(175b) Guidance for Sizing Relief Devices That Are Installed below Liquid Level in an External Fire

API 520 Part I & API 521 provide guidance for designing emergency relief devices for the external pool fire scenario for pressure vessels built to ASME Section VIII, Div.1. Current API standards provide vapor generation and/or two-phase flow sizing models assuming that the relief valve is installed in the top of the vessel. ASME Section VIII, Div.1, UG-135, provides guidance in the installation of the relief device. However, the current standards do not limit the location of relief devices to only the top of the vessels. Certain pressure vessels, such as filters or vertical heat exchanger shells may have relief devices installed at a location below the liquid level for maintenance access, space limitations, or piping constraints.  The use of traditional sizing methods can grossly undersize the valve as the relief device will need to pass fluids at an equal volume of vapor generated by the fire. Existing literature provides limited guidance on sizing and disposal requirements. This paper provides general guidance, sizing method strategies, mitigation, and design and documentation of relief devices used in such services.