Ovens, furnaces and boilers play important roles in many industries. They heat products in petroleum, chemical and pharmaceutical industries and produce or handle molten products in glass, steel and other industries. In most cases, if only because of their high operating temperatures and their capacity to cause injury or death as a result of some failures, ovens, furnaces and boilers should be included in predictive maintenance (PdM) programs that monitor their condition while they operate.
The purpose of a PdM program is to detect and prevent imminent failures before they occur to avoid the shutdown of critical equipment. One powerful tool for monitoring the condition of ovens and boilers is thermal imaging, which captures two-dimensional images of the temperature profiles of objects. Thermal images can reveal potential points of failure in ovens, furnaces and boilers and help extend the life of their refractory insulation. Using thermal imaging or thermography to troubleshoot furnaces and boilers -- especially the refractory insulation directly inside a unit’s exterior wall or the insulating lining of vessels handling or conveying molten material -- can help prevent unplanned downtime due to equipment failure.
What to Check?A thermal imager can be used to check any critical furnace, process heater or boiler. Of course, thermographers should prioritize those whose failure could threaten human health or safety, property, productivity or the product itself. Equipment conditions that pose a safety risk should always receive the highest repair priority. Clearly, one of the most potentially dangerous situations that might occur is the failure of a furnace or ladle for a molten material such as glass or steel. Monitoring such equipment serves a two-fold purpose: to maximize the life of the unit’s refractory and to guard against a breakout that discharges very hot materials into a facility.
Highly skilled thermographers report some success checking the tubes of furnaces and boilers for hot spots, which can signal a potential failure. Clearly, a breach in the wall of a tube containing very hot water, steam or hot product could be catastrophic, but those who would attempt to use thermography for such monitoring must realize that to do so is difficult and dangerous, putting both the thermographer and the imaging instrument at risk. Also, it requires substantial knowledge, training and experience to get reliable results in environments as harsh as the inside of a furnace or boiler.
By contrast, as long as a unit does not have a shiny surface, exterior thermographic inspection of furnace or boiler is relatively safe and easy, and can help determine the unit’s health.
What to Look For?To protect personnel and property, furnaces, boilers, process heaters and other heat-generating units have insulation or refractory lining their external walls. Using a thermal imager, technicians can look for hot spots on the walls. The hot spots will reveal where the refractory is less effective. The goal is to maximize the useful life of the refractory and to schedule repairs before a burn-through of the unit’s wall results in fire, injury or worse. Of course, a secondary concern with ineffective insulation or refractory is energy loss, which increases operating costs and can jeopardize process efficiency due to heat loss.
A sound approach to oven, furnace and boiler inspections is to create regular inspection routes that include all key ovens or furnaces, boilers, process heaters and other heat-generating equipment. A good approach is to determine the frequency of inspections based on the nature and function of the equipment. For example, you might perform quarterly inspections on indispensable equipment operating under severe conditions and annual inspections on equipment operating under less severe conditions.
When determining the inspection schedule, keep in mind the potential cost of failure. For example, a catastrophic failure in the glass or steel industry would constitute a multi-million dollar production stoppage, even if there were no injuries or deaths. Consider some representative hourly downtime costs for selected industries in which boilers, ovens, furnaces and process heaters are key to production:
- Pharmaceuticals, $1 million.
- Food and Beverage, $800,000.
- Chemicals, $700,000.
These figures are tied to loss of IT performance, but are cast in terms of general downtime.1
After the ScanWhen a thermographer discovers a problem using a thermal imager, he can use the associated software to document the findings in a report that includes a thermal image and a digital image of the equipment. It is a good way to communicate the problems you found and any suggested repairs.
In general, if a catastrophic failure appears imminent, the equipment must either be removed from service or repaired while operating. For example, in the steel industry, both strategies are employed. When it comes to ladles for molten product, mills generally have enough ladles to take a failing one out of service for repairs and replace it with a sound one. However, the refractory in some kinds of ovens, furnaces and heaters in the steel industry can be repaired during operations using a grout pumped onto areas of weak or damaged refractory (as identified in a thermal image). In either case, following repairs, new thermal images can be used to assess the effectiveness of repairs and evaluate the repair materials used.
Sidebar: 3 Ways to Use InfraredIn general, there are three ways to use any kind of infrared, noncontact thermometer for predictive maintenance:
1. To measure the absolute temperature at a spot. This type of measurement is used to measure and track temperature at a single spot. It is useful for trending the temperature of an object or comparing a measurement to a specification. For a predictive maintenance thermometer, look for high repeatability to help ensure consistency.
2.be used to check the functioning of a steam trap by measuring the temperature of the inlet and outlet. No change indicates the trap has failed open. A very large change indicates the trap has failed closed.
3.To scan an object and detect changes within a continuous area on it. This capability allows the user to find hot or cold spots on housings, panels and structures. For example, one can check the heat sink of air-cooled transformers for cool tubes that indicate a restricted flow or a lack of flow.