers are a high fire risk due to the extremely large amount of power that is continuously flowing in the cables.  Although the cables are have minimal voltages with relatively low power, when grouped together and overlaid, the risk of damage increases, particularly during cable mining and replacement operations.

The Cable Chamber area is typically located below ground level and is a confined space with a low rate of airflow.

The most efficient way for a VESDA system to protect Cable Chambers is to install ceiling mounted pipework with multiple pipe runs.  The sampling holes are usually configured in a code-based format. VESDA’s high sensitivity and cumulative sampling allows very small levels of smoke to be identified.

It is common for Cable Chambers to have support beams that support the floor above (usually a Switch Room).  It may be necessary to sample within the voids produced by these beams for code compliance. This is most easily achieved by incorporating ‘walking stick’ style capillary sampling, rather than using pipe bends.  This method of sampling allows the shortest possible smoke transport time while allowing a longer pipe run than would otherwise be possible. (Refer Section 6.2 – Interbeam Sampling)

The Battery Room houses lead acid or nickel cadmium batteries that supply the site’s uninterrupted power supply (UPS).  The UPS is used as a back-up power source and to start the facility in the event plant shutdown.   The size and configuration of the Battery Room is dependent on the power required by the UPS to operate the facility.

VESDA’s system design detects the incidence of overheating cells and links, and failures in high current cables and bus bars.

The environment within the Battery Room may become explosive due to the build up of high concentrations of hydrogen gas.

                                                            Monitored fans that operate on a continual basis are usually installed within the area to counteract the atmospheric build-up of hydrogen and to provide ventilation.

It is recommended that either ceiling mounted and/or Exhaust Air Sampling be installed to protect Battery Room environments.

The pipework is mounted approximately 25 to 100mm (1 to 4”) below the ceiling.  The spacing, diameter of the sampling holes, and the end cap vent size are determined using ASPIRE calculations.

Exhaust Air Sampling can either be positioned at the exhaust air grille of the duct (Exhaust Air Grille Sampling), or located in the duct (In-Duct Sampling).

Incipient smoke generally tends to travel with the natural airflow. By positioning pipework with sampling holes across the air grille of an exhaust ventilation system, any smoke generated in the Battery Room is detected at the earliest stage.

    Duct Sampling is achieved by locating a sampling pipe across the entire width of the duct, and along its centre line.  In this particular application, the sampling pipe is commonly referred to as a ‘probe’.  (Refer to Figure 10)

the airflow stream.  The sample air is then exhausted from the detector back into the duct. To avoid air disturbances from the inflow of air, the in-take probe should be diagonally offset at least 300mm (1ft).  The exhaust probe is typically inserted to a third of the duct's width.

NOTE: It is important to ensure that the points where the probes enter the duct are properly sealed and made airtight.

High/Low Voltage Annexes contain the voltage control and transmission equipment, which is necessary for the operation of any facility. These areas typically consist of rows of cabinets which may be either fully enclosed, or have a top mounted ventilation grille.

A failure of the circuitry or wiring within a cabinet may cause it to overheat; presenting as a significant fire risk.  Cabinet detection and protection can be maximised by installing a VESDA detector to detect overheating wiring and equipment in the pre-combustion (incipient) stage of a fire.

The low thermal energy produced by a smouldering cable and/or poor ventilation of a cabinet can impede the external detection of an in-cabinet fire condition.

VESDA can protect high/low voltage cabinet equipment by continuously sampling the internal cabinet environment. There are two methods, which may be implemented to protect high/low voltage cabinets.

Sampling pipe is positioned over the cabinet and capillaries are attached to the sampling pipe at appropriate intervals.  It is recommended that the capillary tube enter the interior of the cabinet to a depth of 25-50mm (1-2”).

Sampling pipe is installed under the floor void and capillary tubes or riser pipes are attached at appropriate intervals.  Holes are then drilled in the floor and the base of the cabinet.  Capillary tubes or riser pipes are run through the bottom of the cabinet to the cabinet roof by a mounting clip.  It is recommended that the sampling hole faces downwards and should be 25-50mm (1-2”) below the interior of the cabinet top.

Above cabinet sampling is achieved by locating the sampling pipe directly over the ventilation grille of the cabinet.  Stand-off posts are then used to mount the sampling pipe 25-50mm(1-2”) above the grille.  Each cabinet must have at least one dedicated sampling hole that faces the direct airflow from the cabinet.

Electronic programming equipment that allows the user to control machinery positioned both on-site and at remote locations.

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