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 140°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 40°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.