According to Paratherm Corp., W. Conshohocken, Pa., a maker of heat transfer fluids, water lurking in a system can show up in more than one way. Here's the company's advice on discovering, removing and preventing water in your hot-oil system.
Small water infiltrations of less than 300 to 400 ppm show up as pump pressure fluctuations (cavitation). Sometimes, the fluctuations are misinterpreted or ignored because a system's heater-outlet temperature operates well above water's atmospheric boiling point. However, the fluid temperature is lowest at the pump suction, and that temperature determines whether the water is liquid or gas at that point in the system. If the fluctuations start suddenly while the system is heating up, there are low levels of water in the fluid.
High levels of water -- much like the free water visible at the bottom of a decanted sample of fluid are difficult to misinterpret. Once water residing at a system's low point reaches its boiling point, its volume increases suddenly by up to 1,600 times, depending on the fluid temperature and pressure at that point -- as it flashes to steam. The displaced fluid is quickly forced into the expansion tank and out the vent. Hot fluid and steam spurting out of a vent are hard to ignore. Serious injury and even fire can occur if an open drum is used as the catch tank.
Be on the lookout for any sudden change in your system, whether it is pump cavitation, increased expansion-tank volume, unusual sounds or increased pressures, especially during startup. All of these can indicate water in the system.
If the system has significant free-liquid water, as much liquid as possible must be drained from the system low points. Operators may think that running the system with the vent open until the pump stops cavitating will solve the problem. But that might not be the case.
Any steam that does not vent to atmosphere will condense in the expansion tank. Because these small droplets are covered with fluid, they will remain undetected on the bottom of the tank until they:
- Get pulled into the main loop as the system cools down.
- Turn to steam when they get hit with hot fluid during a fast startup.
Because the change in volume when water turns to steam is about 1,000 to 1, it doesn't take much water to create a lot of steam. To ensure complete removal of water from a thermal fluid system, keep the expansion-tank temperature above 212oF (100oC) to prevent vapor condensation, and add nitrogen to the expansion tank's headspace to sweep the water vapor from the tank as it is generated.
The boil-out should continue until the temperature at the pump suction is above 212oF. Once the system is stable, check for water at all the low-point drains in the expansion tank.
It's impossible to keep your hot oil system 100 percent free of moisture, but even minimal precautions help keep water at bay.
- Do not hydro-test a new system with water. Save the water for testing equipment from which it can be removed easily. Besides, most thermal-fluid leaks occur when the system is hot, so an operator would have to repeat the entire bolt-tightening process at running temperature, anyway. If a system component is being replaced, make sure the vendor dries it thoroughly if he has hydro-tested it.
- Do not store drums of fluid outdoors where water can collect in the drum head. The expansion and contraction of the fluid with temperature changes can pull water in through the bungs. If drums must be stored outdoors, lay them on their sides. After unsealing a drum, don't use the fluid if there is rust on the seal itself or on the drum head.
- Protect the pump used to charge fluid into the system with a big sign that says “for heat transfer fluid only” to minimize the chances that it will be used for other purposes.
- Consider installing a nitrogen blanket on the expansion tank if your system is vented and your plant is located in a humid part of the country. If tank temperature drops below the dewpoint, condensation will form on the outside and inside of a vented expansion tank.
For more information, contact Paratherm at (800) 222-3611.
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