Keeping samples accurate

Louise Davis

Clive Jones discusses the importance of conducting accurate heat transfer fluid sampling

The rate of decomposition of a heat transfer fluid (HTF) in normal production is influenced by a number of factors, including the type of heat transfer fluid used, the heat transfer system design, the extent of oxidation and the operating temperature of the heat transfer system.

The lack of guidance on how often HTF should be replaced means that regular sampling tests are vital in maintaining a safe environment for plant workers.

Over time, it's normal for a HTF to decompose through thermal cracking because of its high operating temperatures.

During thermal cracking, molecules break down and form highly volatile components (light ends) that become flammable when the fluid comes into contact with atmosphere, for example in the case of a leak.

Hot HTFs can also decompose as a result of chemical reactions in the presence of oxygen, a process commonly referred to as oxidation.

The product of oxidation will cause sticky carbon deposits of sludge, which may be corrosive to a heat transfer system.

To maintain a safe working environment it is essential that the by-products of decomposition are monitored routinely and measured accurately. This is easily done by implementing a continuous maintenance programme, such as Global Heat Transfer's Thermocare.

Effective sampling

There are standardised tests in place to measure thermal decomposition on a regular basis, so the process of sampling is often overlooked. In the testing process, sampling is only effective if done accurately. An unrepresentative sample can lead to an engineer drawing incorrect conclusions from the test results.

The single most important parameter that is affected by incorrect sampling is the open and closed flash point temperature of the HTF.

Open flash point tests the likelihood of a flammable reaction by removing combustible vapours using the most volatile escape route.

In opposition, closed flash point examines the vapour reaction when maintained in the hot HTF, meaning that it is usually lower than the open flash point temperature.

Taking a closed hot sample

The two principle ways to sample are by taking a live hot sample, or a live cooled sample. This can be done while the sampling container is open or closed to air.

Taking a closed hot sample provides an accurate flash point temperature and is therefore the only way to gain a true representation of your thermal fluid. An incorrect high rating can mask potential hazards.

Total acid number

HTF molecules that have decomposed as a result of thermal oxidation become organic acids that can be measured in terms of total acid number (TAN).

TAN is a measure of the concentration of acidity in a heat transfer fluid and is measured in terms of the volume of alkaline reagent needed to neutralise the acid.

One limitation of measuring TAN is that it provides a measure of the acids generated by both oxidation and by those acids produced by contaminants during the process, so bear this in mind when you receive your final sample result report.

The accurate and repeated measurement of carbon, flash temperatures and total acid number is critical in the assessment of a heat transfer system's safety.

Limiting the HTF's exposure to air using closed flash point sampling will reduce the risk of false measurements, ensuring the HTF is replaced before plant safety is compromised.

Clive Jones is managing director of Global Heat Transfer

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