By Martin Gollin
Under certain conditions, some reactions can undergo an exothermic runaway. Such a reaction occurs when the rate of heat generation exceeds the rate of heat removal. The temperature of the system then increases, which accelerates the rates of reaction and of heat generation. The temperature then increases further, and the reaction experiences a rapid (exponential) temperature and pressure increase. In some cases, the exothermic reaction may generate compounds that can result in secondary consequences that must also be mitigated (e.g., peroxide decompositions can generate oxygen with the potential for the formation of flammable vapor mixtures).
The potential for an exothermic runaway reaction, and its severity, depend upon several factors. These include the:
Initiating events that may result in an exothermic runaway reaction include fire, loss of cooling, contamination, loss of upstream reaction, control system failure, catalyst deactivation, etc. In order to analyze the response of the system and the requirements of the safety systems, it is important to clearly define the scenarios that could require safety systems to operate.
The effect of using a pressure activated relief device to provide protection in the event of an exothermic runaway reaction is that, when the relief valve opens it is the “light” material that is vented preferentially. This has the effect of changing the Vapor-Liquid Equilibrium (VLE). As a consequence, if the reactants are the “heavies” in the system, when the relief valve closes the temperature is higher than at the same pressure before the relief valve opened. Therefore, if no action is taken, the exotherm will continue, but at an accelerated rate. The cycle of relief valve opening, closing and the increasing reaction rate will continue until, eventually, the relief valve cannot provide adequate cooling and the temperature and pressure of the system accelerate exponentially.
There are systems where, no matter how large a conventional pressure-activated relief valve is installed, it may not be possible to demonstrate analytically that sufficient venting can be provided to protect the system from exceeding the Maximum Allowable Working Pressure (MAWP). Such systems may include polyol reactors, oxidation reactors, epoxidation reactors, etc. Indeed, any system where the reactants are high boiling point materials should be examined for the potential that a conventional spring activated pressure relief valve system might be unable to prevent an overpressure following an exothermic runaway reaction. If protection against overpressure of the system cannot be provided by a conventional spring activated pressure relief system, then some type of instrumented system or a pressure relief system that does not re-close (e.g., a rupture disk) must be used.
The potential for the situation where conventional pressure-activated relief valves may be unable to fully protect a vessel or system from overpressure is recognized by the ASME Code. ASME approved Code Case 2211 (ASME, 1995) which allows pressure vessels and systems to be protected by system design in lieu of mechanical (i.e., pressure-activated) relief devices, subject to the following conditions:
CAUTION: This is a short summary of the discussion and guidance provided by Code Case 2211. Any organization wishing to utilize this approach is advised to study the Code in detail before proceeding with this practice.
Code Case 2211 does not define the level of protection that must be provided by an instrumented system in order that the frequency of a scenario that a mechanical relief device would be required to work is “tolerable.” This is designated (implicitly) as the responsibility of the user/owner of the system.
Some governments define acceptable risk levels for facilities constructed within their jurisdiction. This is not the situation in the USA. Various companies have adopted their own internal objective risk tolerance criteria that allow them to determine at what frequency a given consequence should be tolerated. This issue is discussed in the books “Layer of Protection Analysis - Simplified Process Risk Assessment1” and “Guidelines for Evaluating Process Plant Buildings for External Explosions and Fires2.” Other companies rely upon analyzing each scenario individually and assessing qualitatively whether adequate protection has been provided. However, unless some endpoint is defined, it is difficult for an organization to assess in a consistent manner whether adequate protection has been provided for a given scenario.
Conventional pressure activated relief valves may not be able to provide sufficient venting to protect a system from exceeding its MAWP. In such cases, overpressure protection may be provided by system design. A subsequent article will highlight some ways that this may be done.