Flammability Combustion/Decomposition of Refrigerants

Flammable refrigerants present an immediate danger when released into the air. The refrigerant can combine with air at atmospheric pressure and ignite, causing a flame and possibly an explosion to occur. Because of the obvious hazards, the use of flammable refrigerants is restricted to controlled environments that have monitors, proper ventilation,
explosion-proof equipment and generally few people near the equipment (refineries,
storage warehouses, breweries, etc.).

Some refrigerants can burn with oxygen, but only at higher pressures or temperatures and never in air at atmospheric conditions. These are called ‘combustible’ refrigerants. Underwriter’s Laboratories (UL) lists these refrigerants as ‘practically non-flammable.’

R-22 and R-134a fall into this category. R-22 was found to cause a combustion hazard during a pressurised leak test with air. For this reason, most refrigerants should be used only with pressurised nitrogen for leak testing. As long as refrigerant is not mixed with large amounts of air, there should be little hazard from these refrigerants during normal handling and use.

Figure 1: Minimum ignition energy and lower flame limit for selected refrigerants

Decomposition can occur with any refrigerant when it gets hot enough (generally above 7000° F). Refrigerant can decompose in systems or containers exposed to fire or other extreme heat, electrical shorts (burnouts), or in refrigerant lines being soldered or brazed without being cleared first. Obviously, refrigerant containers or charged systems
should never intentionally be exposed to a flame or torch.

When a refrigerant is decomposed or burned, the primary products formed are acids: Hydrochloric acid (HCl), if the refrigerant contains chlorine, and hydrofluoric acid (HF) and fluorine. These products are certainly formed when hydrogen is present such as from the
breakdown of oil, water or if the refrigerant has hydrogen attached (like R-22 or R-134a). If oxygen also is present (from air or water), then it is possible to form carbon monoxide, carbon dioxide and various unsaturated carbonyl compounds — the most notorious of which is phosgene.

Being extremely toxic in small amounts, phosgene formation was a real concern when traditional refrigerants (R11, R- 12, R- 113, R- 114) decomposed. Phosgene contains two chlorine atoms and an oxygen atom. It will only form when oxygen is present and only the refrigerants with chlorine attached will produce phosgene (not HFCs). R22 has only one chlorine atom per molecule, so, it is extremely difficult, chemically speaking, to get another one attached to form phosgene. Decomposition of R-22 or HFCs may form other carbonyl fluorides, however, they are not as toxic as phosgene.

The standard practice for handling decomposed refrigerant is to collect the gas, treat the refrigerant and/or the system for acid contamination, and appropriately dispose of the burnt gas. Please note that any cylinder or system component exposed to high heat or fire should be retested or discarded. Cylinders used to recover burnt gas should be checked and cleaned before being put back into service, especially the valve and/or pressure relief
device.

Flammability Characteristics of Refrigerants

Flammability is a property of a mixture in which a flame is capable of selfpropagating for a certain distance. Generally, flammability of a refrigerant is its ability to burn or ignite, causing fire or combustion. The degree of difficulty required to cause the combustion of a substance is quantified through fire testing and dependent on a number of parameters discussed below:

When the substance is flammable depends upon the upper and lower flammability limits and the supplied energy for ignition. The consequences of the flammability event depend on the burning velocity, heat released and by-products of combustion.

Mixtures of refrigerant and air will burn only if the fuel concentration lies within well-defined lower and upper bounds determined experimentally referred to as flammability limits.

Figure 2: Heat of combustion and burning velocity for selected refrigerants

Lower Flammability Limit (LFL, percent by volume or g/m3): minimum concentration of the refrigerant that is capable of propagating a flame through a homogeneous mixture of the refrigerant and air under the specified test conditions at 23.0°C and 101.3 k Pa. At a concentration in air lower than the LFL, gas mixtures are too weak to burn. Methane gas has a LFL of 4.4 per cent. If the atmosphere has less than 4.4 per cent methane, combustion cannot occur even if a source of ignition is present.
Upper Flammability Limit (UFL) is the highest concentration of a gas or a vapour in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). Concentrations higher than UFL are ‘too rich’ to burn.
Burning velocity is velocity relative to the unburnt gas (normally in cm/s), at which a laminar flame propagates in a direction normal to the flame front at the concentration of refrigerant with air giving the maximum velocity.
Heat of combustion is the heat evolved from a specified reaction of a substance with oxygen.

Standards & Classification

Standards provide limitations and recommended practices on how to properly handle different refrigerants, including flammable ones.

