HRSG inspection, maintenance and repair
Paul D. Gremaud, Nooter/Eriksen, Inc., Fenton, MO, United States
Abstract
A heat recovery steam generator (HRSG) is a large, complex piece of equipment and, as such, requires regular inspection and maintenance, and occasional repairs to keep it functioning in a safe, efficient, and reliable manner. Hot inspection and maintenance that is conducted while the HRSG is operating, cold inspection and maintenance that is conducted when the HRSG is shut down and allowed to cool, and common repairs to HRSGs are reviewed in this chapter.
Keywords
Expansion joint; burner runner; penetration seals; high-temperature; liner system; steam drum
16.1 Introduction
A heat recovery steam generator (HRSG) is a large, complex piece of equipment and, as such, requires regular inspection and maintenance and occasional repairs to keep it functioning in a safe, efficient, and reliable manner. Although many people in the boiler industry think of inspection, maintenance, and repair occurring at the annual shutdown of the facility, a well-run plant will also utilize daily “walkdowns” of the equipment to proactively search for potential problems. They also take advantage of scheduled and unscheduled shutdowns for additional inspection and maintenance to keep all systems functioning properly.
16.2 Inspection and maintenance
HRSG inspection and the maintenance associated with it can be divided into two categories: hot inspection and cold inspection. Hot inspections are performed on the outside of the unit when the HRSG is either operating or has been recently shut down and is still hot. Hot inspections should be performed at regular intervals with the daily walkdown considered an abbreviated hot inspection. Cold inspections take place when the HRSG is shut down and has cooled off so that it is possible to enter the HRSG.
There are various types of maintenance programs that have been studied and developed in the recent past: preventative maintenance, predictive maintenance, reliability centered maintenance, etc. This chapter will cover a practical approach that can easily be used by HRSG maintenance personnel to take a proactive approach to maintenance and understand the important aspects of maintaining a reliable HRSG.
Each component of the HRSG should be listed and a maintenance plan for each of these components should be developed so that maintenance will be routinely and consistently performed at the appropriate time. The Operating and Maintenance Manual provided with the HRSG is a useful document to use when developing this document. The HRSG supplier should also be able to help.
A list of and inventory of critical spare parts is also necessary for an effective maintenance program and to minimize the impact of unplanned outages. The operating and maintenance manual and HRSG supplier are helpful in developing this list. The list should include at a minimum:
• two sections of tubing (including bends) for every coil and material in the HRSG
• two tube plugs for each configuration of tubing
The most common mechanisms for HRSG component failures are flow-accelerated corrosion (FAC), thermal fatigue in superheaters and reheaters, and under-deposit corrosion in HP evaporator tubing as described very well in Ref. [1]. Inspection and maintenance should therefore place special emphasis on these areas.
16.2.1 Hot inspection
A regularly scheduled (yearly or more often) hot inspection is an inexpensive, proactive task that can help avoid more costly repairs in the future. The inspection should include the use of a thermal camera. The hot inspection will incorporate some of the tasks that are part of the daily walkdown, namely listening to the sounds near the inlet duct, viewing the casing penetration seals in the high-temperature region, and observing the duct burner flame pattern. A thorough hot inspection will take approximately half a day for the typical HRSG. Performing a hot inspection 2–3 months before a scheduled cold inspection can be very useful in preparing for the cold inspection and maintenance.
16.2.1.1 Inlet expansion joint
Begin the inspection at the connection between the combustion turbine diffuser and the HRSG inlet duct. This will be the region that is most susceptible to damage and other issues related to the high velocity and turbulence in the combustion turbine exhaust. The fabric expansion joint, which is the interface between the combustion turbine and the HRSG, should be viewed in its entirety. A thermal camera should be used to ensure that the fabric temperatures are below the design temperature of the outer layer material. Any type of material on the external face of the belt will increase the belt temperature; the belt will fail rather quickly if its temperature exceeds 350°F. If a local area is hot, the issue may be as easy as exhaust leakage due to a loose backing bar at the outside of the expansion joint. The seam where the fabric expansion joint is field bonded is a typical location of failure, so this area should always be closely inspected.
