An analytical API ‐type method for estimating fire relief capacity of compressed‐gas tube trailer cylinders

Abstract The fire relief sizing of compressed‐gas tube trailer cylinders is commonly performed using simplified engineering methods. This study revisits the API‐type analytical formulation for gas‐filled vessels and develops a modified steady constant‐pressure relief equation for hydrogen and natural‐gas tube trailer cylinders. The model is derived for a rigid open control volume after relief activation and couples the mass balance, internal‐energy balance, natural convection inside a horizontal cylinder, and real‐gas thermodynamic properties. The method is intended as an analytical sizing and comparison tool under a prescribed maximum wall‐temperature condition, rather than as a transient fire simulation. A real‐gas volumetric expansion coefficient is introduced to avoid the ideal‐gas approximation. The resulting coefficient is evaluated for hydrogen and methane over pressure and bulk‐gas temperature ranges relevant to tube‐trailer service. A case study for a hydrogen tube trailer cylinder is then compared with API 521 and CGA S‐1.1. For the selected hydrogen‐cylinder case, the predicted relief area is larger than that obtained from API 521 but smaller than the empirical requirement from CGA S‐1.1. The limitations of the method are discussed, including the absence of wall thermal inertia, external flame‐to‐wall heat‐transfer dynamics, localized fire exposure, jet‐fire impingement, and wall failure.