By Garry Bennett, John Frank, Marc Van De Velde, XL Global Asset Protection Services
Vapor releases at industrial facilities handling flammable liquids or gases can result in some of the most challenging incidents an industrial responder can face. There are many ways such a release could occur. For this article, we will focus on process vapor release at petrochemical plants. Process vapor releases can result in three primary types of incidents:
• A vapor release without ignition
• A vapor release with delayed ignition resulting in a vapor cloud explosion (VCE)
• A vapor release with immediate ignition resulting in a jet fire
These are highly complex incidents because of a wide range of potential outcomes and potential exposure to a wide variety of process equipment, each of which has its own unique fire fighting problems. This is intended to be starting a point for discussion and to illustrate how existing chemical safety analyses can be used in pre-planning. VCEs will be addressed in this article. Vapor clouds that do not ignite and jet fires will be addressed in part two.
VCEs occur when vapors are released, and ignition occurs after a period of time. Prevention of ignition is beyond the scope of this article; we are assuming that an ignition source has been found.
Most facilities conduct some kind of VCE analysis. The output is expressed in overpressure rings (in psi or millibar (mbar)) with associated damage. Figure 1 shows overpressure rings for a simple model overlayed on facility diagram. Table 1 shows damage expected with various over-pressures. This example assumes the following:
• the main constituent is isobutane,
• process conditions 102° Celsius @ 40 bar (216° Farenheit @ 580 psi),
• entire reactor is filled with isobutane,
• release through a 100 mm (four inch) flange break; in 120 seconds the entire mass was released,
• calculated flash fraction is 81 percent or a vaporized mass of 31950 kg (70440 lbs),
• yield factor for isobutane of four percent, • calculated with ExTool v3.06 under license from SwissRe using default atmos-pheric conditions,
• vapor drift based on varying atmospheric conditions is not considered in this model,
• shrapnel projectiles are not accounted.
There are many VCE models available with varying degrees of sophistication. The more sophisticated the model, the more variables can be accounted for. Besides the model’s variables, there will be a wide range of input assumptions. It is important for responders to understand the assumptions. Responders should ask what would happen if the release scenario was more severe. Additional models may be appropriate.
It is also important to understand that these are just models. An actual incident can unfold in a number of ways. For the purposes of this article, the model is intended to be the basis of discussion about what might be encountered. For example, if the site fire station is in one of the damage rings, its equipment may not be available. It may even be decided that the station should be relocated or a satellite station established so everything would not be lost at once.
The explosion (overpressure or shrapnel) might also shear off fire protection risers or hydrants resulting in loss of water pressure. These damaged systems would need to be isolated to preserve water for other fire fighting operations. A working knowledge of fire protection valving is needed to shut down as little of the system as possible to restore pressure and still leave adjacent fire protection in service. If this cannot be done due to radiant heat, it may be necessary to fall back to more remote valves or to establish auxiliary water supplies. Auxiliary supplies might include drafting from reservoirs and using large diameter hose to supply trailer mounted monitor nozzles.
The explosion might also shear off process piping, in turn dumping more flammable liquids or gases (or otherwise hazardous materials) into the fire. Remote shutdown for these lines, along with the potential loss of control equipment must all be consid-ered.
Finally, an idea of what to expect in terms of damaged structures and equipment can assist with triage, technical rescue, and casualty treatment decision making.
For this scenario, we found that smaller processing plants would be entirely within the severe damage rings including the loss of all fire protection. This is important information to know. Containment efforts might have to be focused on protecting property outside the fence with off-site fire protection water supplies. In one case, on site fire pumps were taken out as well. If a rescue effort was going be mounted under the protection of fire hose streams or monitors, the on-site water supplies might not be available. Hose relays from off-site, tanker shuttles, apparatus booster tanks, etc. would need to be used.
This article is a simplification of a very complex topic. Even the most sophisticated models cannot determine exactly how an incident could unfold. Combinations of VCEs and then subsequent jet fires can occur. These jet fires can then in turn expose pressure vessels with a resulting BLEVE (boiling liquid expanding vapor explosion) threat. The issues discussed here, along with this basic model, can help guide discussions and serve as a starting point for more in-depth evaluation. Flexibility during an operation is essential, but the skills and thought processes developed in preplanning sessions can guide decisions during an operation, no matter how it unfolds.
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