Pressure Boundary Inspection Quick Guide

Effective HRSG inspections focus on damage mechanisms that could affect each of the specific components; not all components are susceptible to all damage types.  Focusing an inspection only on components affected at other plants (scope defined by “industry experience”) has in some instances resulted in wasted resources or ineffective utilization of resources.  This is because high-risk locations for some mechanisms (for example, FAC) vary from plant to plant.  Blind inspections that do not consider plant specific conditions often yield sub-optimal results. The table below shows is a quick summary guide to the damage mechanisms affecting pressure parts and the most likely locations where one might look first for that type of damage, if time is limited and/or in the absence of any clear indication that this damage might have occurred in any particular area of the HRSG.  Once the pressure boundary is breached then water and steam will leak out.  If the leak is large enough, it will be detected by excessive makeup water consumption and/or by noting steam or water exiting the casing during operation. During an internal inspection, the location of leaks and the pressure boundary breach are typically identified by the presence of water traces or staining on the inside of the unit.  Nevertheless, it is important to note that the source of most staining or discoloration is not pressure boundary leaks.  Other common sources include: rainwater or vented steam/water leakage from roof, water leaks during testing and tube exposure to elements during construction

Damage Type

Usually Found In

Occasionally Found In

Best Location to First Inspect

1

Corrosion Fatigue 

Economizers, Evaporators, Preheaters

Superheaters, Reheaters

Selected tube-to-header weld areas on economizer or preheater row that see greatest thermal transient when during operation (typically startup/shutdown cycle)

2

Creep or Creep Fatigue 

Superheaters, Reheaters,

 

Selection of hottest tubes that might have exceeded creep threshold temperature, particularly if tubes are bowed

3

Graphitization

Superheaters, reheaters

 

Selection of tubes running hot (>800°F/425°C ) for long period (>100K Hours) and only if material is carbon steel

4

Deposition & Underdeposit Corrosion 

Evaporators

 

Selection of horizontal or bent tube sections

5

Erosive Wear and FAC 

Economizers,
FW Heaters (FAC); LP Evaporator Tubes (FAC and EW); LP and IP Drum Internals (FAC or EW)

LP & IP Evaporators (FAC or EW)

Sizeable sample of accessible bends in carbon steel tubes, jumpers and risers, particularly if operating at temperature range of 200 °F – 400 °F,  (100 to 200°C);

Baffles in LP or IP drums above risers

6

External Corrosion and Oxidation 

FW Preheaters, LP Economizers

Superheaters, Reheaters,

Selected gas side surfaces, particularly toward colder end of unit. 

7

Acid Dewpoint Corrosion 

FW Preheaters, or LP Economizers

 

Coldest Tubes Near Stack

8

Fatigue 

Superheaters, Reheaters

Economizers, FW Preheaters

Vibration-induced (high cycle fatigue) in first row tubes or near gas baffles;

Thermal transient induced (low-cycle fatigue) at tubes on inlet header of bundle on cycling units

9

Pitting 

Drums, Economizers, FW Heaters, Drain Lines

Evaporators, Superheaters, Reheaters, Vents

Drum interior surfaces at each outage.

Inspect selected locations on tube or header interiors if:

·         frequent or long layups have occurred prior to outage

·         feedwater dissolved oxygen was 2 or 3 times above plant chemistry limit for extended periods

·         drum(s) show significant pitting

10

Stress Corrosion Cracking 

Economizers, FW Heaters

Superheaters, Reheaters, Evaporators

Systematically inspect significant sample of tubes if a failure has occurred or if there is reason to believe (history at similar plants) that SCC may be occurring in a particular region

11

Thermal Overstress 

HP Superheater, Reheater

 

Check all tubes for any sign of bowing or further bowing since last outage

12

Tensile Overload 

HP Superheater, Reheater

 

Bowed tubes or all tubes in area if failure has occurred

13

Thermal Quench 

HP Superheater, Reheater

 

Bowed tubes or all tubes in area if failure has occurred

14

Wear 

All tubes

 

Tubes near supports having bent fins or where support is damaged

15

Weld & Fabrication Defects 

All areas

 

Full survey of similar welds as accessible upon failure occurring

 

About Us

Established in 1988, Tetra Engineering has more than 25 years experience providing solutions to the power industry. We specialize in solutions for HRSGs, conventional boilers & steam-cycle balance of plant.

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