Explosion overpressure relief

A common cause of a BLE T] in plants of the hydrocarbon-chemical industry is exposure to fire. With an external fire below the liquid level in a vessel, the heat of vaporization provides a heat sink, as with a teakettle evolved vapors exit tnrough the relief valve. But if the flame impinges on the vessel above the liquid level, the metal will weaken and may cause the vessel to rupture suddenly, even with the relief valve open. The explosive energy for a BLE T] comes from superheat. This energy is at a maximum at the superheat hmit temperature. (SLT is the maximum temperature to which a hquid can be heated before homogeneous nucleation occurs with explosive vaporization of the hquid and accompanying overpressure.) The SLT... 

We would never knowingly tolerate a situation in which accidental operation of a valve resulted in the overpressuring of a vessel. We would install a relief valve. In the same way, accidental operation of a valve should not be allowed to result in explosion or runaway reaction. 

Rupture disks are used for the same purpose as safety valves and, in addition, serve to relieve internal explosions in many applications. If the pressure rise can be anticipated, then the volume change corresponding to this change can be calculated by simple gas laws, and the capacity of the disk at the relieving pressure is knowm. The system must be examined and the possible causes of overpressure and their respective relief capacities identified before a reliable size can be determined. See Figure 7-14. 

P2 = back pressure or downstream at outlet of safety relief device, psig, or psia, depending on usage p = rupture pressure for disk, psig or psia p° = overpressure (explosion), lb force/sq in. p = pressure, psi abs... 

Explosion calculations, 499-504 Estimating destruction, 501 Overpressure, 502 Pressure piling, 501, 504 Relief sizing, 505 Scaled distance, 502, 503 Schock from velocity, 503 TNT equivalent, 499-504 Explosion characteristics of dusts, 515 Explosion suppression, 518 Explosion venting, gases/vapors, 504 Bleves, 504 Explosions, 482 Blast pressure. 496 Combustion, 482 Confined, 482 Damage, 498-501 Deflagration, 482 Detonation, 483... 

Blowout panels are designed to provide the proper relief area, depending on a number of design factors. These include the explosive behavior of the dust or vapor, the maximum overpressure allowable in the structure, and the volume of the structure. Design standards are available.15... 

First, we need to define generally what we are talking about A pressure relief device is any device that can purge a system from an overpressure condition. More particularly, an SRV is a pressure relief device that is self-actuated, and whose primary purpose is the protection of life and equipment. Through a controlled discharge of a required (rated) amount of fluid at a predetermined pressure, an SRV must prevent overpressure in pressurized vessels and systems, and it operates within limits which are determined by international codes. An SRV is often the final control device in the prevention of accidents or explosions caused by overpressure... 

If the plant safety shutdown is not rapid enough and an overpressure situation develops, then the pressure relief system is activated. Pressure vessel design codes such as the ASME Boiler and Pressure Vessel Code require relief devices to be fitted on all pressure vessels (see Section 13.17). If the relief system has been properly designed and maintained, then in the event of an overpressure incident, the plant contents will be vented via relief valves or bursting disks into the relief system, where liquids are recovered for treatment and vapors are sent to flare stacks or discharged to the atmosphere if it is safe to do so. The pressure relief system should allow the plant to be relieved of any source of overpressure before damage to process equipment (leaks, bursting, or explosion) can occur. 

Potential hydrocarbon losses from the overpressuring of operating vessels are controlled first by staged computer alerts and/or manual alarms to provide for correction of the condition. If the overpressure exceeds a second set point, pressure relief valves vent the vessel contents to a flare release system. The flare system provides a means of controlled burning of hydrocarbon vapors at a nonhazardous point to avoid fire or explosion risks. Smoke problems from flares are avoided by more efficient designs that use multiple nozzles and low pressure operation to promote clean combustion [57]. Greenhouse gas concerns should more frequently stimulate an interest in energy recovery options from flared hydrocarbons. 

Obviously, the best plan is to prevent an explosion from occurring, but appropriate plans should be made in the event one should occur. For example, it is recommended that all personnel be located away from the furnace during light-off in case there is an incident. It may also be advisable to have some type of relief system built into the furnace, for example pressure relief doors (sometimes referred to as explosion doors) that can open in the case of overpressurization. 

The two major explosion types can be divided further into fonr snbtypes that result in the release of high-pressure gas. The first occurs as the result of the physical overpressurization of a container, causing the container to burst, as in the case of a child s balloon bnrsting when too much air is placed in it. The container fails because it can no longer hold the pressure built up inside. This can occur in containers that may not have pressure-rehef valves, or if the pressure-relief valve fails to operate. This overpressure does not have to occur as a result of filling the container. As heat is apphed to a container from ambient temperature increases, or from radiant heat, the pressure increases inside the container. If this increase in pressure is not reheved, the container may fail. 

As much as possible, enclosures such as silos, tanks, and materialhandling structures are designed to relieve overpressures due to explosions. Design provisions include doors, windows, relief panels, and light gauge coverings... 

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