Cracked heater tubes
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.
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.