How do you know when you need to clean out your hot oil system, and what things should you keep in mind as you do it? This article describes:

  • The signs that you need to clean out your system.
  • What causes heat transfer fluids to break down.
  • Several clean out procedure options.
  • Manpower and length of time required to perform the clean out.
  • Pump and seals.
  • Disposal.
  • What you should expect when you are finished.

Let’s get started. The obvious indication that your fluid has degraded to the point that you need to clean out your system is that it cannot maintain the operating temperature of the process. This is evidenced when you find yourself dialing up the temperature on the heater, and you may notice that the flow rates have dropped off as well. All these are indicators that the fluid has degraded to the point that it is starting to sludge up the system and possibly coating the internal components. This coating can act as an insulator to keep the heat away from the oil in the heater as well as keep the heat in the oil at that process.

Whether the heat transfer fluid has begun to degrade due to oxidation, thermal cracking or contamination, it will need to be cleaned out. However, there are enough differences with each system and situation to make a single detailed clean out procedure impractical or even incorrect for some cases. Instead, several general options are presented. Talk with a thermal fluid specialist to determine which option is best for you.

1. Flush with a Flushing Fluid

Procedure suggested for use when:

  • System is not badly fouled.
  • Fluids are chemically compatible.
  • System is not difficult to completely drain.
  • Time is short.

To begin, if possible, filter the existing fluid. Then, run the system at 225 to 250°F (108 to 122°C) - or just below the flashpoint - to mix well and suspend all the loose particulate in the thermal fluid. Stop the heater and, with the circulating pump going, allow the temperature to drop. Once the system has reached an acceptable temperature, stop the system pump and quickly drain the thermal fluid from all low points.

Please note, personnel should exercise appropriate caution when working around hot fluid - remember, safety comes first. The benefit of hot draining is that it prevents sludge and tars from re-depositing in the system.

Next, recharge the system 100 percent. Flushing the fluid from all of the low point drains helps prevent air pockets from developing. Start the pump (circulation) without heat to help remove any air pockets. You will hear them working their way into the expansion tank. When all of the air pockets are removed, apply heat.

Circulate the flushing fluid for the manufacturer’s recommended time at temperatures above 200°F (94°C) or at operating temperature. Then, reduce the heater outlet temperature to 225°F (108°C) at end of the circulating time before draining out the fluid to ensure smooth removal of all fluid and particulates gathered with the flushing fluid.

Turn off the heater and allow the temperature to drop while keeping the circulating pump on. When at an acceptable temperature, stop the system pump and drain quickly.

Recharge the system with new heat transfer fluid and restart. Take a fluid sample within one week of startup and send it to your heat transfer fluid supplier for analysis.

2. Flush with a Process System Cleaner

Procedure suggested for use when:

  • Fluids are not compatible and are different types.
  • System has particulate and loosely adhering carbonaceous deposits.
  • System is badly sludged up or flow is restricted in some areas.
  • Treating a large system, where a full charge of flushing fluid is not acceptable.

To use a system cleaner, follow the general procedure used for the flushing, but add anywhere between 4 percent and 20 percent process system cleaner - depending on how badly degraded the system fluid is  - to the existing fluid. Add the process system cleaner by using a secondary pump on the suction side of the main system pump - do not add in through the expansion tank. Circulate the cleaner for 48 hours at operating temperature.

Then, reduce the heater outlet temperature to 225°F (108°C) at end of time before draining out. Reducing the temperature before draining helps to ensure smooth removal of all fluid and particulates. If the system fluid has been severely degraded, filtering the fluid is recommended to remove as much particulate as possible before draining the system.

Once the system is drained, recharge with new heat transfer fluid and restart. As with any recharge, take a fluid sample within one week of startup and send it to your heat transfer fluid supplier for analysis.

3. Professional Cleaning

If you do not have the manpower or equipment, or if you have special clean out requirements, there are companies that can perform thermal fluid system cleaning. They will clean out your system using the procedures already outlined and can meet any specific requirements. In addition, if your system requires a chemical clean out, there are companies that provide this type of specific experience.

In a typical procedure, the following cleaners may be used:

  • Water-borne cleaner, frequently based on inhibited phosphoric acid or sodium hydroxide with surfactants.
  • Solvents and/or chelates.
  • Sodium hydroxide with potassium permanganate.

Caustic cleaners are hard on copper, aluminum and other alloys and should be used with caution. Chlorinated solvents such as 1, 1, 1, trichloroethane or perchloroethylene should not be used due to the potential for environmental damage or system damage from residual solvent.

Once the system is chemically cleaned, flush it with plenty of water. (Of course, it is very important that all water be removed with a dry-out procedure or air prior to startup.) Perform a final rinse, which may contain inorganic rust inhibitor, and drain the system hot. Then dry out the system completely with heated nitrogen gas or air, as a second choice, to a -20°F (-29°C) dewpoint with the help of vacuum.

Other Factors

When performing a clean out, keep in mind that time, pumps and seals, and disposal also must be considered. A common mistake is to try to squeeze a clean out into a weekend with a Monday morning production startup. This is practical only for small, non-complicated systems. Considerable time at temperature is required for cleaning. Good flushing is not done quickly.

Also, pumps and seals must be considered. For flushing fluid and system cleaner options, be sure the cleaning fluids are compatible with the system components. When using the chemical clean out procedure, the pump and seals on heat transfer fluid systems are made for organic heat transfer fluids and will probably not be satisfactory for water-borne cleaners. An auxiliary pump will probably be required.

Remember, uncontaminated organic heat transfer fluids can be taken by a waste oil hauler (oil recycler) and disposed of like used motor oil. If your system is thousands of gallons, most waste oil haulers will offer to remove the oil from the system and pay you a small amount per gallon. If your fluid is classified as hazardous waste, contaminated fluid must be handled according to the contaminant type. Local and federal regulations must be consulted and followed.

When you have selected the correct procedure and reviewed it with your hot oil supplier, you should expect to see your system clean and free of any soft sludge buildup. What you cannot expect is that, when using flushing fluid or system cleaner, any of the solid or hardened buildup will be removed. Chemical cleaning will remove the hardened buildup but also will remove any carbon molecules that has plugged a pin hole leak you might have had from the pipe rusting or breaking down from the outside in. If this happens, you can expect to see your new heat transfer fluid seeping out these holes when you recharge your system.

Given all of these considerations, and how essential thermal fluid heating systems are to their facilities, the best idea is to contact your current fluid provider to help chose the right procedure.