Plant & Works Engineering
Hidden energy
Published:  26 February, 2014

Over a five-year year period, a properly focussed maintenance plan will save at least half the cost of unplanned production stoppages caused by thermal fluid breakdown. However, the frequency of gathering sampling data for a heat transfer fluid, on which the maintenance plan depends, is determined by whether the fluid is mineral or synthetic. Clive Jones, managing director of Global Heat Transfer explains.

In the nineteenth century, the London Hydraulic Company had 181 miles of hydraulic cable running underneath the UK capital, providing power to its factories. Yet these are rarely referenced in the history books.

This illustrates that the provision of energy to manufacturing, irrespective of its form, has always been hidden. In the process industry, the transfer of energy in the form of heat is so hidden that it is extremely common for a plant manager or chief engineer to have no idea of the correct methods of thermal fluid maintenance.

System maintenance work on thermal fluid may not be the first thing that comes to mind for the average plant manager, but without periodic system drain, flush and full charge of new thermal fluid or planned preventative maintenance, systems have a habit of breaking down at the most inconvenient and expensive time.

Regular sampling

Regular thermal fluid sampling serves as an early warning system for future events and enables us to plan effectively how to increase the productivity of a system so that it runs efficiently at all times. Plant managers want to operate a safe business knowing that thermal fluid is managed in line with DSEAR and ATEX requirements to monitor its condition.

Proper maintenance reduces carbon deposition and fouling which damages systems. Average capital expenditure costs for replacement parts are up to £21,000 ($32,657, €25,000) for a heater coil and £45,000 ($70,000, €53,378) for a new heater - based on a system of just 50,000 litres. This does not include the cost of draining the system, flushing it and replacing the thermal fluid.

By proactively managing thermal fluid, focussed maintenance also lowers the environmental impact of a plant by reducing waste, and decreasing health and safety risks while complying with insurance requirements.

Degraded oil in a system wastes energy because pumps have to work harder. As a result, a maintenance plan will regularly assess the extent of system wear by incorporating regular thermal fluid sampling. Regular fluid sampling may also highlight an increase in carbon which will ultimately reduce the efficiency of the thermal fluid because carbon acts as an insulator. When carbon is present in thermal fluid it demands more energy to maintain temperature which results in higher fuel costs.

A plant manager can ensure continual output and a safe and legal system by proactively taking thermal fluid samples and using the results to prompt a series of corrective actions, including periodic dilution, filtration and regular venting to ensure a safer system. This negates the need to flush the system and, as such, reduces waste and responsibility associated with waste management.

Top-up of thermal fluid is sufficient in some cases but proper thermal fluid management is preferable. The minimum annual requirement for system safety surveillance, to check for things like leaking flanges, provides a baseline service from which a maintenance plan can be developed. This can be achieved by switching the frequency to a half yearly cycle, or to a three monthly cycle in an ideal situation.

Analysis and sampling to check for flashpoint, viscosity and carbon build up should be more frequent; twice a year minimum, but every three months is better, and less than three monthly is best.

Leaks and cavities

In the case of top-ups, the analogy of a central heating system can be quite useful. Radiators are bled and water is topped up every year before the cold season to remedy any leaks and air cavities that may have formed through an air collector. If you don’t do this, the flow of a system can easily be compromised, making it perform less efficiently.

Similarly, leaks and air cavities in a thermal fluid system can typically account for a five percent fluid loss, which needs to be topped up. However, top-up is a poor substitute for a complete fluid management system, which will ensure that the fluid is in a safe and healthy condition.

Complete fluid care is a lifecycle management process, involving fluid sampling and analysis of the results, as well as getting fluid to the site and correct disposal of old fluid.

Synthetic or mineral?

Synthetic heat transfer fluids are designed to resist thermal cracking and oxidation better than mineral fluids. As a result, a minimum maintenance plan may be appropriate for a synthetic fluid, whereas more frequent sampling may be needed for mineral fluids, which are subject to quicker thermal cracking.

Light ends are a common result of thermal cracking. Thermal oil with excessive light ends will build an explosive atmosphere in the expansion tank, in the drain tank, the boiler room or factory, in the event of a leak. Light ends are the lower-boiling components of a mixture of hydrocarbons, such as those evaporated or distilled off easily in comparison to the bulk of the mixture. Examples include low molecular weight organic compounds such as methane, ethane and propane which can be included as components in the oil.

Frequent sampling

Another reason for frequent sampling and regular maintenance - which should include a safety survey and safety training - is to produce trend data from the results. In an ideal world, there would be continuous measurement of temperature fluctuations, carbon build-up and other important parameters which affect thermal fluid performance and present health and safety risks.

The annual fluid analysis should be complemented by adequate environmental allowances for workers, which may necessitate fitting flash guards or similar equipment. This is particularly true if there is activity in a zoned explosive risk area. Ideally, the entire site should be covered by an engineer who is trained to take thermal fluid samples and staff who are fully briefed in how to read and interpret analysis results.

While heat transfer fluid might be a hidden form of transferring energy from one point to another, and perhaps not one most associated with the concept of sustainability, it’s essential that we begin thinking about it in this way. They have to be sustainable from a maintenance perspective, sustainable from an energy management standpoint and sustainable economically.

The correct choice of fluid and the correct maintenance will go a long way towards achieving this. We may have forgotten the London Hydraulic Company and its 181 miles of cable, but we can’t afford to forget the maintenance of our own heat transfer fluids.

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