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.

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 40Â°F to 100Â°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.

To avoid incidents resulting from loss of flow, low-flow shutdown should be included in the burner safety interlock. Flow detectors that are immersed in the fluid are not recommended because they might fail in the open position. Pressure sensors have proved to be the most reliable for long-term service. To provide effective indication of a no-flow situation, plants can install pressure sensors across a fixed restriction such as an orifice plate or the heater itself to measure pressure drop, or as high- and low-discharge pump pressure monitors. Catch drums should be located inside a fire-proof cabinet, away from the exit door of the heater room.

Cracked heater tubes

Serious fires caused by cracked heater tubes are relatively rare, but can occur. Cracks are formed by excessive thermal cycling or near hot spots that develop from internal fouling or flame impingement. Leaking fluid will burn off immediately while the heater is operating. However, when the system is not in operation, fluid will continue to leak into the combustion chamber as the result of head pressure from the expansion tank and overhead piping. In the most serious cases, fluid forms in a large pool inside the heater during a prolonged shutdown. When the heater is restarted, the entire pool ignites and destroys the heater.

To prevent excessive thermal cycling of the heater tube bundle, oversized heaters should be derated by the manufacturer. Flame impingement will cause severe thermal cracking of the fluid that can be detected by routine fluid analysis. Heat tube fouling often is caused by deposits that result from fluid oxidation. Oxidation occurs if the expansion tank remains hot (more than 140Â°F) during normal operation and is open to air. The reaction of the hot fluid and air forms tars and sludge that coat surfaces and reduce heat transfer. These deposits could create heater hot spots that ultimately cause cracks. Oxidation, which also is detected by routine fluid analysis, could be prevented by keeping the expansion tank cool (lower than 40Â°F) and by keeping air out.

Large-volume leaks

Large-volume leakage from the thermal fluid system might be a direct cause of fire if the hot fluid contacts an ignition source. Most major leaks result from component failure. Expansion joints, flexible hose and rotary unions are among the components that might fail. If the ignition source is part of the failing component or is the source of the leak, a significant fire may occur. For example, if a pump seal seizes up, the resulting friction could heat the fluid up to ignition. Proper system design prevents most of these types of leaks.

Although small flange leaks usually are more of a nuisance than a hazard, they do create a mess, as well as potential safety problems. This type of leak can be minimized with the use of 300-pound flanges and gaskets made of graphite or fiber-reinforced Teflon material.

To prevent leaks, plants should:

Allow expansion joints and flexible hoses to move along their axes, never sideways.
Maintain lubrication systems for rotary unions and supply these systems with the correct lubricating oils.
Install isolation and bleed valves in the piping for each piece of equipment so maintenance can be performed without draining the whole system.
Use the appropriate recommended stem packing for globe, ball or plug valves in thermal fluid service.
Install valves with their stems sideways so any leaks run down the stem and away from the piping.

Proper operation and maintenance of thermal fluid systems also reduce the risk of fire. Maintenance should include daily and weekly inspections for signs of smoke from potential leak points, especially valves, flanges, welds, instrument ports and threaded fittings. By performing timely inspections and understanding the fire risks, plants can increase safety dramatically.

Oetinger is a sales manager for Paratherm, Conshohocken, Pa. Contact him at (800) 222-3611.