Keywords

1 Introduction

Fires on shipboard may lead to a loss of capability, function, and life and, in extreme situations, the loss of the ship. Machinery spaces are high fire risk areas where flammable liquids are pumped and piped under pressure in the presence of ignition sources. Thus, installed fire protection systems are required in these spaces. As a very effective extinguishing agent, Halon 1301 has been widely used for protecting ship machinery spaces since the late 1970s [1]. But Halon 1301 has ozone-depleting effect, which could decompose into chlorine, fluorine, and carbon components that react with the ozone in the atmosphere to eliminate it. In 1987, the provision of the Montreal Protocol was adopted to ban the use of Halon 1301 [2]. Faced with the inability to utilize Halon 1301 for future designs, considerable efforts were expended on halon replacement. One excellent candidate of the halon alternatives is water mist. The abundance of water, its lack of toxicity, and its environmental acceptability offered attractive incentives to pursue a water mist system.

The initial studies of water mist for fire protection applications started from 1978 to 1980 by the US Navy [3, 4]. In this period, the research was focused on calculations that illustrated how small water droplets, as contained in “water fog” or “fine water spray,” could achieve fire extinguishment by gas phase cooling. Then some experiments were carried out to validate the theoretical calculations.

From 1990 to 1994, water mist research entered into a prosperous period; the US Naval Research Laboratory conducted a series of small-scale testing in a 3 × 3 × 2.4 m steel compartment [5]. Parameters such as fire size and location, nozzle spacing, mist application rates, mist characteristics, ventilation, degree of obstructions, corner effects, and oxygen depletion were analyzed.

Following small-scale tests, full-scale tests were conducted to develop fundamental water mist system design parameters for protecting ship machinery spaces. But most of these tests were conducted aboard the US Navy’s fire research vessel, the Ex-Shadwell [6–13]. The Shadwell tests were run in a two-level compartment having a gross volume of 960 m3. Thus, a question was posed: “if the shape of the test compartment changes and its volume is larger than 1000 m3, are the conclusions of the Shadwell tests still working?” Meanwhile, the performance of nozzles and the technique of piping works are developing. Recently, a new water mist system was designed, whose fire suppression performance needed to be tested.

For the reasons above, a test compartment having a volume of 1200 m3 was built, which simulated a real shipboard machinery space. Five full-scale tests were conducted to study the suppression performance of the newly designed water mist system in the large compartment.

In this article, the test compartment, the configuration of the newly designed water mist system, the test fire scenarios, and the measure instrumentation were introduced. Then, the impact of water mist injection on extinguishment time, compartment temperature, and concentrations of combustion products was studied. A series of full-scale test data was obtained, providing relating reference for such engineering applications.

2 Experimental

2.1 Experimental Compartment

The tests were conducted in a full-scale compartment which was built to simulate a typical machinery space of ships (Fig. 90.1). The space was roughly 21 × 12 × 6 m (1200 m3) and was bounded by metal bulkheads.

Fig. 90.1
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Machinery space mock-up: (a) elevation view, (b) plan view of the higher catwalk level, (c) plan view of the lower catwalk level

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This space had three levels, i.e., a bilge level and two levels of catwalks, as shown in Fig. 90.1. These catwalks allow easy access to critical areas in the space and, to some degree, serve as obstructions for the water mist systems. The catwalks located high in the space were constructed with metal grating, and the lower catwalks were constructed of steel plating. Typical machinery space equipments, such as engines, were simulated using sheet metal mock-ups as shown in Fig. 90.1. A 2 × 2 m door was added in the aft bulkhead on the lower catwalk level, and a similar door was added in the forward bulkhead on the lower catwalk level. Four 1.2 × 1.3 m vents and two 1.8 × 1.1 m vents were added on the roof.

2.2 Water Mist System

Three types of nozzles were used in this water mist system. All the nozzles were researched and developed by self, and their specifications are listed in Table 90.1.

Table 90.1 Specifications of the water mist nozzles
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In order to improve the suppression capabilities of the system, the piping network was divided into three layers. The upper layer consisting of 31 nozzles was installed under roof. The middle layer consisting of 23 nozzles was installed under catwalks of grating, i.e., higher catwalks. The lower layer consisting of 16 nozzles was installed under catwalks of plate, i.e., lower catwalks. The piping network with nozzle locations was shown in Fig. 90.2.

Fig. 90.2
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Piping network with nozzle locations: (a) higher catwalk level, (b) lower catwalk level, (c) bilge level

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The nozzle placement rules were followed. Under each platform of the compartment, a layer of nozzles should be installed. The spacing between nozzles should be less than the effective operating diameter of the nozzle. More nozzles should be considered in zones at high fire risk.

2.3 Fire Scenarios

To select fire scenarios reasonably, on the one hand, general standards, such as MSC/Circ.848, MSC/Circ.1165, and so on, were adopted. On the other hand, the possible fire scenarios of the simulated machinery compartment were analyzed.

