News & View, Volume 47 | Surface Preparation – A Pivotal Step in the Inspection Process
By: Ben Ruchte, Steve Gressler, and Clark McDonald
Properly inspecting plant piping and components for service damage is an integral part of proper asset management. High energy systems constructed in accordance with ASME codes require appropriate inspections that are based on established industry practices, such as implementation of complimentary and non-destructive examination (NDE) methods that are best suited for detecting the types of damage expected within the system. In any instance where NDE is used to target service damage, it is desirable to perform high quality inspections while at the same time optimizing inspection efficiency in light of the need to return the unit to service. This concept is universally applicable to high energy piping, tubing, headers, valves, turbines, and various other power and industrial systems and components.
Industry experience shows that waterwall tubing in conventional boilers can be susceptible to fireside corrosion, depending on fuel type, firing practice, etc. In boilers where fireside corrosion has been identified as a maintenance issue, wastage rates of 5 to 25 mils/year are not uncommon. Since the mid 1990s, the installation of low NOx burner systems designed to lower NOx emissions has significantly increased the wastage rates in some boilers. Operators of subcritical boilers have reported wastage rates as high as 30 mils/year, while those operating supercritical boilers have reported rates exceeding 100 mils/year in the worst cases. These higher damage rates have resulted in an increase in tube failures, and operators have struggled to accurately define the extent of the damage and install the appropriate mitigating technologies.
Plant engineers must often answer ‘what damage is this piece of equipment susceptible to, and what can we do about it?’ or ‘how much longer can this component continue operating’? 
High Energy Piping systems, including main steam and hot reheat piping, are typically very reliable and can often operate trouble-free for decades.
Structural Integrity and GP Strategies recently announced an agreement to bring SI’s technology for calculating, tracking, and trending life consumption of piping and boiler components to GP Strategies EtaPRO real-time monitoring platform (Press release here).
Attemperators (aka desuperheaters) are used in fossil and combined cycle plants to protect boiler/HRSG components and steam turbines from temperature transients that occur during startup or load changes. The attemperator sprays water droplets into the superheated steam to ensure that the downstream, mixed, steam temperature will not adversely affect downstream components.
As plants age, the need for inspection for service related damage to ensure unit reliability increases. There are several approaches that plants can take to reduce the risk of premature failures and proactively manage their DMWs. First is metallurgical sampling. Based on temperature profiles across the boiler, operating conditions, and operating history, DMWs can be selected for laboratory analysis. This will provide some insight into possible damage accumulation; however, the better approach, if damage is suspected, is to perform an ultrasonic inspection of the DMWs. This allows inspection of all the DMWs, and only requires access and surface preparation. If indications are detected, then tube sampling should be performed. It is critical to perform a metallurgical analysis of several of the DMWs suspected of containing service damage to confirm that the indications are service related and to help establish the extent of the damage compared to ultrasonic testing results. Typical DMW damage is described in the Featured Damage Mechanism article. The importance of the metallurgical analysis is demonstrated in the three following case studies.