Quest Relief Valves and Vents: How Exit Conditions Affect Hazard Zones

Pressure relief valves and vents in the petrochemical industry are often the last line of defense in averting a major accident. Recent design standards (API 520/521) have been developed which have reduced the recommended exit velocities for hydrocarbons from pressurized storage. There are computer models available which predict the release and dispersion of high velocity gas jets. In some instances, these models have been modified to account for the formation and dispersion of aerosol clouds. This paper will compare the API recommended practices with actual test data and current model predictions. INTRODUCTION Within the hydrocarbon and chemical processing industries, the use of pressure relief devices to protect process equipment and storage tanks from excessive internal pressure is commonplace, and is often required by government or industry codes. In many cases, discharges from pressure relief devices flow through a closed piping system to an elevated vent or flare. One primary function of an elevated flare or vent is to safely dispose of flammable and/or toxic gases released by the pressure relief devices, either by burning the gas to create less hazardous products of combustion, or by releasing it at an elevation that is sufficient to ensure that hazardous concentrations of the gas will not return to grade level. This type of system generally works well and is often the preferred method for disposing of hazardous gases released from pressure relief devices. However, such systems are not, and cannot be, used universally. There are many situations where it is not practical to connect pressure relief devices to a vent or flare header. For example, pressure safety valves (PSVs) on storage tanks for refrigerated liquefied gases relieve at low pressures, typically 2.0 psig or less. To safely dispose of this low pressure gas through an elevated flare or vent would require a very large diameter header system, otherwise the pressure drop in the header would create excessive backpressure on the relief valves. A similar argument applies to vent valves on API cone roof tanks.