GUIDELINES FOR CONTROLLING CATASTROPHIC RELEASES BY MEANS OF REMOTELY ACTUATED VALVES

摘要

Modern world-scale process plants often hold significant amounts of potentially hazardous materials. This goes against the principles of inherent safety and is often also undesirable for financial and quality reasons. The main reasons for the large inventories are the super-sized equipment in modern world-scale plants and process control requirements. Loss of containment (LOC) can cause major incidents in the process industry and lack of quick isolation is often cited as a contributing factor by investigators of such incidents. Some regulators have therefore prescribed the use of emergency isolation as in the case of storage of liquefied petroleum gases (LPGs). While it might be true that in storage yard settings the benefits of quick isolation usually outweigh the adverse effects, this cannot be said for a process plant setting. A desire for quick isolation circumvents the necessary discussion about the wisdom of isolation, the speed at which this isolation should occur and also the practicality. Quick isolation of a flow, in a process facility, can create unanticipated and undesirable process conditions some of which might prove to be more hazardous than the initiating LOC event. For example, emergency isolation of LPG systems under fire conditions can cause boiling liquid expanding vapor explosions (BLEVEs). Pressure effects or missiles from such BLEVEs can easily damage adjacent equipment and cause domino effects. Draining energy from such systems, e.g., through depressurization, might be safer than relying on the integrity of the containment system during fire situations. Root causes for LOC include random equipment failures, age-related failures, human error and systematic failures. Factors that greatly increase the consequences of a LOC incident include equipment congestion and encroachment by third parties. Encroachment is often beyond management's control; however, a buildup of congestion within the plant is under management's control. An uncontrolled and haphazard provision of quick isolation valves can significantly increase the number of leak points as well as congestion in a process plant. Such congestion can potentially turn a flash fire incident into a vapor cloud explosion (VCE), which has significant knock-on effect potential. Issues surrounding the design, installation and operation of remotely actuated valves in a process plant need therefore to be evaluated on an individual basis and compared with the pros and cons of other alternatives. Installation of emergency isolation in a process plant based on a simple summation of inventories is therefore ill advised. Emergency isolation should be one possible solution to a potential LOC design scenario. Any solution should be based on the application of risk-derived criteria that distinguish between safety, environmental and nuisance events and also between spills and airborne releases. Within this framework it is particularly important to address performance criteria that a solution has to meet; e.g., retention of hazardous material, spurious trips, etc. Performance is strongly dependent on the intervention by the operator and unrealistic expectations can yield unsatisfactory designs. There are at present no practical industry guidelines for designers that address the circumstances in which remotely actuated valves should be installed. This paper aims at providing practical rules for the installation of remotely actuated isolation capabilities.