16.2.1.2 Inlet duct
The inlet duct region of the HRSG is the key location for using the “watch and listen” approach. The loads on the liner system due to the high velocity and turbulence in the turbine exhaust can cause pulsation of the casing and liner systems. This movement of the casing will ultimately cause fatigue failure of the liner support system resulting in a potential forced outage. Liner system failures will not only cause high casing temperatures and personnel safety issues, but will also permit the liberation of insulation, which will coat all heating surfaces and equipment downstream. The concern for a forced outage comes into play if there is a CO or SCR system. The loose insulation will block the open spaces/channels in the CO or SCR blocks, cause a large increase in differential pressure across the equipment, and can even cause failure of the support system. Pumpable insulation, which can be installed through a hole in the casing, can be an effective temporary fix for a hot spot until a permanent repair can be made during an outage.
16.2.1.3 Duct burner
A duct burner is frequently incorporated into a HRSG to increase output. The efficiency and flexibility provided by the duct burner and the additional steam production that can be delivered have made it common for the duct burner to be cycled multiple times daily. Review of duct burner operation is an item that should be included in a daily walkdown schedule. Burner viewports are typically provided with the duct burner system, however, a sufficient number to easily view the flame pattern are often not available. Viewports should allow for viewing all burner runners in their entirety and allow for the viewing of flame impingement on the face of the coil immediately downstream of the burner. Fig. 16.1 shows a typical duct burner flame pattern as viewed through a viewport.
Issues arising from improper duct burner operation or design are unfortunately a common occurrence. Damage to liner systems, vibration supports, heating surface, and burner runners occur frequently. Although this damage often does not directly cause a forced outage, significant damage could be avoided by viewing the duct burner flame patterns during the daily walkdown. If the duct burner is operated at various combustion turbine loads, the flame pattern should be viewed on a more frequent basis.
16.2.1.4 Casing
Inspection of the casing is much like inspection of the inlet duct; however, gas velocities and turbulence are lower in this area. Hot casing is not an uncommon occurrence in HRSGs. However, the typical scenario is a very local hot spot, usually around a penetration seal, test port, or at structural members. The HRSG casing should be viewed for regions with discolored paint or distorted sections. The important areas to view are the casing sections nearest the combustion turbine, i.e., the inlet duct through the reheater/HP superheater coils (Fig. 16.2).
16.2.1.5 Casing penetration seals
Penetration seals in the hot region of the HRSG must perform in a severe environment. They are utilized where an inlet nozzle, outlet nozzle, or drain line for a header must pass through the casing. They can be required to seal 1700°F exhaust and allow for large vertical movements of the component within the penetration. The lateral design movement of the penetration seals can also be difficult.
Casing penetration seal design has improved tremendously in the past ten years. This is especially true for fabric penetration seals that are used with high-temperature components such as HP superheaters, reheaters, and HP evaporators. Although there are several companies that provide excellent products, thorough viewing of the high-temperature penetration seals during a hot inspection is necessary. A thermal camera should be used to measure the temperature of the outer fabric. The penetration seal supplier should provide the appropriate temperature. It is important that the exterior of the fabric seal be free of insulation so it is cooled by the ambient air. The inspection should include a check for gas leakage. Caution must be taken due to the high exhaust temperature. It is critical to identify and replace damaged or leaking penetration seals as the hot exhaust can cause injury, failure of adjacent seals, or damage to other equipment. These penetration seals should thus be viewed during a daily walkdown (Fig. 16.3).
16.2.1.6 High-energy piping and support system
The high-energy piping is an area prone to issues due to high operating pressure and temperatures and the corresponding large thermal expansions. The support system combined with proper fabrication and installation of the system is critical to long-term reliability. Damage due to creep and fatigue can occur and is exacerbated if materials were not fabricated and heat treated with great care.
The typical hot inspection of high-energy piping would entail visual inspection of the piping system with special care taken to view all the supports. The support condition should be compared to the pipe support drawing from the original designer. It is critical that the support functions as designed and that the pipe line is not overly restrained by the support. Spring supports should be viewed to confirm that the position indicator is in the proper “hot” location (Fig. 16.4).
