Even though thermal-fluid heating systems have been widely used for over 80 years, they still provoke a certain amount of fear and trepidation in users. This anxiety is reinforced by any number of horror stories that usually involve systems that suddenly develop a "problem" after years of trouble-free operation. Maybe it's a fire around a pump or another piece of equipment. Or maybe the heater room fills up with smoke. What makes these incidents into oft-repeated horror stories is the lack of a readily apparent explanation for the event. Accident investigators might want to know why the pump seal was leaking, when the flash/fire/autoignition of the fluid was last tested, or whether a higher-temperature thermal fluid should have been specified. Then, whoever has the most experience with these systems will recite some myths and old wives' tales about thermal fluids and systems, and blame the incident on one of them.
Because thermal fluid systems do involve a combustible liquid under moderate pressure at an elevated temperature, they should inspire some degree of caution. However, few plants have people who've had time to develop a real understanding of how such systems work; thus, the myths and old wives' tales get accepted. So here, to help you reduce the potential for safety incidents, let's dispel five of the more common myths.
Myth 1 — New thermal fluid always contains water. It's easy to see how this myth keeps going. After new fluid is added to the system, suddenly hot fluid "geysers" out of the expansion tank vent into the catch tank and possibly onto the heater room floor. Or the pump begins to cavitate at every startup. Typically these incidents result in calls to the supplier, berating it for sloppy quality control and requesting an immediate shipment of "good" fluid. Ignoring the plant's own role in this problem (e.g., storing drums of fluid unprotected outside), the truth is all new fluid does contain trace amounts of water, typically less than 150 ppm. This is why new system startup instructions include a boil-out ("seasoning" or "conditioning the fluid") step. A proper boil-out not only removes any water in the new fluid but also eliminates water remaining in components from hydrotesting and in piping left exposed to weather prior to installation.
Unfortunately, often the boil-out is terminated prematurely, as soon as the pump cavitation stops or the scheduled time is up. Any water left in the system will collect in the bottom of the expansion tank and in other cold (less than 212°F) low points such the thermal buffer tanks. The fluid samples shown in Figure 1 come from the main loop and thermal buffer tank of a system that had been operating steadily at 550°F. Covered by the thermal fluid (because it's heavier and insoluble in water), water can't evaporate and, as a result, can remain hidden for years. Documented incidents testify to water remaining in place long enough to corrode through the metal, causing a thermal fluid leak (which creates its own excitement because these tanks are usually installed above the heater) even though the system has been run continuously. Normal operating-temperature swings might not generate enough flow past the hidden water to pull it into the main loop. However, a large drop in temperature at shutdown or adding new "good" fluid through the expansion tank may cause enough flow to dislodge the water. Problems might begin at the expected temperatures during startup (when the heater outlet temperature reaches 220–250°F) or might not occur until the outlet temperature exceeds 350°F in a large multiple-loop system. The most effective way to prevent geysers is to take the time to do a thorough boil-out after every cold shutdown. Circulate fluid through all branches and all heat exchangers. Leave the boil-out valve open until the expansion tank temperature is above 230°F and no more steam is visible from the vent. Justification for this extra time is simple — water will keep a system from operating.
Myth 2 — The thermal fluid must be bad because my pump seal is leaking. Leaking pump seals should be a concern because they are involved in a majority of thermal fluid fires. Seals are the only components of a thermal fluid system where there isn't a solid piece of metal between the hot combustible fluid and air. So while the ignition source isn't always identifiable, the fuel source is always the leak from the pump seal. What perpetuates this myth is that leaking seals often will have carbon buildups on the outer surface. While this carbon can force the seal faces apart and cause a leak, it doesn't necessarily indicate the fluid needs replacing.
Mechanical seals are lubricated by a thin film of fluid that seeps through the gap between the faces. Once this fluid hits the air on the outside surface it acts just like the fluid around a leaking flange, producing some smoke and lots of carbon. Worn or poorly installed replacement seals will allow more fluid to seep out between the faces, producing more carbon. While it's true that carbon is formed in the fluid as it degrades, these particles typically are soft, soot-like and small enough to pass between the faces (think graphite). Even larger pieces of "coke" that flake off heater tubes can't cause seal leaks because they're still soft enough to be ground up between the tungsten carbide and silicon carbide faces typically specified for thermal fluids. Only metal particles are hard enough to score a seal face; this is why a 60+ mesh screen should be installed in the pump suction Y-strainer on a new system or on any system that has had the piping opened up or new components added.
Myth 3 — An open-top drum makes a great catch tank. This seems like a good idea until the heater room suddenly fills with smoke or worse. Every thermal-fluid heater installed inside a building requires a catch tank to collect the discharges of the expansion tank vent and system relief valves. Whether the expansion tank is open to atmosphere or blanketed with nitrogen, these collectibles typically will include water and volatile byproducts of fluid degradation ("low boilers"), along with the occasional fluid geyser from entrained water. This is why it's not a good idea to re-use any fluid from the catch tank; re-use can allow collected water and other volatiles to vaporize again and end up back in the tank (or on the floor around the tank when it bubbles over).
