A relief system designer is primarily guided by the expected “relief load.” That is, the maximum flow rate expected through the relief system under credible failure scenarios. An overestimate of the relief load will lead to oversized equipment (wasted capital and higher operating costs) and an underestimate will result in an ineffective and unsafe system. In the aftermath of the March 2005 major incident at the BP Texas City refinery, on-plot relieving of hydrocarbons to atmosphere has come under intense criticism, and rightly so. It can be stipulated that the remedial measures being considered include routing of any existing on-plot atmospheric relieving devises to closed relief systems. This change, if brought about without due care, can lead to compromising the design capacity of the relief system, and to potential disaster. In the sizing of individual relief valves protecting equipment or process or system, it is a common practice not to take cognizance of any immediate operator action or the action of any mitigating devices. However, an increasing number of consultants and practitioners are recommending not to apply the same philosophy when it comes to designing an overall refinery flare system to cope with common mode failures (e.g., loss of power, cooling water supply failure, etc.). They propose taking credit for the action of devices such as unit emergency shutdown (ESD) systems, trips (for example, fired heater fuel supply cut-offs), or auto-starts of pumps whose actions reduce the potential load on the overall refinery flare system. Savings can thus be realized in the sizing of flare headers and other ancillary equipment. While there is no objection, in principle, to taking credit for ESDs in the design of relief systems, its application in practice deserves careful scrutiny. There are still many related issues that have not been adequately addressed by the proponents of the credit-taking approach. This paper highlights these concerns and offers practical advice to those facing relief system design decisions.
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