Calculators
PSV Relief Valve Sizing — Gas / Vapor
Required effective orifice area for a pressure safety valve in gas or vapor service per API 520 Part I, Equation 14 (critical flow). Selects the smallest standard API 526 orifice above the required area.
Method: API 520 Part I (10th ed.), Equation 14, critical-flow gas sizing. Assumes: critical flow (verify condition), single-phase gas, valid Z, Kd per vendor cert (0.975 typical). Back-pressure correction Kb = 1.0 for conventional PSV when built-up back pressure < 10% of set. For fire case use 21% overpressure per ASME UG-125. Standard: API 526 orifice letters.
© 2026 Inflection Point Engineering, LLC. All rights reserved. The content of this page — including calculation methods, reference data, written analysis, interactive tools, and source code — is the intellectual property of Inflection Point Engineering, LLC and is protected under applicable copyright, trademark, and trade secret laws. Unauthorized reproduction, redistribution, modification, or derivative use in whole or in part is prohibited without prior written consent.
Disclaimer. This material is provided for informational and educational purposes only and does not constitute professional engineering advice. Calculations, reference data, and methodologies are based on published standards and accepted engineering practice but are not a substitute for engineering judgment, site-specific analysis, or review by a licensed Professional Engineer. Inflection Point Engineering, LLC makes no warranties, express or implied, regarding the accuracy, completeness, or fitness for a particular purpose of any content presented here, and shall not be liable for any direct, indirect, incidental, or consequential damages arising from its use. Users assume all risk associated with applying this content to real-world design, operations, or decisions.
© 2026 Inflection Point Engineering, LLC. All rights reserved.