Protection from runaway

Containment systems are only rarely designed with sufficient pressure and temperature rating to fully contain a runaway reaction. For this reason, overpressure protection is of obvious critical importance as a last line of defense against loss events that can result from runaway reactions. The latter sections in this chapter on Pressure Relief Systems and on Emergency Relief Device Effluent Collection and Handling address design basis selection, relief calculations, and effluent treatment system configurations for reactive system overpressure protection. 

The system can prevent explosion, fire, and venting with fire under conditions of abuse. These batteries have a unique battery chemistry based on LiAsF6/l,3-di-oxolane/tributylamine electrolyte solutions which provide internal safety mechanism that protect the batteries from short-circuit, overcharge and thermal runaway upon heating to 135 °C. This behavior is due to the fact that the electrolyte solution is stable at low-to-medium temperatures but polymerizes at a temperature over 125 °C... 

A measure is preventive if it prevents the occurrence of a runaway, a decomposition, or a hazardous secondary reaction. The system conditions remain close to operating conditions. Excessive increases in temperatures and/or pressures are avoided. Preventive measures include feed rate control systems, interlocks to prevent the reaction from starting unless sufficient diluent is present or the cooling system is working, and tests for the presence of catalysts or unwanted impurities. Preventive measures are always to be preferred over protective or mitigating (defensive) measures. 

The protection principle of pressure relief is based on limiting the pressure to which equipment might be exposed by the removal of gaseous or multi-phase material flows from, in the case of an explosion or a runaway reaction by allowing certain predetermined openings to be opened in such a way that the pressure in the vessel does not exceed a predetermined permitted value. 

Where the hazard is an exothermic runaway reaction, there are number, of alternative measures that may be used either to prevent or control runaway. Informatipn on the various options is in Annex 1. In the UK, one of the most commonly selected measures used to protect reactors from exothermic runaway is an emergency relief system. These have a number of advantages ... 

In the event of abuse cases, such as short circuit, exposure to air, or overcharge or overdischarge, it is assumed that solid state Li batteries based on polymeric electrolyte systems should be much less dangerous than liquid-based batteries. It is expected that the former battery systems would be much better protected in the above cases from thermal runaway than are the liquid-based batteries. 

Rupture disks are often used upstream of relief valves to protect the relief valve from corrosion or to reduce losses due to relief valve leakage. Large rupture disks are also used in situations that require very fast response time or high relieving load (for example, reactor runaway and external fire cases). They are also used in situations in which pressure is intentionally reduced below the operating pressure for safety reasons. 

A rate-limiting circuit is incorporated into the timing circuit to prevent the pacing rate from exceeding an upper limit, should a random component failure occur (an extremely rare event). This is also referred to as runaway protection and is typically 180-200 pulses per minute. 

Most annealing of stainless-steel strip is done without a protective atmosphere in the furnace. However, combustibles must be avoided to prevent their effect on the surface chemistry of the strip. Likewise, high excess air at low fuel inputs may necessitate more aftercleaning, but some excess air protects the strip from a runaway furnace temperature condition. A simple cross-connected regulator with a low-flow tension spring (fig. 4.12) is ideal for this. Figure 4.13 shows a more accurate control. 

It is clearly not safe to test unknown reactions or compounds in a full-size reactor, as a vigorous exotherm may overcome the protection systems provided. Various theoretical techniques and small-scale tests have therefore been devised to provide data on the likelihood and severity of a runaway reaction. They vary from simple calculations and basic heating tests to sophisticated simulations of full-size plant. This chapter describes the main theoretical techniques and experimental tests available for identifying chemical reaction hazards, and suggests how to select a suitable test regime. 

Few chemical reactors are operated without any pressure relief system, but it is not uncommon for the relief system to be sized only to deal with overpressures from service fluids or fire engulfment. In such situations, where the relief system does not protect against the consequences of a runaway reaction, the safety of the reactor system must be assessed thoroughly to demonstrate compliance with legal requirements and recommended standards. The company operating the process should be able to justify its decision not to provide adequate overpressure relief. ... 

Thermal stability testing showed that the reaction mixture could decompose exothermically with self heating occurring on the plant scale from 145°C (the boiling point of the mixture is about 160°C). Decomposition of the reactant mass would lead to a runaway reaction with the generation of a toxic and irritant gas. This would be vented safely, as far as protecting the reactor from overpressure is concerned, by the emergency relief vent, but would cause a serious toxic and corrosive aerosol emission. 

In order to limit the dangers inherent to thermal runaway, these external safety devices are supplemented within the cell itself by internal protective mechanisms, such as shutdown separators (discussed in section 5.5.1), which are effective for temperature ranges from 135°C to 160°C beyond this temperature, they become ineffective, because they lose their mechanical integrity, leading to the two electrodes coming into direct contact and an internal short-circuit. This type of separator tends to be found generally in high-capacity elements. 

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