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The importance of automatic fire suppression systems

Posted: 22 January 2013 | Jonas Brandt, Project Manager, SP Technical Research Institute of Sweden | No comments yet

This is the second and final part of an article to provide a background look at bus fires and to underline the importance of automatic fire suppression systems for buses and coaches. Bus and coach1 fires are a common issue in society. Given the fact that most fires start in the engine compartment, adequate active fire protection systems for engine compartments are advantageous both in terms of passenger safety, carrier and insurance company economy, and general public resource management. However, there is still no legislative demand for this course of action, nor an international standard for testing bus engine compartment fire suppression systems. For this reason, SP has recently developed a new standard named SP Method 4912.

SP Method 4912 describes a new method to test and validate the fire suppression performance of different suppression systems in a repeatable and reproducible way. A broad reference group including more than 80 companies such as bus manufacturers and operators, transport auth – orities, insurance companies, fire investigators and suppression system manufacturers have given valuable feedback to the development of the method. More than 450 pre-tests have been performed involving 10 fire suppression system manufacturers and with several different types of suppression agents, such as ABC- and BC-dry chemical, water mist, water spray and foam systems, aerosol and clean agent.

This is the second and final part of an article to provide a background look at bus fires and to underline the importance of automatic fire suppression systems for buses and coaches. Bus and coach1 fires are a common issue in society. Given the fact that most fires start in the engine compartment, adequate active fire protection systems for engine compartments are advantageous both in terms of passenger safety, carrier and insurance company economy, and general public resource management. However, there is still no legislative demand for this course of action, nor an international standard for testing bus engine compartment fire suppression systems. For this reason, SP has recently developed a new standard named SP Method 4912. SP Method 4912 describes a new method to test and validate the fire suppression performance of different suppression systems in a repeatable and reproducible way. A broad reference group including more than 80 companies such as bus manufacturers and operators, transport auth - orities, insurance companies, fire investigators and suppression system manufacturers have given valuable feedback to the development of the method. More than 450 pre-tests have been performed involving 10 fire suppression system manufacturers and with several different types of suppression agents, such as ABC- and BC-dry chemical, water mist, water spray and foam systems, aerosol and clean agent.

This is the second and final part of an article to provide a background look at bus fires and to underline the importance of automatic fire suppression systems for buses and coaches. Bus and coach1 fires are a common issue in society. Given the fact that most fires start in the engine compartment, adequate active fire protection systems for engine compartments are advantageous both in terms of passenger safety, carrier and insurance company economy, and general public resource management. However, there is still no legislative demand for this course of action, nor an international standard for testing bus engine compartment fire suppression systems. For this reason, SP has recently developed a new standard named SP Method 4912.

SP Method 4912 describes a new method to test and validate the fire suppression performance of different suppression systems in a repeatable and reproducible way. A broad reference group including more than 80 companies such as bus manufacturers and operators, transport auth – orities, insurance companies, fire investigators and suppression system manufacturers have given valuable feedback to the development of the method. More than 450 pre-tests have been performed involving 10 fire suppression system manufacturers and with several different types of suppression agents, such as ABC- and BC-dry chemical, water mist, water spray and foam systems, aerosol and clean agent.

Requirements on a test method

In order for the new test method to be valuable, it must be repeatable, which means that the test method must be able to produce the same result at another time. The test also needs to be reproducible, meaning that it must be possible to reproduce the test setup at another location and get the same test results. This requires a test apparatus that is not too complicated to reproduce. It should not involve, for example, a specific engine type as this might go out of production and be difficult to obtain. Further – more, it must be possible to obtain the same pre-test condition before every new test. The test apparatus should not be damaged after a series of testing making the 20th test different from the first. Additionally, the test should have a realistic fire challenge, which means that the test must re-create potential fire scenarios from the real world that the fire suppression systems should be able to handle. Moreover, it is important that the test does not wrongly favour or disqualify a certain agent or suppression technology. The test should also be designed in a way that a modification made on the suppression system in order to receive a better test result also provides a better suppression performance in a real fire event. Furthermore, it is preferable if the test method not only gives the information: pass or fail, but can rate systems in a manner that makes it possible to compare different systems with the possibility to choose a better system if a higher safety level is desired. This also motivates manufacturers to improve systems and get a higher rating. In this case, the rating system of hand extinguishers is a model.

Investigation of engine compartments

In order to develop a realistic test apparatus, several buses were examined2. The survey was limited to heavy diesel fueled buses with rear mounted engine compartments as those represent the vast majority of buses. Engine compartments were found to have several properties challenging a fire suppression system and some observations are to follow further in this article. As a part of this investigation, fire tests in real bus engines have been performed in four different buses.

Geometry of bus engine compartments

The size of heavy bus and coach engine compartments is highly variable and the gross volume usually differs between 2-6m3. Engine compartments can have different shapes, and do not need to be rectangular. In profile, the floor is often slightly inclined and the ceiling may have a stepped shape. The test apparatus have a typical engine compartment size with a gross volume of 4m3.

