The first five years of HRSG operation are critical for building the foundation necessary to assure a reliable and satisfactory service life, said engineers from HRST Inc participating in an editorial roundtable with CCJ staff.
In the early years, they continued, control and organization of supplier documentation will benefit current and future decision-making. Plus, timely inspections and analysis can help ward off potentially damaging conditions or identify items to allow contract closure with suppliers.
As time goes on, analysis also can determine the effects of any current or impending changes to operations that were not planned when original specifications were prepared. Finally, there are commissioning and early operating practices and conditions that can harm the HRSG. Many examples are in evidence industry-wide, the experts said. Several are identified below.
Focus plant personnel in Year One on gathering, organizing, and protecting documentation. Proper and efficient management of maintenance and operation require comprehensive and complete documentation, Team HRST said.
It’s important to confirm that all suppliers have submitted complete design packages. Developing a reconciled list of documents with the latest revisions and dates will prove invaluable as time goes on and personnel change. Obtaining copies of the ASME Manufacturer’s Data Report forms can verify component design pressures and materials of construction, and aid in future repairs.
ASME B31.1 requires every facility to have an inspection program for covered piping systems (CPS). This primarily constitutes pipe 4 in. diam and larger between the HRSG and steam turbine or user. The program calls for a comprehensive list of documents. B31.1, Article 141, “Operation and Maintenance,” provides guidance.
Confirm that the documents are complete, current, and well protected. Guard against catastrophic loss and assure ready availability by keeping complete sets of documents in multiple accessible locations.
Inspection, testing, analysis. Your pre-first-fire inspection should focus on design details, use of correct materials, and proper installation, and be supported with ample photography.
Inspectors should go through the gas-side access and crawl spaces, paying close attention to the inlet duct and firing duct. Violent exhaust flow is known to take down components in the inlet duct if they are not installed properly. Note interferences from restrained thermal expansion of tube bundles and duct liner systems.
Inspect each drum, paying close attention to the final (secondary) steam separator that covers the steam outlet to the superheater. The HRST engineers said it limits carryover of water droplets and impurities that can quench or plate out on downstream tube surfaces. Excessive carryover can lead to loss of superheat and elevated risk of tube damage from high temperatures. In severe cases, carryover may quench the tubes, causing fatigue damage at the header joints.
All pipe and pipe supports external to the HRSG must be in the correct position and any hydro-stops removed before fired operation. This is a good time to document the hot and cold marks, before they fade or fall off. If low points in steam pipe cannot be drained, consult the OEM or contractor.
First two years
Near the end of Year One, or just before expiration of warranty periods, a comprehensive inspection and test program is highly recommended by the HRST experts. It will provide baselines for future inspections. Photographic documentation is especially valuable.
Within the gas path, inspectors should look for signs of restrained expansion, interferences, and wear. Some cracks and twists can be expected and are often self-limiting. Others are signs of imminent or future trouble. Improper material selection or the occasional installation of the wrong material will often be revealed as a difference in oxide color or scale formation. If damage is found, this is the time to address the mechanisms conducive to future component failures.
Examples of inspection findings by Team HRST include the following:
Stressed tubes. Warped tubes are not necessarily a liability, but they are signs that the tube has been stressed and has yielded. Additional fatigue may cause failure. Cracking might be apparent in tube joints. Stress is common in the superheaters, reheaters, and economizers. Failures have been known to occur shortly after commissioning.
External tube wear. Tubes are susceptible to oscillation from forced vibration attributed to exhaust flow. A tube-tie support lattice typically is installed at multiple points along the height of the tube bank to limit this oscillation. If the tube ties are spaced too far apart or if they are loose, tube vibration causes fin wear, and eventually wear on the tube wall.
Overheated materials are those that have been subjected to a sufficiently high temperature for a length of time to cause changes in microstructure, or excessive scale formation. Perhaps the materials are mismatched to their location in the HRSG. Overheating typically is confined to the inlet duct and firing duct. Inspectors may find one liner sheet, gas baffle, or tube that looks out of place. If not corrected, early failures can be expected.
Duct burners. Look for signs of flame impingement on the walls or downstream tubes. Long flames or excessive localized heat can be caused by failed burner nozzles, inadequate fuel distribution, inadequate exhaust distribution, or improper controls.
Online inspections. There are several ways to identify areas of concern with the HRSG in operation, including these:
Infrared imaging inspections of the ducts—in particular the roof, doors, and pipe seals of the first three modules (HP evaporator and forward to the gas turbine)—can point to excessive casing temperatures where issues are likely to exist.
Monitor operation of the steam separators to verify their proper operation. Do this by testing for the purity of steam leaving the drums to ensure downstream equipment is protected against carryover.
Examine flames through the burner viewports to verify flame lengths are not excessive.
Review data in the plant historian to identify excessive cycling of valves, valve leakage, excessive desuperheater spray water, and other harmful conditions.