Where the open catch tank myth really falls apart is when the same catch tank also serves as the collection point for the heater relief-valve discharge. A heater relief valve doesn't lift very often but when it does it discharges the hottest fluid in the system into the catch tank. The hot fluid shooting into the tank can explosively vaporize any water lying in the bottom — this, in turn, will vaporize the rest of the tank contents and fill the heater room with a potentially ignitable cloud. Preventing this requires containing these types of discharges and draining the tank frequently. Both requirements can be met if the catch tank is a closed-head dish-bottom steel tank with a drain valve, pressure-rated sight glass and a minimum 4-in.-dia. vent to a safe location (preferably outside of the building). If the tank must be vented inside the heater room, locate the vent as far away as possible from the doorway, control panel and fresh air inlet.
Myth 4 — Never operate a thermal fluid above its autoignition temperature (AIT). Autoignition (spontaneous ignition) is the most frightening potential source of fire because all you theoretically need is temperature and a leak to turn the heater room into a furnace. As long as the fluid temperature stays below the stated AIT, so the myth goes, the risk of fire is minimal. The truth actually is pretty simple. With the exception of most vapor-phase products, a few high-temperature aromatic-based liquids and many low-temperature non-aqueous fluids, most thermal fluids don't have a true AIT. The current standard test method, "Autoignition Temperature of Liquid Chemicals" (ASTM E 659-78), involves injecting 100 microliters of liquid into a heated 500-ml flask. If no ignition of the sample is observed within 10 minutes, the temperature of the flask is raised and another sample is injected. The critical piece of information regarding this test appears in the "Significance and Use" section (5.5): "This test method is not designed to measure the autoignition temperature of materials which are solids or liquids at the test temperature (for example, wood, paper, cotton, plastics, and high-boiling-point chemicals). Such materials will thermally degrade in the flask and the accumulated degradation products may ignite." Because most liquid-phase thermal fluids have boiling points (i.e., vapor pressure exceeds 14.7 psia) above 690ºF, any reported AIT less than the boiling point is by definition not a true AIT.
Fluids can and do spontaneously ignite, though. Insulation fires can result from spontaneous ignition, as can some pump fires. However, the material that ignites isn't the original thermal fluid but fluid that's been exposed to air for an extended time. In porous insulation such as fiberglass, rock wool or perlite, the fluid wicks away from the leak point, leaving a thin film through the insulation. The limited supply of air and constant high temperature from the pipe cause the fluid to decompose. Eventually the decomposition products reach their AITs and ignite. Use of nonporous insulation or judicious use of porous insulation can prevent these fires. Leaked fluid that pools under a pump or some other uninsulated piece of equipment can undergo a similar decomposition. The most effective way of preventing these pool fires is to ensure any leaked fluid drains away from the hot equipment.
Myth 5 — My flash point has fallen so I can't start up. This myth is widely repeated by corporate safety departments and rogue insurance representatives. It assumes that operating a thermal fluid above its flash point will result in an immediate and thoroughly destructive fire if the fluid leaks out. Actually liquid heated above its flash point produces combustible vapor and the "flash" ends as soon as the vapor has been consumed. For a thermal fluid to burn, it must remain hot enough to continuously generate vapor. Because liquid-phase thermal fluid systems operate under moderate pressure — typically a maximum of 100 psig at the circulation pump discharge — a leak will result in a fluid discharge into the surrounding area. Most thermal fluids have a flash point between 300°F and 450°F (some have a flashpoint as high as 530ºF but their high viscosity limits their usefulness) and can't produce ignitable vapors below 300°F. So unless a combustion event (like a fire) already is in progress around the leak, even a heater room in Arizona in August is cool enough to reduce the temperature of the leaking fluid rapidly below its vapor region.
The potential for vapor ignition becomes the most problematic when either the air volume is small or the leak volume is large. In enclosed areas such as ovens with limited air to cool and react with the vapor, even small leaks can pose risks. What often misleads investigators is that fires in these confined spaces can result from autoignition caused by the partial degradation of the leaked fluid. Large leaks become a problem when the quantity of vapor released overwhelms the capacity of the ventilation available to cool the leaked fluid as well as react with the vapor. If the vapor cloud, which resembles gasoline vapors in appearance, gets large enough, it may eventually reach an ignition source. So why don't insurance companies, international fire codes or government regulators prohibit thermal fluids? The truth is that one of the requirements for a fire — air — actually can prevent these fires from occurring. As most users know, the telltale sign of a thermal fluid leak is smoke. Smoke is the visible byproduct of the hot (>300°F) thermal fluid vapor reacting with air. Good ventilation not only cools the fluid so vapor no longer is produced but also increases the quantity of vapor that can be safely converted without an actual fire. While no current specifications exist for "good ventilation," a useful rule of thumb for a heater room is that room temperature shouldn't exceed ambient temperature by more than 40°F.
DON'T BE MISLED
The common thread through these myths is that thermal fluid systems are mysterious and unpredictable and, as a result, potentially dangerous. Like any myths, these can be debunked with an understanding of what really causes the problems and, more importantly, how to prevent them.
JIM OETINGER is technical director of Paratherm, West Conshohocken, Pa. E-mail him at firstname.lastname@example.org.