Thermal Fluid Changeout, Part 4
Changing the fluid in your hot oil system can be dangerous. For this Safety Zone series (click here for Part 1), 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 dangerous task to ensure the safest change.
In this month's column, I conclude this series with specifics about filling the system.
Filling the SystemWhen filling the system with new fluid, there are several mechanical considerations, according to Dowlen. Most system circulation pumps are centrifugal type -- they have little or no suction ability. They do have a net positive suction head (NPSH) minimum requirement, which means that the fluid must have a minimum positive pressure to the pump inlet or the pump will not pump any fluid. “This is another reason that an expansion tank is required,” says Dowlen. “The thermal fluid will need to be pumped into the expansion tank in order to be pumped into the system.”
If the fluid is delivered to the plant site by bulk delivery truck, Dowlen suggests you request that the delivery company have an unload pump on the delivery truck. And if this is not possible, he says, you will need to supply a transfer pump that will be adequate for the job at hand. “In other words, do not use a 10 gal/hr pump to fill a 7,000 gal system,” says Dowlen.
While filling the piping system with fluid, Dowlen says the use of block and bleed valves can be a savior. “Just as most customers do not think about how to remove fluid from a system, they also do not think about how to adequately fill a system with new fluid,” Dowlen says. By using block and bleed valves, Dowlen says, you can ensure the system can be filled 95 percent or better with new, clean fluid (figure 1).
Dowlen says that by closing the block valve and opening the bleed valve while filling the system with new fluid, the new fluid will force the old fluid and air out of the system. “Once clean fluid is discharging into the catch drum, close the bleed valve, open the block valve and go to the next block/bleed point in the system and repeat. If block and bleed valves were not originally installed in the system, they can be installed prior to filling the system with fluid,” he says.
Once the fluid is in the system and the circulation pump and heater burner are running, Dowlen says to be sure to bring the temperature of the fluid up in increments. “Never take the temperature of the fluid over the flashpoint without closing the expansion tank off from atmospheric conditions -- close off the vent -- and applying inert gas to the expansion tank such as a nitrogen purge system,” Dowlen says.
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.