Road tank vehicles overfill protection system fails during testing

What is the incident?

WorkSafe recently attended a full function test of a dangerous goods tanker, which included testing the compartment overfill protection system with water. During testing, the overfill protection system did not stop the filling process. As a result, the compartment overflowed from both the vapour recovery line and the compartment vent.

Testing identified the overfill protection fault. Without testing, this fault could have later resulted in the loss of containment of dangerous goods, risking the driver, filling operator and the public's safety.

What was the testing process?

Equipment functionality

The road tank vehicle was fitted with an overfill protection system as part of the earthing integrity system. The following were tested and were functional before use:

• The road tank vehicle and the overfill protection system.
• The filling-gantry earthing system.

Filling

The road tank vehicle compartment was filled to 90% of the safe fill level as part of the full function testing. The filling flow rate for these road tank vehicles is typically 500-700 litres per minute.

Findings

The investigation identified the overfill protection probe was too short (in length). As a result, it did not detect the fluid in time to shut down the transfer before overflowing the compartment. This fault was only detectable through full-fill testing.

What are the issues?

A loss of containment event can occur if an overfill protection probe is fitted incorrectly to the road tank vehicle or if the length does not comply with the manufacturer's design to prevent overfilling.

A loss of containment places the driver, the filling operator, and the public at risk of exposure to hazardous substances and dangerous goods.

What are the control measures?

Inspection and testing of overfill protection systems

Overfill protection systems need to be tested and inspected in accordance with Australian Standard (AS) 2809 - Road tank vehicles for Dangerous Goods, Part 1: General requirements for all road tank vehicles.

The full-fill function testing should occur as part of the inspection and/or testing of the compartment hatch and its assemblies. The testing needs to occur regularly on each road tank vehicle.

Note: Testing the overfill protection system without a full-fill test will not identify this fault. Tests that will not identify the fault should be avoided, including, only using the overfill/earth system testing unit, or placing probes in a bucket of a suitable liquid testing medium.

The road tank vehicle inspection requirements are set out in the Australian Standard AS 2809.1 – Road tank vehicles for dangerous goods, Part 1: General requirements for all road tank vehicles, Section 3 Vehicle Inspection.

Installing or replacing overfill protection systems

Where overfill protection systems or probes need replacing, the work should be undertaken:

• by a suitably qualified service technician
• in accordance with the manufacturer's specifications
• in a workshop environment (avoid undertaking work in the field)

Maintaining safe systems of work

Duty holders with management and control need to have a system of work in place to ensure the overfill protection probe is operational. This includes ensuring:

• the overfill protection system is installed as per the manufacturer's installation specifications
• the overfill protection probe is set to the correct length for each road tank vehicle compartment (measured from bottom of hatch to bottom of probe)   
  Note: If the manufactures information is not available, the length can be determined through engineering calculations. 
• the overfill protection systems probe length is accessible to service technicians and others who may need to know this information
  • the probe length detail, of each road tank vehicle, can be recorded on a label adjacent to each compartment

Legal duties

The Dangerous Goods Act 1985 and the Dangerous Goods (Transport by Road or Rail) Regulations 2018 set out the obligations of people involved in transporting dangerous goods by land and gives effect to the standards, requirements and procedures of the Australian Code for the Transport of Dangerous Goods by Road & Rail (ADG Code).