Suppression system

Alternatives to Arresters Alternatives to the use of flame arresters include fast-ac ting isolation valves, vapor suppression systems, velocity-type devices in which gas velocity is designed to exceed flashback velocity, and control of the flammable mixture (NFPA 69 standard, Explosion Prevention Systems ). The latter alternative frequently involves reduction of oxygen concentration to less than the limiting oxygen concentration (LOC) of the gas stream. 

Deflagration pressure can be reduced substantially by suppression. Figure 26-30 shows the pressures measured in an ethylene-air explosion and a sodium bicarbonate-suppressed ethylene-air explosion. Fike Corporation, Blue Springs Missouri, and Fenwal Safety Systems, Marlborough, Mass., supply explosion suppression systems. 

To reduce the chance of false activation of the suppression system by vibration, a flexible pressure detector standoff is often used. Also, two detectors in series may be employed to reduce further the possi-bihty of false activation. 

Explosion-Pressure-Resistant Design for Reduced Maximum Explosion Overpressure with Explosion Suppression Explosion suppression systems provide one means to prevent the buildup of an inadmissibly high pressure, which is the consequence of explosions of combustible material in vessels. They operate by effectively extinguishing explosion flames in the initial stage of the explosion. An explosion of combustible material can generally be regarded as successfully suppressed when the maximum explosion overpressure can be lowered to a reduced explosion overpressure of not more than 1 bar (see Fig. 26-40). 

Depending upon the design criteria of the installed suppression system, an unsuppressed explosion overpressure of around 7 to 10 bar is reduced to a suppressed reduced explosion overpressure which lies in the range of Fred,max = 0-2 to 1 bar. Thus, vessels need to be explosion resistant for an overpressure of maximum 1 bar (ISO Standard 6184/4, Explosion Protection Systems Paii 4 Determination of Efficacy of Explosion Suppression Systems, Geneva, 1985). 

The best advantages of explosion suppression systems is that they can also be used for explosions of combustible materials with toxic properties and that there is no penetration on the location of the process equipment for safe application. 

Explosion suppression systems comprise explosion detec tors, pres-... 

FIG. 26-42 Influence of the activation overpressure and suppressant upon the effectiveness of a suppression system (constant test condition). 

Design of explosion suppression systems is clearly complex, since the effectiveness of an explosion suppression system is dependent on a large number of parameters. One Hypothesis of suppression system design identifies a limiting combustion wave adiabatic flame temperature, below which combustion reactions are not sustained. Suppression is thus attained, provided that sufficient thermal quenching results in depression of the combustion wave temperature below this critical value. This hypothesis identifies the need to deliver greater than a critical mass of suppressant into the enveloping fireball to effect suppression (see Fig. 26-43). 

Explosion hazard definition Suppression system hardware definition... 

For any proposed suppression system design, it is necessary to ascribe with confidence an effective worst-case suppressed maximum explosion overpressure Pred.max- Provided that the suppressed explosion overpressure is less than the process equipment pressure shoclc resistance and provided further that this projected suppression is achieved with a sufficient margin of safety, explosion protection security is assured. These two criteria are mutually independent, but both must be satisfied if a suppression system is to be deployed to provide industrial explosion protection. 

Suppression system design parameters fall into the two categories of explosion threat definition and suppression system hardware definition. The various influences are summarized in Table 26-23. 

Provide automatic sprinkler system/inerting gas Provide deflagration vents Provide deflagration suppression system Monitor flammable atmosphere/fire Provide nitrogen blocks (nitrogen injection to stop flame propagation) or other explosion isolation measures... 

Check for plant integrity/flammable leaks periodically or continuously on-line, as appropriate Install appropriate fire/smoke detection, audible alarms Provide adequate fire suppression systems Deal with mishaps such as spillage immediately... 

Ensure the adequaey (in terms of quality, quantity and reliability) of serviees/utilities, e.g. steam, proeess/eooling water, eleetrieity, eompressed air, inert gas, fire suppression systems, ventilation. Stand-by or emergeney serviees may be required. Some general safety design eonsiderations are summarized in Table 12.11. 

The flare tip assembly shall be furnished complete with smoke suppression system, pilot and igniter assemblies, windshield, and flame retention assembly. 

The scope of supply includes all the required equipment, piping and instrumentation for proper flaring of gaseous effluents with smoke suppression system. The scope of supply shall include at least ... 

Flare smoke suppression system consisting of an air duct mixing section, air nozzle ring header and associated duct work from ground level to fire tip. 

It should be recognized that deflagration suppression systems have a number of shortcomings, such as ... 

Figure 3-12 is a schematic of a deflagration suppressant system for process equipment. Each application requires experimental validation of the suppression system design. 

Suppressant systems for piping are called suppressant or chemical barriers and are discussed in Section 3.8. 

Additional information on deflagration suppression systems can be found in NFPA 69 (1997), CCPS Guidelines (1993), Bartknecht (1989), Schofield and Abbott (1988), and Eckhoff (1997). 

FIGURE 3-12. Schematic of a deflagration suppression system for process equipment. 

This type of isolation device (also called a chemical barrier) is similar to deflagration suppression systems used on process equipment. This barrier system consists of an optical sensor, installed in the pipeline or duct between two items of equipment, that detects an oncoming deflagration... 

The unique combination of properties associated with certain fluonnated methanes and ethanes has led to their widespread use in fire suppression systems The three halogenated fire suppression agents in general use today are bromo-trifluoromethane (CF Br, Halon 1301), bromochlorodifluoromethane (CF2BtCl, Halon 1211) and, in small volumes, primarily in the republics of the former Soviet Union and Eastern European nations, 1,2-dibromotetrafluoroethane (BrCF2Cp2Br, Halon 2402)... 

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