Walk-down high-energy piping systems to locate unwanted movement/displacement.
Second through fifth years
Every year or two after COD (commercial operating date), HRST engineers recommend a standard 10- to 20-hr inspection to look for wear issues throughout the HRSG. As conditions develop, they can be identified and the underlying cause corrected before large-expense outlays are required. After four years’ time, issues that tend to be time-dependent should get more focused attention—including corrosion, expansion-joint failures, pipe seal failures, and fatigue cracking of pressure parts and liner systems.
Analysis for vulnerabilities and changes. Flow-accelerated corrosion (FAC) may be the most detrimental and expensive of tube and pipe failures encountered by HRSG owner/operators. It is highly dependent on water chemistry. Damage from FAC can develop rapidly, with visual wear occurring in less than one year.
HRST engineers told the editors that the HRSG should be evaluated for FAC risk before the third year of operation. A follow-on FAC inspection plan can then begin. However, your mitigation program should begin earlier if there are telltale signs on FAC in the LP or IP drum in Year One.
While HRSGs are now constructed using FAC-resistant materials in high-risk areas; in those cases, the scope or frequency of inspection can be reduced—perhaps—but never neglected.
Operational changes. Several years after COD, the owner may need an operating profile different from that specified for the original design. This could involve a change in cycling frequency, operation at a lower load, etc. A thorough engineering evaluation of the new operating parameters on the HRSG is highly recommended by HRST engineers. Here are some things to think about as you go through this process:
Cycling study. Many specs require that the HRSG be designed for a certain number of starts per year. Each start contributes to wear and tear. A cycling study will look at those components vulnerable to thermal transients, high temperatures, and low flow rates that often accompany startup and shutdown conditions.
Bear in mind that cycles affect the allowable HP steam-drum ramp rate, so if startup time must be reduced or is in question, the cycling study will reveal the balance between cycle frequency and ramp rate.
Low-load operation. Operation of the gas turbine at low loads affects steam and water flowrates. This can lead to unstable flows in economizers and poor flow distribution in superheaters and reheaters, all affecting tube reliability. High exhaust temperatures from some turbines require excessive desuperheater spray that damages downstream pipes and tubes
Gas-turbine modifications primarily affect the HRSG through a change in exhaust mass flow and temperature. This changes the ratio of HP/IP/LP steam generation as well as overall heat recovery. Many components must be examined for suitable operation. Modifications with high exhaust temperatures may result in excessive desuperheater spray demand as well as superheater and reheater tube metal temperatures above the design temperature.
Avoid ongoing operations that damage the HRSG, including the following:
The desuperheater is responsible for a significant share of damage to superheaters, reheaters, and steam pipes. In some cases, very few fatigue events are needed to initiate tube failures and cracks in the steam piping. There are two underlying causes:
First, there may not be enough energy in the steam to evaporate the spray water. This causes restrained thermal expansion and potentially high stresses in components quenched by the water droplets. The steam temperature downstream of the spray nozzle should be 50 deg F or more above the saturation temperature.
Second, damage to pipes and tubes often occurs when water flows through the spray nozzle without proper atomization. This is caused by a leaking block valve during periods of no spray water demand or from a broken spray nozzle. Water then can flow upstream or downstream through the steam pipe.
A drain pot with automatic detection and drainage is required on all new HRSGs with a desuperheater. In such cases it is important to verify that the drain pot will collect any leakage (sometimes it does not) and is functioning properly.
Burner operation. Excessive heat can damage downstream tubes and duct-wall liner systems. Burner failures result in high localized heat input while faulty control logic can cause a wall of flame pushing through the tube banks.
Burner system controls must be checked as well. Burner output should be controlled to the limitations of the HRSG, not to the burner design maximum. This is especially important when multiple HRSGs have burners controlled to provide a minimum steam generation rate in a common header. If one unit goes offline or if some of the burner elements are isolated, how is the additional heat distributed among remaining units? It is important to verify that controls distribute heat as intended. Burner management system permissives and fuel-skid set points must prevent excessive fuel flow to any one HRSG.
Piping and header drain control. It is critical to verify that low points in the steam pipe and tube circuits are properly drained and that there is no water leakage through steam conditioning systems. If water hammer occurs, it is important to inspect the damage and the position of the pipe. Water hammer could create a low point that did not exist previously. The reheat system can cause extremely expensive pressure-part damage if not maintained and operated properly (including sizing, location, and control).
After shutdown of the HRSG, steam will condense and flow to the lower headers of the HP superheater and the reheater. If this water is not removed prior to startup, steam can push water droplets up tubes or water can restrict steam flow. These conditions cause high stress resulting in tube failures.
HP-drum pressure ramp rate. HP steam drums in cycling service are susceptible to fatigue cracking of nozzles, particularly the downcomers. Exceeding the maximum ramp rate will increase the risk of such cracking.