Five types of fires were used in this study. The parameters of Fire #1–5 were listed in Table 90.2. Fire #1–4 were diesel fires to simulate Class B fires. Fire #1 was low-pressure horizontal spray with nominal fuel pressure 8 bar and fuel flow 0.16 ± 0.01 kg/s. Fire #2 was an unobstructed small tray fire. Fire #3 was an unobstructed large tray fire. Fire #4 was obstructed by an engine mock-up. Fire #5 was a wood crib fire to simulate Class A fire. The locations of Fire #1–5 were shown in Fig. 90.1. With the combination of Fire #1–5, five fire scenarios are concluded, as shown in Table 90.3.

Table 90.2 Parameters of fires
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Table 90.3 Fire scenarios
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2.4 Instrumentation

Three thermocouple trees were installed in the compartment. The thermocouple tree (TC Tree #1–3) layout was shown in Fig. 90.1. Each tree consisted of nine thermocouples. In order to measure the temperature at bilge, one thermocouple was set 0.5 m below the plates of the lower catwalks. The other eight thermocouples were poisoned at 0.5 m increments starting 0.5 m above the lower catwalk level. In order to determine the extinguishment time of each fire, thermocouples were located in the flame region of the fires. The thermocouples used in those tests were inconel-sheathed type-K thermocouples with 1.0 mm in diameter and could measure temperature up to 1200 °C.

In order to study the impact of water mist discharge on combustion products, gas analyzer was set up to measure the concentrations of carbon monoxide, carbon dioxide, and oxygen at three elevations in the compartment. The three measure points were poisoned at 1.0 m, 2.5 m, and 4.0 m above the lower catwalk level. The gas measure point (GM) layout was shown in Fig. 90.1.

2.5 Experimental Procedures

The data acquisition system was activated prior to the test. Then the test fires were ignited leaving the compartment vents open. Fires were allowed a certain preburn period before the water mist is discharged. A 120-s preburn time is required for the tray fire and 5–15 s for the spray fire. For the wood crib fire, the preburn time is 30 s for Fire Scenarios #4 and 360 s for Fire Scenarios #5. After the preburn, the compartment was then sealed, and the mist system was activated. The mist system remained activated for a period of 15 min during each test or until all of the fires had been extinguished, whichever came first. At the completion of the discharge, the mist system was secured marking the end of the test. The space remained off-limits until cleared by the safety officer and the test director.

3 Results and Discussions

3.1 Flame Temperature and Fire Extinguishment Time

Extinguishment time was an important parameter to evaluate the suppression performance of a water mist system. In this article, the extinguishment time was defined as the time between the injection of the water mist and the instant of fire extinguishment. The extinguishment of fires was determined based on temperature measurements recorded in the flame during each test. The flame temperature of each fire for each fire scenario was shown in Fig. 90.3. Extinguishment time for each fire scenario was shown in Fig. 90.4.

Fig. 90.3
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Time history of flame temperature: (a) Fire Scenario #1, (b) Fire Scenario #2, (c) Fire Scenario #3, (d) Fire Scenario #4, (e) Fire Scenario #5

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Fig. 90.4
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Extinguishment time comparison for all the fire scenarios

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  • For Fire Scenario #1, the flame temperature of the spray fire was relatively low, about 160 °C before the system activation. After water mist injection, the temperature decreased while oscillating. This indicated that the water mist formed a relatively sealed space around the spray fire and the flame burnt as in a poor ventilation enclosure. Thus, the spray fire turned to be a small fire which was more difficult to extinguish than larger fires. So the extinguishment time for this scenario was relatively longer, about 115 s.

  • For Fire Scenario #2, the flame temperature of the small tray fire was about 900 °C before the system activation and dramatically decreased in a short time after water mist injection. The extinguishment time for this scenario was about 10 s.

  • For Fire Scenario #3, the 2.0 m2 tray fire was unobstructed, and the 4.0 m2 tray fire was obstructed by an engine mock-up. The extinguishment times of the two fires were both very short, 17 s for the 2.0 m2 tray fire and 22 s for the 4.0 m2 tray fire.

  • For Fire Scenario #4, the flame temperature of the wood crib fire was about 900 °C before the system activation. After water mist injection, the temperature decreased relatively slowly. The extinguishment time was about 241 s. Why Class A fires were so difficult to extinguish? Perhaps because the water mist could only put out the outer surface flame of the wood crib, and it was still high temperature in its inner part.

  • For Fire Scenario #5, fire resources included spray fire, unobstructed tray fire, obstructed tray fire, and wood crib fire. After water mist injection, all the fires extinguished in a short time, approximately 30 s. This indicated that although fires like Fire #1 and Fire #5 were difficult to extinguish alone, in the presence of larger fires, they were much easier to extinguish.

