Insulating Hot Oil Systems
Material selection and installation of steam pipe insulation can reduce the potential for fires.
Insulation fires from leaked thermal fluid are one of the least understood potential hazards facing hot oil systems. The main theory suggests that the fluid degrades in an oxygen-starved environment until its chemical makeup reaches its autoignition temperature. Academic papers have studied the kinetics of the spontaneous ignition inside the insulation and developed estimates of the activation energy required. The bottom line, however, is that thermal fluid systems have two of the required items necessary for autoignition to occur: a combustible liquid and heat. Material selection and installation can significantly reduce the potential for these fires.
Even if energy costs were zero, there would still be the need to protect company personnel from exposure to hot pipes. In boiler and heater rooms where insulation is absent or insufficient, operators have complained that they must use metal tools with extra caution because they get so hot they can cause first-degree burns. In these conditions, wire-rimmed glasses are best left in a case on the desk.
A rule of thumb for specifying thermal insulation at elevated temperatures is to size the insulation to maintain a temperature at the outer surface not exceeding 140°F (60°C), according to Lee’s Loss Prevention in the Process Industries.
Hydrocarbon-based heat transfer fluids present a unique insulating problem because fluid that leaks into the insulation can become a fire hazard. The very conditions that insulation create - for instance, preservation of heat and exclusion of air circulation - can lead to circumstances that promote self-ignition. The vast surface area within common insulation types - all those interstices - can further complicate the picture. The continued exposure to high temperatures inside the insulation and the limited fresh-air supply combine to partially oxidize the fluid into very different material. Autoignition occurs when either:
- The molecular rearrangement produces a compound that ignites at the existing temperature and oxygen level.
- A sudden increase in oxygen allows ignition as is.
But, there are materials, procedures and practices you can use to control the risk.
Minimizing the Fire Hazard
Shown before cladding and coating, cellular glass insulation such as Foamglas from Pittsburgh-Corning must be purchased in either blocks or shapes to fit specific components, but its closed design reduces fire hazards in hot oil systems.
Minimizing the fire hazard from insulation is straightforward, but the materials can be more expensive than conventional insulation.
Insulation is nothing more than a large number of air pockets that are held in place by some type of material. For high temperature systems pumping combustible liquids, these materials may consist of mineral fibers, compressed particles (calcium silicate or perlite) or cellular glass. When considering autoignition conditions, the cellular glass delivers some clear advantages.
Because of the way the material is produced, the air pockets in the cellular glass material are not connected. So, any fluid entering the insulation remains isolated near the leak point, preventing the type of auto-ignition that causes fires. Often, weep holes are drilled through the material to allow pooled fluid to drain out, and this further reduces the fire hazard - unless, of course, the drain is right over an ignition source.
In the other types of insulation, the hot fluid may spread throughout the vast interstitial surfaces. There, it shares the space with thousands of air pockets at elevated temperatures maintained by the insulation itself - the very conditions that can promote self-ignition.
Because cellular glass is rigid, it must be purchased in either blocks or shapes to fit specific components such as valves or piping Ts. This increases the material cost. Installation costs also are higher because of the onsite cutting required to fit one rigid component to another.
The trick to optimizing and minimizing insulation costs is to utilize cellular glass where it is necessary, and then use less expensive material where there is minimal potential for fluid leaks. For instance, if you get a leak on an uninterrupted pipe run, you have bigger problems than soaked insulation. Typical high-potential fluid leak areas include valves, Y-strainers and any pressure taps or other connections. Fiberglass or calcium silicate can be used elsewhere.
Transitions from one type of insulation to another are critical. Horizontal pipe runs can transition at flanges or 12 to 18" from the potential leak point. If you want to keep the cladding intact, it is a good idea to leave 1 to 2" of space between the insulation and the flange, so you can retighten the bolts once the system is at operating temperature. The open end of the insulation should be sealed to keep any spills from above from leaking into the mat. Sealing the end is critical where the transition occurs on vertical pipe runs because any leakage will run along the pipe under the cellular glass.
For short vertical runs, optimizing the insulation might dictate skipping the transition until the horizontal run starts - where thoroughly sealing the open end is less critical. And, as a reminder, never insulate flanges in a hot-oil system. If the bare flange presents a safety issue, install a protective sheet-metal cover with a drain hole. As an alternative, removable modular cladding/insulation systems that are non-permeable can be installed over flanges. These also work well for valves and Y-strainers.
Once the equipment is up and running, the insulation should be inspected periodically to make sure that there is no fluid leaking into it. Any visible smoke or odor of hot fluid should be investigated. Weep holes should be drilled into the bottom of the cladding to allow leaked fluid to escape. Any darkening of the insulation or cladding indicates that fluid is leaking. Insulation that has been soaked with fluid should be removed carefully because any sudden increase in the oxygen level can result in autoignition.