The first inspection of a heater or boiler is necessary to confirm the anticipated rate of deterioration and to identify any unanticipated deterioration. Typically, a comparison is made with the initial inspection at the time of construction and with design records that detail considerations of corrosion, erosion, and other factors. The first inspection also helps to maintain the safety and efficiency of continued operation and forecast maintenance and replacements, based on the indicated deterioration rate. In the same way, all subsequent inspections are compared with the preceding inspection of the same specific purpose. The determination of the physical condition and the rates and causes of deterioration in the various parts makes it possible to schedule repairs or replacements prior to compromising mechanical integrity and resulting failure. Many of the parts that make up a boiler or fired heater depend on some other part, and when deterioration and serious weakening occur in one part, some other part may become unprotected or overstressed. This can shorten service life.
Heater reliability often depends on periodic internal inspections and routine, on-stream monitoring/inspection. Typically, heaters are an integral section of a process unit such that internal inspection can only be accommodated during unit outages. However, the length of time between internal inspections should consider the historic and predicted deterioration rates for components (including the impact of any process change), the historic inspection findings, the results of on-stream monitoring/inspection, previous maintenance activities and their quality.
Similar information can be inputted into a risk assessment, which considers the probability of failure and the consequence of failure. The inspection strategy and interval could be modified by a risk assessment. Additional information on risk-based inspection can be found in API RP 580. Routine, on-stream monitoring/inspection is a necessary component for improved reliability. Some common on-stream inspections include:
a. Visual inspection of the firebox and in particular burner flame patterns by operations personnel on a routine basis.
b. Installation and monitoring of tubeskin thermocouples for tube metal temperatures.
c. Periodic Infrared inspection of tubes for “hot spots”, and heater ducts/air preheater casings to determine if refractory or insulation degradation has occurred.
d. “Tell-tale” holes.