Page 314 - CHEF Guide

Mitigating Safeguards and Protection Layers

Low Pressure Tank Relief
Devices

 Reclosing devices

Figure 20-3 Low pressure relief devices

A common type of relief device is the Rupture Disk. Advantages include:
 Immediate opening
 Available in very large sizes
 Less costly if very large size or corrosion resistance is needed
There are several disadvantages including:
 Not re-closing
 High risk of nuisance failures in liquid services
 Process Safety Event (PSE) and plant reliability risk
 Subject to metal fatigue failure (cycling services or frequent SU/SD)
 Burst pressure is sensitive to temperature

20.2 Design Basis for Pressure Relief Devices

In many cases, a relief device is required for protection of vessels and equipment. All codes intentionally leave it to the
user to determine the sizing basis (sizing scenarios) for relief devices. The user is to identify all the potential causes of
overpressure and then determine which of those are credible scenarios for which the relief device is to be sized. American
Petroleum Institute standard 521 provide industry-standard guidance. However, selecting a design basis often requires
judgment decision.

There are several industry and regulatory organizations with published guidance and requirements for relief devices.
The rules often depend on the specific application such as in protection of a boiler versus protection of a pressure vessel
versus protection of a storage or low-design pressure tank. Pressure protection designs are influenced by a broad range of
rules (codes, standards, and local governmental requirements). The applicable rules depend on the equipment type
(pressure vessel, storage tank, pipe) and the geographic location.

There are many considerations in the selection of the appropriate pressure relief device including:
 Rupture disk versus Pressure Safety Valve
 Vacuum operations
 Pressure cycling

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