Safety of tank strorage facilities

Over the past few years there has been a steady drive to improve the standard of level-measurement systems and technology used at oil storage facilities around the UK.

Many of these enhancements can be linked back to the devastating Buncefield accident in December 2005, which caused the largest ever explosion in peacetime Europe.

While there were no direct fatalities, estimates suggest that as many as 3,000 people could have been killed if the 3am blast at the Hertfordshire oil storage site had happened at a different time of day.

The incident was essentially caused by a huge overfill, linked to the failure of a level-monitoring device on an oil storage tank.

Many sites are still relying on old equipment that could and perhaps should be replaced by more modern technology

Chris Brennan

Investigators found that the level-measurement gauge reading had not altered during a three-hour period in which the tank was being filled with unleaded petrol via a pipeline from the Lindsey Oil Refinery in Lincolnshire.

Subsequent reports from the Health & Safety Executive - through the Major Incident Investigation Board (MIIB) - set out a series of recommendations to prevent such an incident from ever happening again.

The final report, published in December 2008 included a mandatory requirement that overfill be prevented by “independent and automatic means” and so not rely on human intervention.

However, since then industry’s progress with implementing these recommendations has been difficult to track.

This is particularly because the timescales for implementation are linked to when sites plan to install overfill protection - as agreed with the relevant authority.

One of the main industry bodies driving this process safety agenda is the UK Petroleum Industry Association (UKPIA), which represents the UK’s main refining and marketing companies - BP, Essar, Esso, Murco, Petroineos, Phillips 66, Shell, Total and Valero.

“Many sites have now installed automatic overfill protection systems,” says Peter Davidson, UKPIA safety director, commercial and projects.

“The reason not all sites [including those of non-UKPIA companies] will be complete yet is because the work needs to be scheduled to take account of factors such as other upgrade/ improvement works, tank outages [and] scheduling work with equipment suppliers.”

However, Tank Storage Association (TSA) executive director Dr Hugh Bray adds: “All member companies with in-scope gasoline tanks are now understood to comply with the MIIB’s recommendation regarding level measurement and control.”

Out with the old

As well as crude oil and petroleum, TSA members also provide third-party bulk liquid storage of products such as chemicals, potable liquids, edible oils and fats.

These companies are also committed to ensuring that all level sensing on their sites comply with the Buncefield recommendations, says Bray.

From a supplier’s perspective, the drive to adopt automated level measurement systems has been somewhat patchy across the process sector as a whole - particularly away from the larger oil storage facilities.

“The use of automation has certainly increased at many of the larger fuel storage sites since the Buncefield enquiry,” says Endress+Hauser level product specialist Chris Brennan.

“However, to the best of my knowledge, this hasn’t happened at the lower end of the industry, on the smaller sites. What’s also clear is that many sites are still relying on old equipment that could and perhaps should be replaced by more modern technology.”

Brennan adds that the recommendations from the Buncefield inquiry should extend beyond fuel storage facilities and “should be taken on board by other process industries”.

In terms of the types of systems being adopted, radar is now the most widely used technology in the fuel-storage industry.

This is mainly because of its reliability due to the removal of mechanical components, which are subject to the effects of wear and tear and corrosion.

As one operator commented: “Radar-based level measurement technology has improved significantly, allowing higher resolution of level measurement. The technology avoids moving parts used in servo-based level measurement and eliminates the problem of freezing.”

While servo gauge systems are widely used in level measurement and are highly accurate, these devices can be damaged by corrosive biofuel additives.

This is an increasing problem where tanks are used to store both biofuel and gasoline at different points in their service life.

Radar roll-out

According to Doug Anderson, marketing manager of device supplier VEGA, many sites with large storage tanks are now looking at overfill monitoring using non-contact Safety Integrity Level 2 (SIL2)-qualified radar for internal
or external floating roof detection.

These offer an effective backup to the main level reading when linked with the automatic tank gauging system.

Away from the larger storage facilities, Anderson sees less activity on the floating roof-radar front. However, guided wave radar (GWR), non-contact radar and tuning fork level switches do feature on smaller projects being carried out via third-party system suppliers.

Testing of level-measurement systems can present a major challenge for many tank storage operators. Here, Anderson says there is significant interest in a new, qualified five-step proof test developed by VEGA.

This allows radar-based devices to be prooftested in situ with minimum disruption to the process.

At the TSA, Bray points to a series of recent and current projects to implement advanced level measurement systems among member companies within the association.

One approach is based on a pre-Buncefield standard that uses two analogue gauges - a custody transfer gauge and an independent safety gauge - to indicate the tank level.

The analogue devices constantly compare their indications and generate a differential alarm if either gauge drifts.

Bray also cites a terminal that already had two independent high level product alarms linked to the control room and ‘live’ data via tank radar system connected to computer software alarms.

This was backed up by robust management, engineering and competence systems.

Following a recent safety review, the terminal also installed remotely operated shut-off valves (ROSOVs) to protect all ‘high-risk’ storage tanks.

On activation of the product’s highest alarm level, called high-high, the valves shut off pipelines safely to prevent a spill and/or vapour cloud explosion.

Integrated control technologies are also a central part of the solution - as evidenced by the Experion automation platform being installed to improve the efficiency, reliability and quality of level technology at one UK site.

Minimising the scope for human error at its oil storage facilities is the target for another TSA member company, which is now installing tank-side ROSOVs on crude oil storage tanks.

The aim is to eliminate the need for a human response to any failure to terminate the transfer of product into the tank when specified levels are reached.

Another site is currently installing a gasoline storage and export facility. Again, this company has installed a high-high alarm system that on activation will close the ROSOVs installed both on tank-side and import pipelines.

Lights, buzzers and computer graphics will also be activated in the control room and the site emergency shutdown device (ESD) will close the entire site and jetty. Tank interlocks will also prevent transfers.

Bray adds that there is also a project installing an Enraf Entis inventory management system (IMS). This is designed to improve the control of batch transfers by focusing the operator’s attention on the transfers in order of completion.

The IMS system allows the operator to set up a batch transfer, which is analysed by the IMS to ensure there is sufficient ullage in the import tank for the transfer.

It then calculates the transfer completion time so the operator can see instantly whether the new transfer conflicts with the completion of another parallel transfer and alerts the team when the operation needs to be terminated.