Several laboratory methods are available that can be used to measure water in thermal fluids. However, such methods are only useful to measure fluids that have not been charged into the system. Water can be easily detected as soon as it enters the system.
Small water infiltrations of less than 300 - 400 ppm can appear as pump pressure fluctuations or even as cavitations. These fluctuations are occasionally ignored or misinterpreted as the heater outlet temperature of the system is well above the atmospheric boiling point of water.
However, the temperature of the fluid is lowest at the pump suction and this temperature establishes whether the water is in liquid or vapor state at that point in the fluid system. When there are sudden fluctuations while heating up the system, it means there are low water levels in the fluid.
It is very difficult to misinterpret high levels of water, which is similar to the free water visible at the base of a decanted sample of fluid. As soon as the water present at a system’s low-point reaches its boiling point, its level rises by approximately 1600 times, based on the temperature and pressure of the fluid at that boiling point, as the water flashes to steam.
The steam bubble quickly forces the displaced fluid up into the expansion tank and out of the vent. It is difficult to ignore hot fluid as well as steam that spurts out of a vent. If an open drum is used as a catch tank, it can lead to serious injury and could possibly cause a fire.
Users have to be more vigilant to identify any changes that occur in the system, whether it may be a pump cavitation, unusual sounds, increased pressures, or increased expansion tank level, particularly during the initial stage.
Removing Water from the System
If water is identified to be present in the system, the source is tracked down and eliminated. If a significant amount of free-liquid water is present in the system, the maximum amount of water that can be removed is drained from the system low points. In this situation running with the vent open until there is no cavitation in the pump does not always resolve the issue.
If steam does not vent to atmosphere, it will condense in the expansion tank, developing pinhole leaks in the bottom of it. As these small droplets are covered with fluid, they cannot be detected until:
- The droplets are drawn into the main loop when the system cools down
- The droplets turn to steam when they come into contact with hot fluid during a fast start-up
As the volume change is around 1000 to 1 when water turns to steam (or consider that a 7-ounce glass of water expands to 55 gallons of steam), it does not take much. In order to ensure that water is completely removed from a system:
- The expansion tank temperature is maintained above 212°F to avoid any vapor condensation.
- Nitrogen is added to the headspace of the expansion tank to sweep the water vapor from the tank when it is formed.
The boilout needs to be continued until the pump suction temperature is higher than 212°F. Once the system is stable, water is tracked down at all the low point drains in the expansion tank.
Preventing Water in the System
Water is present everywhere. About three-quarters of the earth’s surface is water. Approximately 60% of the average human body is water.
It would be difficult to keep all of this water out of a thermal fluid system, but with minimal precautions, this water can be kept at bay. Some of these precautions are as follows:
- Avoid using water to “hydro-test” a new system. Save the water to test the equipment that it can be easily removed from. As most thermal-fluid leaks take place when the system is hot, the complete bolt tightening process will have to be repeated at running temperature. Ensure that any component that is being replaced is dried thoroughly by the vendor if it is being subjecting to a hydro-test.
- Avoid storing fluid drums outside where water can accumulate in the drum head. With changes in temperature the expansion and contraction of the fluid can draw water in through the bungs. If the drums have to stored outside, ensure that they are placed on their sides. Do not use fluid from a “new” drum if rust can be found on the drum head or the seal itself when the seal is broken.
- Use a big “for heat transfer fluid only” sign with some threats involving fire arms to protect the pump used to charge fluid into the system. While this sounds like a no brainer, if the pump is left unprotected, it will get borrowed.
- Consider installing a nitrogen blanket on the expansion tank if the system is located in a humid area and is vented. Condensation will form on the inside and the outside of a vented expansion tank if the tank temperature is less than the dewpoint. If the tank is insulated and it is hard to confirm if the tank is sweating, water may not be the only problem.
This information has been sourced, reviewed and adapted from materials provided by Paratherm.
For more information on this source, please visit Paratherm.