Real time corrosion monitoring for HRSG

 

Flow accelerated corrosion (FAC) can cause iron loss in steam piping and lead to lethal accidents. As a result, programs are necessary to reduce metal loss and to monitor the loss which does occur. A team from Danaher analyzed the problem in a Power Engineering article earlier this year.

 

The reducing environment produced by oxygen scavengers is the prime ingredient for single-phase flow-accelerated corrosion (FAC) of carbon steel. The attack occurs at flow disturbances such as elbows in feedwater piping and economizers, feedwater heater drains, locations downstream of valves and reducing fittings, attemperator piping; and, most notably for combined-cycle heat recovery steam generators (HRSGs), in low-pressure evaporators, where the waterwall tubes, aka harps, have many short-radius elbows. In fact, FAC is typically the leading on-line corrosion mechanism in HRSGs.

 

Based on the method of oxygenated treatment (OT) that arose in Europe in the early 1970s, EPRI developed a program to replace AVT(R) for drum units, known as AVT(O), which stands for all-volatile treatment oxidizing. If the condensate/feedwater system contains no copper alloys, which is true for virtually all HRSGs, then AVT(R) is not recommended, rather AVT(O).

In brief, with AVT(O) chemistry the oxygen scavenger feed is eliminated, and a small residual concentration [5 to 10 parts-per-billion (ppb)] of dissolved oxygen is maintained at the economizer inlet. Ammonia or an ammonia/neutralizing amine blend is still utilized for pH control.

 

A combination of a simple colorimetric total iron laboratory analysis with a sensitive laser nephelometric analyzer can also provide a method for cost effective, quantitative, real-time corrosion monitoring. When properly calibrated, the nephelometric units provided by a nephelometer can be correlated to total iron concentration values. The iron concentration of the process water is a direct indicator of steel corrosion.

 

As the process waters used in power generation are extremely pure, it can be assumed that almost all insoluble matter present in a ferrous metallurgy process stream is due to steel corrosion in the form of particulate or colloidal iron oxides. Corrosion of steel components in power generation is generally found as iron oxides and hydroxides, primarily, iron (II, III) oxide (magnetite), α-iron (III) oxide (hematite), or dissolved iron. Each of these species produces a different nephelometric response to visible light.

https://www.power-eng.com/2019/01/21/the-integral-benefits-of-iron-monitoring-for-steam-generation-chemistry-control/#gref