Wednesday, September 9, 2009
CCPS and Fire as a Weapon III
This weekend I received my monthly fix of process safety information from the Center for Chemical Process Safety (CCPS); they sent out their September CCPS Process Safety Beacon. Each month the Center takes a one page look at a particular process safety issue taken from the recent news. This month the Beacon was shone on overfilling flammable liquid storage tanks and the potentially resulting fires. To be perfectly fair, the CCPS Beacon does not say anything about using fire as a weapon, they are strictly looking at process safety issues. They point at an operational error or equipment malfunction leading to a tank being overfilled and an existing ignition source turning the spill into a conflagration. This is certainly a more likely occurrence than a terrorist attack, but given yesterday’s blog about fire being used to turn a pressure tank into a bomb I had to tie the two together. BLEVEs Require Fuel Source Pressure tanks holding flammable gasses or flammable liquids are potential explosive devices if they are not properly protected against what is known in the process safety community as the ‘Fire Case’; a long burning, large fire with flames directly impinging on the storage tank. Yesterday’s blog looked at the details of the BLEVE (boiling liquid expanding vapor explosion). The video showed a constant flame on the tank apparently coming from a gas line that had broken free, but is that really reasonable? Surely someone would shut off the feed for that line when the temperature alarms started going off. There would likely be a number of safety systems that would prevent such a gas stream from causing a BLEVE. High flow valves on the line would automatically shut down the fuel feed. Temperature and pressure alarms on the pressure tank would require someone to investigate the situation and they could manually shut off the line. Fire sensors could be arrayed around the tank. What is a more likely fuel source for a BLEVE is a flow of flammable liquid or gas from a nearby tank that is not easily shut off. One example would be the overflow situation described in the CCPS Safety Beacon. While the actual overflow would probably be stopped in relatively short order the resulting fire described in the Beacon could result in the flow of burning liquid into the containment area holding the pressure tank. The risk of this type fire causing a BLEVE could be reduced by ensuring that pressure tanks are in separately diked areas. Terrorist Caused BLEVE First off, why would a terrorist want to cause a BLEVE; why not just place an IED or VBIED to cause the same effect? The most obvious potential reason would be that security measures in place at the site might be stringent enough to make it unreasonably difficult to emplace such a device. Or it might be an attack by an insider with knowledge of the facility but no knowledge of how to construct a reliable explosive device. Since there is a need to have a constant supply of fuel to the fire causing the BLEVE there are only a limited number of ways that this technique could be used. The easiest way would be to flood the pressure tank containment area with flammable liquid and then igniting the liquid. This could be done by deliberately short circuiting the safety systems preventing a tank from being overfilled and then pumping excess material into the tank. It would be difficult to calculate the amount necessary to cause a BLEVE so a generous amount would be required. Another way to do this would be to puncture a large flammable liquid/gas storage tank near the intended BLEVE tank. The leaking stream of liquid/gas would be pointed at the BLEVE tank, preferably actually hitting the tank, but at least landing near or below the tank. The stream would then be ignited. If the point of ignition were near enough the source tank it would be very difficult for most facilities to plug the leak. This would leave pumping the source tank empty as the only potential response. Finally, someone could run hoses from a flammable liquid storage tank to the containment area where the BLEVE tank was located. A large quantity of the liquid would be pumped into the area. It would then be ignited. Preventing Terrorist BLEVEs There are some relatively simple actions that a facility can take to prevent terrorist caused BLEVEs. Identification of potential BLEVEs should be the first priority. Then the facility can look at a combination of isolation, safety systems and emergency response procedures to prevent both accidental and terrorist caused BLEVEs from occurring. Finally, the potential attractiveness of the BLEVE as a terrorist target needs to be reduced. First, pressure tanks containing flammable liquids or gasses should be isolated from other flammable liquid tanks. When ever possible such tanks should be individually diked to reduce the likelihood of a flammable liquid pool occurring in the diked area. Flammable liquid and gas transfer lines not going to the BLEVE tank should be routed clear of the diked area around the BLEVE tank. Pressure tanks should be far enough from other flammable liquid/gas storage tanks that a fire in nearby tanks cannot cause the catastrophic failure of the BLEVE tank. A wide variety of safety systems can be used to help prevent both accidental or deliberate BLEVEs. An appropriate fire suppression system around the tank will go a long way to preventing a BLEVE. Where practical, a deluge water system trained on the tank can help to keep the tank cool enough to prevent a catastrophic failure of the tank. A temperature and pressure alarm system on the BLEVE tank will provide advance warning of a problem and increase the potential response time. Finally, a properly sized venting system will go a long way to preventing a catastrophic failure of the BLEVE tank. Finally, while a BLEVE certainly produces an impressively large fuel-air explosion and will probably damage large portions of the facility, that will seldom be enough to make the tank a terrorist target. Unless shutting down the facility is the terrorists objective (possible in some cases) there will have to be a potential significant off-site affect of the BLEVE for it to be a terrorist weapon. Such an effect could be as simple as a residential area within the blast effect area of the BLEVE. A more round about effect could be the blast caused release of a toxic chemical from a nearby storage tank. Thus the location of the potential BLEVE tank with respect to potential targets might need to be adjusted. While the CCPS and Dr Saraf were looking at process safety issues in their discussions of BLEVEs and storage tank overflows, those safety issues have potential security implications. Actually any safety issue that can result in a catastrophic consequence can potentially be used as a trigger for a terrorist attack. Many of the safety measures used to prevent the catastrophic consequence can also be considered part of the security plan to prevent the associated terrorist attack.