A Critical Review of ISO 4126-10 Second Edition 2024

This paper provides a critical review of the second edition of ISO 1 4126-10, an international standard titled “Safety devices for protection against excessive pressure”. Part 10 of ISO 4126 focuses on “Sizing of safety valves and bursting discs for gas/liquid twophase flow”. The second edition of ISO 4126-10 was published in February of 2024. The first edition of ISO 4126-10 was published in October of 2010. This review is primarily focused on items that significantly deviate from other recognized and generally accepted good engineering practices (RAGAGEP).

Applicability

A single parameter w method is used to calculate the relief flow requirements for different types of applicable scenarios. Both the first and second editions of ISO 4126-10 place strict limitations on the applicability of the single parameter ! method (section 5) used to establish the relief flow requirements. These limitations are summarized in Table 1.

ISO 4126-10 indicates that the use of the sizing method outside the designated limits can lead to “unacceptable errors” and/or in general the “oversizing” of the relief device. In many cases, oversizing a pressure relief valve is just as “unacceptable” because oversizing can induce twophase flow, add more safe discharge location challenges, and/or lead to destructive chatter.

We note that critical properties for mixtures are not simply linear averages of the individual mixture components critical properties (see Figure 1). ISO 4126-10 does not provide a method for the calculation of mixture critical properties. An equation of state is required to generate mixture critical properties. If an equation of state is needed to generate critical mixture properties then the same equation of state can easily be used to generate suitable properties data sets for direct numerical vdP integration.

ISO 4126-10 methods also require the evaluation of mixture latent heat of vaporization, mixture surface tension, mixture liquid heat capacity, mixture liquid and vapor mass densities, mixture bubble point or saturation temperatures, and mixture viscosity. During pressure relief caused by a runaway chemical reaction the mixture properties will dynamically and continuously change. Providing a consistent set of these thermodynamic and transport properties already dictates the use of detailed thermophysical and transport properties models. This important fact contradicts the use of the simplified single parameter ! method for establishing the mass flow rate and choking conditions. The w method is a simplified equation of state with a limited range of applicability which has an implicit imputed value of the mixture speed of sound for nozzle and pipe flow calculations.

The limitations shown in Table 1 render the sizing based on the single parameter ! method outlined in ISO 4126-10 essentially useless (or at best minimally useful) for many industrial systems and applications [7, 12]. The use of the w method has introduced the need to perform iterative complex calculations that would otherwise be much simpler and more accurate using direct numerical vdP integration for different types of flow and where condensation, degassing, body bowl choking, and/or multiple chokes are possible.

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