ISO 21013-3:2016

Overview

ISO 21013-3:2016 - "Cryogenic vessels - Pressure‑relief accessories for cryogenic service - Part 3: Sizing and capacity determination" gives engineers and safety professionals standardized calculation methods to determine the required mass flow that must be relieved from cryogenic vessels under a range of credible upset conditions. The standard covers vacuum‑insulated and non‑vacuum vessels and addresses normal operation, loss of vacuum, air/nitrogen ingress, fire engulfment, and combinations (e.g., fire with loss of vacuum). It also provides guidance for cryostats (Annex A) and points to related safety device standards.

Key technical topics and requirements

• Scenarios covered: vacuum‑insulated intact system, pressure‑build‑up regulator fully open, loss of vacuum, air/nitrogen condensation in the vacuum space, fire engulfment (with insulation in place or lost), and total loss of insulation.
• Heat transfer calculation: methods to calculate total heat input from the hot (outer jacket) to the cold (inner vessel) wall including:
  • heat leak through insulation using overall heat‑transfer coefficients (U) and thermal conductivity (k),
  • heat from pressure‑build‑up devices (e.g., ambient air vaporizers),
  • heat through supports and piping in the interspace,
  • effects of air/nitrogen condensation and increased gas thermal conductivity.
• Mass‑flow (relief) sizing: procedures to convert heat input to the mass flow that pressure‑relief devices must discharge at the relieving pressure and temperature (P, T). The standard distinguishes subcritical and critical flow regimes.
• Pressure‑relief device sizing and piping: sizing rules for pressure‑relief valves and bursting discs, accounting for pressure drop, backpressure and heat transfer in relief piping.
• Conservative design guidance: where a specific calculation method is not applicable, the standard requires application of good engineering practice based on established physical science.

Practical applications and users

Who uses ISO 21013-3:

• Pressure‑vessel and cryogenic system designers
• Safety and process engineers performing relief system analysis
• Manufacturers of pressure‑relief valves and bursting discs
• Plant operators, commissioning engineers and regulatory authorities

Typical uses:

• Sizing relief devices for liquid helium, nitrogen, hydrogen, oxygen and other cryogenic fluids
• Calculating required discharge capacity after loss of vacuum or fire scenarios
• Ensuring compliance with cryogenic pressure‑relief safety practices and interfacing with procurement/specification of relief devices

Related standards

• ISO 21013 series (Parts 1, 2, 4) - overall guidance on pressure‑relief accessories for cryogenic service
• ISO 4126‑1 and ISO 4126‑6 - safety valves and bursting disc application, selection and installation

Keywords: ISO 21013-3:2016, cryogenic vessels, pressure‑relief accessories, sizing, capacity determination, loss of vacuum, fire engulfment, mass flow, heat transfer, pressure‑relief valves, bursting discs.