Failure of Boiler Tube in Heat Recovery Steam Generator (HRSG) System by Flow-accelerated Corrosion

I would like to show my case study of failure analysis of boiler tube. It failed by FAC .

Introduction


Flow-accelerated corrosion (FAC) is metal loss through the dissolution of the protective oxide film by localized turbulence in water or wet steam piping system [1]. The metal corrosion is accelerated by the continuous erosive destruction of the protective layers. Under certain water chemistry, fluid velocity, and operating conditions, FAC can cause internal wall thinning of condensate and feed-water piping, heater drip and drain lines, and other carbon steel piping [2]. In some cases, this thinning has led to catastrophic failures and fatalities.

Fig.1 Boiler tube for failure analysis

In this case study, the failed component was the evaporator tube, which had been in service for nearly 4 years. The tube was the component of the evaporator outlet of the low-pressure steam in the heat recovery steam generator (HSRG). The tube was used for feeding water to the continuous process for boiler. During operation, it was exposed to hot water (120-200 C) and the pressure of 4-6 Bar (depends on the flow rate). The water solution has pH around 8.4-9.4, conductivity 3.8-7.5 micro-S/cm, silica content 4-20 microgram/l and iron content 3-24 microgram/l. The fluid flow velocity was about 0.3 M/s (flow rate 0.217 kg/s). The liquid ammonia was added into the water to control the pH value, and the hydrazine was also added to reduce oxygen content. The outer part of the tube was exposed to exhaust gas from the gas turbine (520 C and atmospheric pressure). The tube material was made of carbon steel grade ASTM SA-178A, which was 1.5 inches diameter and 0.094 inch thick. One section of the tube was replaced with a new ASTM SA106B steel tube by welding.

Fig.2 Longitudinal cross cut showing metal loss patterns

Fig.3 Horse shoe patterns, indicative of FAC was found on the inner tube surface

The bended surface of the tube revealed the damage from an erosion-corrosion type mechanism which is known as flow-accelerated corrosion (FAC). The attack was caused by a combination of flow velocity (mechanical factors) and corrosion (electrochemical factors). Especially at the site where a continuity surface was uneven such as weld bead at the inner wall of the tube during welding process, the erosion corrosion was extremely severe. Also the metal on the surface was removed and became particles. The most significant effect of erosion corrosion is the constant removal of protective films from the metal surface, thus resulting in localized attack at the area at which the film is removed [3]. This phenomenon is usually caused by movement at high velocities, and will be particularly prone to occur by the solution containing solid particle (e.g. silica, iron, insoluble salts, sand and silt) that has an abrasive action. Affected tube surface will often contain horseshoe pattern, grooves, or wavelike marks that indicate the attack direction. From the internal inspection of evaporator tubes using SEM, it revealed that the tube had an internal weld bead. This defect can be a potential source of small particles when these sites are attacked by erosion corrosion. In addition, hydrazine, a reducing agent, was added to the feed-water system in petrochemical plant to maintain a reducing environment in the steam generators. Hydrazine is reactive and unstable. It reacts with oxygen forming water and nitrogen. Un-reacted hydrazine would thermally decompose to form ammonia. FAC rate increases with increasing hydrazine level as the oxidizing–reducing potential becomes more reducing. The service temperature around 120-200 C is susceptible to FAC [1]. High flow rates, turbulence, and flow impingement increase water-steel contact and thereby increase the rate of iron dissolution and corrosion. FAC is most severe at abrupt flow discontinuities. Corrosion rate is higher at turbulence promoters such as irregular welds or changes in pipe diameter that is consistent with our results. In the area of FAC, the interaction between flow and the electrochemical processes of corrosion produces a characteristic abrasion pattern. The surface appearance ranges from smooth to pitted, but a distinctive wave-like appearance or horseshoe pattern with smooth scallops and sharp crests is found on the severe site as confirmed by microstructural analysis. The metal particles in the working fluid combine with silica also cause wear damage.

Fig.4 Macrostructure showing the wave patterns

Suggestions

Most cases of erosion corrosion can be minimized by

1. Redesigning or replacing the bended section with a larger radius tube.
2. Controlling the weld bead inside the tube during and after welding process.

3. Operating under homogenization of the water temperature and flow rate.

4. Maintaining the cleanliness of the fluid.

5. Selecting the material with high erosion corrosion resistant

6. Choosing the same grade and diameter of the pipe to be joint together

7. Avoiding overfeed of hydrazine and control the feed rate to match the water rate.

References:
[1] R.B. Dooley, V.K. Chexal, 2000, “Flow-accelerated corrosion of pressure vessels in fossil plant”, International Journal of Pressure Vessels and Piping, Vol.77, pp.85-90.
[2] Crispin Hales, Kelly J. Stevens, Philip L. Daniel, Mehrooz Zamanzadeh, Albert D. Owens., 2002, “Boiler feedwater pipe failure by flow-assisted chelant corrosion”, Engineering Failure Analysis, Vol.9, pp.235-243.
[3] R.D. Port, 1998, “Flow Accelerated Corrosion”, Nace International, paper no.721.