Investigation of Air Foam and Heptafluoropropane Foam Fire Extinguishment for Storage Tanks Containing Low-Boiling-Point Flammable Liquids

Abstract

Due to the high saturated vapor pressure of low-boiling-point flammable liquids, it is difficult to make fire extinguishers for storage tanks containing them. Air foam extinguishing technology has been recommended by several standards. However, the effectiveness of air foam against low-boiling-point flammable liquid is still limited due to a lack of experimental data. To validate the reliability of air foam, fire-extinguishing measures for three low-boiling-point flammable liquids including propylene oxide, n-pentane, and condensate oil were carried out for the first time in this work. The results show that air foam fails the fire extinguishment of the studied liquids even at higher supply intensities. To address the challenge of fire extinguishment in storage tanks containing low-boiling-point flammable liquids, a novel method using heptafluoropropane (HFC227ea) phase change foaming to substitute air was proposed in this work. The experimental system of HFC227ea foam fire extinguishment was constructed. In addition, two low-boiling-point flammable liquids propylene oxide and n-pentane were selected as the research subjects, the fire extinguishment measures were conducted. The results show that the proposed method can realize rapid and effective extinguishment of flames for the studied liquids.

1. Introduction

In the petrochemical industry, pure flammable liquids with boiling points below 45 °C or hydrocarbon mixtures with a molar fraction of Cn (n ≤ 5) exceeding 30% are classified as low-boiling-point flammable liquids (LBPF liquids) [1]. Due to the high saturated vapor pressure of the liquids, it is difficult to make fire extinguish when the storage tank containing such liquids is burned. In 2003, a full-surface fire incident occurred in a light naphtha storage tank (42.7 m diameter) at a Hokkaido refinery in Japan. The tank contained 26,000 m³ of light naphtha with a 31.5–78.5 °C boiling range. Despite full-scale firefighting efforts, the blaze remained uncontained and ultimately self-extinguished after 44 h when the fuel was completely consumed [2]. Typically, air foam fire extinguishment system is installed in such kind of tanks [3,4,5]. However, the effectiveness of air foam against LBPF liquid fires is still limited due to a lack of experimental fire suppression data. Furthermore, the existing standards do not provide clear specifications for fire suppression parameters in foam systems for such tanks. For example, the NFPA 11: 2024 “Standard for low-, medium-, and high-expansion foam” [6] outlines that “flammable liquids with boiling point below 37.8 °C require higher supply rates, the specific parameters should be determined through testing”. Similarly, European Standard EN 13565(2): 2018 “Fixed firefighting systems. Foam systems-design, construction and maintenance” [7] and China’s standard GB 50151: 2010 “Code of design for foam extinguishing systems” [8] also state that flammable liquids with boiling points below 40 °C should have higher supply intensities, and the specifics should be determined experimentally.

In order to validate the reliability of the air foam fire extinguishment system and to obtain detailed fire suppression parameters for storage tanks containing LBPF liquids, the fire extinguishment measures for three LBPF liquids including propylene oxide, n-pentane, and condensate oil were carried out for the first time. The results show that it is difficult to extinguish the fires completely. Due to this reason, a new foam extinguishing technology was proposed based on heptafluoropropane (HFC227ea) phase change foaming in this work, and the experimental system for fire extinguishment was developed. A series of fire extinguishment experiments were also conducted for storage tanks containing propylene oxide and n-pentane liquids, the comparison with air foam technology was carried out.

2. Air Foam Fire Extinguishment Experiments

In this work, three representative LBPF liquids including propylene oxide, n-pentane, and condensate oil were selected. Propylene oxide is a water-soluble liquid with a boiling point of 34.24 °C, saturated vapor pressure of 75.86 kPa (at 25 °C), flash point of −37 °C, density of 0.84 g/cm³, and an explosion limit of 2.80–37.00%. At 20 °C, the solubility of propylene oxide in water is 40.5 g, while the solubility of water in propylene oxide is 12.8 g [9]. For n-pentane, the boiling point is 36.10 °C and the saturated vapor pressure is 53.32 kPa (at 18.5 °C) [10]. The components of the condensate oil are hydrocarbons and detailed information is listed in

Table 1. Components of condensate oil.