ASHRAE standard 34 “Designation and Safety Classification of Refrigerants” classifies refrigerants based on their flammability and toxicity characteristics. It recognises several flammability classes from non-flammable A1 to highly flammable A3, depending on refrigerant’s LFL value, heat of combustion and maximum burning velocity.

Similarly, to ASHRAE standard 34, standard ISO 817 ‘Refrigerants — Designation and Safety Classification’ provides an unambiguous system for assigning designations to refrigerants and its flammability on international levels.

Figure 3: Flammability classes of flammable refrigerants according to ASHRAE standard 34

European Standard EN 378 ‘Safety and Environmental Requirements for Refrigeration Systems and Heat Pumps’ aims to reduce the number of hazards to persons, property and the environment caused by refrigerating systems and refrigerants. It, therefore, regulates the usage of flammable refrigerants in systems depending on system location, occupancy
level, system type and refrigerant used. Current edition of EN378 standard was published in 2008 and do not directly recognises the A2L flammability refrigerants. It can be expected that the standard will be accordingly updated in its new edition that is currently under development.

In other countries the safety on refrigeration system level is regulated by ASHRAE Standard 15 ‘Safety Code for Mechanical Refrigeration” (US) and ISO 5149 “Refrigerating systems and heat pumps – Safety and environmental requirements” (internationally). On the equipment level safety is regulated by, for instance, European standards EN 60335- 2-34 and EN 60335-2-40.

Lower Flammability Refrigerants

To be deemed mildly flammable, a substance must burn at a velocity no greater than 10 cm/s. By comparison, Usain Bolt’s world record 100-metre time equates to 1043 cm/s, while hydrocarbons burn many times faster.

The need for more precise flammability index was proposed in ISO 817 revision working group (WG) in 1999. This proposal was to extend the relaxed antiexplosion requirements for ammonia, which was already well-known as difficult to ignite substance, to all similar or lower flammability refrigerants. The WG concluded to employ burning velocity as an additional category in 2002 with the upper boundary of 10 cm/s. This category was named 2L to distinguish from conventional flammable class 2. ASHRAE34 adopted this concept in 2010, while ISO 817 finally adopted in 2014.

Figure 4: R134a A/C system (VW Lupo)

Figure 5: Test adaptations to R134a A/C system (VW Lupo)

In order to ensure safe use of refrigerants with this flammability class and to open up a path for lower GWP refrigerants in the class, experts on the issue have conducted research and development for more than 10 years. Many risk assessments were conducted or being conducted. They indicated that 2L refrigerants’ flammability is acceptable for air conditioners and heat pumps when these systems comply with standards for equipment safety such as EN 378.

It can be seen that flame of mildly flammable refrigerants R-32 and ammonia do not propagate horizontally due to their low burning velocities. Additionally, the range of impact of the combustion of 2L refrigerants is limited due to their low heat of combustion (that is specifically visible for refrigerant R-32).

Flammability Investigation of Different Refrigerants

A flammability investigation of three different refrigerants was carried out. In a first step the actual safety level of R134a was investigated. The results were put in relation to the two possible alternatives R744 and 2,3,3,3 –Tetra fluoro propene (also called HFO-1234yf).

Test Setup

A VW Lupo system was used to do the testing. The system was charged with refrigerant (500gr for R134a and HFO1234yf) and operated under real operating conditions (Pd ~15bar). As lubrication medium PAG oil ND8 (135ml for all tests) was used. A possible leak of the refrigerant caused by a front-end collision was simulated by a release of refrigerant through a manually operated valve onto a hot surface. Such a front-end collision can rupture a refrigerant line and release the refrigerant-oil mixture into the engine compartment. The released refrigerant-oil mixture was directed to a hot surface simulating a turbo charger or hot exhaust manifold. The surface temperature was measured and the temperature was adjusted through a controller of the electrical powered heat source.

Base Line Test

The base line test with R134a (ignition temperature >743°C) and ND8 (flash point 204°C) was carried out to define the existing safety level in the vehicle. A surface temperature of 970°C was chosen. Under this condition, it was possible to show that the R134a prevents the mist of oil and refrigerant vapor from ignition. During this test no flame propagation was observed. It could be concluded that the use of R134a and ND8 oil is safe up to surface temperatures of 970°C.

Second Test

In a second test, R134a was replaced by R744 (charge of 50gr and additional R744 container of 20kg was connected to the system) and the test was repeated at the same surface temperature of 970°C. Since the oil circulation rate in a R134a system is higher (~4 per cent) and more refrigerant is solved in the circulating oil than in an R744 system, the risk of mist ignition and the causing of a pilot fire is reduced with the inflammable refrigerant R744. In a normal operating, R744 system pressures are higher than in the one used during the test. However, the lower-thannormal pressures of the R744 system are considered to increase the risk of ignition since a higher pressure would cause also higher release speeds and therefore, reduce the probability to ignite the mixture. In the test, the release pressure was stabilised to 20 bar. No misting was observed and no ignition was observed.

