Venting storage tanks is essential for safety and reliability but is a practice that has long been poorly understood and regulated, according to tanks expert Assentech. The specification and maintenance of critical components here – breather valves – often falls short of what’s required, and tank leakage is a widespread problem. But change appears to be afoot, as Envirotec discovered
When managing atmospheric storage tanks a key priority is regulating the internal pressure – in relation to the pressure outside. Perhaps the most dramatic consequence of failure in this respect is when a tank actually implodes or collapses – video footage of which can be found easily on youtube. Assentech’s website notes that incidences of tanks failing – in the sense of this kind of structural failure – is more often a consequence of vacuum or low-pressure conditions inside, as opposed to over-pressure.
Breather valves are an important component here, which – as the name implies – allow a tank to “breathe” in the sense of providing a controlled opening that will prevent the build-up of pressure or vacuum inside. The valves are normally mounted on the nozzle opening at the top of a tank.
Of course, since these valves provide an opening to the outside world, if not specified correctly, they can be a source of vapour leakage – both chemicals escaping from the tank, and water vapour getting in. The latter might be particularly problematic when storing something like aviation fuel, given the dangerous consequences of introducing water to a jet engine.
As Debbie Pearce of Assentech points out, the difficulty is that many tank owners have a poor understanding of the function of a breather valve. “Most people don’t understand why it’s there,” she says, adding that “if they did, they would be a bit more prudent about measuring its tightness.”
Fugitive emissions are an increasingly unignorable risk factor, whether it be the danger presented by flammable vapours collecting on a site, or the low level of toxicity of chemicals like VOCs, which is becoming increasingly well understood, and subject to litigation.
Tank owners have to comply with local authority pollution control measures. Councils often put monitoring apparatus around the perimeter of these sites. But this is still not very well policed. Some industries also have specific requirements to declare certain kinds of emissions. Tank operators, particularly COMAH registered ones are required to arrange inspection of their tank assets to ensure they are not blocked or damaged, but this is just a visual check and does not confirm the function of the device.
Formal guidance and legislative structure regulating the management of ageing tank venting equipment is not as comprehensive as It could be in this industry. The fact that many manufacturers still supply new vents without certifying leakage and calibration in 2019 when this has been defined as a requirement in standards since 2008, or that they manipulate the test criteria to hide poor performance, are clear indicators that despite increasing focus on the environment operators are unknowingly being drawn to cheap non-compliant equipment rather than that designed and tested according to the standards.
Certification is a particular area of weakness where breather valves are concerned. The API2000 standard (also known as ISO28300:2008) was the first to require testing for leakage and correct calibration, including the specification of a maximum allowable leakage rate, when Edition VI was published in 2008. While some manufacturers specify components in accordance with API2000, many still don’t. Some will provide this kind of certification as an option – available at extra cost – while many low-cost manufacturers don’t issue certificates at all. As an industry it still “pretty unregulated”, says Pearce.
“Every customer should insist on individual certificates for each device device,” said Pearce. If an incident does occur then HSE will want documentation proving that equipment was fit for purpose.
The problem is especially acute with tanks where there is a differential pressure value of less than 0.5 barg, since pressure system regulations have tended to apply only to systems specified with pressure values exceeding this level. More recently, the Engineering Equipment and Materials Users Association (EEMUA) has released a document, EEMUA 231, which outlines good practice and guidance on managing devices used in the storage of dangerous substances. The standard also covers best practices for <0.5 bar systems – if the tank has a critical safety function then it must be treated as though it were >0.5 bar.
The emergence of best practice documentation like this is making things clearer for tank operators, but is also increasing the pressure on them to manage these assets properly.
An average storage tank will be holding around 100m3 of liquid, depending on its specific gravity – and this might be potentially toxic or flammable material. A tank rupture – which might arise via a fault with the valve or over-filling – could lead to serious human injury or a pollution incident.
“As people are becoming more litigious, a lot of companies will have to review not only what they are doing now but [what they were doing] 20 years ago,” said Pearce.
There are many reasons why tank operators might be recalcitrant about the measurement and testing of tanks and breather valves. Sometimes it might be fear that a tank will be pulled in. When you engage a customer and find them very receptive, it is often because they are being hammered by their local authority, said Pearce.
Assentech works with firms looking to review or tighten up their practices with tank calibration and maintenance. Such as has been the absence of testing practices and equipment to date, said Pearce, that Assentech has had to develop its own testing procedures and equipment.
A customer might be puzzled as to where all the water is coming from. Or there might an obvious odour problem, which will have prompted them to request a tank vent service. At one recent premises visit the group were faced with a large number of vessels storing a highly flammable product – the odour of which was very strong throughout the facility. Assentech had been asked to service the breather valves, but noticed they were all at very high settings, presenting a risk of tank rupture. Instead of going ahead with a service, they invited an engineering firm to come in and reverse engineer the tanks to assess design pressures before proceeding.
Assentech’s approach, she said, is to give the customer “all the information they need to decide if it’s worth investing in new valve or whether they can live with it.” They will often agree upon a written period of examination, and a timetable for inspecting valves, which might be every few months.
Storage tanks containing fuel and hazardous materials have received much attention from regulators and safety bodies in recent times, notably following the fire at the Buncefield depot in 2005. But it remains an undertaking where best practice and the latest technology might seem more of a luxury than a necessity. With the increasing potential for litigation, it seems complacency is an increasingly unaffordable option. Her firm, she says, “has a major campaign to educate customers” on this point, and to make them aware of the critical aspects of specifying and correctly operating these valves.