Components	Mole Fraction (%)	Components	Mole Fraction (%)
C3	0.03	C6	27.64
iC4	0.08	C7	26.43
C4	6.38	C8	12.22
iC5	8.97	C9	3.21
C5	14.02	C10	1.03

. The main components are C5~C8 with a mole fraction of 89.28%. The foam concentrates used include 3% aqueous film-forming foam concentrate (3% AFFF), 6% aqueous film-forming foam concentrate (6% AFFF), 3% fluoroprotein foam concentrate (3% FP), 6% film-forming fluoroprotein foam concentrate (6% FFFP), 3% alcohol-resistant aqueous film-forming foam concentrate (3% AFFF/AR), 6% alcohol-resistant aqueous film-forming foam concentrate (6% AFFF/AR), and 6% alcohol-resistant synthetic foam concentrate (SLPK-HB, 6% S/AR). All of them were purchased from Jiangsu Suolong Fire Science and Technology Co., Ltd. (Taizhou, China).

2.1. Fire Extinguishing Results for Propylene Oxide

The fire-extinguishing measures for propylene oxide using the air foam fire suppression were carried out in an oil pan with a diameter of 1.48 m and a storage tank with a diameter of 3.5 m [8] (as indicated in Figure 1a), respectively. Three foam concentrates including 6% S/AR, 6% AFFF/AR, and 3% AFFF/AR were used. Air was used to generate foam with the foam concentrates. Detailed equipment and facilities have been described in our previous work [11]. All the fire suppression operations were initiated 1 min after the ignition of the low-boiling-point liquids.

With respect to the fire-extinguishing measures in the oil pan, the depth of the oil pan was 0.2 m, and the rated flow of the foam-making nozzle was 11.4 L/min according to the requirement of the standard ISO 7203-3: 2019 [12]. Moreover, the volume of the test liquid injected into the oil pan was 120 L. The measures were carried out four times (ambient temperature: ~35 °C and foam solution temperature: ~32 °C). Figure 1b–d shows the photo of the fire extinguishment measures. The detailed results are listed in

Table 2. Experimental results of air foam extinguishment for propylene oxide in oil pans.

Type of the Foam Concentrate	Expansion	25% Drainage Time (min)	Wall Cooling	Foam Solution Supply Rate (L/min·m2)	Results
6% S/AR	~7.5	~8.8	with water jacket	6.6	Fire controlled at 50 s, extinguished at 185 s
			without water jacket	6.6	Fire controlled at 60 s, fire in the edge of the oil pan still existed at 300 s
6% AFFF/AR	~6.9	~9.5	with water jacket	6.6	Fire controlled at 150 s, flame existed above the foam coating at 300 s
3% AFFF/AR	~7.4	~7.5	with water jacket	6.6	Fire controlled at 240 s, flame existed above the foam coating at 300 s

.

For the fire-extinguishing measures in the storage tank, 6% S/AR was used. Considering the possibility of a partial opening of the tank during the fire, a semi-open tank was used in this work. Additionally, to prevent accidents and liquid leakage, a protective water pool was built around the tank. Cooling water with a rate of 2.5 L/min·m² was applied to the tank walls during the measurement [13].

Before the test, the propylene oxide with 2000 L was added to the test tank. After ignition, the foam generator combined with air was activated to supply foam to the tank, and fire control and extinguishing times were recorded (ambient temperature: ~25 °C, foam solution temperature: ~24 °C, expansion: ~7.6, and 25% drainage time: 9.5 min). For the 6% S/AR, when the foam solution supply rate was 11.6 L/min·m², the flames inside the tank could not be controlled within 180 s. Then, the foam solution supply rate was increased to 22.3 L/min·m², the fire still could not be controlled. This is mainly due to propylene oxide vapor penetrating the foam layer, leading to sustained flames in multiple areas on the foam surface. Figure 2 shows test photos of air foam extinguishment.

2.2. Fire Extinguishing Results for n-Pentane

Based on the requirement of ISO 7203-1: 2019 [14], the diameter of the experimental storage tank was 2.4 m, and the depth of the tank was 0.8 m. The cooling water ring pipe was installed around the wall of the tank. Cooling water with a rate of 2.5 L/min·m² was applied to the tank walls during the measurement. Two foam-making nozzles were used to supply foam, and the flow rate of the foam-making nozzle was 11.4 L/min. The 3% FP and 6% AFFF concentrates were used for the present measurement. The volume of the test liquid injected in the storage tank was 149 L. Figure 3 shows the test tank and liquid storage tank. The measurement results are given in

Table 3. Experimental results on air foam extinguishment for n-pentane in storage tank.

