Explosion Protective Design Arrangements

Research on water explosion inhibiting systems is providing an avenue of future protection possibilities against vapor cloud explosions. British Gas experimentation on the mitigation of explosions by water sprays, shows that flame speeds of an explosion may be reduced by this method. The British Gas research indicates that small droplet spray systems can act to reduce the rate of flame speed acceleration and therefore the consequential damage that could be produced. Normal water deluge systems appear to produce too large a droplet size to be effective in explosion flame speed retardation and may increase the air turbulence in the areas. 

The following are typical design practices that are employed to prevent vapor cloud explosions. 

Enclosed locations will not receive adequate ventilation and could allow the build-up of combustible vapors or gases. Vapors with heavy densities can be particularly cumbersome as they will seek the low areas that are normally not provided with fresh air circulation. 

Walls or roofs tend to block vision and access, trap sand, debris, and reduce ventilation so that flammable vapors are not as quickly dispersed. They may also collapse if there is an explosion or deflagration. They can therefore contribute to secondary effects by falling onto pipes an equipment that may substantially exceed damage from the original explosion or deflagration. They can also lead to a false sense of security. 

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Facilities that are deemed critical or highly manned should be relocated out of the overpressure circles or provided with explosion protection measures. Other systems within these overpressure zones should be evaluated for the specific benefits of providing explosive protective design arrangements. An example is provided in Figure 16.1. 

Most petroleum and chemical facilities rely on inherent safety and control features of the process, inherent design arrangements of the facility, and process safety ESD features as the prime loss prevention measures. These features are immediately utilized at the time of an incident. Passive and active explosion and fire protection measures are applicable after the initiating event has occurred and an adverse affect to the operation has been realized. These features are used until their capability has been exhausted or the incident has been controlled. 

Siting - The preferred arrangement of bulk transport pipeline systems is for burial underground. This provides for enhanced protection from overhead events. This is even the case for offshore pipelines where there have been numerous incidents of dragged anchors from fishing vessels to pipelines exposed on the seabed. A radius of exposure from a pipeline can also be easily calculated for fires and vapor explosions based on the commodity, pressure, release opening, etc. From these calculations a restricted zone or similar can be designated. 

Optical flame sensors can be used for detecting the initial explosion, provided interference of environmental conditions can be reliably prevented. Pressure transducers are often used because the pressure wave travels at the speed of sound and can be detected at various angles. Vibrations and other mechanical movements can interfere pressure sensors. Therefore, efforts have to be made to minimize the influence of these interferences. The suppressant must be effective in flame quenching and compatible with product and the material of the plant. The suppressors must be designed and arranged adequately, so that the suppressant is rapidly and uniformly injected to the gas mixture in the protected enclosure. 

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