Relief Device Sizing and worst case scenario of Over-Pressurisation

Thanks to all for overwhelming responses to the recent update on Partial Inerting and Operator Earthing.

Looking at recent incidents that have happened in India, this topic was long due to being covered here on my blog. If we look at the Hyderabad Reactor Explosion or Tarapur Explosion or the more recent Moosapet Explosion, all have been suspected to be reactor explosion because of overpressurization.

The companies who are into manufacturing sector usually size their relief devices in the designing stage, when the process to be followed is not finalized and hence they assume a base solvent and size the relief devices for its vaporization in case of an external fire.

Now if we consider the T2 Laboratories accident where MCMT (Methylcyclopentadienyl Manganese Tricarbonyl) was being manufactured, an explosion which was equivalent to 1400 pounds of TNT was a case of a runaway reaction and killed 4 employees of T2 laboratories and injured 32 others. The impact was heard till 15 miles, the debris was found up to one mile away & buildings were damaged till one-quarter of a mile.


There were three simple problems in the system:

1. There was no scale-up study carried out to change from one-liter reactor to 2450 gallon reactor and not even after a further increase by one third.
2. The cooling system employed by T2 was susceptible to single-point failures due to a lack of design redundancy. 
3. The MCMT reactor relief system was incapable of relieving the pressure from a runaway reaction.

The outcome was a runaway reaction, which caused an explosion and killed multiple. 

The most difficult aspect of the design and sizing of pressure relief devices is ascertaining the controlling cause of overpressurization. This is sometimes referred to as the worst case scenario. Overpressurization of equipment may result from a number of causes or combination of causes. Each case must be investigated for its magnitude and for the probability of its occurrence with other events.

Hence it is imperative that relief device sizing is reviewed for the worst possible case of overpressurization which can include: 

1. External fire; 
2. Blocked outlets (Liquid outlet/vapor outlet);
3. Utility failures, such as Power failure, Loss of instrument air, etc.;
4. Loss of cooling duty, such as Loss of cold feed, Loss of top or intermediate reflux.;
5. Thermal expansion; 
6. Abnormal heat input; 
7. Abnormal vapor input; 
8. Loss of absorbent flow; 
9. Entrance of volatile material; 
10. Accumulation of Non-condensable; 
11. Valve malfunction, such as Check valve malfunction, Inadvertent valve operation (open/close/ bypass), Control valve stuck open/ fails closed.;
12. Process control failure;
13. Mechanical equipment failure;
14. Exchanger tube rupture;
15. Upstream pressure relief;
16. Runaway chemical reaction;
17. Human error.

It is also important to review single-phase or two-phase sizing for the relief devices. Most widely accepted sizing methodology is AIChE DIERS (Design Institute for Emergency Relief Systems).

Also, a scale-up study is an important factor to ensure adequate measures to prevent lack of cooling and to ensure adequate relief sizing.

Summarizing the discussion, the adequacy of any safety relief system is subject to its capacity to relieve the overpressure generated in a system. Pressure relief devices should be designed to passively protect against a predetermined set of “worst case” conditions and should be installed to react to these conditions regardless of daily operation activities. For each piece of equipment requiring overpressurization protection, a credible worst-case scenario should be defined. For a given vessel, several plausible scenarios may exist – from external fire to various operating contingencies, such as overfill or runaway reaction or vessel swell conditions. System overpressurization is assumed to be caused by the controlling scenario. Most controlling scenarios are loaded with conservative assumptions that are never achieved in actual operating conditions. It is the controlling scenario relieving rate that dictates the pressure relief device size. 

Hope you can gather the right crux of the discussion, which is, that relief sizing should be based on worst case scenario instead of a considering only a single external fire case scenario. 

Please do not forget to share this with your friends and colleagues. Do follow the blog and keep writing to me for any questions and issues on himanshuchichra@gmail.com.