Fire risks at filling stations for liquefied natural gas

Published:  14 November, 2014

SP has carried out tests to investigate the effect of heat from leaks of burning gas on hoses carrying liquefied natural gas (LNG).  The results will be used as a basis for new rules for the design and construction of fire resistant filling stations for LNG powered vehicles.

Natural gas consists mainly of methane, and is widely used as a fuel.  If it is cooled to -160 °C it liquefies, with a corresponding density increase of 640 times.  Conveying the gas in liquid form is therefore an effective way of transporting it in areas where there is no distribution infrastructure.  In recent years, vehicles (particularly goods vehicles and cargo ships) have started to use LNG directly in their fuel tanks, with the result that a network of LNG filling stations has been established in Europe, America and (particularly) China.  Once LNG has returned to the gaseous state, it burns at a concentration of 5-15 % in air.  Nevertheless, today there are no safety rules on how such filling stations should be built in order to ensure fire safety.  SP has therefore, in conjunction with The Swedish Gas Association, performed tests to simulate a hose leakage in a filling station in order to provide data for safety distances of various types of buildings and equipment for newly-built filling stations.

The ‘standard damage case’ and tests

The ‘standard damage case’ defined by the industry and by the Swedish Civil Contingencies Agency is that hoses, whether from a tanker that supplies LNG to the filling station, or from a dispenser to the customer vehicle, are abraded from the inside, resulting in a crack around one third of the circumference of the hose.  The cracked hose is surrounded by a flexible braid made of stainless steel wire, which is regarded as undamaged; see Figure 1.

Figure 1: a flexible cryohose of the type used in the trials, showing the external braid of woven stainless steel.

This default case is used for liquefied petroleum gas hoses, originating from a study carried out by Shell during the 1990s.  However, there has been some uncertainty concerning when it can be applied to hoses operating at cryogenic temperatures, as used for LNG, where the crack may become larger in the event of a burning leak.

The trials measured the thermal effect of a number of fires, based on different hose sizes and extent of damage.  In addition, the effect of the surrounding braid was also investigated.  Real hoses, together with steel pipes, with and without braid, were used, exposed to a pressure of 10 bar.

Results

The results show that the radiant flux intensity varies with time, as LNG finds different ways out through the external braid.  However, the variations lie within 25 % of the maximum radiant flux intensity.

Trials using 2” hoses (representing those from a road tanker vehicle) resulted in a large fire plume (see Figure 2), but not in any jet flame as had been forecast by earlier simulations. 

Figure 2: a picture of the trial of a 2” hose with a crack around one third of its circumference, representing damage to the hose between a tanker vehicle and the filling station.

The heat flux decreased rapidly with distance from the flame:  at a distance of 6 m from the leak, the maximum incident flux on a vertical surface was 10 kW/m².  The corresponding thermal flux level at 9 m was 6 kW/m².  The flame in this case was about 8-10 m high and 3 m wide.  Conducting the test with a smaller hose of 1” internal diameter (corresponding to the hose from a fuel dispenser to a customer vehicle) halved the radiant flux levels in comparison with those from the larger hose.  The flame in this case was about 1-2 m high and 2 m wide.

The trials also showed that without the external braid, larger damage sites on the hose can result in jet flames.  The effect of the braid is significant, as it prevents the creation of a jet flame, spreading the flow of methane in various directions and reducing the maximum radiant flux intensity by a factor of 2-3.  The results also show that the cracks in the hoses did not increase in size from the artificial damage for the trials, indicating that this standard damage case is also suitable for LNG systems.  Figure 3 shows how the maximum radiant flux intensity varies with distance from the leak for different sizes of damage and with an intact external braid. Apart from the most severe damage case (2” hose and a 5 x 45 mm opening), the radiant flux level is less than 15 kW/h² for all distances above 4 m.

Figure 3: maximum measured radiant flux at the same height as the leak, for different sizes of damage.  The circles represent damage to a 2” hose from a tanker vehicle and a 1” hose to a customer vehicle.  These results apply for leaking hose/tubes with an intact external braid.

IR documentation

Observing the trials with an IR camera also showed that no larger pools of unburnt gas were formed in connection with a burning leak.  However, smaller pool fires arose beneath the leak, contributing to the overall fire intensity.  However, it must be pointed out that before the leak was ignited, combustible accumulations of gas arose from a leak and slowly spread from the area.  If subsequently ignited, these particular cases can cause very high fire loads, although over only a limited time, over a wide area around the leak.  Such scenarios have not been investigated in this study.  The IR analysis also shows that the flame has a maximum temperature of about 1000 °C.

The results from this investigation are now being used as a basis for the tables setting the required safety distances in the rules for the design of new LNG filling stations that are being drawn up by the industry and public authorities concerned.  A full description of the investigation has been published in SP Report 2013:61, ‘Thermal exposure from burning leaks on LNG hoses:  Experimental results’.

  • Operation Florian

Sign up: eMagazine & eNewsletter

The latest issues in your inbox.

Company Profiles

Advancing rescue technology

The specialist supplier of quality PPE and Rescue Equipment to Emergency Services.

Revolutionizing fire fighting foam technology

The one-stop resource for fire fighting foam concentrates and custom-designed foam suppression systems hardware.

The ultimate in innovation, quality and service

For 60 years Lehavot has been delivering the world’s most advanced fire detection and suppression automatic systems

Trust the best, let us be your foam solution

AUXQUIMIA is a Spanish company whose main activity is the design, manufacture and commercialization of firefighting foam concentrates.

Williams Fire & Hazard Control offers a full line of specialized fire response equipment for oil and gas platforms

From storage tanks and pipeline emergencies to offshore platforms and vessels at sea, Williams' response personnel and specialized equipment quickly address adverse fire emergencies.

Foam fights fire

Europe’s foremost fire fighting foam manufacturer has been developing and producing foams since the 1920s.

Optical flame and open path gas detectors

Spectrex Inc. is a world leader in optical flame and combustible gas detection with over 30 years experience.

The leader in truck-mounted hydraulic platforms

Our mission is to provide the best and the safest solution to professionals that work at height.

The independent alternative

Dafo Fomtec AB is a privately owned company with head office in Stockholm Sweden and manufacturing in Helsingborg in the south of Sweden.

If you want quality, you want Zico

Since its inception Ziamatic Corp has provided the men and women of the fire service with products designed to make their jobs safer and easier.

Calendar