The bursting of a pressure vessel due to exposure to fire is known as a Boiling Liquid Expanding Vapor Explosion, abbreviated to BLEVE.
The exposure of pressure vessels to high temperatures, due to fire or other causes, has not always been considered a major risk. Investigations of accidents in which pressure vessels have burst often assume that the relief valve system was at fault. After the relief system has been shown to be satisfactory, the possibility of high. temperature is then considered as a possible cause of the vessel failure.
BURSTING OF PRESSURE VESSELS
Pressure vessels are designed for a specific maximum working pressure. The pressure in the vessel may not exceed this maximum except when the safety relief valve or valves are discharging. When the safety valves are discharging, the maximum allowable working pressure may be exceeded by a certain percentage, depending on what codes, etc. a vessel is fabricated to.
The working pressure and the stress allowed for the material are used to determine the vessel wall thickness. The allowable stress depends on the maximum operating temperature for a particular type and grade of material. Any increase of pressure or temperature over the maximum working limits will overstress the material. Excessive increases in temperature or pressure will ultimately lead to bursting of the vessel.
When a pressure vessel is exposed to intense heat, the following occurs:
- The temperature of the material increases.
- The tensile strength of the material decreases as the temperature rises.
- The ultimate strength of the material decreases as the tensile strength decreases.
- The pressure within the vessel will increase unless the temperature rise is prevented.
A pressure vessel containing petroleum products, liquefied natural gas (LNG), or other products which expand with an increase in temperature is likely to explode when exposed to fire.
Experiments have proven, and common sense tells us, that thin materials, such as steel plate, will experience a more rapid temperature increase than thicker plates made of the same material. For example, a 25 mm thick steel plate, if exposed to fire, could reach a temperature of approximately 750ºC in about twenty minutes. A 13 mm thick plate of the same material would reach that temperature in about ten minutes. Similarly, experiments using different thicknesses of different materials have made it possible to make fairly accurate estimates of how long a pressure vessel will be able to withstand very high temperatures.
1. Case One
A leak occurred in a spherical tank containing propane at 860 kPa. The propane ignited and a fierce fire burned beneath the tank. The fire fighters used water to cool the adjacent tanks. The relief valves operated and it was believed that this would protect the vessel from exploding. After 90 minutes, the tank exploded and a wave of burning propane killed the fire fighters and then spread unchecked.
The bursting of the vessel was due to the upper, unwetted portion of the sphere being heated to a temperature that the steel was unable to withstand. Below the liquid level, the boiling liquid absorbed the heat and prevented the steel from excessive temperatures and consequent failure.
2. Case Two
A storage tank supplied fuel oil to an electric utility generating plant. After ignition and 6 hours of intense burning, the contents erupted violently. The incident caused the death of more than 150 people, 40 of whom were the fire fighters., None of those actually fighting the fire survived. As a consequence, many of the details of the fire fighting strategy and the progression of events remain unknown.
Causes of Damage in the Vicinity of the Vessel
When a BLEVE occurs, injuries and damage can be caused in several different ways.
- Parts of the pressure vessel are projected due to the strong force of the explosion, and can cause injuries and damage up to 300 m from the vessel location.
- The escaping gas or liquid burns, and the heat can cause injuries to persons and cause buildings to be set on fire.
- The escaping gas or liquid can mix with the air and cause a secondary explosion and pressure wave. This secondary wave can cause further injuries and damage.
- The pressure of the escaping gas or liquid can cause injuries to people, and damage buildings or structures.
The injuries and damage from items 1 and 4 can occur even if the vessel contents are not flammable. For example, they can occur when a steam boiler, an air receiver, or a similar piece of equipment bursts.
The Role of Relief Valves
When a vessel is subjected to excessive heat and the temperature rises, a relief valve system will not prevent the vessel from bursting. Some plant operators believe that a relief valve that is properly sized, installed and maintained will always prevent a vessel from bursting. This is only true if the vessel is at or close to the design temperature.
A propane vessel may be designed for 1725 kPa and a temperature of 52ºC and have a shell wall thickness of 8 mm. If this vessel is subjected to fire, in approximately 3.5 minutes the unwetted metal temperature will have reached 538 ºC, more than 10 times its design temperature. The pressure can reach 8 times the original pressure, in this case 13 800 kPa.
Safety relief valves cannot reduce the pressure in the vessel to atmospheric pressure. The valve can only reduce it to a point below its popping pressure. As the LPG will always be above its normal boiling point, there will always be pressure in the vessel.
In the case of direct flame contact, the ultimate strength of the material decreases to the point where it may be overstressed and fail. This can occur even though the safety relief valve is discharging and limiting the pressure increase.
METHODS OF PROTECTION
There are four main methods of protecting a pressure vessel against fire:
- Sloping the ground.
- Water deluge.
- Vapor depressurizing.
Sloping the Ground
The area around the pressure vessel base should be impervious and should be sloped so that any spill is collectable. The collecting area should be far enough from the vessel so that flames from a fire in the collecting area will not impinge on the vessel. The above can generally be applied to pressure storage vessels using a ground slope of 1 in 40. In the case of process vessels, a slope of 1 in 40 may not be practicable, but the gradient should not be less than 1 in 60.
Insulation on a vessel will provide an immediate barrier to heat input prior to water cooling being applied. Insulation is not an alternative to water cooling; insulation will slow down the rate of heat input into the vessel, but it does not ultimately prevent the vessel from overheating.
Various materials can be used for fire insulation purposes. Concrete using low conductivity aggregate such as vermiculite, calcium silicate thermal insulation, and some mastics have been used. Low temperature insulation such as plastic foams are likely to be rapidly destroyed. Where plastic foam is used for low temperature thermal insulation, it should be covered on the outside with a fire protective layer using a vermiculite concrete or similar fireproofing material.
