Heat Recovery Steam Generators (HRSGs) are energy recovery heat exchangers used to produce steam for use in various applications. Caustic attack, or “gouging,” is an important under-deposit corrosion mechanism associated with HRSG units. This form of corrosion usually occurs in the pre-boiler part of the process and can potentially have a catastrophic effect on production.
Caustic gouges are formed when the active corrodent sodium hydroxide — from poor-quality feedwater — concentrates at the base of corrosion product deposits in the internal tube wall surface. Beneath the deposits, the tube surface temperature increases, resulting in a high-sodium hydroxide concentration within the water. Corrosion product deposits are usually in the form of metal oxides, which are produced from the corrosion within the steam condensate and boiler feedwater system. The high concentration of sodium hydroxide dissolves the protective magnetite (oxide) layer on the internal tube wall, where an area of potentially catastrophic internal corrosion, or gouging, ensues.
It should be noted the microstructure of the metal does not change throughout this process. The typical characteristics of a caustic gouge are smooth localized contours elliptical in shape and surrounded by heavy oxide depositions. The typical rate of attack can be extremely high, frequently greater than 0.08 inches per year. If undetected, the thinning damage can continue to the point where a local rupture occurs. With downtime being a considerable factor in any industrial process, it’s imperative premature failure is prevented.
Caustic attack can be highly localized in nature. Moreover, the tubing arrangements within HRSG boilers are commonly arranged in a very tight bundle arrangement, which can lead to a very low probability of detection when using traditional NDT techniques such as manual ultrasonic testing. A new generation of narrow-bore, untethered smartpigs has given rise to a new, reliable method of detection for these metallurgic defects. Using a series of radially mounted ultrasonic immersion transducers, the smartpig is able to axially and circumferentially locate the area of caustic attack as well as provide a full contour profile of the defect for use in Level 2 Fitness for Service Assessments. This method of inspection negates the need for internal access to the process tubing and is widely regarded as the most accurate and efficient method for inspecting HRSG process tubes.
Caustic attack is a major concern for HRSG processes, but is avoidable if preventative measures are taken. Its essential corrosion deposition is minimized by applying a suitable chemical treatment and monitoring arrangement within the feedwater supply system. The prevention of excessive sodium hydroxide concentrations within the feedwater system can be carried out by using adequate boiler water treatment and accurate monitoring of the water supply. Regular descaling and smartpigging operations are also recommended for total preventative maintenance.
For more information, contact Dr. David Thewsey at dthewsey@cokebusters.com.