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Ignition risk from oil tank washing

A series of large tank explosions in the 1960s brought to light the problems of electrostatic build-up during tank cleaning. One of the leading authorities in the area of electrostatic research, physicist Dr. John Chubb, describes in his detailed report how extensive research into the cause of these explosions has made today’s tank cleaning technology the safest ever. Read Dr. Chubb's biography.

Download or read the report:
Read Dr. John Chubb’s in-depth report (PDF-file 820 KB) detailing electrostatic ignition risks from tank washing operations. (If you can't read the file, download Adobe Reader).

Introduction to the report:
Is tank cleaning still a dangerous industry?
In the late 1960s, a number of very large crude oil carriers were severely damaged by explosions while being cleaned. The washing operation itself was cited as the cause of the ignition of inflammable vapours inside the tank. But how could washing the tanks cause such a devastating result? And how has the answer affected safety management in cleaning of carriers and aboveground storage tanks today?

Electrostatic ignition risks from water jets
Studies showed that it was the water itself that was the cause of the explosions. It was discovered that the spray mist from the water jets used in the cleaning process was electrostatically charged. On its own, this was insufficient to create any lightning-like electrostatic charges to ignite residual fumes present in an empty tank. However, studies showed ‘lumps’ or ‘slugs’ of water could become charged by the mist and create electrostatic sparks on contact with projections on the inside surface of a tank such as bulkheads or girder work.

From water mist to ignition:
Electrostatic mist created from the impact of washing jets on the tank walls
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Electric fields created at the edges of all projections on the tank walls
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‘Lumps’ or ‘slugs’ of wash water become charged by these electric fields
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Electrostatic sparks occur when these ‘lumps’ of water contact projecting tank structure
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Flammable vapours in the tank ignited if sparks have sufficient energy

 

From water mist to ignition:

Electrostatic mist created from the impact of washing jets on the tank walls

Electric fields created at the edges of all projections on the tank walls

‘Lumps’ or ‘slugs’ of wash water become charged by these electric fields

Electrostatic sparks occur when these ‘lumps’ of water contact projecting tank structure

Flammable vapours in the tank ignited if sparks have sufficient energy

Manual operations pose an additional hazard
Another aspect of tank cleaning which still poses a potential safety hazard is human activity. External activities with equipment inside tanks containing highly charged mists or cleaning activities and body movements (including rubbing of clothing) inside a tank could create an electrostatic build-up on the body and create a risk of an incentive charge leading to an explosion.

Electrostatic build-up – still a major safety focus
Physicist Dr. John Chubb is one of the leading authorities in the area of electrostatic research. His findings have answered many questions concerning electrostatic problems – and how to control them. Without a proper understanding of the problems, risks of explosion are as real today as they were in the 1960s. At greatest risk are large crude oil tankers because of the large volumes of tank vapours. But there is a risk of explosion in any situation where high-pressure water jets are used for cleaning a space where flammable gas mixtures could be present or created.

Technology that keeps explosions a thing of the past
Automated tank cleaning solutions, such as the systems offered by Oreco, focus on reducing the risks of electrostatic ignition of tank fumes. Stabilising tank environments with inert gas and using fully automated cleaning systems which don’t require human entry into a tank are just a few of the advances which have been developed thanks to an increased awareness of the problem.

Published: 2004-12-27
List of all Industry articles.

Read more about washing of crude oil tank here

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