In partnership with KRA Brown and Shark ESMS Ltd
Notes for Bitumen Silo DSEAR assessment
Standard data sheets do not list Bitumen as an explosive, only a flammability risk and that is very low.
The Bitumen flashpoint of >230°C it is significantly above the upper limit of 60°C for a liquid to be considered as a hazardous source under the ATEX/UKEX regulations (unless it is heated to the flashpoint) and therefore does not directly come under the ATEX/UKEX standards.
However, when heated in storage silos (to maintain as a liquid) other gases (not Bitumen) can be given off even if the temperature is well below the Bitumen flashpoint. These can be collected in the void above the bitumen in the tank and build up to explosive levels.
The risk then, is when tanks are filled the rising level of Bitumen pushes any flammable gas out of the vent pipe, conversely as the tank is being emptied air is pulled in via the vent pipe diluting the gas in the void.
The consequence is that the timing of an explosive hazard at the vent pipe exit is generally predictable. In most cases it will only occur during a silo filling operation.
Of course this should be confirmed as part of the DSEAR assessment for each location.
There seems to be minimal available information as to the exact nature of the trapped gas and would not necessarily have the same characteristics as the Bitumen.
Carbon monixide (CO) and Hydrogen Sulphide (H2S) are probable candidates but certainly there is the potential of a wider cocktail of gases as well as bitumen particulates which may affect flammability levels and the assessment.
It must be noted both CO and H2S have toxic limits far below any explosive levels which is the primary concern for personnel in the vicinity.
The Hazardous area standards require this gas to be analysed, (or technical assesed for estimate of constituents). A risk assessment should be conducted for any equipment within the tank headspace or any gas vent pipes and possibly the use of Hazardous area certified sensors etc.
In the absence of contra information, it could be assumed the silo void is a Zone 0 area to err on the safe side.
It is clear that bitumen would not release sufficient vapours to create an explosive mix via pipework etc. and therefore flanges for example would not be sources for a hazardous area Zone. Similarly pumps and other components would generally not need to be ATEX/UKEX certified unless there was further external explosive gas sources or possible items in a confined space subject to heating with regular and persistent bitumen leakage. In which case good housekeeping to remove the potential hazard should be the primary solution.
These are general comments and every site should be assessed by a proven 'competent' person
Bitumen Data
| Auto-ignition Temperature |
Flash Point |
Gas Group |
|---|---|---|
| > 300°C | >230°C | N/A |
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.
- Analysis of gas mix to calculate, ignition temperature (T group) , ignition energy (gas group) lower explosive limits
- Measure concentration in the void, by analysis or flammability sensor
Carbon Monoxide CO
CO is a colourless, tasteless, odourless, non-irritating gas with a long term short term (15 minute) exposure limit of 100ppm.
This compares to the much higher lower explosive limit of 12% v/v
EN 60079-20 part 1 specifically lists Carbon Monxide as Gas group IIB.
Hydrogen Sulphide H2S
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 the vent pipes.
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 with flamepaths would not be recommended where bitumen is the vapour source and particulates could be present.
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
It is known that Carbon Monoxide gas as is likely to be given off within the silo. EN 60079-20-1 clause 5.2.3 details Carbon Monoxide as a Gas group IIB which rules out using IIA certified equipment.
If Hydrogen (and possibly other hydrocarbons) is present it may be that group IIC equipment is required.
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, most equipment is certified to T4 (135°C) which should cover all the expected gases from Bitumen.
The bottom line is the DSEAR consultant and site competent person must accept responsibility for the assumption and outcome.
Zone assessment
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. This includes instrumentation with other certification where only the sensor inside the Zone 0 is intrinsic safety protected.
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
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. Although, as the worst case release is during silo filling, physical monitoring during filling should give a good guide as 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 particularly when it impregnates insulation.
"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.
If these are identified as Carbon Monoxide and Hydrogen Sulphide then toxicity rather than explosive hazard is probably the primary concern for personel.
Working with KRA Brown Electrical Services Ltd we offer advice on DSEAR report interpretation and implementation as well as Equipment ond general compliance surveys. Refer here for a detailed list
References
The Dangerous Substances and Explosive Atmospheres Regulations 2002
Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 2016 (updated March 2023).
The above as amended by
The Product Safety and Metrology etc. (Amendment) Regulations (2019 and 2021)
EN 1127-1 Explosive atmospheres - Explosion prevention and protection - Part 1: Basic concepts and methodology
EN 60079-10-1 Explosive atmospheres. Classification of areas: Explosive gas atmospheres
EN 60079-20-1 Material characteristics for gas and vapour classification
Neste Oy Bitumen Safety data sheet no 10507
Shell MSDS 02459536 Bitumen (Asphalt)