Notes for Bitumen Silo DSEAR assessment

silos Standard data sheets do not list Bitumen as an explosive, only a flammability risk. It is not considered a risk in terms of ATEX/UKEX no information seems to be available as to the vapour released . When heated in storage silos there is a potential for vapour to be produced, therefore a risk assessment is required for equipment within the tank or any gas vent pipes.
The Bitumen itself may be a flammable hazard but does not represent an explosive risk in its own right, pipework and equipment associated with the inlet or outlet will contain heated Bitumen with leakage, possibly being a fire risk, but if well below the flashpoint temperature common sense would suggest that the volumes of vapour from any leaks are unlikely to come even close to the lower explosive limit. The possible exception being any leaks into a confined space which can build up over a period of time.

Bitumen Data

Boiling
Point
Auto-ignition
Temperature
Flash
Point
Gas
Group
>230°C > 400°C >200°C Unknown

In a storage silo the bitumen is generally heated to about 160°C, making it a liquid to enable pumping.
This is still below the Flash point of >200°C but there could be other factors e.g. localised hot-spots and there is evidence of release of vapours via evaporation. Common sense would suggest these are low level emissions but they could collect in the confined space of the void above the storage tank.
In the absence of contra information it should be assumed the silo void is a Zone 0 area. However, there is minimal information as to the exact nature of the trapped gas other than it is a mix of hydrocarbons, these would not necessarily have the same characteristics as the Bitumen. The vapour must be analysed to confirm the contained gas mix. Other factors may affect its explosive characteristics such as levels of particulate.

In order to assess properly samples would need to taken and analysed from the void the silo to ascertain the nature of the gas.

  1. Analysis of gas mix to calculate, ignition temperature (T group) , ignition energy (gas group) lower explosive limits
  2. Measure concentration in the void, by analysis or flammability sensor

Hydrogen Sulphide
Bitumen safety data sheets report a H2S potential, this is not generally a specific concern in the hazardous area assessment per se.
H2S can be smelled from 0.1ppm, has a time dependant permitted exposure limit 10-50ppm and potentially life threatening effects at 200ppm. This compares to the much higher lower explosive limit of 4.5% so it is considered primarily to be a toxic risk rather than an explosive risk

Bitumen particles
These are present in the vapour, as can usually be seen in the residue from any vents.
The Bitumen dust particulate is of concern for some equipment protection techniques as, although not necessarily a hybrid environment, Bitumen’s highly adhesive nature is of concern when using flameproof ex d protection. It has the very real potential to affect or even block the flamepath causing protection failure.
Certainly Ex d enclosures would not be recommended where bitumen vapour is expected

Pragmatic Assessment
As information on the gas/vapour is not available a more pragmatic approach could be investigated and, subject to justification, reasonable assumptions made.

Gas Groups
Based on limited information the vapours could be treated as gas group IIC which covers all gases but this would limit equipment selection.
If the group IIC gases (Hydrogen, Carbon Disulphide, Acetylene, Ethyl Nitrate) are eliminated, with Hydrogen being the most likely IIC gas of the four to be present, then the vapour could be defined as Gas group IIB making equipment selection much simpler. It must be noted this is a higher risk strategy.

Temperature - T rating
Although the auto ignition temperature of Bitumen is around 400°C, although likely, there is no guarantee that the vapours would be the same. Using T6 for equipment would cover all options. However, with a flashpoint of 200°C using T3 (200°C) may be a reasonable safe assumption.

The bottom line is the DSEAR consultant and site competent person must accept responsibility for the assumption and outcome.

Zone assessment
silo Silo void
In the absence of detailed gas testing or datasheet information it should be assumed the void is Zone 0 and Category 1 equipment must be used.
Equipment going into the silo void space Using Intrinsic Safety for sensors which generally have rating IIC T6, and every eventuality would be covered. It may be feasible to use other protection methods for associated instruments on the outside of the silo.
Note sensors in welded pockets e.g. temperature sensors would not be considered in the Zone 0 region.

For this application the Silo’s are not pressurised so it is possible leakage via sensor entries etc would be minimal.

Silo Vent Pipe
vent Without knowing the nature of the vapour, assessing zones around vents is pure guesswork and it would have to be assumed vapours could have both heavier and lighter than air constituents.
Due to particulate residue often the vent is at low level.
As the tank is not pressurised and evidence suggests rate of vapours generation is low, significant volume of gas emission from the vent is likely to only occur during silo filling. By the same argument regular filling and emptying of the silo would reduce the potential concentrations of vapour in the void. Without knowledge of the vapour and estimate of potential release it is not possible to put realistic distances to the extent of the zone.

Every assessment comes down to knowing details of the vapour.
The general guidance would be to move the vent point to a well ventilated area and minimise ignition points.

Flammability
Although bitumen leakage in most instances is unlikely to result in explosive levels of vapour, and zoning may not be relevant, Bitumen can still be a fire risk..

The Neste data sheet reports:
"Selfheating leading to auto ignition at the surfaces of porous or fibrous materials impregnated with oils or bitumen, can occur at temperatures as low as 100°C.

Consideration should be given to using Category 3 equipment for items such as motors eliminating the risk of heat and spark triggering a fire.

References
EN 1127-1Explosive atmospheres - Explosion prevention and protection - Part 1: Basic concepts and methodology
EN 60079-10-1 Explosive atmospheres. Classification of areas: Explosive gas atmospheres
Neste Oy Bitumen Safety data sheet no 10507

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