Third Test

The third test was carried out with 2,3,3,3-Tetrafluroprop-1-ene (also called HFO-1234yf and having a flash point of ~400°C. Miscibility and solubility characteristics with ND8 are considered to be similar to R134a. During the testing an ignition was observed and the flame propagation and the flammability envelop was judged to be substantial.

Conclusions

R134, having an A1 rating (not flammable according to ASHRAE safety standard), can not be ignited during an accident simulation (vehicle front end) and under realistic operating conditions including circulating oil. R744, having also an A1 rating, behaves similar to R134a and the risk of ignition can be judged to be lower than with R134a. R744 is, therefore, generating an improved safety level. The HFO-1234yf, with a probable ASHRAE safety rating of A2 or an even lower safety level in terms of flammability and toxicity (toxicity testing not completed) clearly increases the risk of a pilot fire after a front-end collision.

Incident 1: 1992 – At a meat packing plant, a forklift struck and ruptured a pipe carrying ammonia

In 1992 incident at a meat packing plant, a forklift struck and ruptured a pipe carrying ammonia for refrigeration. Workers were evacuated when the leak was detected. A short time later, an explosion occurred that caused extensive damage, including large holes in two sides of the building. The forklift was believed to be the source of ignition. In this incident, physical barriers would have provided mechanical protection to the refrigeration
system and prevented a release.

Incident 2: 1999 – An explosion in the machine room of chiller 1A 100 level refrigeration plant

On Sunday 24th October 1999, an explosion took place in the machine room of chiller 1A 100 level refrigeration plant. At the time, 14 people were working in close vicinity of the compressor where the explosion occurred and seven people were hospitalised for smoke inhalation. The refrigeration plant originally operated on CFC12, was purchased in 1967, and would have been commissioned during1968-1969. The plant is located in the first chamber (1A) of 5 chambers situated on 100 level Tau Tona. Each chamber houses
two refrigeration plants. The plant was not retrofitted; however, a HFC134a ‘drop-in’ was done by the mine at an unknown date. The compressor and original relay logic was replaced with a programmable logic controller (PLC). The compressor was fitted with a non-original equipment manufacturer (OEM) stainless steel impeller for use with CFC12.

Two possible explanations for the explosion emerged.

i. Air entered the compressor and a hightemperature, high-pressure mist comprising of HFC134a, oil and air ignited resulting in the explosion.
ii. A second possibility is that the pressurised HFC134a and oil mist escaped past the red-hot journal bearing metal of the damaged compressor and mixed with the surrounding air. Under these conditions, the mixture ignited causing an explosion.

Incident 3: 2017 – The Grenfell Tower Fire

The Grenfell Tower fire occurred on 14 June 2017 at the 24-storey Grenfell Tower block of public housing flats in North Kensington, Royal Borough of Kensington and Chelsea, West London. It caused at least 80 deaths and over 70 injuries.

Investigation revealed that the fire started accidentally in a fridge-freezer on the fourth floor. The fridge was using R 600 (n Butane) as refrigerant though it was not confirmed if leaking refrigerant was the cause of fire.

The rapid growth of the fire is thought to have been accelerated by the building’s exterior cladding, which is of a common type in widespread use. An independent review of building regulations and fire safety has been launched.

Emergency services received the first report of the fire at 00:54 local time. It burned for about 60 hours until finally extinguished. More than 250 firefighters and 70 fire engines from stations all over London were involved in efforts to control the fire. Over 100 London Ambulance Service crew on at least 20 ambulances attended, joined by the specialist Hazardous Area Response Team. London’s Air Ambulance sent teams of HEMS doctors and paramedics by road in support.

Fire fighters rescued 65 people. It was presumed that the building’s structure could contain a fire within a single flat, but in this case the fire was spreading rapidly via the building’s exterior.

Figure 6: Grenfell Tower Fire

General Fire Safety Precautions

• Keep torches away from combustible materials
• Always keep a fire extinguisher nearby when working with flammable materials
• Use a fire shield when soldering near combustibles
• Never solder tubing on a sealed system as pressure could develop
• Take proper precautions when working near motors and hot pipes
• Take proper precautions against heat related illnesses when working indoors or outdoors in extreme heat
• Explore the possibility of heat accumulation