Fire and Explosion Protection

There are many reasons for fires and explosions in a process plant. They result from a chemical reaction where a combustible material reacts with oxygen and heat is released. Fire and explosion properties of materials were already dealt with in Chap. 2. 

A fire occurs if a heat source enters into contact with a combustible material. If a combustible solid or hquid material is heated vapours evolve first. If their concentration is sufficiently large, a flammable mixture with the oxygen of the air is formed. If this mixture is heated further to its ignition point, combustion starts. The same apphes to mixtures of combustible gases or vapours, if mixed with oxygen and heated. Hence, there are three conditions, which are essential for a fire. 

The fire triangle shows how to combat a fire. In the first place, the fuel supply can be interrupted. This is especially important for fires due to leaks in process plants. Secondly, heat can be removed. This is frequently done by extinguishing a fire with water. The third approach is to cut off the oxygen supply. This can be done in several ways, for example by covering the fire with foam or an inert gas. 

A fire is maintained only, if its heat generation is equal or larger than its heat losses. The heat stems from the combustion of the material involved. If that is solid or liquid it has to be evaporated in the first place, which implies a heat loss. If liquids or solids burn there usually is a positive feedback. The heat produced by combustion leads to evaporation of the material and thus to a further spread of the fire. 

Acceptable protection techniques for electrical and electronic valve accessories used in specific class and division locations include explosion-proof enclosures intrinsically safe circuits nonincendive circuits, equipment, and components dust-ignition-proof enclosures dusttight enclosures purged and pressurized enclosures oil immersion for current-interrupting contacts and hermetically sealed equipment. Details of these techniques can be found in the National Electrical Code Handbook, available from the National Fire Protection Association. 

Certified testing and approval for control valve devices used in hazardous locations is normally procured by the manufacturer of the device. The manufacturer typically goes to a third-party laboratory for testing and certification. Applicable approval standards are available from CSA, CENELEC, FM, SAA, and UL. 

Environmental Enclosures Enclosures for valve accessories are sometimes required to provide protection from specific environmental conditions. The National Electrical Manufacturers Association (NEMA) provides descriptions and test methods for equipment used in specific environmental conditions in NEMA 250. IEC 60529, Degrees of Protection Provided by Enclosures (IP Code), describes the European system for classifying the degrees of protection provided by the enclosures of electrical equipment. Rain, windblown dust, hose-directed water, and external ice formation are examples of environmental conditions that are covered by these enclosure standards. 

The centrifugal pump directly driven by a variable-speed electric motor is the most commonly used hardware combination for adjustable-speed pumping. The motor is operated by an electronic motor speed controller whose function is to generate the voltage or current waveform required by the motor to make the speed of the motor track the input command signal from the process controller. 

8-89 Pressure, flow, and power for throttling a process using a control valve and a constant-speed pump compared to throttling with an adjustable-speed pump. 

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Industrial fire protection and safety engineers attempt to eliminate hazards at their source or to reduce their intensity with protective systems. Hazard elimination may typically require the use of alternative and less toxic materials, changes in the process, spacing or guarding, improved ventilation or, spill control or inventory reduction measures, fire and explosion protective measures - both active and passive mechanisms, protective clothing, etc. The level or protection is dependent on the risk prevalent at the facility versus the cost to implement safety measures. 

Due to the destructive nature of hydrocarbon forces when handled incorrectly, fire and explosion protection principles should be the prime feature in the risk philosophy of any hydrocarbon facility. Vapor cloud explosions in particular are consider the highest risk at a hydrocarbon facility. Disregarding the importance of protection features or systems will eventually prove to be costly both in economic and human terms should a catastrophic incident occur without adequate safeguards. 

In general the fire and explosion protection engineering design philosophy for oil, gas and related facilities can be defined by the following objectives (listed in order of decreasing importance) ... 

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