Blowdown Tool and Vessel Survivability

Depressurisation of vessels or isolated sections is a key element of the safety case for a given process design. The objective is to remove the flammable hydrocarbons from isolated process areas in a timely manner.

The blowdown event is normally separated into two types of cases:

  • Blowdown with ambient heat transfer, often initiated as part of the normal emergency shutdown procedures from typical causes like gas detection, high pressure, high temperature or high level
  • Blowdown under fire attack, where the process has entered into an emergency shutdown mode, however, a given isolated section has a pool or jet fire impinging on the vessels or pipes

The Genesis blowdown tool, SAFFIRE, provides engineers with a rigorous means of performing blowdown studies with and without fire. The mass and energy balances of the individual phases are specifically accounted for in the modelling. The tool has been verified against well documented experimental data, validated against the leading academic research and used on many projects to provide detailed analysis of blowdown events. SAFFIRE is compliant with API 521 Edition 6.

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Normally ambient heat transfer blowdown sets the minimum metal design temperature for a given isolated section due to the associated Joule-Thomson cooling or sets the maximum relief rate consistent with a given minimum metal design temperature. In contrast fire attack blowdown would normally set the minimum required relief rate to the flare system due to the requirement that a given defined level of depressurisation is required before vessel or pipe rupture occurs. Preventing rupture mitigates the risk of escalation of the fire and risk of explosion. Sometimes these minimum and maximum relief requirements cannot be met simultaneously and this is why Genesis recommend that the two types of depressuring simulation be considered together; hence the need for the Genesis blowdown tool, SAFFIRE.