Changing the fluid in your hot oil system can be dangerous. Find out how to ensure the safest change.

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 Oetin- ger, 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 continue this series with specifics about removing the old fluid and what to do while the system is drained.

While preparing to remove the old 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.

“Continuous circulation of the fluid ensures a uniform temperature and no unpleasant surprises,” Irvine says. “Continuous circulation until just prior to draining also ensures that any free solids or other contaminants remain suspended in the fluid so that they can be drained from the system with the used fluid.”

When it comes to fluid-specific concerns, i.e, glycol vs. synthetic, Irvine says that after the fluid is drained, the system should be flushed using a compatible flushing fluid. “If the new fluid is of a different and incompatible chemistry, all of the old fluid needs to be completely flushed from the system. In high temperature systems that use non-aqueous heat transfer fluids, users should avoid using water or water-based cleaners where possible and avoid hydrotesting the system for leaks. Water can cause operational and safety issues after the system is restarted,” Irvine says.

Paratherm's Oetinger says that synthetics typically require more extensive safety equipment than mineral oils. He also notes that disposal of spilled material can be more involved with glycols because they are water soluble.

Take Advantage During Changeout

While performing a fluid change, preventive maintenance and checks should be performed. “This is a prime opportunity to inspect and clean the expansion tank, heater, heat exchangers and valves,” says Irvine. “Any sludge or deposits observed should be removed.”

He says that at the same time, the user should be questioning the source of any sludge or deposits found. Are they the result of poor system design or operation, system contamination, or the fluid being used beyond its design limits?

Oetinger says that all temperature, pressure and level sensors, switches, etc., should be cleaned and calibrated, too.

In addition, Irvine says that all flanges and connections should be checked for leaks, and any oil-soaked insulation should be replaced. Irvine cautions that care should be taken when replacing oil-soaked insulation if the system is still hot. Slow oxidation of any fluid that has seeped into the insulation, plus heat retained from the system, can result in significant temperatures in the insulation.

“Fluid that has soaked into the insulation can have degraded to the point where its autoignition temperature is below ambient conditions,” Irvine says. There may be a slow smoldering fire present due to the low levels of oxygen present in the insulation. The inrush of oxygen that occurs when insulation is cut open for removal can be enough to change that slow smoldering fire to a raging inferno, Irvine asserts.

Just after a system has been cleaned and recharged with fresh fluid is a prime opportunity for taking some baselines of system performance. Irvine says that knowing how the system performs when clean is key to tracking degradations in system performance and identifying fouling.

“Users should take note of key system temperatures, operating pressures and process heating rates,” Irvine notes. “In addition, a sample of the freshly charged fluid should be taken after 1 hr of circulation as a baseline of the fluid condition.” It can be used to assess any contamination of the new fluid during charging (e.g., residual cleaning fluids or used heat transfer fluid) and used to assess any future fluid degradation (e.g., thermal or oxidative).

I'll conclude this series (click here for Part 4) with a look at tips for filling the system.

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.