Several engineering and consulting companies have developed inspection plans for high-energy piping. These inspection programs are typically performed during an outage and include nondestructive examination and other material testing that is not appropriate for a hot inspection.
16.2.2 Daily walkdown of equipment
If there is one aspect of an inspection program that is underutilized, it is the daily walkdown of the HRSG by plant personnel. This is very unfortunate, as the daily viewing of plant equipment is an important, proactive task that can significantly reduce maintenance spending and capital costs over the life of the HRSG. The daily walkdown also allows personnel to understand operational norms so they can better identify when something is amiss.
The daily walkdown is an abbreviated version of the hot inspection described above but it should not be performed in haste or carelessly. Notes should be taken during this daily exercise. A standard document can be created to make this an efficient process. The notes can be an important reference when issues arise. Thermal scans can be performed for areas of interest such as the high-temperature casing penetrations in the reheater and HP superheater sections on a regular (not daily) schedule. These scans can be compared to previous scans to help identify gradual degradation of equipment where repairs or replacements can be planned before failures occur. Any steam/water leakage should be noted and corrected at a subsequent outage.
Drain line temperature downstream of stop valves should be checked in order to determine if drain valves are leaking. Leaking drain valves are a common problem and operators must understand that these valves are not to be used as blowdown valves.
16.2.3 Cold inspection and maintenance
The cold inspection is the best method to verify the current condition of the heating surfaces of a HRSG. The cold inspection is also the only way to effectively inspect several other components, such as the liner systems, distribution grid, duct burner, and the catalyst systems.
There are several acceptable options regarding the inspection of your HRSG. The inspection can be performed by plant personnel experienced in the maintenance of HRSGs. Plant personnel can be effectively trained by the HRSG supplier and provided with a basic inspection program including critical inspection items. Most HRSG suppliers also have competent personnel to perform the inspection service.
To prepare for the cold inspection, the HRSG must be isolated from all steam headers and feedwater sources. All gas duct access doors should be removed and the stack damper should be placed in the open position. Access door surfaces should be cleaned so new gasket materials can be used when the doors are closed.
Once the unit has cooled and all plant safety requirements have been completed, the HRSG can be safely entered.
Use of conventional terminology is useful for effective communication when working in and around a HRSG. The upstream end of the HRSG is at the end where the gas turbine is, i.e., downstream is near the stack. Right and left sides are determined when standing at the upstream end and looking downstream.
The following tools are necessary for a detailed inspection:
16.2.3.1 Inlet duct
Standard practice is to begin the HRSG gas path at the interface between the combustion turbine diffuser and the inlet duct. The expansion joint at this location is particularly susceptible to failure due to the extremely high velocities and turbulence in this area. The inlet duct, especially the floor liner system, should be viewed in great detail, looking for damaged or missing liner plates, pins or clips, guide vanes or deflectors and insulation. Areas often in need of maintenance are field seams, corner angles, and access doors. Exposed insulation at the liner system should always be replaced and covered to minimize the amount of foreign material in the HRSG gas flow and provide adequate insulation for the casing (Fig. 16.5).
Each liner pin should be viewed for structural integrity. Any liner pins where the connection to the casing has failed should be repaired. Also, gaps at liner pins should be repaired if the gap is greater than 1/8″. A shim should be added to minimize deflection of the liner system, and potential failures during operation. The shim can be a slotted plate, slightly larger than the washer, that is slipped between the liner and washer and welded to the existing pin and washer. The shim must not be welded to the liner plate.
Repairs of the liner system damage are important to the reliability of the HRSG especially if there is CO or SCR equipment as discussed previously.
16.2.3.2 Distribution grid
A distribution grid, if present, should be inspected due to the extremely difficult operating conditions in the inlet duct of a HRSG. The typical distribution grid has several components that should be inspected. Many different restraint systems have been utilized over the years. The recommendations below are specific to a grid that is designed to rest on the floor of the inlet duct with a fixed restraint at the center line. This distribution grid is designed to expand vertically from the floor and also expand horizontally from the centerline of the gas path toward each sidewall of the HRSG inlet duct.