Ignition sources

A fire in an engine compartment can start due to several reasons. It might, for example, be leaking fuel or material in contact with hot surfaces as well as electrical failures or overheated components. A fire can start at many locations, in particular: at the turbo charger or exhaust system, particle filter, alternators, starter motor, electrical cables and boxes, and ping tank etc. To simulate the many possible ignition sources in SP Method 4912, multiply fire sources are located in different areas of the test apparatus.

Obstructions

Engine compartments can sometimes be cluttered and potential fire sources can be hidden behind obstructing engine compo – nents, cylinders, boxes, tubes and balks. If the obstructions are hiding the fire source, it might be very difficult for the suppression agents to reach and suppress the fire.

The SP 4912 test apparatus is equipped with a dummy engine mockup and exhaust system. Since an engine compartment fire source can be concealed to different degrees, the test apparatus are equipped with obstructions concealing the fire sources at different degrees.

Air flow

Engine compartments are usually well venti – lated by the engine fan. However, depending on the bus construction and driving conditions, the ventilation rate in the engine compartment can range from very small to very high air flow. If the suppression system activates while the engine is off, the air movement will only come naturally through the apertures of the engine compartment, but if the engine is running with high rpm, the engine fan may create a very high air flow through the engine compartment. The rate of air flow will have a great impact on the suppression performance as the air flow may carry away the suppression agent and introduce more oxygen to the fire. To simulate these conditions in the test method, a fan is used with the same diameter and air flow as fans found on some coaches.

Flammable liquids and solids

Engine compartments include many flammable liquids and solids. Depending on the con – struction it may contain tens of liters of oil. Some of the oil is stored under pressure. A leakage of pressurised diesel or hydraulic fluid might result in a spray that wets hot engine components and, if ignition occurs, creates a quickly develop ing fire. Engine compartments may also include flammable solids such as plastic containers, rubber hoses and acoustic insulation made, for example, of foam or lump material. SP Method 4912 includes diesel pool and spray fires, fires involving oil leaks and deposition, as well as fires in solid materials.

Hot surfaces

The presence of hot surfaces in the engine compartment is an important fire challenge. The external surface temperature of turbo chargers and exhaust systems can be in excess of 600°C during certain conditions. This temperature is high enough to ignite the flammable liquids present in engine compart – ments. It also creates the risk of fuel re-ignition after extinguishment. In order to re-create the hot surface challenge in a repeatable and reproducible way, a hollow tube with a thermal mass similar to an engine compartment turbo charger is heated from the inside with a propane burner. The tube is designed to have the same temperature profile as a turbo charger on a modern Euro 5 bus engine.

During the re-ignition fire test according to SP Method 4912, the tube is heated to 615°C and after that the pre-heating burner is removed, simulating a hot engine which has been turned off and thus slowly starts cooling. When the tube has cooled to 600°C a small leakage of engine oil is applied. Due to the high surface temperature of the tube, the engine oil ignites immediately and the suppression system is activated after 10 seconds. While the suppression system has a short discharge time, the oil continues to drip on the heated tube. The suppression system can only prevent a re-ignition if it cools and keeps the surface at a temperature roughly below 400°C or maintains a required concentration of suppression agent for a time long enough to prevent re-ignition or coats the hot surface with a layer of suppressant. The suppression agent is able to cool the surface, but when the stored heat in the tube is distributed back to the surface, the temperature increases again which finally might cause a re-ignition.

Fire scenarios

As the size and character of an engine com – partment fire can greatly differ depending on the actual circumstances, the fire scenarios in SP Method 4912 are divided into different categories. In order to cover both small and large fires, the total heat release rate of the different fire scenarios is ranging from 5 kW to approximately 1 MW. Figure 5 shows a fire test performed during the pre-test period.

Certified suppression system

The main goal of the project is to develop a test method that can be included in an amendment of UNECE Regulation 107 and thus making it mandatory to install approved fire suppression systems in all new European buses. Legislative changes is a long process and fire suppression systems that have been tested and are meeting the tough requirements of SP Method 4912 and SPCR 183 can meanwhile be certified with the P-mark which is SP’s quality sign. Transport Authorities and insurance companies can now start citing P-marked systems in their technical requirements for ‘procurement of fire suppression systems.

 

References

1. Buses and coaches are referred to as ‘buses’ in this article

2. More about this process can be found in SP Report 2011:22 “Testing active fire protection systems for engine compartments in buses and coaches – a pilot study” (J. Brandt, M. Försth) or FIVE Proceedings 2012: “New International Test Method for Fire Suppression Systems in Bus and Coach Engine Compartment” (Brandt, Försth)

 

About the author

Jonas Brandt is a Project Manager at SP Technical Research Institute of Sweden. Having studied fire protection engin – eering and with a background as a bus driver, Jonas has a special interest in the field of bus fire safety.