Jun-2023

Explosion in analyser house

Following an incident at a refinery, design modifications made to compressors means any future explosions will be prevented.

Rajib Talukder
Aramco

Article Summary

In 1999, the author witnessed an explosion in a refinery’s analyser house. The root cause analysis revealed that hydrogen ingress occurred through the Instrument Air (IA) line, leading to the explosion. The source of hydrogen was traced back to the Diesel Hydrotreater (DHT) recycle gas compressor, which happened to be a reciprocating compressor.

The author is pleased to observe that the vendor responsible for supplying reciprocating compressors to the aforementioned refinery has made design modifications in the new machines installed in a recently commissioned refinery. These modifications aim to prevent incidents like the 1999 explosion in the analyser house.

However, the author is deeply concerned to discover a similar arrangement in the reciprocating compressor of a different vendor at a refinery commissioned in 2018.

The purpose of this article is to raise awareness and emphasise the importance of avoiding such recurrences by addressing the design and operational aspects related to hydrogen ingress and its potential hazards.

Findings and  Analysis
On the morning of 1999, an explosion occurred in the analyser house due to hydrogen ingress through the Instrument Air (IA) line. The purpose of pressurising the analyser house with IA was to maintain a non-explosive environment inside.

Prior to the explosion, one of the recycle gas compressors in the diesel hydrotreater (DHT) was taken out of service for maintenance due to abnormal noise. The reciprocating compressor, which had eight loading/unloading valves, underwent a thorough overhaul by the maintenance team. Each valve had IA tubing connections for loading/unloading the valve actuators and flare tubing connections for collecting valve gland leakage. Both types of tubing, IA and flaring had the same size and the same direction of rotation for tightening/loosening. However, there was no colour coding for the tubing.

During investigation, it was discovered that during the machine’s overhaul, one IA and flare tubing connection had been inadvertently swapped.

After the completion of maintenance, the recycle gas compressor was restarted on the previous night. Unfortunately, due to an incorrect tubing connection, the recycle gas from valve gland leakage, containing 95 mol% hydrogen and 0.05 mole% H₂S, continued to flow into the Instrument Air (IA) header. It’s worth noting that the analyser house was located in close proximity to the compressor house, where the reciprocating machine was housed. This proximity allowed the hydrogen and H₂S-laden gas to reach the analyser house, leading to the subsequent explosion.

The following morning, a distinct smell of hydrogen sulphide (H₂S) was detected in the vicinity of the analyser house. To identify the source of the H₂S, a technician entered the analyser house. However, shortly after entering, a loud explosion occurred in the analyser house.

Indeed, the sequence of events highlights the gravity of the situation and the potential dangers involved in the presence of hydrogen and H₂S within the confined space of the analyser house. The unintended flow of hydrogen and H₂S-laden gas into the Instrument Air (IA) system posed a substantial risk, which ultimately resulted in the subsequent explosion.

This incident underscores the importance of effective preventive measures and diligent maintenance practices to mitigate the potential hazards associated with such gas leaks. It also serves as a reminder of the critical need for proper safety protocols and awareness to prevent similar occurrences in the future.

Recommendations
In response to the incident and as a short-term measure, changes were implemented in the refinery. Flare tubing was painted red, while Instrument Air (IA) tubing was painted blue, providing visual differentiation between the two types of tubing. This colour coding aimed to prevent the incorrect connection of IA and flare tubing in the future.
For a long-term solution, modifications were made to the tubing system. Different sizes of tubing were implemented, ensuring that IA tubing and flare tubing were not interchangeable. Additionally, the directions of rotation for tightening and loosening were modified to be different for each type of tubing. These changes were intended to prevent the inadvertent swapping or misconnection of IA and flare tubing during maintenance or overhaul activities.

By implementing these short-term and long-term measures, the refinery aimed to enhance safety and prevent recurrence of incidents related to tubing connection errors, reducing the risks associated with hydrogen ingress and potential explosions.