“I have always fancied that the end of the world will be when some enormous boiler, heated to three thousand millions of atmospheric pressure, shall explode and blow up the globe… They [the Americans] are great boiler makers.”  — Jules Verne

I was recently asked about using hot oil instead of steam to heat process equipment, in terms of safety.

My immediate thought was that either can be used safely. As I thought about it, though, I realized that the two heating media have very different hazards, and that their safe use depends on understanding those different hazards and dealing with them appropriately.

If all you have ever used is steam, hot oil can seem very scary. Likewise, if you are primarily familiar with hot oil system, you may wonder why we’re still using steam after 300 years of boiler explosions.

300 Years of Industrial Steam

Steam boilers have been used in industrial applications since early in the 18th centuries. Since almost the beginning, industrial steam boilers have been blowing up. That’s because steam depends on pressure to achieve elevated temperatures. To operate at 300°F requires steam at slightly over 50 psig. Pushing the pressure up to 100 psig only increases the temperature another 40°F.

However, we’ve been using steam for so long that we have experienced every way it can go wrong. We are not using steam the same as we were in 1709, or 1830, or 1990. We’ve been making improvements all along.

NFPA 85, Boiler and Combustion Systems Hazards Code has its origins in NBFU 60, Regulations of the National Board of Fire Underwriters for the Installation of Pulverized Fuel Systems as Recommended by the National Fire Protection Association. Even this storied code is predated by the American Society of Mechanical Engineers Boiler and Pressure Vessel Code; its first edition was the 1914 edition.

We’ve had a lot of time to get comfortable with steam. Hot oil systems have not been around as long, and for many, they still seem scary.

Some Hazards are Common to Both

Both steam and hot oil must be heated before they are of any use. Either type of system can be designed to use any of three sources of heat: direct fire, electric heat, and waste heat recovered from exhaust systems. The heat sources have their own hazards, and those hazards are the same, whether used to heat water to make steam, or to heat oil.

Some Hazards are Different

The difference in hazards is in the fluid that carries the heat to process. Steam, like water in all its forms, is non-combustible. If a steam system is involved in a fire, it will be because of the fuel in use at the boiler, not because of the steam which piped throughout the facility. Heat transfer oil, on the other hand, is combustible. In fact, at the temperatures it is used, it is extremely flammable. So, not only is it vulnerable to the same type of fires to which steam systems are vulnerable, it is vulnerable to fire at any place in the system where the oil might leak.

Steam is not without its own essential hazard. To achieve elevated temperatures, steam must be at pressures well above atmospheric pressure. Steam boilers don’t blow up because they are hot; they blow up because they cannot contain the pressure they are subjected to. Hot oil systems don’t need pressure to achieve high temperature. They operate at very low pressures, most of which is just the pressure necessary to circulate the hot oil through the system. A typical hot oil system can achieve temperatures approaching 600°F at just a few psig. A steam system can only achieve those temperatures at pressures approaching 1300 psig.

Using Hot Oil Safely

The manufacturers of hot oil systems are quick to insist that hot oil is safer than steam systems. They may be right, but only if their systems are designed, installed, operated, and maintained safely. Here are some suggestions for the safe operation of hot oil systems:

  • Unlike steam, hot oil creates a fire hazard wherever it leaks. That suggests that piping, as much as possible, should be all-welded, with as few separable joints as possible. Also, unlike steam, hot oil is a pollutant, so the loss of containment poses not just a safety issue, but an environmental issue.
  • Pump fires resulting from seal leaks are still too common. Sealless pumps are preferred, and essential when operating above the normal boiling point of the oil. When operating below the normal boiling point of the oil, double mechanical seals may be acceptable, as long as they are never allowed to run dry.
  • Good ventilation in areas where hot oil is used can disperse vapor clouds that form from leaks before they find an ignition source, which has catastrophic results.
  • Temperature control. There is a wide variety of heat transfer fluid to choose from. Take boiling point, flash point, fire point, autoignition temperature, and operating temperature into account when choosing the heat transfer fluid, then stay within the recommended limits. Overheated oil decomposes, forming low-boiling constituents which in turn lower the flash point and other properties of the oil.
  • When steam soaks into insulation, the result is wet insulation. When hot oil soaks into insulation, the result can be a flash fire when the oil self-ignites. Do not use combustible insulation, and where possible, use closed-cell types of insulation.
  • Start-up and shut-down. Take the necessary time to gradually bring an oil system up to temperature during start-up, and then to gradually cool the system down during shut down. Short-cutting these steps can lead to disaster.

Let’s Not Repeat the 19th Century

No system is free of hazards, and no system can be safe if it is not used safely. Steam systems have hazards, but after centuries of use, we have figured out how to use them safely. Hot oil systems have different hazards and don’t have centuries of use. Still, it’s possible to use them safely. Never forget that hot oil, when in use, is extremely flammable and needs to be treated accordingly. Do that and you can reap the many benefits of these systems. Don’t and you may find yourself looking that equivalent of an early 19th century steamboat explosion.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.