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Cooling condensers are among the most critical equipment in plants with steam-turbine-driven equipment, where the turbine steam exhaust is condensed by means of cooling water. (Typical industries include steel, pulp and paper, power and food processing.) The main purpose of these heat exchangers is to maintain vacuum on the turbine exhaust and to recycle steam as high purity condensate. The steam condenses within closed-loop systems, creating a partial vacuum near absolute zero pressure. The resulting high purity condensate is pumped back into the cycle for return to the boiler.

The condensers are designed to prevent direct mixing between the shell-side steam and the cooling water flowing through the tubes. Isolating the steam from the cooling water maintains the high purity water, allowing the condensate to be returned to the boiler. This minimizes the water purification costs for the feedwater. Figure 1 shows a simple example of the basic process flow for typical condensers.

FIGURE 1. In this condenser flow diagram, a horizontal tube cooler is joined between tubesheets. Leaking can occur at the tube-to-tubesheet joints and from the tubes due to corrosion, biofouling, vibration and other problems. Image provided by Corrosion Monitoring Services (Click on the image to enlarge.)

Water Impurities and Biofouling

When cooling condenser performance is compromised by corrosion, biofouling and other problems, system performance suffers. Because the condenser plays a key role in performance, hidden problems can unexpectedly result in de-ratings.

Some operations draw cooling water from nearby rivers. Screening processes at the intake, however, may be insufficient to remove all sediment and other impurities. Untreated cooling water has the potential to support biological growth and leave deposits that can collect inside and around the cooling condenser tubes and tubesheet.

Unit reliability is affected if the cooling water leaks into the condensate system and carries with it dissolved minerals and biological impurities. If the boiler water impurities cannot be managed by water treatment and boiler blowdown, system impurities may result in boiler and turbine damage.

Tube inside-diameter (ID) growth due to mineral deposits or fouling reduces the surface area available for heat transfer. When the surface area lost to plugged tubes or reduced heat transfer exceeds this threshold, a significant decrease in the unit’s overall efficiency and capacity rating is common.

Thus, plant operators continuously monitor condensate/feedwater purity. This allows them to detect significant condenser-tube leaks and other problems within minutes of occurrence — and provides the opportunity to plan solutions. The response to indications of these leaks can be delayed, however. Usually, it is a function of following established limits, evaluating steam-load requirements and any impacts on operating costs.

Loss of vacuum affects the efficiency of the entire thermal process. In addition, impurities in the condensate can damage the boiler and turbine components, resulting in reliability and capacity issues.

FIGURE 2. Tube-to-tubesheet damage from pressure cleaning can be seen in this condenser. Image provided by Corrosion Monitoring Services (Click on the image to enlarge.)

FIGURE 3. High flow tube leaks are temporarily marked so that the technicians know where to install tube plugging prior to condenser repair. Image provided by Corrosion Monitoring Services (Click on the image to enlarge.)

Other Failure Modes

The condenser-tube service life also can be reduced by other modes of deterioration. Failure may occur due to corrosion, erosion, vibration and even chemical attacks from cleaning agents.

As cooling condenser tubes age, the susceptibility for failure at the tube-to-tubesheet plate roll-joint interface increases. It is common for condenser tubes to lose the seal at the tubesheet plate and for leakage to occur around that joint. The process of decay at the roll joint can be thought of as corrosion fatigue. This method of failure is related to mechanical stress loading at the tube-to-tubesheet plate joint, combined with any other damage mechanisms.

Due to the location and nature of corrosion or other defects at the mechanical roll joints, predictive nondestructive test (NDT) methods are of limited value. (This also is due to instrument limitations near the tubesheet.) Close monitoring of the condensate water-quality limits is required.

Any detrimental effect on the condenser performance varies based on the location and extent of the problem. For example, corrosion of the condenser tubes can allow the cooling water to leak into the hot well and contaminate the condensate systems. Alternatively, fouling can reduce heat transfer, or roll-joint leakage can impact the shell-side vacuum and turbine capability.

Protection Starts with Inspection

When water-chemistry parameters indicate significant deviations, and inspectors observe significant failures during hydrostatic testing, many plants rely on service providers who conduct monitoring to identify leaking or failed tubes.

The hydrostatic testing process involves filling the shell-side of the condenser with water, on the steam side, and visually inspecting conditions at the tubesheet plate. If water is leaking from the tube ID, this indicates that condenser tubes have failed. The failed tubes may not be the total source of the contamination, however.

Instead, once any leaking tubes are plugged, removing the main flow of water, the inspection may find water leaking around or through the tube-to-tubesheet-plate interface. The tube may not be defective at all. Mechanical (e.g., vibration) or other issues could have caused the tube to separate from the tubesheet.

If that is the case, repair of the leak at the roll joint is a possibility. Re-expansion of tubes into the tubesheet plate often seals many of the leaks. Of course, the overall success of these repairs depends on a variety of circumstances, including a highly technical expansion process to prevent overexpansion of the tube. This includes a detailed assessment of “as-found” dimensions in all cases.

Case in Point. A brief case history illustrates the approach.

Personnel at one site observed multiple failures during a hydrostatic test. This included major water flow streaming from the shell side into the water box. It was estimated that up to 50 tubes had failed (figure 2). No additional information was available regarding failure modes or the condenser maintenance history. Figure 3 shows yellow flags inserted in the failed tubes.

