How safe are heat transfer systems?

Questions are being raised over the safety of heat transfer systems used in the process industries. The thermal fluid used in these units can be a serious fire and explosion risk, especially if the oil falls short of the required specification and/or is not regularly monitored to ensure it is in good condition.

There have been a number of incidents involving heat transfer fluids in the UK recently, reports Heat Transfer Systems (HTS), a company involved in the development of systems to condition thermal fluids as well as in audits, risk assessments and training to support thermal fluid users in meeting safety requirements.

According to the Derbyshire-based company, the UK Health and Safety Executive (HSE) has recently issued a prohibition notice to a UK company following a major thermal fluid incident and significantly, following that incident, has identified thermal fluid systems as a fire and explosion risk.

There have been other serious thermal fluid-related incidents this year, including outside the UK, an HTS spokesman added. These, he said, include an accident at the Egger Timber Products’ plant in Brilon, Germany, where three workers were killed in a boiler explosion at a biomass CHP station at the chipboard factory on 5 Feb.

Need for sampling

Meanwhile, another supplier in this arena, Global Heat Transfer (GHT) of Stone, Staffordshire cites an explosion and thermal oil fire which destroyed a biomass power plant in Sittard, Netherlands, back in 2007. This, it said, underlined the need for sampling and analysis to monitor areas like the presence of carbon, flash points, fire points, autoignition temperatures and acidity, as well as systems for fire damping and automatically isolating the energy source.

GHT suggests that the Sittard blast might have followed a leak in the heater tubes where the heat transfer fluid circulated. Degraded fluid, it noted, can deposit carbon insolubles out onto the coil interior; once baked on, this has an insulating effect and can lead to hot spots appearing and, in turn, to coil breaches.

However, rather than responding to any particular incident, a HSE spokesman said: “Issues relating to DSEAR compliance are addressed as part of the HSE’s routine intervention activities across the industry sectors that it regulates. Where the control of risk is found to be inadequate, HSE will take appropriate action.”

Based on its experience of numerous systems both in the UK and across Europe, HTS believes many thermal fluid system components will be defined as Zone 2 under the ATEX Directive (1999/92/EC). As such, area classification is required, together with a full risk assessment to identify the risks and any required actions, which must include specialist training for relevant employees.

HTS estimates that only a small percentage of the estimated 4,000 UK companies that operate thermal fluid systems are meeting their requirements in relation to these fluids.

“Regular thermal fluid testing and the results obtained will indicate the physical condition of the fluid and the degree of risk in the event of a fluid release,” said HTS. “Testing alone is not enough to comply with DSEAR and HTS is concerned that the majority of companies operating thermal fluid systems may not be aware of this.”

“There does not seem to be much understanding,” said Simon Adams, a spokesman for HTS.

The Health and Safety Executive has a range of different regulations to cover the whole issue properly, so it can be difficult for companies to act comprehensively under the current regulatory regime.

“What tends to happen is that HTS is called by new clients in a distress situation; often a case of locking the stable door after the horse has bolted,” according to Adams.

Asked to comment on these concerns, the HSE spokesman replied: “HSE is aware of the hazards and risks associated with heat transfer fluids and is satisfied that the provisions of the Dangerous Substances and Explosives Accidents Regulations 2002 (DSEAR) are sufficient to address the sort of incidents mentioned. substances - including liquids handled above their flashpoint - to carry out a risk assessment, a hazardous area classification exercise and install appropriate equipment.”

Alongside the HSE implications, Adams points to the financial considerations of using organic thermal fluids, which degrade over time and are likely to lead to loss of efficiency, increased flammability, potential pump failure, system downtime and associated cost.

“Regular monitoring helps keep the oil at optimum condition while reducing downtime, including due to the need to drain systems, said Adams. “By not maintaining the condition of their fluids, the vast majority of companies are running inefficiently, as well as falling foul of ATEX regulations.

On average, added Adams, these systems have capacities of 20,000 litres of fluid - typically costing 50-60p a litre - with the fluid replaced every two years. This, he said, adds up to major costs, even before factoring in loss of system efficiency, system downtime and the cost of disposal or re-processing of the used fluids.

Alternative viewpoint

Process Engineering’s e-newsletter recently included a comment item based on industry’s concerns over fluid heat transfer systems. Among the responses was the following:

The vast majority of correctly installed and well maintained thermal fluid heat transfer systems are very safe. Generally, they do not fall under the Pressure Systems Safety Regulations and do not present the risks associated with pressurised heat transfer media.

A thermal fluid when heated above its flashpoint can require mechanical joints in the immediate proximity that might pose a leak risk to be rated zone 2, according to DSEAR. The risk of an explosive atmosphere resulting from a leak, however, is normally considered to be less than that from a gas leak, as the leaking fluid soon cools to a temperature at which vapours are no longer released, while gas remains in its gaseous form.

As with gas pipework systems, where correct installation, ventilation and maintenance are applied, the extent of any zone 2 area around joints will generally be negligible, so Zone 2 NE under BS 60079-10 generally applies.

Problems can arise with a thermal fluid installation if adequate precautions are not taken. However, there is no reason for this to be any more difficult to prevent, or any more hazardous, than an industrial gas installation - it generally falls into the same category - and it has been found that gas installations can be easily managed to prevent such situations arising.

It is, therefore, very important that the results from a small number of rare incidents, which may have occurred in badly managed plant or as a result of hazards other than the heat transfer medium being present, do not cause an over-reaction to a well proven and generally safe and simple-to-operate technology.

Robert Webster, engineering & manufacturing manager, Babcock Wanson UK Ltd