Explosion pressure relief

With explosion pressure relief the maximum possible explosion pressure is lowered by opening a relief vent after explosion initiation. This can be a bursting disc, explosion panel or explosion vent, with the latter allowing repeated use. In extreme cases the side wall or the roof of the industrial hall may serve as vent opening. By explosion relief the confined explosion becomes a partially confined one with a corresponding lower maximum pressure. Pertinent calculation procedures can be found in [12, 13]. 

Explosion valves working without auxiliary sources of energy close at the higher velocities of flow occurring during explosion pressure relief. Thus they automatically block the pipe and prevent the explosion from propagating. 

Among other things, one prerequisite necessaiy to calculate the pressure relief openings needed on the apparatus is knowledge about the explosion threat definition and venting system hardware definition. Tne various influences are summarized in Table 26-22. 

Consider need for inerting, flame arresters, pressure relief valves, explosion vents (venting to safe location)... 

Protective features Provide protective as well as control features, e.g. pressure and vacuum relief, explosion suppression relief, advance inerting, containment... 

At 2 20 A M., another explosion occurred, the BLEVE of sphere 407. Its fireball was less intense than the earlier one. The sphere s top section traveled 190 m (620 ft) and caused the destruction of a firewater tank and one of the plant s fire pumps. Other sections further damaged other units. The pressure relief valve of this sphere traveled 500 m (1600 ft). The damage from projectiles was much greater than that caused by the first sphere failure because they traveled farther and in more damaging directions. 

The driver of the tank truck decided to take the coastal route to Puerto-tollano, which passed close to several campsites. During the journey, the pressure in the tank built up and, because of the absence of a pressure relief valve, the weakened tank cracked. The propylene that was released ignited, and a flash fire burned near the tank. Eventually this ruptured and an explosion occurred close to a campsite, killing 210 people. 

Another required safety feature on water heaters is a temperature and pressure relief valve. This is mounted near or on top of the tank. The temperature and pressure valves are designed to vent water if the temperature or pressure in the tank becomes too high. If there is no relief valve and the controls fail to limit water temperature, there is a danger of a powerful explosion from superheated, pressurized water flashing to steam as the water heater bursts. 

Rupture disks may be used to satisfy the requirements of the code for conditions such as corrosion and polymer formations, which might make the safety/relief valve inoperative, or where small leakage by a safety valve cannot be tolerated. They are particularly helpful for internal explosion pressure release. 

WTien the relief device relieves, the explosion pressure falls off, but then it increases faster than in the beginning due to the opening of the relief when the flame front is distorted creating an acceleration of the combustion process [54]. Thus, there are two pressure peaks in the course of the relieving (1) at the activation of the relief device due to pressure buildup in the vessel, and (2) at the end of the combustion reaction. The first pressure is always greater than the second. The second pressure rise is created by turbulence during the venting process [54]. 

Using a nomograph requires only the vessel volume in meters, selecting the dust class. St-1, St-2 or St-3 from Table 7-28. Using Tables 7-29 or 7-30 select the Kst value determined experimentally. The reduced pressure, Pfed. (maximum pressure actually developed during a vented deflagration, termed reduced explosion pressure) must not exceed strength of vessel (see earlier discussion) and the Psut, i.e., the vent device release pressure. Note that the static activation pressure, Pjj, must be determined from experimental tests of the manufacture of relief panels such as rupture disks. 

The required relief area. A], for a volume V] = 1 cu m obtained from the nomograms. The same reduced explosion pressure, P,.ed, and same static activation pressure, Psiau of the relief device are the same for both volumes Vi and V2, and therefore constant. WTien the mechanical strength of the vessel, is changed, the maximum volume and height will change with the hazard class, St-1, St-2, or St-3. 

Donat, C., Selecting and Dimensioning of Pressure Relief Devices for Dust Explosions, Staub-Reinhaltung der Luft, 31, No. 4, 1971, pp. 17-25. 

Emergency Pressure Relief Fires and Explosions Rupture Disks... 

Provide pressure relief and explosion relief (or suppression) and systems for emergency venting where relevant... 

The term engineered safety covers the provision in the design of control systems, alarms, trips, pressure-relief devices, automatic shut-down systems, duplication of key equipment services and fire-fighting equipment, sprinkler systems and blast walls, to contain any fire or explosion. 

Interaction is exothermic and the mixture was normally maintained at 60°C by stirring and cooling. Malfunction caused a temperature increase to 70° C and cooling capacity was insufficent to regain control. The temperature steadily increased to 120°C, when explosive decomposition occurred. This was attributed to thermal instability of the reaction system and inadequate pressure relief arrangements. 

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