Layout of equipment in vapor phase oxidation unit before and after explosion in reactor. The reactor operated in the explosive range but below the autoignition temperature. The designers realized that hot spots might form in the reactor and ignite the reaction mixture, so they strengthened the reactor and provided explosion vents. The flame arrestor was installed to prevent the explosion passing back into the heat exchanger.
There was no need, they decided, to strengthen or vent the vaporizer, mixer, or heat exchanger as there was no source of ignition in them, or so they thought. After two years of operation, an explosion demolished the mixer and damaged the heat exchanger. The probable source of ignition was an unlikely one. The vaporizer had to be cleaned from time to time. Various agents had been used including acids, which had attacked the vaporizer and deposited a mixture of metal and organic residues in the mixer.
These oxidized and became hot enough to ignite the flammable mixture of reactant vapor and air in the mixer.
THE GAP IN THE FENCE
When the plant was repaired, the reactant vapor and air were mixed immediately before entering the reactor. A flammable mixture was then present only in the reactor. This is an inherently safer solution . This could have been done in the original design if someone had realized that flammable mixtures are easily ignited and that we should therefore avoid the need for them when possible and assume they might explode when their presence is essential.
In addition to the necessary repairs, the whole plant was strengthened. Bond  summarizes many fires and explosions caused by unsuspected sources of ignition. See also Section Trevor Kletz, in What Went Wrong? Fifth Edition , Many incidents have shown that sources of ignition are likely to turn up even though we have tried to remove all those we can foresee. The reactor operated in the explosive range but below the auto-ignition temperature.
After a two-year operation, an explosion demolished the mixer and damaged the heat exchanger. This is an inherently safer solution [ 17 ]. Bond [ 18 ] summarizes many fires and explosions caused by unsuspected sources of ignition.
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Other examples of little-known knowledge are presented in Chapter Vapor cloud explosions are caused by the rapid combustion of flammable gas, mist, or small particles that generate pressure effects due to confinement; they can occur inside process equipment or pipes, buildings, and other contained areas.
A vapor cloud explosion can be either a deflagration or a detonation the distinction is important when deciding on whether or not to use a flame arrestor in pressure relief systems. A deflagration occurs when a flame front propagates by transferring heat and mass to the unburned air-vapor mixture ahead of the front. The combustion wave travels at subsonic speeds to unburned gas immediately ahead of the flame front.
Flame speeds range from 1 to meters per second. At low speeds there is little effect from the blast overpressure while at high speeds, peak overpressures can be as high as 20 times the initial pressure. Most vapor cloud explosions are deflagrations. A detonation occurs when the flame velocity reaches supersonic speeds above meters per second they are generally in the to meter per second range.
Peak overpressures can be 20 to times the initial pressure. Detonation can be initiated either by use of a high explosive charge or from a deflagration wave that accelerates due to congestion and confinement.
Sign In Help. Download as PDF. Set alert. About this page. Learn more about Flame Arrestor. Piping and valves Ian Sutton, in Plant Design and Operations , Flame arrestors A flame arrestor is installed just upstream of a pipe vent when it is possible that the material being vented could catch fire and then send a flame front back down the pipe into the equipment below the vent tip. Piping Ian Sutton, in Plant Design and Operations Second Edition , Flame Arrestors A flame arrestor also spelled arrester is installed just upstream of a pipe vent when it is possible that the material being vented could catch fire and then send a flame front back down the pipe into the equipment below it.
Figure 4. Thermal flame arrestor.