As I discussed in last month's column, changing the fluid in your hot oil system can be dangerous. David Dowlen, assistant service manager at Heatec Inc., Chattanooga, Tenn.; Doug Irvine, engineering manager, thermal fluids, at Petro-Canada Lubricants, Mississauga, Ontario; and Jim Oetinger, sales director and chief chemical engineer at Paratherm Corp., Conshohocken, Pa., talked with me about specifics actions that can be followed while performing this task to ensure the safest change.
In this month's column, I continue this series with specifics about the vapor pressure curve, a nitrogen purge system and draining the system. In addition to MSDSs (discussed in last month's column), a copy of the vapor pressure curve for the fluid being used also should be available, according to Dowlen. “When dealing with high temperature [typically above 375oF (191oC)], it is necessary to apply an inert gas such as nitrogen to the expansion tank. The higher the operating temperature, the higher the nitrogen pressure will be needed to keep the fluid in a liquid state rather than flashing into a vapor,” he says.
Also, Dowlen says that an adequate nitrogen purge system is needed for the expansion tank. A nitrogen system would consist of a pressure regulator and a backpressure regulator. If the expansion tank is to have more than 15 psi applied to it, the tank would have to be an ASME- certified pressure vessel. Therefore, the nitrogen backpressure regulator would be adjusted to keep the tank pressure from ever reaching 15 psi. However, Dowlen says, some states or municipalities may require that any thermal fluid system (piping included) be ASME certified.
“For example,” says Dowlen, “the technical data sheet for Marlotherm SH heat transfer fluid indicates on the vapor pressure curve that at a temperature of 644oF [340oC], 8.12 psi of inert gas be applied to the expansion tank to keep the fluid in a liquid state.”
Another factor that sometimes gets overlooked when performing a hot-oil change in a thermal fluid heater is the percent of fluid expansion. “This is especially important if a customer decides to change the brand of fluid when replacing the fluid in a system,” Dowlen says. It is quite common to put 5,000 to 7,000 gal of fluid in large industrial heating systems. Dowlen offers this example:
Suppose that a fluid with 20 percent expansion rate were being put into a system with 5,000-gal capacity. The expansion tank should be filled to a point high enough to keep the low-fluid level switch closed. The position of minimum level varies per manufacturer. The expanded fluid volume of fluid would now be 6,000 gal total capacity. If the total volume capacity of the expansion tank were only 500 gal, a serious problem exists. The fluid would not be able to be operated at the correct temperature. This happens quite frequently, Dowlen says, even on new installations.
Removing Old FluidIn this area, the customer is limited as to what can be done, according to Dowlen. “Almost 99.9 percent of customers do not consider fluid removal when installing a piping system,” he says. Due to the multiple elevation changes of a piping system, it is physically impossible to drain 100 percent of the old fluid from a system. Another method must be used. The most common method is to use compressed air to blow the fluid out of the system. Another method is to hire an approved company with a vacuum truck to suck the old fluid from the system. If a vacuum truck service is used, be sure the company you hire has an approved disposal method.
On smaller systems, the customer may find low points in the system to drain the fluid. If you drain the system yourself, be sure you have an approved containment and disposal method.
Irvine agrees, “In case of spills, users should be aware of local environmental guidelines. They should also be aware of local waste disposal guidelines that apply to the used fluid.”
Irvine notes that it is always good practice to circulate the fluid after the heater has been shutdown to prevent localized overheating due to heat retention in the heater's refractory. This practice of circulating the fluid after shutdown also is important during fluid changeouts to prevent pockets of fluid that can be significantly and dangerously hotter than the rest of the fluid.
Go to Part 3
Editor's Note: Neither Process Heating nor any of the sources contributing to this article are liable for the suggestions or recommendations made. As with any engineering project, you need to consider the specifics of your applications as well as local codes and restrictions.