Fix a Hot Spot
During Summer months, power plants are running full tilt to satisfy the peaking demand for electricity due to air conditioning. Oil refineries too find themselves running equipment at the highest production level, so they can provide enough gasoline for automobile travel. When hot weather hits, equipment that may already be pushed to the limit, can become problematic. Scantly lined steel skins on boilers, furnaces and high temperature reactors overheat causing paint to blister and steel to distort and buckle. Heat exchanger efficiency drops because cooling water is hot. These problems become serious when temperatures exceed tolerable limits. And when production shutdown means huge profit loss, the pressure to keep equipment running gets intense.
Many techniques have been devised to protect equipment from premature shutdown due to overheating. Here are several "on-line" methods for cooling down equipment when the heat is on.
When a refractory lining deteriorates or gets damaged, the steel skin of a boiler, furnace and the like can overheat. If steel gets too hot it distorts, glows red, and can even melt. Also referred to as a "hot shot", the hot gunning method is used to coat the internal surface of an overheating steel casing while equipment is running. A custom fitted extension nozzle is used to reach the hot spot. It is inserted across from or next to the area to be coated. If necessary, wire mesh, metal lath ceramic fiber insulation or other material is combined with the spray application to increase effectiveness and to make the temporary fix last longer. This application is most effective on equipment that is not pressurized. Use of hot gunning on a pressure vessel is limited and always a significant challenge. Nonetheless, it has been used successfully for many years.
When hot gunning is too difficult or dangerous, usually on pressurized equipment, another option is to weld a steel box to the hot shell. The box is constructed so when installed the top side will be open to allow it to be filled with refractory. Before installation, refractory anchors are welded to the inside of the box, designed to hold the refractory in place. After the box, or other suitably shaped container, is welded in place over the hot spot the cavity is filled with refractory. After the treatment the temperature of the original skin increases since it is covered with an insulating material and shielded from the wind. In time, the original steel beneath the refractory filled box will disintegrate exposing the new refractory patch to the internal environment of the equipment, which it is designed to do. This is generally a longer lasting fix then a "hot shot" but can cause several time as much.
Perhaps the simplest method of treating hot spots in boilers and furnaces is to weld on injection nozzles, drill through the shell and inject refractory material directly into the equipment. This is appropriate when the injection material can be contained on the inside of the hot shell by boiler tubes or other nearby blockage. The obstruction acts as a dam and the material backs up toward the exposed steel and eventually quenches the hot spot. There are many types of injection materials including pumpable ceramic fibers and insulating and erosion resistant refractory castables. Each is available in 5 gallon buckets, caulk tube and bags.
Over time, a scale builds up on the internal surfaces of tubes, shells and fins of heat exchangers. The coating acts as an insulator which reduces heat flow and decreases heat exchanging efficiency. That's why a car's radiator, also a heat exchanger, should be flushed out as part of your car's preventive maintenance procedure. Left untreated, the resulting increase in temperature can halt a process, whether it's your car burning gasoline, or an exchanger used in the process of making the fuel. If shutdown and clean out is out of the question, then one way to cool down is through rock salt injection. Rock salt is injected, using high pressure air, into the cooling stream as it enters the exchanger. The agitative mixture of air, fluid, and rock salt, loosens the scale which is then carried away in the coolant stream. The salt dissolves and dissipates. The process continues uninterrupted, the fouling scale is removed and the heat exchanging efficiency is increased during the process.
On-line cooling treatments can be safe and effective when chosen carefully and performed properly.
Kraemer Gunite, Inc.
PO Box 305
Pitman, New Jersey 08071
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