A common place to start is with the grid restraints on the floor of the inlet duct. The most important floor restraint will be the fixed support at the center of the duct (Fig. 16.6). Inspection of the condition of the weld at the fixed support is key. If the grid is not fixed and is allowed to move freely perpendicular to the exhaust gas at this point, binding at other restraints and subsequent failures can be expected. The floor guides to the sides of the fixed restraint (Fig. 16.7) should also be inspected to ensure that they allow the grid to move perpendicular to the exhaust gas yet provide support in the direction of the exhaust flow.
Next, the sidewall restraints (Fig. 16.8) should be inspected. The lowest sidewall restraints can be viewed from the floor of the duct or from a ladder. There are a couple different styles, such as a pin and retainer lug, or a horizontal bumper. It is important that these restraints provide support in the gas flow direction, but also allow the distribution grid to grow vertically and horizontally several inches in the direction perpendicular to the gas flow. The lower sidewall restraints typically withstand the highest loads and are most prone to failure. If damage is found on a component of the support system, it would be prudent to then closely inspect the adjacent supports. Any support damage must be repaired. Failure of one support can very easily propagate to others and cause failure of the grid section.
The distribution grid perforated plate and frame sections should also be inspected at each outage. Deformation of the perforated plate can be common but is not cause for great concern. It is appropriate to document the distortion and continue to monitor at each subsequent outage. Cracks in the ligaments of the perforated plate should be repaired to ensure that sections of the grid do not fail and cause collateral damage. Repeated cracking in a specific area will require strengthening of the grid in that area. Grid designs using perforated plate thicknesses less than half an inch in the lower sections historically have not been very reliable.
16.2.3.3 Duct burner
The duct burner, a critical component in the HRSG, is subjected to very high temperatures and should be inspected carefully during the cold inspection. The ignitor, flame scanner, burner runners, and baffles should be viewed carefully. Viewports should be inspected and cleaned, gaskets under the glass should be replaced, and seams should be sealed with high-temperature sealant.
Close attention to the burner spuds or holes in the runner is important as cracks can be common in the runners at the holes. Coking and other buildup on the runners is also a frequent issue that typically stems from incomplete combustion due to lack of oxygen. Viewing the flames during operation can help assist with possible solutions to this problem.
Another common occurrence is severe distortion of the lowest runner, or lowest two runners (Fig. 16.9). This distortion is a classic result of quenching of the runner with condensate when the gas valve is opened to begin duct burner operation. During operation of the combustion turbine, before the duct burner is ignited, condensate is created in the external burner piping due to hot exhaust gas flowing into the burner runner and subsequently migrating into the external piping. When the exhaust hits the cold external piping, condensate is formed and fills the external gas piping. This cold condensate is forced into the lowest runners once the gas valve is opened. A distorted runner should be viewed to see if it is still supported adequately and whether it will expand and contract as required.
In addition to inspecting the burner, the surrounding equipment should be viewed to see if the heat released from the burner is causing collateral issues. The sidewall, roof, and floor liner panels should be viewed for distortion or discoloration, which would signify an overheating condition (Figs. 16.10 and 16.11) and the vibration supports on the coil immediately downstream of the burner should also be viewed. If the duct burner flame impinges on a vibration support, overheating can occur (Fig. 16.12).
16.2.3.4 Heating surfaces/HRSG coils
The heat transfer coils are the backbone of the HRSG and are the most expensive components; thus a thorough inspection is warranted. There should be particular focus on the tube-to-header joints and drain connections to look for damage due to thermal stress. Special attention should be paid to superheaters and reheaters upstream of the HP evaporator and the coldest row of the feedwater preheater. A thorough inspection will include a hydraulic test of the coils under pressure to look for leaks. In the event a leak is detected, it must be repaired before bringing the unit back online and a root cause analysis should be performed. The root cause analysis should, at a minimum, focus on normal operation, upsets, and excursions of the HRSG, auxiliary equipment, chemical treatment equipment, conditions during previous lay-up, and outages and previous repair work in the vicinity of the leak.
