Preventing Fires in Thermal Oil Heat-Transfer Systems

The chemical processing industry has used heat-transfer fluids for more than eight decades. Because thermal fluid heating systems include fuel, air and an ignition source, the risk of fire always is present. However, plants can minimize the risk of fire by strictly observing proper installation, maintenance and operating procedures.

Evaluating fire risks
Fire safety in thermal fluid systems depends on three measurements — flash point, fire point and autoignition temperature.

Flash point. The flash point of a fluid is the temperature at which sufficient vapor is generated for the fluid to flash when exposed to an ignition source. Two common methods of determining a flash point use a heated container with a fluid sample and a temperature probe. The Cleveland Open Cup (COC) test method, which complies with American Society for Testing and Materials (ASTM) D92, uses an open cup partially filled with a fluid sample. The sample is heated at a fixed rate. A small flame continually passes back and forth just above the fluid surface until the fluid’;s vapor ignites.

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The Pensky-Martens Closed Cup (PMCC) method, which complies with ASTM D93, uses a container that is closed except for a small opening through which the fluid’;s vapor is exposed to a flame. Results of this method usually are several Fahrenheit degrees cooler than the COC method because the concentration of vapor in the closed cup is higher.

Fire point. The fire point is the point at which a fluid generates sufficient vapor to support continued combustion. The fire point typically is 40°F to 100°F hotter than the flash point. The COC is used most frequently to find the fire point.

Data collected from these tests must be interpreted in the context of actual operating conditions for thermal fluid systems. For the vapor to be ignited, the fluid must be at the flash or fire point temperature with a source of ignition close enough to the surface to ensure a minimum vapor concentration.

In actual conditions, however, leaking fluid will cool quickly when exposed to air, dropping below the flash point. Any vapors produced will turn to smoke if the area has adequate ventilation. This smoke is most noticeable around small-volume leaks known as weepers.

The most important use of flash and fire points is to provide an indication of the fluid’;s volatility or its ability to generate vapor under a given set of conditions. If a significant leak occurs, a fluid with a lower flash point will generate more vapors, creating a greater potential for fire.

Autoignition temperature. The temperature at which a fluid will ignite without any external source of ignition is the autoignition temperature (AIT). The current ASTM E659-78 standard superseded the popular ASTM D-2155 standard several years ago. ASTM E659-78 calls for an injection of sample fluid into a test beaker filled with hot air. The temperature of the air at which the fluid sample ignites is the AIT. Not all users agree this test method is applicable to assessing risk for thermal fluid systems because the air is heated, not the fluid.

Even though these tests provide useful data, none should be applied as the only selection criterion. Heat transfer systems typically and routinely operate at temperatures well in excess of their fluid’;s flash and fire points, but never in excess of their AIT. Relatively few fires have originated in thermal fluid systems. Most of those that do occur are insulation fires, or are caused by loss of flow, cracked heater tubes or leakage.

Insulation
Insulation fires occur when heat-transfer fluid leakage from valves, gaskets, welds or instrument ports infiltrates porous insulation such as calcium silicate or fiberglass wool. The porous installation’;s open structure allows the heat transfer fluid to "wick" away from the leak and spread throughout the insulation. Spontaneous ignition might result upon the fluid’;s sudden exposure to air if, for example, the protective covering is punctured.

The most effective precaution against insulation fires is the identification of all potential leak points and the specification of high-temperature closed-cell insulation or no insulation at these points. Closed-cell insulation prevents the fluid from spreading throughout the insulation. If necessary, flanges should be covered only with metal caps with weep holes — users should avoid insulating these areas if possible.

Loss of flow
Loss of flow occurs when a series of equipment failures interrupts the flow of thermal fluid to the heater. A pump motor loss, a coupling failure, a system pressure control valve failure or a blinded full-flow filter might cause the initial failure.

The second failure then occurs when fouling, burnout or poor location causes the high-temperature cut-off device to miss the sudden temperature increase. As the burner or electrical element continues to put energy into the now stagnant fluid, the temperature increases rapidly beyond the AIT. If a crack develops in the heater coil or the piping connected to the heater, hot fluid is discharged into the hot atmosphere, where the fluid spontaneously ignites.

If the piping remains intact, the vaporized fluid either discharges through a relief valve into the catch tank or pushes fluid up into the expansion tank, which then discharges the fluid into the catch tank. Violent discharges have caused fires when the hot thermal fluid vaporizes the volatile material in the tank, and the vapor then is ignited by the heater.