Screw Plug Heaters Keep Petroleum Products Flowing
Another obscure place that screw plug heaters are used is on tank farms. Tank farms are warehouses for petroleum in varying stages of refinement. Commonly these farms consist of numerous large vertical tanks holding 10,000 gallons (47,850 liters) or more. Some of the tanks do not require heat, but those holding heavy petroleum substances do require heat in order for the products to be pumped.
The heaters used are commonly 2.5-inch NPT screw plug heaters. If the heater is directly immersed into the petroleum, then a low watt density steel sheath can normally be used.
In some cases a dry well is installed in the tank. The dry well is merely a horizontal pipe welded in place through the lower portion of the tank's wall. The heater is then installed in the dry well, and actually acts as an air heater. The heated air then heats the pipe (dry well). The advantages of the dry well are that it allows replacement of the heater without draining the tank and the pipe surface acts as a low watt density heat transfer surface preventing "coking" or charring of the petroleum. If a dry well is used a low watt density Incoloy® sheath should be utilized. In all cases the heaters should be located near the bottom of the tank, but far enough from the bottom to keep the heaters out of any sludge that may accumulate.
Since the heaters are normally located outdoors a NEMA 4 (moisture resistant) housing is normally used. In some cases a NEMA 7 (explosion resistant) housing may be necessary. Temperature control can be accomplished by mounting a bulb and capillary thermostat directly on the heater. An alternative is to mount the thermostat in a separate thermowell two-three feet above, and at 90 degrees to the heater. This arrangement will tend to allow a larger portion of the tanks contents to be at temperature before the heater begins to cycle.
Ability to provide a custom thermal solution.
Incoloy® is a registered trademark of Special Metals Corporation.