An explosion occurred at a chemical plant in an analysis room containing various analyzer instruments, including a gas chromatograph supplied with hydrogen. A contract operator was performing work to install a new vent line to a benzene analyzer that was part of a group of CO2 analyzers, but separate and unrelated to the gas chromatograph. During the process of this work, a plant supervisor accompanying the contract operator doing the work had an indication of flammable gas present on a portable detector. This was in conflict with the fixed gas detector in the analysis room that was indicating that no flammable gas was present. As a precaution, the plant supervisor immediately cut off the hydrogen supply and, along with the contract operator, began the normal task of determining if there was a gas leak in the room. While investigating the potential gas leak, the gas chromatograph that was housed in a cabinet exploded and the nearby contract operator was injured in the chest and hand when the cabinet door burst open and received minor burns to the hair and face from the fire. See Attachments 1 and 2 for diagrams and photos.
The plant control room was immediately notified by radio of the incident and subsequently called emergency services to send an ambulance to treat the injured contract operator. The ambulance arrived at the plant in approximately 15 minutes and, as a precautionary measure, the contract operator was transported to a nearby hospital for further assessment. There was no internal damage or damage to the airways and no additional medical injuries were found. Minor damage to the gas chromatograph and cabinet occurred from the explosion/fire.
The flame ionization gas chromatograph (used in this application to measure the quality of specific hydrocarbon) was installed in a small cabinet that was not rated for explosive atmosphere application, and the natural ventilation through cabinet louvers was insufficient. In addition, the cabinet louvers were not connected to an exhaust duct, allowing gases to escape into a room not rated for explosive atmosphere. The analysis room's fixed gas detector was located too far from the cabinet and was not mounted high enough to detect the small gas leak from the gas chromatograph system. The valve that shuts off the hydrogen supply was located near the gas chromatograph's inlet, and it would be better located outside of the analysis room. The origin of the leak has not yet been pinpointed, but the explosion occurred either in the gas chromatograph or in its cabinet. These deficiencies have since been corrected and are shown in the diagrams presented in Attachment 1. Photos of the gas chromatograph and associated cabinet are shown in Attachment 2.
- It is important to understand the requirements and standards associated with safe equipment design (especially electrical equipment containing an internal ignition source with flammable gas) in potentially explosive atmosphere environments. Misinterpretation of requirements and standards can lead to serious consequences. If the application of a standard is not fully understood, it is advisable to contact the author of the standard to remove any misunderstanding and not try to interpret the rules.
- Gas detection instrument location is critical to proper functioning. Light gases like hydrogen rise in air, and gas detection needs to be at the high point in the potential source area so that even small leaks can be detected. Various alarm/action thresholds below the lower flammability limit (LFL) of the flammable gas give additional warning of a possible problem in the event of a gas leak. The following is a summary of how equipment involved in this incident should have been installed.
- The gas chromatograph should not be installed in a sealed cabinet, but should follow explosive atmosphere design standards to have forced ventilation with a minimum flow rate of 12 times the cabinet volume per hour and to exhaust outside the building. With this change, the analysis room can remain in its current configuration and it does not fall under the explosive atmosphere regulations.
- The fixed gas detector must be installed in the cabinet sealed volume, and must comply with explosive atmosphere regulations. The gas detector must be connected to an interlock system and set with two threshold levels; the first at 25% of the LFL (which sends an alarm) and the second at 50% of the LFL (which closes the hydrogen isolation safety valve). For this gas detector, the 100% LFL threshold level is set at 4% hydrogen in air.
- To minimize the consequences of a possible leak of hydrogen inside the cabinet, it is recommended that the hydrogen isolation safety valve be installed outside the analysis room.
- The explosive atmosphere regulations also require the installation of a door switch that stops the supply of electricity and flammable gases whenever the door is opened (for example, when performing maintenance). This door switch limits the risk of creating an explosive atmosphere in the room that is not regulated under explosive atmosphere standards.