Condition monitoring of thermal oil reduces explosion risk and damage to equipment
13 July 2012
The Abergavenny Fine Food Company discovered an oxidation issue in its thermal oil, which was causing considerable carbon fouling of its equipment and a build up of acidity within the fluid. Andy Burns, technical business manager at Global Heat Transfer, reports on the search for a solution.
Based in Gwent, The Abergavenny Fine Food Company is a medium-sized family business operating from a 40,000 sq. ft. facility in Blaenavon, offering products across a variety of categories including high-end finger food, ready meals, dairy and desserts.
“We were having a problem with our thermal oil - it was frequently damaging oil circulation pumps,” explained Paul Sanders, site engineering manager at Abergavenny Fine Foods.
“We use thermal oil as a means of indirectly heating fryer oil for our breaded party foods line. I had been in the business for a few months when, about three years ago, we approached Global Heat Transfer. They carried out and analysis of the oil in the system and found that there was a build up of carbon around the pipework because the oil had not been tested or maintained for a number of years,” he continued.
Thermal oxidation occurs when an oil temperature of over 65 degrees Celsius and air meet in the same space. This starts a chemical process from the reaction with oxygen, the products of which are organic acids, carbon and sludge. Furthermore, for every increase of 10 degrees C, the reaction rate doubles. Abergavenny’s header tank was in excess of 100 degrees C and had no nitrogen blanketing system to protect the overheated oil from the air above it.
DSEAR & ATEX
“The carbon was coming loose and damaging the seals in the circulation pump. Global Heat Transfer put forward their recommendations to firstly clean the system, then to refill it, and finally to comply with the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) Act 2002 by using monthly testing and oil analysis,” Sanders said.
DSEAR legislation requires employers to control fire and explosion risk by carrying out stringent tests on a regular basis. An effective preventive maintenance program should include regularly scheduled and representative analysis of thermal fluids, which benefits users by providing early warnings about process problems, helping maintain a safer working environment, increase system efficiency and saving money while keeping the operator legal under ATEX 137 regulations.
ATEX 137, which was introduced in July 2006, applies to users of equipment, plant operators and anyone handling or processing potentially explosive dusts or vapours. The requirements involve an assessment of all areas of a process plant to determine their classification. In addition, all existing equipment has to be assessed to check if it is suitable, and all new equipment must comply with ATEX 137 standards.
External view of the oil-based heat transfer system
Flushing and cleaning
A team of Global Heat Transfer engineers drained, flushed and cleaned the system at Abergavenny using the company’s exclusive GlobalTherm C1 product, before replacing the oil. C1 is a dual-action preparation that rids a heat transfer system of potentially harmful contaminants such as old or oxidised residual fluids, carbon deposits, loose debris, water and volatile light ends.
It is specially formulated to scour away harsh by-products of synthetic and mineral-based fluids and effectively displaces and flushes out waste, leaving behind a clean and safe operating system, ready to accept a new charge of heat transfer fluid. C1 is also compatible with most heat transfer fluids.
“Now we have the condition of thermal oil under control, and when we carry out our monthly analysis we can pick up an issue before it starts to cause problems,” said Sanders. “We can monitor the carbon content and pick up any excess build-up of carbon, as well as acidity levels, which can cause corrosion if they fluctuate too much.
“We can also keep an eye on the flashpoint, which was very low in this case. When I joined the company, the oil had not been analysed or changed for seven years prior to Global Heat Transfer getting involved. By that time the damage had been done from the carbon build up in the pipework, tubes and boiler.”
Global Heat Transfer carries out a standard suite of 11 tests and has found from this that around 80% of customers are carrying out irrelevant checks and incorrect sampling, if any at all. The three main assessments the company undertakes are:
* checking the carbon level and amount of insoluble particulates,
* determining the acidity level
* finding the closed flash point
Plant room inspection of the oil-based heat transfer system
If the carbon level (heavy ends) is too high it will result in system fouling, which means carbon insoluble particles will stick to the system internals and eventually bake on hard if not flagged, cleaned and flushed in time. Carbon is an insulator and a build up will ultimately reduce efficiency at the process end and result in higher running costs. The most common cause of heater coil failure is when the insulating effects of carbon fouling do not allow the thermal oil to carry enough heat away from the burner flame. This can result in hot spots and the coil burning through, at which point the combustion triangle of ignition source, fuel and air are present. This can lead to a serious fire in the thermal oil if there are not adequate or correctly operating safety systems in place.
The acidity level is determined by the total acid number (TAN), which is the amount of potassium hydroxide in milligrams that is needed to neutralise the acids in one gram of oil - an indication of any oxidation or acidic contamination that may be present.
The closed flash point must be managed under DSEAR legislation. Global Heat Transfer takes a hot, circulating and ‘closed’ sample, as open samples allow light ends to flash off to atmosphere giving inaccurate readings, which are outside DSEAR and insurance industry requirements. Other checks include viscosity, water content, ferrous and particulate quantities, as well as open flash and fire point, which are accessed in conjunction with the other sample results.
If thermal fluid samples are not collected in a representative method, artificially high flash point values will be returned. This results in the end user perceiving a lower risk from flash points than is actually correct. This has obvious and important insurance, health and safety and DSEAR/ATEX legislative implications.
Representative thermal fluid samples must be collected hot at operating temperature. In its document entitled ‘Monitoring Heat Transfer Fluids: The Sampling Bomb’, dated December 1980, BP Oil states, ‘A truly representative sample of the complete charge can be taken only when the fluid is hot and circulating.’
The document then goes on to describe use of ‘the bomb’ – a closed sampling device designed to capture the volatile light ends that would otherwise be boiled off if the sample was to be taken to atmosphere. System design varies widely from site to site in terms of pipe diameter, fluid velocity, pipe layout and so on. Also, aged oil can be significantly more viscous.
Thermal heat controls
Therefore if the sample is taken with the system running and at normal working temperature, it is far more likely that turbulent flow is occurring, thus ensuring that a homogenous mix of fractions within the bulk fluid is sampled. Any insoluble contaminants will, for the same reason, be more likely to be suspended within the bulk fluid. Both high and low molecular weight ranges of fractions will be detected.
Representative thermal fluid samples must also be collected in a closed manner. A closed sample device such as a ‘bomb’ must be used to ensure that the fluid does not pass through atmosphere. Light ends or volatiles consist of a homologous mix of hydrocarbons with different boiling/flash points.
Where an ‘open’ sample is collected, the most volatile (lowest flash point) species will automatically escape and flash off to atmosphere, instead of being allowed to cool and condense back into the sample, where it can be decanted under lab conditions. In this case, as the lowest flash point material has been vented off, incorrect (too high) flash point values will be returned.
Finally, representative thermal fluid samples must be collected from a circulating system. This is again to ensure a homologous mix of hydrocarbons. Light ends will ‘pond out’ in still fluid.
“Boiler efficiency has improved significantly and has seen our heat-up time reduced by 50%. It is flowing more efficiently and will show a saving in pump life,” Sanders said.
“At least we can now keep an eye on these things before either a major disaster or further damage occurs. We now change the oil annually, but we are continually finding improvements in the system which will allow us to extend this period longer and longer,” he added.
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