Bringing Condensers Back from the Brink

This case points to the importance of regular inspection and capturing results for further review. The more details that can be supplied to a contractor prior to inspection and repair, the better the chances for successful repairs. The following activities should be considered in advance of an outage to assist in preparing for future repairs to condensers:

• Review cleaning processes used currently and supply this information to your contractor. There may be opportunities to improve current practices and the removal of biological deposits and scaling.
• Perform a third-party laboratory failure analysis. This requires submission of the design criteria and operating information. In addition, it can assist in analyzing the useful or remaining service life of the equipment.
• Gather design data, including drawings that identify the tube materials and wall thickness; original tube expansion specifications; current tubesheet maps; and photos of access areas at both ends of the condenser.

FIGURE 4. Easy-to-install plugs can be used and left in place on most condenser systems or easily removed. During the initial search and repair of tube-to-tubesheet roll joints, these plugs stop leaks, allowing technicians to locate defects, which are often near tube ends. Image provided by Corrosion Monitoring Services (Click on the image to enlarge.)

There are times when a turbine can no longer continue to operate due to steam-surface condenser problems that have accelerated almost overnight. In such cases, quickly obtaining and installing a long overdue bundle or hundreds of new tubes is not realistic. When units may have reached such degraded performance for the steam-surface condenser, availability of the entire steam system requires immediate attention.

For example, if the steam-side water chemistry limits were exceeded by the intrusion of contaminated cooling water, over time, the turbine may lose the capability to maintain an ideal condenser vacuum. In addition, the maximum capacity rating (MCR) also could be compromised. When units reach this critical juncture, it is no longer about maximizing efficiency. The focus shifts to restoring and sustaining operations until a long-term repair plan is approved.

Condenser damage typically develops gradually. When a high number of condenser tubes are plugged, boilers can limp along, operating with marginal — but controllable — water quality for long periods. When things finally reach a tipping point, problems can escalate quickly.

Consider this hypothetical case: A routine inspection during a plugging operation could reveal that more tubes have failed since the last outage. Several tube-to-tubesheet joints may appear to be leaking. Although this situation is not unusual, leaks and other mounting damage often go undetected — and untreated — until they grow into a bigger problem.

In such cases, the loss of surface area for heat transfer due to an excessive number of plugged tubes, extensive process changes due to steam load input, and the general effects of erosion and corrosion on both the steam and water sides could ultimately require the replacement of components to extend the operational life of the condenser. In the meantime, immediate action may be warranted to keep plant equipment operating. When time and budget are limited, one alternative is to remove the leaking tubes from the damaged condenser and return as many as possible to service, allowing continued operation until more expensive solutions are an option.

FIGURE 5. Tooling is inserted into a leaking joint for tube-to-tubesheet repair. Yellow barrier tape was used as the location marker of the tube being expanded. Image provided by Corrosion Monitoring Services (Click on the image to enlarge.)

The Repair Process in Practice

When plant personnel or contractors identify tube leaks and roll-joint failures as the major problems within a condenser, the goal often becomes repairing as many of these failures in as short of a period as possible. The first action in the repair process is to identify and plug all significant leaks with expanding-type plugs that can be removed easily for testing or attempts to repair (figure 4). Once all of the leaking tubes are sealed at both ends, the identification and repair of the tube-to-tubesheet joint leaks can begin.

The following example shows the repair process of inserting specially designed tooling into leaking tube-ends to attempt to seal leakage through the process of tube-to-tubesheet plate re-expansion. Figure 5 shows tooling being inserted into a leaking tube. Technicians then apply the torque-controlled rolling motor and roll the joint until seepage around the tube ceases.

In cases where specific tube expansion and tube-wall reduction specifications for a condenser are not available, contractors must consider using industry standards when selecting the tooling and the force to be applied during re-expansion.

During tube re-expansion, field technicians examine the condition of tube-to-tubesheet plate ligament spaces and the tubesheet plate. If faults in the tubesheet plate or ligament spacing are not observed after re-expansion, it is an indication that the most obvious sources of water/leakage may have been eliminated.

The first image in this article provides a look inside a sample condenser, where deformed tubes were repaired and tube-to-tubesheet expansion was successfully completed. (Note, this photo was taken while the condenser remained pressurized with water.) In this example, approximately 70 tubes were re-expanded into the tubesheet plate. This process was successful in sealing the seepage and leaks into the water-box on these tubes.

It is important to recognize that visual inspections have limited reliability. While all obvious sources of roll joint failure or fatigue were addressed in the project mentioned, it should be expected that seepage may not be identified on several tubes. Acoustic pulse reflectometry (APRIS) or other NDT methods can help determine causes and other leakage mechanisms in condensers (figure 6).

FIGURE 6. When the propagated pulse wave deviates from the anticipated path (defect-free tubing), the instrument recognizes the change as a discontinuity or fault. This enables the comparison of multiple failure locations and their position relative to baffles and process piping. Image provided by Corrosion Monitoring Services (Click on the image to enlarge.)

In conclusion, the project described was successful because the operations personnel had identified a number of tubes that required plugging prior to the technicians arriving for repair work. Damaged condenser tubes were ready to plug, stopping high flow leaks and enabling the repair team to locate tube-to-tubesheet joint failures. Technicians successfully rerolled approximately 100 tubes into the tubesheet, restoring vacuum and water quality. The unit was released for service in three shifts, and the emergency replacement of the heat exchanger was delayed.

Planning inspections with your contractor can provide the information that will help ensure that the job is done right and on time. By taking these steps, it may be possible to repair your cooler condenser and put it back to work quickly without having to invest the extra time and money on a replacement unit.