Silo effect

The storing of bulk materials is riddled with hidden dangers. Whether you are handling hazardous liquids or powdered ingredients, the design and construction of storage vessels demand careful consideration.

Things tend to go wrong when an inadequate or ‘standard’ storage vessel is selected on the basis of cost, says Richard Farnish, a senior research fellow at The Wolfson Centre for Bulk Solids Handling Technology at Greenwich University.

“The biggest challenge for end users is trying to get something for nothing,” says Farnish.

“With powder feeders or silos, many people try to get maximum value with minimum expenditure. It is a nice idea but where it all falls down is that many people are not in a position to realise what value means.Value is getting the balance right in terms of purchasing equipment or products to serve a purpose where it is supposed to.”

Buncefield in particular has heightened awareness of risks associated with bulk storage infrastructure and risks to life and the environment when things go wrong

Simon Gibbons

According to Farnish, the basis for design of any storage system should always start with an understanding of the bulk properties of the materials it will be required to handle.

“You don’t want to invest in a silo and then discover the material won’t come out – or that 20% of it can’t be reclaimed and is in there permanently,” he says.

“In this case the original equipment supplier will often be called upon to install some kind of discharge assistance to free up materials. That can alleviate the symptoms but may not change the fact that the vessel may only be suited to ‘first-in-last-out’ applications.”

To avoid such issues, Farnish recommends selecting suppliers with good credentials.

“Ideally the supplier themselves would be able to correctly characterise your bulk material,” he says.

The repurposing of existing plant equipment can also be problematic.

“To bring something back into good working condition could be cheaper than buying a new one, but this practice very often ends in tears,” says Farnish.

“It may have been sitting out in the plant, and may be corroded.”

Another area of difficulty is storing recycled materials either for reuse or for calorific value to be used in the combustion process.

“These can throw up a lot more challenges because the particles are very irregular and inconsistent in terms of density, shape and size,” says Farnish.

“It might be clothing or old wood, but if you try to put it through a normal bit of equipment, it won’t come out.”

Biomass, which is increasingly being adopted by many plants that have previously run boilers on oil or LPG, can also present storage problems.

A high-profile convert in the UK is the Drax power station, which announced plans in 2012 to transform itself into a predominantly biomass-fuelled generator.

In April 2013 Drax fully converted its first generating unit to burn biomass instead of coal and in October announced that over 1 million tonnes had been burnt in that unit alone.

Most biomass boiler fuel is made up of wood chips and pellets, and Drax has discovered key differences between coal and biomass in the way you have to store and handle it.

“Biomass is affected by moisture in the air, so we have to store biomass in enclosed storage,” says Drax head of engineering Jason Shipstone.

“Biomass is [also] a very dusty material so it’s very important that we either contain or eliminate the dust when we’re handling biomass within the process.”

The physical properties of biomass also meant that Drax needed to handle a larger volume of biomass to generate a similar amount of heat and therefore electricity.

The space required for the storage of the fuel can be up to four times size of the areas used to store oil.

Bulk storage can also present major safety and environmental challenges at sites handling highly flammable or explosive materials or liquids.

Simon Gibbons, technical director at environmental management group ERM, says the Buncefield fire and the offshore Deepwater Horizon oil spill have substantially lifted awareness of the connection between failures in process safety systems and environmental damage.

“Buncefield in particular has heightened awareness of risks associated with bulk storage infrastructure and risks to life and the environment when things go wrong,” he says.

However, there has been relatively little work to prioritise planning and preventative expenditure reflecting environmental, as well as safety, outcomes, he adds.

But given the number of bulk storage vessels and the volumes stored at individual sites, he says the risks associated with these assets are significant.

Since the introduction of the Containment Policy, the historical approach of selecting ‘representative’ scenarios for analysis of environmental risks is no longer an accepted approach for the Competent Authority when enforcing the Control of Major Accident Hazards Regulations 1999 (COMAH).

Instead, COMAH-registered sites are required to complete an assessment to measure the current tolerability of risk.

This looks holistically at all of the risks at an establishment and impacts to each of the potentially affected receptors.

The aim of this approach is to identify areas of intolerable risk and to assess appropriate measures to lower that risk.

However, this approach has had massive financial ramifications for older sites, says Gibbons.

“This drive for engineering improvement often overlooks the specific risk drivers for environmental impact,” says Gibbons.

“The biggest challenge in the industry at present is therefore being able to demonstrate an understanding of the infrastructure process safety risks and combining that in an appropriate way to demonstrate to the regulator that environmental risk is being managed in an acceptable manner.

Fresh guidance from the Chemical and Downstream Oil Industries Forum (CDOIF) is offering some respite by providing an alternative means to assess risks associated with storage infrastructure, says Gibbons.

This spells out clear procedures that sites can follow to demonstrate their understanding of the environmental risks, building on a more complete assessment of process safety risks.

To illustrate, Gibbons says one issue faced by some plant managers is that routine maintenance often identifies no issues with tank floor corrosion.

Therefore, replacing a tank floor or tank jacking to place a liner might be a cost that is disproportionate to the level of risk.

A risk-based approach, when implemented appropriately, can help sites to demonstrate this disproportionality through an assessment of consequence.

He says this can help them to save costs and also provides the regulator with a clear argument as to why full compliance with the Containment Policy, even when opportunities exist to upgrade, may not always be appropriate.