All coils should be inspected in their entirety from the floor of the HRSG. If a scaffold is in place or access doors are open on the roof, these should be used to view the coils in more detail. In addition to looking for leaks, the coil inspection should include the general condition of the tubes, the tube-to-header joints, and the drain piping. Drain lines passing through casing penetrations often corrode due to trapped moisture in the area and should therefore be inspected carefully.
Nonpressure parts such as the finning, lower coil restraint system, vibration supports, lower gas baffles, and acoustic baffles should also be viewed. It is common to see lower gas baffles out of position or damaged, especially in the front section of the HRSG. These baffles should be repositioned and fixed at the bottom of the tube fins to minimize gas bypass.
It is convenient to divide the coils into categories: HP superheaters and reheaters, the HP evaporators, and the lower temperature coils back through the preheaters for the sake of discussion.
16.2.3.5 HP superheater and reheater coils
Tube-to-header joints at the bottom of the HP superheater and reheater coils should be inspected closely. These coils will be subjected to very large tube temperature changes between startup and normal operation. With a change in temperature of 1000°F or greater, the tubes can expand up to 12″ if the coils are made from stainless steel. This amount of expansion can cause failure in very little time if the expansion is restricted. Look for tube bulging or damage to the oxide layer on the tubes at the connection to the lower headers. This damage will be a sign that there are high stress conditions that will ultimately lead to failures. If the coils are top supported, as is the most common scenario, the lower header gas flow restraints should be inspected closely. Restraints should permit vertical movement of the headers while restricting horizontal movement. Improper restraint or restraint failure can cause significant damage. If damage is found, some type of nondestructive examination of the affected joints should be performed to see if there are indications or cracks that should be repaired. A root cause evaluation should also be conducted.
If there are questions as to whether header restraints are functioning properly, it would be advisable to contact the original equipment manufacturer.
Photographs of the front and back of each coil should be taken at each outage to document the general condition for future comparison. Any bowing or distortion of tubes (Fig. 16.13) should be noted and a simplified root cause evaluation performed to help the plant personnel understand if repairs or operational modification should be implemented. The HRSG supplier should be able to provide drawings (if you do not already have them) with nozzle and drain locations that will quickly help you identify the potential cause to the majority of the damage that will be encountered. Similar to the situation where tube-to-header damage is identified, if bowed tubes are located, some type of nondestructive examination of the affected joints should be performed to see if there are indications or cracks that should be repaired.
If there is no distribution grid in the inlet duct, the first HP superheater or reheater coil can be subjected to high loads due to exposure to the high-velocity and extremely turbulent gas flow. Damage to the tube fins at the vibration supports and bowing or movement of the coils is possible. Additional vibration supports can be installed if necessary.
It is common to have issues with casing penetration seals in this area where temperatures are very hot and the coils expand a considerable amount. Each seal should be inspected carefully, looking for uncovered and missing insulation. Missing insulation should be replaced and liner plates should be repositioned or replaced to ensure that the seal will not overheat upon restart of the unit. This is also a good time to repack any packing glands that are present.
16.2.3.6 Evaporator coils
Evaporator coils operate at lower and more uniform temperatures than the HP superheaters and reheaters, therefore there is much less issue with tube-to-header failures. Even though many rows of evaporator tubes may be connected into the same upper and lower headers, the row-to-row temperature differentials are very small so issues with thermal stress-induced failures are very rare.
The HP evaporator can be susceptible to under-deposit corrosion. If this occurs, it will present itself in the higher heat flux rows near the front of the evaporator. Under-deposit corrosion is very uncommon in the first 10 years or so of operation. However, there are some factors that can make under-deposit corrosion appear much sooner in the life of the HRSG. If the coils were not chemically cleaned properly, so that a proper magnetite protective layer is formed, or if there is a high level of iron in the water, which can deposit in the high heat flux tubes, then the occurrence of under-deposit corrosion is much more likely. The iron in the water could be a result of FAC issues in the LP system or from somewhere else in the steam/water cycle of the facility. If there is concern about under-deposit corrosion, the deposit weight density in a HP evaporator tube should be tested to determine if chemical cleaning of the evaporator is recommended (see Ref. [2]). Section 15.3.1.3 contains additional information related to under-deposit corrosion.