Type of the Foam Concentrate	Expansion	25% Drainage Time (min)	Cooling Water Supply Rate (L/min·m²)	Foam Solution Supply Rate (L/min·m²)	Results
3% FP	~5.4	~4.8	2.5	5.04	flame existed above the foam coating at 15 min
6% AFFF	~7.4	~3.3	2.5	5.04	flame existed above the foam coating at 24 min

(ambient temperature: ~27 °C and foam solution temperature: ~24 °C). It is obvious that both the 6% AFFF and 3% FP concentrates cannot extinguish the fire.

2.3. Fire Extinguishing Results for Condensate Oil

For the fire extinguishing of the condensate oil, the diameter of the tank was 3.5 m and the height was 1.5 m. The cooling water was supplied in the wall of the tank, and the supplied rate of the cooling water was 2.5 L/min·m². Two foam generators were installed in the tank with a rate of flow of 2 L/s. The 6% FFFP and 6% AFFF concentrates were used for the present measurement. The volume of the test liquid injected into the storage tank was 2000 L. The results are listed in

Table 4. Experimental results on air foam extinguishment for condensate oil in storage tank.

Type of the Foam Concentrate	Expansion	25% Drainage Time (min)	Cooling Water Supply Rate (L/min·m²)	Foam Solution Supply Rate (L/min·m²)	Results
6% FFFP	~5.3	~3.6	2.5	12.0/24.0	The fire was almost controlled at 2 min using one foam generator, and the fire still existed at the edge of the tank. Then, the second foam generator was used, the fire in the edge of the tank still existed until the foam filled the tank.
6% AFFF	~6.3	~2.6	2.5	12.0/24.0	The results are similar to the above measures.

and the photos of the measurement are shown in Figure 4 (ambient temperature: ~−15 °C and foam solution temperature: ~12 °C). It can be observed that the fire can be controlled within 2 min when the foam solution supply rate is 12 L/min·m², but the fire cannot be extinguished completely even if the foam supply rate reaches 24 L/min·m².

According to the above fire-extinguishing measures for propylene oxide, n-pentane, and condensate oil with different agents, it can be concluded that the air foam is not effective for extinguishing fires. Complete fire extinguishment cannot be achieved even with higher supply intensities and longer foam times. The main reason for the poor performance of air foam is that LBPF liquids have relatively high vapor pressure which allows them to penetrate through the foam cover. Additionally, air foam has a low density and poor compactness, and it is difficult to completely block the transport of flammable vapor to the combustion zone, resulting in ineffective extinguishment. Hence, it is necessary to find a suitable blowing agent to substitute air to improve the isolation capacity of foam and further improve the fire-extinguishing capacity.

3. Development of HFC227ea Foam Extinguishing Technology and the Fire Extinguishing Experiments

3.1. HFC227ea Foam Extinguishing System

In general, any gas can replace air to generate foam with foam concentrate. However, from an engineering perspective, the choice of air substitutes should be based on the existing low-expansion foam fire suppression system, avoiding significant system changes. In addition, the substitutes should allow rapid foam formation and remain in a single liquid-phase flow within the pipelines of the system. This requirement dictates that the vapor pressure of the potential substitutes should be higher than atmospheric pressure. Otherwise, if the vapor pressure is too low, the vaporization rate will be slow when the mixture enters the foam generator, leading to poor foam performance.

Generally speaking, commonly used gaseous fire suppressants include nitrogen (N₂), carbon dioxide (CO₂), trifluoromethane (HFC-23), hexafluoropropane (HFC236fa), and heptafluoropropane (HFC227ea). The main physical properties of these substances are listed in

Table 5. Physical properties of common gaseous fire suppressants [10].

Category	Boiling Point (°C)	Density (kg/m3)	pC (MPa)	TC (°C)	Vapor Pressure (20 °C) (MPa)
N2	−195.6	810 (−196 °C)	3.40	−147.0	1.03 (−173 °C)
CO2	−78.5	1101 (−37 °C)	7.29	31.2	5.73
HFC-23	−82.0	806	4.84	25.9	4.27
HFC236fa	−1.4	1337	3.20	124.9	0.23
HFC227ea	−16.4	1407	2.91	101.7	0.40

.