3.2 Compartment Temperature

Water mist could remove heat from a system because of its high specific heat and latent heat of vaporization. Thus, water mist systems could be used to thermally manage the conditions in machinery spaces.

In order to analyze the temperature changes in the test compartment, Fire Scenario #1 was taken, for example. The time history of compartment temperature at different heights for Fire Scenario #1 was shown in Fig. 90.5.

Fig. 90.5
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Time history of compartment temperature at different heights (Fire Scenario #1): (a) thermocouple tree #1, (b) thermocouple tree #2, (c) thermocouple tree #3

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With regard to one thermocouple tree, before mist discharge, temperatures of higher thermocouples were higher than those of lower thermocouples. It was in accordance with the fact that the upper room was filled with hot smoke and the lower room filled with cool air.

After water mist activation, the temperatures of higher thermocouples decreased, and those of lower thermocouples increased. This illustrated that the upper smoke layer and down cool air were mixed because of water mist injection.

After mist injection ending, the temperature gradient at different levels reappeared. Perhaps this phenomenon was caused by the air mixing effects of water mist injection.

As shown in Fig. 90.5, for one thermocouple tree, the temperatures at different heights exhibited almost the same trend. Thus, average temperature was employed here. The time history of average compartment temperature of different thermocouple trees for all the fire scenarios was shown in Fig. 90.6. For all the fire scenarios, the compartment temperature reduced below 70 °C. According to US naval ships’ technical manual, human would be incapable in 5 min and died in 30 min, and electronics would be permanent damaged when temperature was above 150 °C. Thus, the compartment temperature in our tests would help manual intervention, minimize thermal damage, and prevent fire spread from the compartment of origin.

Fig. 90.6
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Time history of average compartment temperature of different thermocouple trees: (a) Fire Scenario #1, (b) Fire Scenario #2, (c) Fire Scenario #3, (d) Fire Scenario #4, (e) Fire Scenario #5

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3.3 Gas Concentration

The results of gas concentration measurement indicated that, for all cases, very slight variations were found in the O2 and CO2 concentrations, so only the changes in CO concentrations were analyzed in the following. The concentrations of CO at three measure points were shown in Fig. 90.7. The changes at three measure points exhibited almost the same trend for all the scenarios. The amounts of CO produced during the preburn remained fairly constant at low value, about 30 ppm, because the combustion was oxygen enriched in this period. During the extinguishment process, the amounts of CO were observed to dramatically increase in a certain time and then decreased. This indicated that water mist injection really brought incomplete combustion at the beginning, but it could then dilute CO with the continuing discharge. Compared to all the scenarios, the CO concentration was larger when fire HRR was higher.

Fig. 90.7
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Time history of CO concentrations at different heights: (a) Fire Scenario #1, (b) Fire Scenario #2, (c) Fire Scenario #3, (d) Fire Scenario #4, (e) Fire Scenario #5.

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While the extinguishment of these fires produced significantly higher CO concentrations, the amount of CO for each scenario was below 4000 ppm. According to US naval ships’ technical manual, human would be incapable in 5 min and died when CO concentration was above 4000 ppm. Thus, the CO concentration in our tests was not life-threatening for short-term exposures. It would provide favorable conditions for personnel who remained staying in the space.

4 Conclusions

Water mist fire suppression systems are being seriously considered to replace Halon 1301 total flooding systems in machinery space applications. In this article, a full-scale experimental facility was built to study the fire suppression performance of a newly designed water mist system for large shipboard machinery spaces. A series of full-scale tests was carried out with different fire scenarios. The tests contributed valuable experimental data for water mist systems installed in large machinery spaces. Conclusions can be drawn as follows:

  1. 1.

    These tests have demonstrated the ability of the newly designed water mist system to extinguish both obstructed and unobstructed Class B tray fires, Class B spray fires, and Class A fires in large machinery spaces.

  2. 2.

    The system required minutes to extinguish fires as opposed to seconds for the gaseous halon alternatives.

  3. 3.

    The parameters of the water mist system nozzles and piping networks, the extinguishment experimental data, and the results provided in this article may serve as an available reference for such engineering applications.

  4. 4.

    The water mist system can be used to thermally manage the conditions in the machinery spaces. For all of the fire scenarios, the system dramatically reduced the temperature of the space almost immediately after activation, and the space became well mixed with a uniform temperature below 70 °C. This reduction in temperature will minimize the thermal damage to the compartment and its infrastructure and provide favorable conditions for personnel evacuation and firefighting.

  5. 5.

    For all of the fire scenarios, CO concentrations were below 4000 ppm. It would provide favorable conditions for personnel who remained staying in the space.

In conclusion, this newly designed water mist system has an excellent ability to extinguish fires in the secured large compartment. However, ventilation was not considered in those tests. Thus, in the future, the ventilation effect on the fire suppression capabilities of the water mist system will be studied in the full-scale test compartment.