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Emergency Relief System Design: What You Need to Know

Jens Conzen

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Oct 15, 2019

Recent chemical accidents—including the MDI tank explosions in China and Turkey in 2016 and 2019, respectively as well as the T2 Laboratories incident in 2007—highlight the potential damage when chemical manufacturers fail to recognize the underlying exothermic, or heat-producing, reaction hazard associated with their manufacturing process.

To prevent future incidents, it’s important to be aware of the energy that could be released when mixing chemicals that react exothermically. A complete understanding of the chemistry—under both optimal and abnormal process conditions—is critical. At the end of the day, your facility should have a plan in place to manage chemical reactivity hazards.

What is an Emergency Relief System?

An ERS is a passive system that consists of a pressure relief device (which can be a rupture disc, a pressure safety valve or both); downstream of the relief device is piping and often a retention system to control the release of toxic and flammable effluent. If the system terminates in a catch tank, it is considered a closed system.

As the industry has gained awareness of reactivity hazards through incidents and near misses, the Design Institute for Emergency Relief Systems (DIERS) was formed to reduce the frequency, severity, and consequences of pressure producing accidents. After thorough research and testing, DIERS created a methodology to improve and streamline the design of emergency relief systems.

The Right Design and Documentation

While it’s required for most chemical manufacturing plants to have an ERS, many production plants are older, so documentation can sometimes be lacking when validating an existing ERS design. It’s also possible that the design basis of an existing system is outdated. This can occur where the process has changed (e.g. greater batch size and higher temperature) or if the ERS was designed prior to the publication of modern design standards (e.g. legacy plant). So, how can you ensure your existing ERS is adequate? The answer depends on several key factors. First, where is the plant located? Different countries have different rules. In the US, the following documents provide best engineering practices and guidance to achieve compliance:

  • API 520/521
  • API 526
  • DIERS Methodology or ISO Standard 4126-10
  • API 2000
  • ASME B31.3

API 520 and 521 provide guidance on sizing of ERS components and computation of loads that act on the ERS during a depressurization event, while ASME B31.3 assures that the ERS will remain structurally intact.

After confirming which regulations apply to you, the next step is to test the chemical mixture in an adiabatic reaction calorimeter to obtain the safety-relevant design parameters required for sizing. In addition, one must reference the original system design documentation of obtain it through an onsite inspection. At the end of the design phase, it is important to perform a dispersion calculation under the consideration of local air emission standards and acute exposure guideline levels (AEGLs) so that environmental considerations are addressed.

The flow chart below outlines the typical process for ERS design and is further described here:

Next Steps

An adequately designed ERS will provide increased safety for plant personnel and surrounding communities by providing containment of flammable as well as toxic mixtures and preventing the rupture of process vessels. It will also improve plant operational reliability. If your plant lacks ERS design records, has made process changes over the years, or if the ERS shows significant wear and tear, contact us to perform a detailed inspection and design verification. Likewise, if you are scaling up a new process or building a new plant, we can lead and document a detailed design analysis for your ERS.