For a low-expansion foam fire suppression system, the pressure at the foam generator is typically around 0.6 MPa, and the actual working pressure of the foam mixture in the pipeline exceeds 0.7 MPa. As shown in Table 5, the vapor pressures of nitrogen, carbon dioxide, and trifluoromethane are higher than the working pressure of the air foam fire suppression system, indicating that the three substances are unsuitable to substitute air. In contrast, HFC236fa and HFC227ea are two potential substitutes. With respect to HFC227ea, the vapor pressure is 0.4 MPa at 20 °C which satisfies the requirement of the foam generator, and it can be considered as an ideal replacement for air foaming. Additionally, the properties of HFC227ea are also helpful in fire suppression. Therefore, HFC227ea was selected as an air substitute for foam generation in this work.

The HFC227ea foam extinguishing system was developed, and it mainly consisted of a water supply system, foam liquid storage tank, foam proportioning device, HFC227ea storage unit, mixing device, foam generator, valves, and pipelines, as shown in Figure 5. Upon system activation, water flowed into the foam proportioning device, where it mixed with foam concentrates to form a foam mixture. The mixture entered into the mixing device to create an HFC227ea foam solution. Then, the mixture flowed through the foam generator to produce HFC227ea foam at a certain expansion ratio, which was applied to the flammable liquid surface for fire suppression. After passing through the foam generator, the external pressure dropped instantaneously to atmospheric pressure, and the phase of HFC227ea changed from liquid to gas, and the foam could be generated.

It must be emphasized that the HFC227ea foam suppression technology proposed in this study fundamentally differs from HFC227ea gas suppression technology in terms of fire suppression mechanisms and application scenarios. The former essentially belongs to foam fire-extinguishing systems, primarily used for extinguishing flammable liquid storage tank fires. The latter is a gas fire-extinguishing system typically employed for electrical fires. The HFC227ea foam suppression technology utilizes the phase-change characteristics of HFC227ea to generate firefighting foam through self-expansion. When released onto burning liquid surfaces, it forms a foam blanket that (1) blocks contact between flammable vapors and air; (2) inhibits liquid evaporation; and (3) resists thermal radiation from flames to fuel mechanisms identical to traditional air foam extinguishing systems. What makes it unique is that the phase-change self-expansion process enables HFC227ea foam to achieve a more uniform bubble distribution compared to air foam. Combined with HFC227ea’s higher density than air (HFC227ea density ~6 × air), this results in superior coverage performance. Furthermore, when the foam breaks down, the released HFC227ea gas can additionally provide chemical fire suppression. This dual mechanism integrating foam blanketing and chemical inhibition makes it particularly suitable for protecting low-boiling-point flammable liquids. Notably, HFC227ea’s saturation vapor pressure remains below the operating pressure of foam systems throughout pipe transportation. This ensures single-phase liquid flow when mixed with foam solution (unlike compressed air or nitrogen-based foam systems), significantly simplifying hydraulic calculations for system design.

3.2. HFC227ea Foam Extinguishing Results

To evaluate the performance of the proposed HFC227ea foam extinguishing technology, two representative LBPF liquids (propylene oxide and n-pentane) were selected in this work. Because propylene oxide is a water-soluble liquid and n-pentane is a non-water-soluble liquid, 6% S/AR and 3% AFFF were used in propylene oxide and n-pentane, respectively.

The diameter of the studied storage tank was 3.5 m and the depth was 1.0 m. The tank was semi-open and a protective water pool with a diameter of 6 m was built around the tank. Cooling water with a rate of 2.5 L/min·m² was applied to the tank wall during the measurement. The schematic diagram of the test system is shown in Figure 6, while Figure 7 illustrates the test tank and liquid storage tank.

Before each test, the propylene oxide or n-pentane with 2000 L was added to the test tank. After ignition, the foam generator was activated to supply foam to the tank, and fire control and extinguishing times were recorded. The extinguishing performance of HFC227ea foam for propylene oxide or n-pentane is shown in

Table 6. HFC227ea foam extinguishing test results.

