Thermal oil systems have proven exceptionally safe over the many years they have served industry. Like any other industrial system
however, the proper design, installation and maintenance of these systems is critical to their smooth functioning and extended
operating life.
With the exception of rare, freakish situations, the limited number of fires that do occur in thermal oil systems occur in insulation.
Fluid leaking from valves, gasketing, welds or instrument ports finds its way into porous insulation and wicks through. Remaining as hot
as the system itself, the fluid comes into intimate contact with the air in the insulation's millions of pockets.
As it enters each pocket, the fluid oxidizes and decomposes-in the process using up the existing air and creating heat. Confined within
the insulation, the heat has little chance of escaping. The continued oxidation causes temperatures to rise. In some cases temperatures
may exceed the autoignition point of the fluid.
Should the insulation be opened up when the system is hot, fresh air will immediately enter. Coming into contact with the hot, partially
oxidized fluid, fresh air can cause spontaneous ignition resulting in a smoldering fire, or a flash.
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Note: Thermal oil vapors leaking from a system can be highly flammable. You should take immediate action. |
Let's review component selection, installation and maintenance.
Component Selection
We strongly recommend that the use of threaded fittings be minimized. With the large degree of thermal expansion and contraction in high
temperature systems, and the limited temperature range of many pipe sealants, leakage is almost guaranteed.
In leak-prone areas-near valves, pumps, flanges and instrument ports, for example-use
only high temperature closed-cell insulation. Or remove insulation entirely at these sites.
For valve stems (or "packed" pumps), we suggest packing sets consisting of end rings of braided carbon or graphite fiber, and
middle rings of pre-formed (pressed) graphite foil. Good choices for flanges are spiral-wound carbon or graphite-filled gaskets (you may
want to investigate a Teflon® material called Gore-Tex Joint Sealant®).
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When installing gasketing, be sure to closely follow the manufacturer's recommended torquing and tightening sequence. In valves, seat each packing ring fully, and tighten gland nuts slowly while moving the handle back and forth. You should consider
specifying bellows-type valves and seal-less magnetic drive pumps. These will give you good performance.
Installation
During construction and installation, four areas should be addressed: system cleanliness, component orientation, system tightness and
allowance for thermal expansion and contraction.
System Cleanliness
Care must be taken to assure that the system is clean and dry. Both the "hard" and "soft" contamination are best removed as the system is
being assembled.
Hard contamination such as mill scale, weld spatter/slag, and dirt can cause restrictions that significantly alter fluid flow.
Resulting low fluid velocity through the heater may cause overheat conditions. Overheating of the fluid can lead to "coking" (carbon
deposits in heater tubes), thermal stress on the heater tubing, and possible tubing rupture.
Soft contamination such as quench oil, welding flux and protective lacquer coatings can
dissolve in the fluid. Carried through the heater, these materials degrade at much lower temperatures than the
fluid itself, and can form carbon crust on heated surfaces (particularly the heater tubing). The coke build-up prevents the
fluid from removing heat from the tubing, and results in thermal stress of that tubing.
Component Orientation
Expansion tanks should be located far enough above heaters so that they run at no more than 150°F in atmospheric vented systems.
Warm-up valves should normally be closed. If run hot, and in contact with air, the fluid can severely oxidize. Valves should be
mounted stem sideward so that leakage from the stem or from bonnet gasketing is less likely to enter insulation. Gaskets should be of the
type that can flex with the system's thermal expansion. Porous insulation should be kept away from potential leak points.
System Tightness
We strongly suggest that the system be charged with inert gas once construction is completed. Not only will corrosion be prevented, but
the system can be pressure-tested using simple soap-bubble detection at potential leak points. And, when the system is charged with heat
transfer fluid, dissolved gas will be inert, virtually eliminating start-up oxidation of the heat transfer fluid.
Expansion and Contraction
The average hot oil system experiences wide temperature swings. Metals expand and contract significantly, with different metals
expanding and contracting at different rates. If allowances are not made, piping and welds may not hold up for long, and can result in a
shower of hot fluid when it's least expected.
Note: During initial operation and run-in, all joint areas should be left free of insulation so that leaks can be easily detected and
corrected.
Maintenance
The proper operation and maintenance of your thermal oil system is the most important means you have to reduce the potential for
problems. A program of daily or weekly system checks can assist in keeping the system running at its best. As you walk the system,
examine each potential leak point for signs of fluid leakage. Closely observe valves, flanges, weldments, instrument ports and
threaded fittings. A "smoking" system is a dead giveaway that fluid is leaking.
Check the system vent. Mist or steam coming from the vent can signal water in the system or decomposition of the fluid itself. Check the
catch container at the end of the line running from the expansion tank's relief valve or vent line. It should be empty. If it
contains liquid, you will want to know why (hot fluid splashing into a drum partially filled with water can create a cloud of hot oil
mist--the hot mist can be ignited).
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Note: Thermal oil vapors leaking from a system can be highly flammable. We strongly recommend you immediately respond to this
problem. |
While walking the system, note any unusual vibration or unusual smells. Like your car, each system has its own sounds, smells and
feels. If the system doesn't seem "right" to you, chances are that it is not.
While potential for fire exists in most plants, strong preventive
maintenance programs and common sense can work together to reduce the chance of fire. We encourage you to call us with any question, no
matter how insignificant it seems.
Questions? We'd like to hear from you. Call toll-free, 800-222-3611 -- or fax or e-mail.
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