16.2.3.7 Emissions control equipment
There are often several pieces of equipment that are related to emissions control located in the HRSG that should be viewed during the cold inspection. If there are any issues with this very specialized equipment the most prudent course of action is typically to contact the original supplier for the best recommendation.
The CO catalyst should be viewed to ensure that the face is clean and there is no foreign material, such as insulation, blocking the flow channels. Also the seals around the edges of the catalyst support frame should be viewed to ensure gas bypass is not occurring. See Section 9.5.4 for additional information related to cleaning CO catalysts.
The ammonia injection grid lances upstream of the SCR catalyst should be viewed at each internal inspection. Each lance will have many small diameter holes that can be prone to blockage. If the lowest runners are viewed and there is no blockage, then most likely the entire system is in good condition. If holes are plugged the lances should be inspected with a borescope to determine if they all should be cleaned.
The SCR catalyst should also be checked during the cold inspection. Similar to the CO catalyst, it is wise to view the front face of the blocks to ensure there is no (or minimal) foreign material such as insulation blocking the cells. Checking the seals at the frame for any damage or area where flow may bypass is important, as is viewing the insulation pieces that are typically placed between each catalyst section and along the edges where the catalyst is fastened to the frame. After several years in operation it is relatively common for insulation pieces to be missing. A photo or two sent to the SCR supplier will help them assess whether repair is required. See Section 8.4.4 for additional information related to maintenance of SCR catalyst systems.
If either the CO or SCR catalyst is underperforming, catalyst samples can be removed and evaluated by the catalyst supplier.
16.2.3.8 Coils in the low-temperature region of the HRSG
Even though the operating temperatures in the back end of the HRSG are low (usually <400°F), the coils should be inspected during a cold inspection. Major concerns are FAC in the tubes, headers, and risers of LP evaporators, and in the tubes and headers of low-temperature economizers and feedwater heaters; thermal stress-induced damage in the preheater/economizer coils from the introduction of cold condensate during warm or hot starts; and fouling of the finned tubes in this area. FAC occurs in areas where the velocity of the water or steam/water mixture is high such as bends in tubes (Fig. 16.14) or risers or tube-to-header joints.
The tube-to-header connections at the inlet headers in the economizers should be viewed for distortion or oxide layer damage, which is a sign of high stresses. Viewing the tube field on the water inlet side is also important. A distorted tube can be a sign of a high thermal stress at some period of operation.
External buildup of debris or fouling of the finned tubes is very common in most HRSGs that have operated for more than 5 years. The temperature of the back end exhaust gas and the prevalence of impurities can lead to the precipitation of these impurities.
A modest buildup of rust on the finned surfaces is common. It can be a concern if it is excessive as it can reduce the efficiency of the heat transfer in the fouled coil. Additionally, the rust will cause higher exhaust side pressure, which can reduce the efficiency of the combustion turbine.
Sulfur (Fig. 16.15), ammonia salts (Fig. 16.16), and other contaminants can also precipitate out on the coils in the back end of the HRSG. These contaminants when wet can be transformed into acids that can attack the tubes and cause tube failures. These deposits can be removed by water washing or more effectively and with less collateral damage by blasting with dry ice pellets. An experienced cleaning contractor should be used for removal of ammonia salts.
16.2.3.9 Internal steam drum inspection
The internals of all steam drums should be inspected at each outage. Both manways should be opened and a fan placed at one end to provide a draft and help cool the drum for quicker access. The HP steam drum, due to shell thicknesses that can be as much as 7 in., may take several days to cool to a reasonable temperature for access.
While the drums are cooling, the external areas can be inspected as follows:
• Check that all valves and trim are supported and sealed properly.
• Service and calibrate safety valves every 2 years at a minimum.
• Check for signs of leaking flanges/covers on the manways and replace gaskets.
• Check the general condition of the level control equipment, cleaning the water column gage glass and probes and replacing gaskets.
• View the saddle support and ensure the slide packs are in the proper position and grease any fittings present.
