By Alejandro Rodas, proposal engineer, Bremco
Over the last three decades, combined cycle power plants have replaced many coal units as our country moves to reduce carbon dioxide emissions.
A feature of every combined cycle unit is the heat recovery steam generator, which converts combustion turbine exhaust heat into steam for additional electrical production that adds approximately 20% net efficiency. Load cycling is basically the norm for combined cycle units, which places much thermal, mechanical and water/steam chemistry stress on HRSGs.
This article provides an overview of many important HRSG inspection items during scheduled (and sometimes forced) outages.
While HRSGs come in a number of configurations, the most common design is the triple-pressure drum style, as shown in Figure 1 below.
As is evident from even this basic figure, HRSG water/steam and gas paths can be complex, where regular and thorough inspections of many components are necessary to ensure reliable and safe operation. The following discussion highlights many, but certainly not all, important inspections and inspection techniques. Given the tight spacing of the evaporator tube bundles, aka harps, and other equipment within an HRSG, methods beyond simple visual observation, including non-destructive techniques, are necessary for a thorough evaluation of equipment conditions.
Visual drum inspections should be standard, and can reveal much about the effectiveness of boiler water treatment programs. Large deposits of loose material on the drum floor(s) suggest corrosion in other parts of the system or perhaps contaminant in-leakage from a steam-surface condenser, where the impurities have reacted with boiler water treatment chemicals. Borescope examination of drum riser tubes is also important.
The recommended feedwater treatment program for most HRSGs is all-volatile treatment oxidizing (AVT(O)), which has no oxygen scavengers to help minimize single-phase flow-accelerated corrosion (FAC). If AVT(O) is applied properly, the drums and evaporator tubes should have a solid red color. If black magnetite is visible, the program is not functioning correctly. Additionally, borescope inspection of LP and IP evaporator and all economizer tube elbows can reveal single-phase FAC.
Also important during drum inspections is examination of the steam separators. Broken or missing components can allow moisture transport, with accompanying impurities, to the steam and ultimately the turbine. Even minor contaminant concentrations can cause multiple steam system problems, including corrosion of the blades and rotor in the last stages of the LP turbine. Inspection of the LP upper drum will reveal the presence or absence of two-phase FAC, which, if present, suggests that modifications to the chemical treatment program are needed.
Visual inspection of the water/steam side of other equipment such as superheaters/reheaters, economizers, lower headers, and so forth may be difficult, as these constitute pressure piping and usually do not have readily-removable inspection ports. Borescope inspections from accessible spots can provide valuable data.
Apart from visual inspections, several valuable non-destructive methods are available for additional evaluations. These include ultrasound thickness (UT), liquid penetrant, and wet fluorescent magnetic particle (WFMP) testing. UT testing is a well-recognized method to locate wall thinning from FAC. Liquid penetrant and WFMP are common techniques to check weld integrity. Weld inspections are a critical process, as stress and corrosion can weaken welds to the point of failure.
HRSG components are also, of course, exposed to high-temperatures and stressful conditions on the gas side. Thorough inspections are necessary to identify equipment condition and those areas that may need repair. The following list outlines a number of the most important gas-side inspection items.
- Inlet duct liners: The liners are exposed to turbulent flow that may cause failure. Original equipment is often under-designed, which exacerbates the issue. Liner degradation and failures may also occur from high temperature impingement induced by duct burners.
- Duct burners: During normal operation, the sole heat source for the HRSG is combustion turbine exhaust. However, many HRSGs are equipped with inlet or interstage duct burners to increase steam generation during times of high load. Burner inspection is important to ensure that all are in proper working order. The inability to produce even a few megawatts below maximum load during emergency peak conditions can be quite costly to the plant owner.
- Lead superheater and evaporator tubes: Natural gas is the typical fuel for combustion turbines, and it burns cleanly. However, even the small amount of particulates that enter with combustion air can collect on evaporator tubes, particularly if the tubes have fins to enhance heat transfer. Cleaning all but the outside tubes may be difficult because of the close spacing within and between the harps.
- Expansion joints: Similar to expansion joints in other applications, HRSG expansion joints can degrade over time leading to cracks and spots for flue gas to escape.
- Exhaust duct floor: The flue gas is obviously much cooler at the HRSG outlet. Condensation within the outlet duct can lead to corrosion, particularly of the duct floor. Among other things, this can be a critical safety item.
- Stack dampers: Dampers allow the HRSG to be mostly sealed off from the ambient environment during unit shutdown, as humid and/or cold ambient air ingress can induce corrosion within the HRSG. The dampers also are utilized to reduce heat loss, which allows for faster startups and reduces thermal fatigue in the HRSG. Inspections confirm damper integrity.
- Stack silencers: Many combined cycle units are located in or near residential areas. Accordingly, stack silencers are often required to reduce the noise level to the surrounding neighborhoods.
- SCR catalyst beds, SCR ammonia grid injection system, and CO catalyst beds: Regular testing and monitoring of NOx, CO, and ammonia-slip emissions are regulatory requirements. An increase in concentration of any indicates some problem, whether it be aging catalyst, or, in the case of the SCR, a problem with the ammonia injection system. Inspections can be valuable for identifying mechanical issues.
The discussion above outlined many, but certainly not all, recommended inspection points and techniques for HRSGs. Neglected or haphazard unit inspections can lead to equipment failures, and even more importantly, potentially jeopardize employee safety. The tight spacing of HRSG internals makes some inspections very difficult, where special methods may be required to ascertain equipment condition.
Buecker, B., Shulder, S., and Sieben, A., “Fossil Power Plant Cycle Chemistry”; pre-conference seminar for the 39th Annual Electric Utility Chemistry Workshop, June 4-6, 2019, Champaign, Illinois.
About the Author: Alejandro Rodas is a Louisville, Ky-based proposal engineer with Bremco (a member of the SVI Industrial family) with over 12 years’ experience working with HRSGs.
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