Continuing with our series on the US Chemical Safety Board accident investigation videos, we discuss today the most insidous and hidden hazards in the chemical processing industry-Static Electricity. Why is static electricity so hazardous in chemical plants? Simply because many inflammable liquids (like most solvents), tend to accumulate static charges on their surface, especially when the liquid becomes turbulent. Static charges can then cause sparking and a single spark can then ignite a fire or cause an explosion of a very high magnitude. The consequences can be frequently disastrous.
Prevention of static buildup is done by bonding the metallic parts of all the liquid carrying equipment to each other (known as “bonding”) and then tying these to a single earth point (known as “grounding”). Together this technique is known as “bonding and grounding” and this helps the static charge that is built up on the liquid surface, to flow to the earth (ground), thereby preventing any sparking.
This is why workers are careful to carry out “bonding and grounding” while emptying the contents of a typical tank truck into a storage tank. The question is whether these measures alone can prevent static buildup entirely?
The answer is no. Surprised? Watch the video below. I”ll bet you never thought about “bonding and grounding” inside a storage tank!
Our conclusion? Safety is ensured by multiple and diverse layers of protection. Simply depending on a single protection mechanism does not ensure safety. Thus relying on just the “bonding and grounding” scheme did not help, as shown in the incident.
Hence the plant management should have thought of using Nitrogen blanketing or purging, as an additional layer of protection. Alternatively, a non contact level measurement technique like a Radar level gauge could have been used. Care should be taken to ensure however that the instrument is suitable for installation in a hazardous area, most likely Zone 0.