When the drum is being entered care should be taken to avoid dropping inspection tools or flashlights as they may enter pump suction or downcomer lines located on the bottom of the drum shell near the drum manway that are not covered. Once inside the drum, look for corrosion, erosion, deposits, or mechanical issues. The following items should be carefully inspected:
• the drum manway forging and cover gasket surfaces
• the downcomer-to-shell connection
• the belly pan and the connections to the drum shell
• the feedwater nozzle connections and internal pipe
• the secondary steam separation boxes, including the mesh pads
Depending on the pressure level of the drum there are different concerns. The major concerns for each drum are:
LP Steam Drum
The major concern for the LP steam drum is the possibility of FAC. The primary separation devices such as the belly pan or cyclones should be viewed for signs of FAC. This may appear as shiny metal where the magnetite layer has been removed or actual erosion of the material. FAC damage can also be present at the downcomer nozzles. Viewing the general color of the inside of the LP drum is also important due to FAC concerns. The inside of the drum should be “ruggedly red” due to the presence of an oxidizing environment (hematite). If the inside of the LP drum is not red in color, the plant chemistry program should be reviewed by an expert as soon as possible.
If ports are available in the lower baffle of the drum (belly pan), looking down into the riser nozzles and beyond into the LP evaporator with a borescope for signs of FAC is recommended.
If there are feedwater headers present in the LP steam drum, the spring-loaded spray nozzles should be checked to ensure the springs are still functioning properly. It should be possible to open the nozzle by hand.
IP Steam Drum
Generally, there is much less concern for issues in the IP steam drum than in the HP or LP steam drums. Inspection of the final separator and associated mesh pad is important to ensure IP steam purity. Viewing the manway forging and manway cover gasket surfaces and the separation equipment is recommended. Additionally, depending on the pressure of the IP system, there could be the potential for FAC at the downcomer or primary separators so they should be viewed as they were in the LP steam drum. Inspection for the general cleanliness or any buildup of loose material at the ends of the IP steam drum is also prudent. This may be a sign of improper blowdown or water quality concerns.
HP Steam Drum
The HP steam drum has several areas that should be inspected closely. The gasket surfaces on the manway forging and the cover should be viewed at each outage. Gouges, scratches, or imperfections that lie across the surface (perpendicular to the edge) are most important. Any damage that is 1/32″ deep or greater should be repaired.
Another very important inspection location in the HP steam drum is the downcomer-to-shell connection, especially if the downcomer forging projects inside the shell inside diameter. Units that were originally designed for baseload service can experience cracking in this area if now started and stopped frequently.
Due to the higher operating temperatures, and subsequently greater thermal stresses between the thin-walled internal components and the thick-walled shell than the IP and LP steam drums, the primary and secondary steam separators in the HP steam drum should be inspected closely. The locations where the separator plates are welded to the drum shell can be prone to small cracks. If small cracks are found they can be monitored yearly to determine if repair is necessary. However, if there is concern that the crack may propagate into the shell base material, then it should be repaired as soon as possible.
The mesh pads should also be checked to ensure they are free of debris and cover the entire surface of the separator vanes as in Fig. 16.17. Bypass of the mesh pads as in Fig. 16.18 has the potential to affect the overall steam quality.
16.2.3.10 Stack
Inspection of the stack should complete the internal inspection of the HRSG. The stack floor and lowest shell can should be viewed for general corrosion. Checking that the floor drain is not blocked by rust or other debris is important to help minimize corrosion that may occur due to the presence of condensate in the stack.
The silencer and stack damper should also be viewed from the floor of the stack. If there is concern about the integrity of the silencers, closer inspection is warranted. If it appears that the damper blades are not sealing completely, the movement of the blades should be checked during the outage.
16.2.3.11 Severe service valves
Attemperators (desuperheaters) and pressure-reducing valves between the HP superheater and reheater are severe service valves and should be inspected and maintained annually.
Nonreturn valves in the HP steam outlet piping are subjected to very difficult operating conditions. These are very large, thick-walled castings or forgings with hardened seats. Thermal stresses at startups and shutdowns can cause cracking and failures in the seating surfaces. Plants that typically operate in a cyclic mode should plan to inspect these valves after 5 years of service.
