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 autostarts 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. (c) 2008 Elsevier Ltd. All rights reserved.