Responding to LNG vapour releases & fires
Published: 01 June, 2007
LNG is a cryogenic liquid stored at -162°C in special single- or double-lined insulated concrete tanks. Its composition varies between 83 and 99.5% methane with traces of ethane, propane and butane. As it vaporises the cold gas condenses moisture in the atmosphere into a white vapour cloud. It is in the vicinity of the edges of this cloud that ignition is most likely to occur, where the LNG reaches its flammability range of 5 to 14% in air. Although asphyxiating, the vapours are not explosive unless confined.
What the facility can do:
The goals of the new facility at Texas A&M are to provide a learning environment and practical workshop experience, develop expertise, test new products and techniques and share knowledge for the wider benefit of the LNG industry. It replicates realistic conditions at modern LNG facilities.
For example, it uses modern vertical-sided concrete pit designs in 3 sizes, a small training pit for dry powder extinguishers at 9.3 square metre area, a large oblong 65 square metre pit, a deep 45 square metre pit, plus an LNG shaped pipe trench with metal grating that gets cherry red and can cause an additional flashback hazard.
It features a deep ‘marine’ pit with a marine prop that simulates LNG valve manifold, drip pan and side of vessel with spray system to simulate flange leaks on water to provide more diverse and realistic scenarios for training.
Current standards such as NFPA 11 or 59A do not provide recommendations about foam system application rates for LNG. NFPA 11 recommends that this shall be determined by test, but cautions that tests often give minimum application rates conducted under ideal conditions with no barriers to fire control.
Final design application rates are therefore generally 3 to 5 times the test rates. It also draws special attention to the heat effects of LNG and the importance of choosing equipment that is fit for purpose, suggesting that time to initiate actuation is also a critical factor in controlling LNG fires. Foam quality of uniform bubble size and correct drainage characteristics plays a major part in success or failure when it comes to LNG releases.
Foam is needed wherever LNG can gather as a pool. The main areas where it can be most effective are therefore loading/unloading jetty manifold sumps, transfer lines, pumps and heat exchangers, impounding basins, containment pits, spill trenches and truck loading bays.
On unignited spills foam quickly reduces the vaporisation rate by insulating the pool from ambient conditions and warming the vapours so they rise vertically, minimising the risk of vapour cloud ignition.
This in turn increases safety for operators and emergency responders. Foam needs to be applied as soon as possible to minimise the risk of escalation. If the LNG ignites, severe radiant heat results, typically 3 times the heat output of an equivalent gasoline fire. Foam quickly reduces this radiation, however, providing fire control and protecting personnel and exposures from escalation and allowing a controlled LNG burn-off through the foam blanket until the incident is declared over.
Vapour dispersion and fire control
During testing we found that LNG Turbex foam generators and Expandol foam controlled and dispersed vapours after about 60 seconds of application. Since this is a dynamic situation, the cold gas freezes the foam on contact as it absorbs the heat from the foam. Ice tubes form and migrate to the surface, resembling miniature volcanic vents releasing LNG vapours.
During other tests LNG vapours above the lower flammability limit were detected up to 150 metres away from the white condensation cloud, which led to a gas detector being mounted on the metal workshop to warn of these conditions in future. Frozen foam left overnight had trapped LNG beneath the ice, which when broken released flammable gas levels the next day. Care should therefore be taken during the clean-up operation.
Water sprays can contain the LNG vapours and add heat, forcing the gas to rise. Several units provide only 120 or 150° fan sprays, so gas escapes beneath them into potentially hazardous areas, so 180° fans are essential.
Any watersprays set up to try and contain the vapour cloud and prevent ignition must overlap and reach down to ground level to prevent gas escaping under the water curtain, like the Angus Hydroshield unit. Systems must also be designed to direct water away from entering the LNG containment pit.
Vapour cloud ignition usually occurs at the edges of the gas cloud, with a slow burning rate back to the liquid pool unless it is in a confined space. The BP test work has largely confirmed the historic position that foam insulates and reduces radiation from the fire back to the LNG. Foam expansions of 500:1 are shown to be the most effective.
A frozen open cellular ice layer forms which floats above the LNG and contributes to dramatically reducing fire intensity. Leakage of gas is more evident around the edges of vertical pits and sloping sides can help to provide better sealing. Foam is pulsed on and off to keep the pit full and allow a controlled LNG burn-off.
Foam quality
Individual foam brands will vary in effectiveness on LNG. A fluorine free synthetic detergent based product like Angus Expandol is necessary to produce a stable foam blanket. Expandol foam concentrate applied at the rate of 10 litres per minute per square metre and a 500:1 expansion ratio and delivered through LNG Turbex foam generators has been proven to achieve 90% radiation reduction within 60 seconds.
This recommended application rate includes a safety margin for the unexpected, like a rain storm or water run-off entering the LNG pit, and minimises the risk of plant damage from incident escalation. Lower quality foams with poor stability can make the fire worse as they release their water too fast.
It is also important to test the specific foam with the foam generators being used on LNG as the generators also have a significant part to play in producing a uniform bubble structure. Extinguishment of LNG using high performance dry chemical powders like Monnex is always an option on small spills using portable equipment, although there is no post-fire security with any powders. All this is covered in the BP LNG Fire Protection and Emergency Response booklet, currently being reprinted and expected to be available soon on the IChemE website at www.icheme.org.
Foam-mixing equipment
Foam generators specified for use on LNG must be engineered to enable heat radiation reductions to be achieved. They also need to withstand the cold of LNG vapour down to around minus 100°C and intense heat of an LNG fire at around plus 1,300°C.
A fan driven unit is essential to avoid air starvation during cross winds, an enclosed water turbine motor to drive the fan with special bearings, and an aerofoil fan and insulation materials. Specially designed hoods are required to direct the foam into the pit, protect against wind and avoid reducing foam expansion during production.
Of course it also needs to be rigorously tested by fire exposure on LNG to verify its suitability. We know from other testing that standard industrial units fail to operate, sometimes within seconds, under such extreme conditions. The Angus LNG Turbex has a stainless steel and bronze construction with four tough fan blades that withstand severe conditions and intense heat for long periods of time.
BP and Texas A&M have provided an enduring facility for the whole LNG industry to experience live fire training and control multiple incidents. To date this facility, expertise, research and equipment used, has enhanced the understanding of a wide range of design houses, constructors, project engineers, regulators and operators. Many of those who have attended have seen LNG for the first time and understood implications they did not fully appreciate before.
Many further workshops are planned to maintain this momentum and provide a good example of how best to maintain the reputation of the LNG industry for safety well into the future.
