Why It’s Important to Draw a Representative Heat Transfer Fluid Sample
Heat Transfer Fluid (HTF) analysis begins with collecting a representative sample of the HTF in order to get an accurate picture of overall condition and suitability for continued use and comply with Health and Safety Regulations, including DSEAR and ATEX.
Prior to taking a sample, ensure there is safe access to all ports of the sample device and that the sampling area is safe. Ensure you have a clear standing area with unobstructed access to the sampling site. The height of the sampling port should be below chest height to avoid any potential risk of splashes to the upper-body; the sampling site should be within easy reach; and the area around it should be clutter-free, have a solid floor, and your sampling bucket or tray should be within reach.
During HTF sampling it is important to extract about 0.5 L of fluid from the thermal system, though with smaller plants, obtaining this volume in a single sample may not be possible. Typically, the sampled volume should generally constitute less than 1% of the total volume. Further consideration should also be given to the sampling site (eg low pressure sites) to get a picture of the condition of the HTF in the system as well as the expansion tank (low pressure, low flow) to get a better picture of the fluid’s overall health.
What to test for in the laboratory
Light ends, heavy ends and oxidative state are routinely analysed in the laboratory. The extent of the light ends in a HTF sample can be quantified by measuring flash temperature. The principle is that a high flash temperature reflects a low proportion of light ends in a HTF and low flash temperature reflects a high proportion of light ends. The flash temperature test measures the temperature at which a flammable vapour will ignite when an ignition source is introduced. Flash temperature is measured in two ways – open and closed. Open flash point temperature simulates the mixing of flammable vapours, whereas closed flash point temperature simulates no mixing between vapours and air. Open flash point temperature is higher than closed flash point temperature as vapours are removed by air movement and the most volatile escape.
The extent of heavy ends in a sample is determined by measuring the percentage of carbon residue in a HTF. Carbon can lead to system fouling due to carbon deposition on the internal surfaces of the thermal system.
At levels below 1%, carbon remains soft and can be removed using a suitable system flushing fluid. At levels above 1%, however, it begins to bake on to the internal surfaces of pipes and heating coils, and can no longer be removed easily.
Oxidative state is assessed by measuring the total acid number (TAN) of a representative sample. TAN reflects the acidity of the HTF, specifically weak organic acids. TAN is calculated by quantifying the amount of potassium hydroxide needed to neutralise free acids in a HTF sample. Acids can be very damaging as they lead to the corrosion of pipes. A high TAN (≥1) indicates that a HTF is ageing and oxidising.
How to improve the accuracy of laboratory testing
In a previous article we said that it is imperative to gain a true sample of a HTF whilst it is hot and circulating, and routinely assess light ends, heavy ends and oxidative status so that you get a representation of the state of your system’s HTF over time. It is clear that routine assessments in the laboratory are heavily dependent on gaining a representative sample. So why is this so important? Well, we learnt that open flash point temperatures are higher than closed flash point temperatures because flammable vapours mix with air. In an operational setting, this means that open hot samples would predict a flash temperature that is high when the actual value of the HTF when it is circulating in the heat transfer system is low, ie a false positive. Although, if a closed sample is taken, the flammable vapours would remain in the sample and the test would predict a low flash temperature, which would be a fair representation of the system’s flash temperature – ie a true positive result. Therefore, a closed sample taken at high temperature is recommended to gain a better representation of the light end composition in the fluids and avoid potentially erroneous measurements of flash point temperature.
Managing fluids at high temperatures present a safety hazard especially if done while the HTF is in operation. The personal protective equipment (PPE) worn by engineers is extremely important for health and safety.
Global Heat Transfer engineers are trained and experienced professionals when it comes to drawing live, hot, circulating representative samples and interpreting results to maximise heat transfer fluid longevity and ensure a safe system, site and workforce.
Disclaimer: This article is provided for guidance only. Best practice is to ensure that trained experts take live, hot, circulating samples for the most accurate results.
Read more about wearing the correct PPE.