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From Powder and Bulk Dot Com Air Quality
Protection of equipment and personnel from the consequences of dust, gas or vapor / air explosions within process equipment can be accomplished by the application of effective preventions and/or mitigations. One important protection method is the use of explosion suppression. This active mitigation is intended to limit the maximum equipment pressure by the high-speed detection and suppression of a developing combustion event. High-speed pressure detectors or flame detectors mounted on the equipment detect the initial stage of the explosion when pressures are low, usually less than one psig. Pressure detectors are often used because dust particles can interfere with the effectiveness of flame detectors. The explosion detectors send an electric signal to a control system, which in turn signals the injection of flame suppressant. When triggered by the control system, a special fast acting valve or rupture disk opens releasing the suppressant into the protected equipment. The suppressant overwhelms the flame front and stops the explosion before damaging pressures can be reached. The reduced pressure, Pred, a design parameter, is the final suppressed explosion containment pressure. The detection phase usually occurs during the first 20 milliseconds of the deflagration event and suppression in the following 60 milliseconds. Thus, successful suppression occurs in less than one tenth of a second. As in the use of relief venting, the equipment must be of a design to adequately contain the predicted suppressed pressure. A RECENT CASE STUDY AND SUPPRESSION SYSTEM FAILURE Recently, an explosion occurred within a powdered product receiver (hopper) protected by a Halon 1011 explosion suppression system. The Halon injection system was mounted on the roof of the hopper and two redundant pressure switches were mounted on the roof and upper side of the hopper. The protection system had been in place for more than ten years, but had received routine inspections four times a year as is recommended by NFPA 69-2002. The most recent inspection had occurred less than a month prior to the incident. In spite of the suppression system, when a dust/air mixture was ignited in the hopper an overpressure occurred and the hopper discharge flange, attached to a screw feeder at the bottom of the hopper, failed catastrophically releasing a fireball into the operating area. Although triggered and discharged, the Halon system did not prevent the hopper overpressure. HOW COULD AN EXPLOSION SUPPRESSION SYSTEM FAIL? Explosion suppression systems are subject to several modes of failure inherent in the their design, installation, and maintenance. The first of concern is the design. The design of an explosion protection system cannot be effective unless it is based on a firm understanding of the burning properties of the expected fuel. Such properties as peak closed system pressure; Pmax and rate of pressure rise, (dP/dt)max are crucial to the design. If the wrong properties for the design are chosen, the system cannot be expected to provide the level of protection needed. In addition, the restraints placed on the design by the containment limitations of the protected equipment can affect the system performance. If Pred exceeds the maximum tolerable pressure of the container, it can fail violently in spite of the “successful” trip of the suppression system. This is also true of container attachments such as flexible connectors not isolated by pressure containing elements. In addition to design considerations, performance of the explosion suppression system will depend, like that of other active Safeguards, on periodic inspections and performance testing. The most important items to be checked are the suppressant weight and pressure, pressure switch set points, software logic performance and physical installation. Without an effective inspection and testing program of explosion suppression systems and their components the systems reliability will deteriorate with time. Prior to and during inspections the system must be safely secured to prevent dangerous trips when equipment is opened. This requirement presents an additional reliability issue in that an adequate management practice must be in place to ensure that the system is reactivated prior to resumption of operations. The final element critical to the successful suppression of a developing explosion is the integrity and correctness of the component installation. In the case study mentioned previously, this aspect of the system was most likely responsible for the system failure and the resulting equipment damage. Because a suppression system must complete its actions in a critically short window of opportunity, interference with the timing of the action will have a serious impact on performance. Delay in activation, suppressant dispensing, and/or mixing within the protected equipment will result in an increase in affected Pred, increasing the likelihood of equipment overpressure. In this case, inspection after the fact indicated that the system trip devices caused system activation and the trip set points had been verified within a month of the incident, so it is unlikely that delayed activation was the cause of the excessive hopper pressure. In addition, the Halon inventory had been verified during the latest inspection, so that a reduction in Halon inventory is unlikely to have caused the overpressure. When the Halon diffuser nozzle was removed from the top of the hopper it was found that the installation was “less than adequate”. Apparently, during the explosion suppression system installation most of a C channel roof support had been cut away in order to accommodate the diffuser leaving the bottom plate of the channel in the pathway of the Halon, as indicated in the figure below.
The steel plate interfered with the suppression of the explosion in several ways. The first, and most obvious, is that the plate was in the direct path of the diffuser protection cap, restricting the caps travel path to about 50% of that required for a clean blow off during the Halon discharge. It can be shown that the mass flow rate of Halon entry into the hopper is directly proportional to the available nozzle area, at a fixed pressure. Any reduction in nozzle area will result in an equivalent reduction in mass flow rate, critical to effective suppression. Even if the cap were released, its motion would be delayed. In addition, the downward path of the boiling Halon droplets was restricted and Halon was deflected sideways and upwards away from the hopper center. The combination of these effects resulted in an increase in the reduced pressure within the hopper to a point where the weakest element, the bottom flange, failed. Although this case was an undetected installation failure it can easily be imagined that some materials are capable of coating the diffuser cap and cementing it in place, an alternate possible failure mechanism. LESSONS LEARNED 1. Explosion Suppression Systems are not infallible; 2. Explosion Suppression Systems must be correctly installed; and 3. Explosion suppressant diffusers should be periodically removed and inspected as part of a routine inspection program. Visual inspection from above can determine if the protective cap is in place but is not enough to ensure that diffuser interferences don’t exist. How can Chilworth Technology, Inc. help you manage your fire and explosion hazards? Chilworth Technology has a group of highly qualified Process Safety Specialists, who can help you with all aspects of fire and explosion hazard evaluation. CTI laboratories can provide you with fire and explosion property testing to determine the critical parameters required to design safe facilities and operations. If you have any questions regarding this article or any other process safety concern, please contact us at (609) 799-4449 or email us at safety@chilworth.com. Source: http://www.chilworth.com/ © Copyright 1998 - 2008 Powder and Bulk Dot Com |