A disadvantage of using insulation is that it is difficult to inspect the vessel or its supports for corrosion. If the fire insulation is removed for corrosion inspection, it must be replaced promptly and to the original specifications. If part of the insulation is missing, or there are gaps, then the whole value of the insulation is lost.
Where thermal insulation is used, it should be protected so that it is not damaged by the impact of water streams from fire hoses or monitors.
Water deluge is the most widely used method of preventing vessels from overheating due to fire. It is essential that the water deluge system be brought into operation as soon as is possible.
Water deluge can be provided by fixed installation. This will ensure that no part of the vessel remains dry and will use a minimum amount of water. Strong winds may affect the water spray pattern and the use of supplementary mobile equipment may be required.
Fixed systems can be applied to most pressure storage systems as the temperature of the vessel is ambient and the risk of thermal shock is small. In the case of process vessels not operating at ambient temperature, there is a risk of thermal shock, which may require the use of alternate methods.
The rate of water application to the vessel is recommended as ten litres per square metre per minute (10 L/m2/min) where the vessel is subject to direct flame impingement. If the ground is sloped, and only radiant heat has to be considered, then the quantity can be considerably reduced. The minimum recommended water flow is 2.5 L/m2/min.
The deluge water from fixed equipment can be applied to the top of the vessel in cases where it will be evenly distributed over the total surface. The water should be applied in several places when even distribution cannot be obtained from a single point.
If the pressure in a vessel subjected to fire is reduced, the stress in the material is reduced and the danger of the vessel bursting is also reduced. The rate of flow from a leak will also be reduced and damage may be prevented to other items of equipment.
Vessels can be depressurized through existing pipework installed for process purposes or through a line bypassing the relief valve system. The depressurizing valves must be operable under fire conditions and this usually means remote operation. The operating controls must be remote and can be electric or pneumatic.
Depressurizing valves are normally arranged to open on pneumatic or power failure. Under some circumstances, the depressurizing valves must have power to open them. In such cases, the cables or pneumatic tubing must be fire rated for at least 15 minutes.
The depressurizing equipment should reduce the pressure in the vessel to approximately 700 kPa, or 50% of the vessel design pressure, whichever is the lower. The depressurizing should be complete in ten minutes for bare vessels, and thirty minutes for fire insulated vessels. If the ground is sloped away from the vessel, and the vessel is only subject to radiant heat, then the above depressurizing times can be doubled. It should, however, be kept in mind that one of the purposes of depressurizing is to limit the leakage rate from the vessel or associated pipework. For this purpose, a sixty minute depressurizing time may be too long.
If the vessel is fire insulated, but the pipework is not, then extending the depressurizing time may not be advisable due to the possibility of the pipework bursting.
Vessels which can release noxious chemicals to the atmosphere, or vessels where the metal temperature may be reduced by adiabatic cooling to a point where brittle fracture may occur are situations where power is required to operate the depressurizing valves.
Vapor depressurizing is important for vessels which contain liquids close to their critical point, where the latent heat of vaporization is zero. The vessel may initially have a considerable quantity of liquid which in the wetted area will absorb some of the heat; however, the latent heat of vaporization decreases as pressure and temperature approach the critical point. Thus it is possible for the vessel to overheat below the liquid level and burst if the pressure is not reduced.
TANK CARS AND TRUCKS
In cases where tank cars and trucks have burst due to exposure to fire, it was usually because they were not kept cool by means of water sprays.
Vapor depressurizing is not practical for this type of mobile equipment. The use of fusible plugs has been suggested, but it is doubtful if such devices are of sufficient size, and they may be located on a part of the vessel not exposed to the fire.
In dealing with tank car and truck fires, it is important that they be cooled with water. The water should be directed at the ends, as these are the most likely places to fail. If possible, the water spray should be applied from a monitor, and not by hand-held hoses.
Most BLEVE’s occur due to fire, but there have been some due to other causes such as corrosion, or the force of an impact. A vessel that has been damaged on impact may not BLEVE until the contents increase in pressure due to a rise in the ambient temperature several hours later.
Most LPG BLEVE’s occur when the vessel is from about one-half to three-quarters full. The fireball from a tank car or truck can be several hundred metres in diameter and parts of the vessel can be propelled up to three hundred metres. Persons eighty metres from the larger containers have been killed, injured or burned due to the BLEVE.
Most BLEVE’s have occurred in a time range of eight to thirty minutes, with nearly 60% occurring in less than fifteen minutes.
Where a vessel may suffer a BLEVE, the following points should be addressed:
- There should be a fire fighting procedure developed for each particular location and it should be well understood by all who are working in the vicinity of the vessels. Temporary workers or those from contractors should also be instructed in the hazards of a BLEVE.
- The contents of the vessels should be known. This particularly applies to tank cars and trucks. The possibility of toxic fumes as a result of the accident, or from the combustion of the tank contents, should be known.
- The time the fire starts should be noted.
- The application of water for cooling purposes should be started at the earliest possible time.
- The depressurizing of vessels, where possible, should be started at the earliest possible time. Depressurizing should have priority over water cooling if a choice has to be made.
- Evacuation of nonessential personnel and the public should be carried out immediately after the accident occurs. The evacuation zone should have a minimum 300 m radius.
- The Dow of gas to or from the vessel should be stopped if possible.
- The water flow rate required to deal with the accident will be high and is required for many hours for cooling and general fire fighting purposes. An adequate supply should be ensured by use of fire ponds, storage tanks, and any public fire system that may be available.
- If there is no fixed cooling water system, the cooling water streams should be applied from monitors rather than hand-held hoses.