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Diesel - Process Safety Considerations

Diesel has long been considered a ‘safe’ fuel. At ambient conditions, it is not particularly flammable- if you throw a lit match into a bucket of diesel, it would just sink to the bottom and extinguish itself. This is thanks to its high flash point temperature.

So, what is flash point temperature? The space immediately above any flammable liquid is only ignitable if there is enough flammable vapour that escapes the liquid phase. This is a function of the lower explosive limit (minimum vapour phase concentration required to produce an ignition) and the volatility of the fuel (tendency to evaporate).

For each type of flammable liquid, there is a temperature where enough liquid has evaporated to form a flammable concentration of vapour immediately above the liquid surface. This is the flash point temperature. For petrol, the flash point temperature is around -40oC, while for diesel, the flash point is around 55oC. So, to get diesel to ignite, it needs to be heated well above ambient.

 Diesel in Context

To give some context of where diesel fuel fits into the scale of flammable substances, it is useful to consider flammable substances more broadly. The HSE has defined three categories of flammable liquids (see HSG51 paragraphs 8 & 9):

  • Category 1 fluids have a flashpoint below 23oC and boiling point below 35oC. These are mostly gases at room temperature (like propane) but may also include some refrigerant gases that are effectively a vapour-liquid equilibrium in a pressurised container. These are also referred to as ‘extremely flammable’.

  • Category 2 fluids have a flashpoint below 23oC, but their boiling point is above 35oC. These are liquids at room temperature, but the associated vapour just above the liquid surface will be flammable. This includes acetone, IPA etc, and are correctly referred to as ‘highly flammable’ liquids and carry the H225 hazard statement.

  • Category 3 fluids have a flashpoint between 23 and 60oC. Diesel is a good example. These fluids are described as flammable liquids and carry the H226 hazard statement.

  • Combustible liquids have a flash point above 60oC and includes most HVO’s (hydrogenated vegetable oil), hydraulic oil etc.

The above categories are not to be confused with the North American Hazardous Area Classification systems of Class I, II or III, which are not interchangeable with UK categories.

Flammable Mist Releases

There is ample evidence that if a high flash point liquid is released as a spray at pressure, the resulting mist can be flammable even well below its flash temperature. This is because of the very high surface area of the droplets which are exposed to oxygen in the air.

The exposed surface area is higher for very small droplet size of a few microns or less. This is referred to as an aerosolised or atomised release and is the basis for burning diesel in motor vehicles and generators, where nozzles are designed to release the fuel at the optimal droplet size.

An accidental diesel mist release may require formal DSEAR zoning and is broadly possible from the following scenarios:

  • If storage tanks are filled from a high-level inlet and splashing is possible inside the tank (splash filling), a flammable mist is possible inside the tank. This can be easily mitigated by using a dip pipe, which is orientated towards the inside tank wall and enters the tank below the liquid level.

  • If diesel is pumped through flanged pipe joints, a flammable mist could be released from a flange leak.

There have been some studies attempting to establish exactly what pressure and whether the geometric scenario of a leaking flange might give rise to a hazardous zone, and there is some emerging specific information for diesel, biodiesel (also referred to as hydrogenated vegetable oil, HVO), kerosine and hydraulic oil. What we do know is that (1) HVO is less likely to produce a flammable mist than diesel, and (2) flange guards can eliminate the need for hazardous zoning if used correctly.

Flange guards are essentially a covering around a flanged joint to collect any atomised release and collect the release as a liquid. Flange guards have to be inspected regularly and replaced if they show signs of being used to collect a spray. There are flange guards on the market that change colour when they are holding liquid, indicating they need to be replaced.

Since flange guards are a relatively simple solution, Sigma-HSE would advise their use to prevent the need for hazardous zoning, and to pre-empt any future guidance that might definitively state they are required.

Separation Distances

Diesel storage tanks could be engulfed by fire that come from somewhere else on the site. There are separation distances prescribed by the HSE (HSG176) from property boundaries, buildings etc. The actual distance depends on the capacity of the tank(s).

If your site is constrained, there are ways to get around these, including firewalls or siting your tank(s) underground. You can also use HVO (hydrogenated vegetable oil) instead of diesel. HVO has a flash point above 60oC, which means it is a combustible liquid, not flammable and thus not subject to the separation distances of HSG176. According to the marketing literature, HVO can be used as a ‘drop-in’ alternative to diesel but check the requirements of your generator first. Also useful for sustainability points.

If the required separation distance is still a problem, you can contact your local emergency services and potentially get permission to be non-compliant. For this, you will need a written justification.

Sigma-HSE Solutions

Sigma-HSE can provide a range of bespoke risk based process safety solutions for your operations. This includes process safety testing (Gases & Vapour) and consulting (Quantitative Risk Analysis, DSEAR Assessments and Process Safety Management) to aid you in identifying and mitigating the potential of your processes to release hazardous materials.

We also offer a range of process safey training solutions to support you in the creation of a disciplined framework i.e., maintenance practices, human error and other safe work practices.

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