If you suspect that water may have gotten into your heat transfer system, you need to be able to detect it and be certain about your gut feelings. In this regard, there are a few things to watch out for. If a visual inspection of your heat transfer fluid container is possible, check to see if there are two distinct layers since water and most heat transfer fluid are immiscible. This approach is only practical with a significant amount of water. The most obvious telltale shows up during regular cold startups. Keep an eye on your pump discharge pressure. If you observe a sudden drop as the thermal fluid temperature is raised above the boiling point of water (100°C/212°F), you have water in that system. The low pump discharge is essentially due to the water component in the thermal fluid vaporizing leading to a low reading on the pressure gauge.
If you did not take note of your pressure gauge during a cold startup or if you have a continuously run heat transfer system, there are other ways to detect water. The occurrence of unexplainable pump and valve cavitation is one. As the system temperature is raised past the boiling point of water, steam is formed. Steam bubbles are entrained with the heat transfer fluid as it moves through pumps and valves. These bubbles implode in the pump/valve trim causing damage. Ultimately, you can draw a thermal fluid sample from the lowest point in your system and ask your fluid supplier/manufacturer to carry out a moisture content test on that sample. Relatherm Heat Transfer Fluids offers a complimentary Fluid Test and Analysis that you will find useful in this regard.