16.3 Repair
Most modern HRSGs are well designed and manufactured to very high standards. As a result, major repairs are usually not required. However, unforeseen situations can arise when operating a complex power plant and components can be damaged. Operating conditions and needs of a plant can also change so that the HRSG may require modifications. The cyclic service, with frequent startups and shutdowns, that is demanded of many power plants in today’s environment is also hard on a HRSG that is not designed for this service. A few of the most common repair situations will therefore be reviewed. Detailed repair procedures are beyond the scope of this book. When making repairs such as these, the services of a contractor who is experienced and certified to repair boilers are required, and this contractor should be involved in developing procedures that are appropriate for both the repair and the staff performing the work.
After a HRSG is constructed and stamped in accordance with ASME rules and procedures, any subsequent repair falls under the jurisdiction of the National Board Inspection Code (NBIC). Repairs and alterations are to be approved by a local authorized inspector.
16.3.1 Flow-accelerated corrosion
FAC occurs predominantly in the tubes, headers, and risers of low-pressure evaporators and in the tubes and headers of economizers and feedwater preheaters operating in the 200–500°F temperature range. It occurs in areas where the velocity of the water or water/steam mixture is high such as bends in tubes or risers or tube-to-header joints. Repair involves replacement of the damaged portion of the component and requires the services of very capable tube welders who are certified to the boiler code in use. Accessibility of the area where the repair is needed can be an issue as the finned tubes in a HRSG are spaced very close together. If the components being replaced are carbon steel, consideration should be given to the use of low-alloy chrome steel for the replacement as it is more resistant to FAC. The water treatment program should also be reviewed as FAC occurs as a result of both inappropriate water treatment and high velocities. See Chapter 15, Developing the optimum cycle chemistry provides the key to reliability for combined cycle/HRSG plants, for water treatment solutions.
16.3.2 Thermal fatigue
Thermal fatigue or operational stress occurs primarily at the hot end of the HRSG where thermal growth of components is greatest. It can be the result of improper restraint of an expanding tube or more commonly the result of inadequate drainage of superheater and reheater tubes during startup. Water entering the coil from any source can cause significant damage. Malfunction or improper operation of attemperator valves is especially troublesome. Nonuniform distribution of attemperator spray into a superheater or reheater will fatigue tubes easily. Fig. 16.19 shows tube-to-header joints that failed as a result of operational stress. Repair involves replacement of the damaged components, much as with FAC. Tube welders must again be very capable and certified—even more so than for the previous FAC example as the high-alloy tubes in a superheater or reheater are more difficult to weld. Heat treatment of the welds may also be required. If thermal fatigue occurs, a root cause analysis should be performed to prevent reoccurrence of the problem. Chapter 10, Mechanical design and Chapter 11, Fast-start and transient operation, discuss reasons for and solutions to thermal fatigue problems.
16.3.3 Under-deposit corrosion
Under-deposit corrosion occurs in tubes at the hot end of a high-pressure evaporator where a contaminant concentrates under a deposit on the inner surface of the tube and corrodes the tube. Repair entails replacement of the failed tubes by certified welders and access can also be an issue. A root cause analysis of the failure should be performed and chemical cleaning of the evaporator may be required to remove deposits from tubes that have not failed. Water treatment is an issue when under-deposit corrosion occurs. See Chapter 15, Developing the optimum cycle chemistry provides the key to reliability for combined cycle/HRSG plants, for solutions.
16.3.4 Casing or liner failures
Casing and liner failures are common in HRSGs due to the high velocities and turbulence in the exhaust from modern combustion turbines but can be greatly minimized by a good inspection and maintenance program. Repair usually involves replacing insulation and making sure that the liner covers it securely. If the damage is near a casing penetration as it often is, the expansion joint or packing gland most likely will need service also. When repairing liners, care must be taken to ensure that the liner and those surrounding it remain free to expand. The repairs are usually performed from the inside of the HRSG but can be performed from the outside when access from the inside is difficult. Qualified welders are required but they do not have to be certified to the boiler code.