Test Liquid	Type of the Foam Concentrate	Expansion	25% Drainage Time (min)	Foam Solution Supply Rate (L/min·m2)	HFC227ea Foam Mixing Ratio	Results
Propylene oxide	6% S/AR	~8.2	~13.2	11.7	5.20%	Fire controlled at 60 s, extinguished at 110 s
		~11.5	~14.6	11.8	8.23%	Fire controlled at 56 s, extinguished at 134 s
n-pentane	3% AFFF	~10.3	~6.9	6.7	6.04%	Fire controlled at 55 s, extinguished at 390 s
		~8.5	~5.8	13.1	5.71%	Fire controlled at 30 s, extinguished at 120 s

, and the photos of the test results are given in Figure 8 and Figure 9, respectively. For propylene oxide, when the foam solution supply rate was 11.7 L/min·m² and the HFC227ea foam mixing ratio was 5.20%, the flames inside the tank were controlled within 60 s, and the fire was extinguished at 110 s (ambient temperature: ~28 °C and foam solution temperature: ~25 °C). For n-pentane, when the supply rate of HFC227ea foam was 13.1 L/(min·m²), fire control was achieved within 30 s, and fire extinguishment occurred after 120 s of foam supply, as shown in Figure 9 (ambient temperature: ~20 °C and foam solution temperature: ~18 °C).

Compared with the extinguishing results of air foam and HFC227ea foam for propylene oxide and n-pentane, it is obvious that air foam is not effective for extinguishing fires. The fire-extinguishing efficiency of the HFC227ea foam is significantly higher than that of air foam. It can achieve fire control and extinguishment in a shorter time with a smaller supply intensity.

3.3. Limitations of HFC227ea Foam Extinguishing Technology

HFC227ea belongs to the category of hydrofluorocarbons (HFCs). According to the Kigali Amendment adopted at the 28th Meeting of the Parties to the Montreal Protocol, the production and consumption of HFCs have been subjected to freezing and phasedown measures, with varying control schemes across different countries or regions. For China specifically, HFC production and consumption have been frozen below baseline levels in 2024, followed by annual reductions—10% reduction by 2029, ultimately reaching an 80% reduction by 2045. Notably, these regulations primarily target HFCs that are predominantly used as refrigerants, with HFC227ea constituting only a minor fraction of this group. In scenarios where no viable alternative fire suppression technologies exist, HFC227ea foam extinguishing technology appears acceptable, at least as an interim solution. A critical environmental trade-off emerges: whether uncontrolled low-boiling-point flammable liquid tank fires pose greater environmental damage than the environmental impact of HFC227ea foam deployment. From a long-term perspective, it is imperative to explore or develop environmentally benign alternatives to HFC227ea that maintain comparable physical properties for foam generation. Such substitutes would need to replicate HFC227ea’s phase-change characteristics and foam-forming capabilities while eliminating ozone depletion potential and minimizing global warming potential.

4. Conclusions and Future Perspectives

In this work, a foam fire-extinguishment for storage tanks containing low-boiling-point flammable liquids including propylene oxide, n-pentane, and condensate oil was investigated, and the air foam and the proposed HFC227ea foam extinguishing technology was used in this work. The results of this work include the following:

(1)

The air foam method was used to evaluate the fire-extinguishment performance of storage tanks with LBPF liquids in detail for the first time, and the results show that it is not effective for extinguishing fires.

(2)

The HFC227ea foam extinguishing technology was proposed in this work, and the corresponding experimental system was established. The fire-extinguishing performance of the system was validated. The results show that the fire-extinguishing efficiency is significantly superior to that of air foam, and the fire can be controlled and extinguished in a shorter time with a smaller supply intensity. HFC227ea foam extinguishing technology is very promising for the fire extinguishment of storage tanks containing low-boiling-point flammable liquids.

The application and refinement of HFC227ea foam extinguishing technology warrant further investigation. In terms of fundamental research, investigations into the self-foaming mechanism of HFC227ea foam, structural aging processes (drainage and coarsening), and rheological properties (flow characteristics) are imperative. Regarding engineering applications, the system design of the HFC227ea foam apparatus and the development of controlled-release delivery equipment warrant focused attention. From a long-term perspective, identifying or developing environmentally benign alternatives with comparable physical properties to HFC227ea for foaming applications is critically essential.