Heat exchanger O&M practices - Energy-Tech Magazine: Heat Exchangers

Heat exchanger O&M practices

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Posted: Monday, March 1, 2010 12:00 am

The balance of plant (BOP) category of utility power plant heat exchangers is often overlooked when maintenance managers evaluate the need for preventive maintenance. The sound, reliable operation of the turbine, generator, pumps, fans, air compressors, etc., is often directly linked to the proper care and maintenance of these various types of BOP heat exchangers. Taking proper care of your BOP heat exchangers can often help avoid suffering more costly consequences as a direct result of inadequate cooling, due to clogged or fouled tubes, or from cross-contamination caused by heat exchanger tube or tube joint failures.

This article aims to provide a basic understanding of the thermal, hydraulic and chemical (metallurgical) issues that challenge the responsible system engineer who is charged with their proper care, and offers specific guidelines for operational and maintenance practices to help establish a successful preventative maintenance program.

References for the basics

Two references are strongly recommended as required reading to gain a working knowledge of the various types and configurations of shell and tube heat exchangers. The first is the Standards of the Tubular Exchanger Manufacturers Association (TEMA). Secondly, the Heat Exchanger Institute guidelines for Power Plant Heat Exchangers, which complements the TEMA standards because it specifically deals with and contains information about utility power-generating heat exchanger applications.

Common BOP heat exchangers

BOP heat exchangers are of various types and configurations, from basic shell and tube exchangers to plate and frame exchangers, finned coils, double-pipe exchangers and many others. Each has its own advantages and disadvantages and each can require different maintenance approaches.

These auxiliary exchangers differ widely, using various fluids to be heated or cooled, as well as which medium is on the tube side and which is on the shell side. It also is important to know which side is operating at the higher pressure, since any failures will result in leakage from the higher to the lower side. Because of this, each exchanger offers different challenges in the operation and maintenance programs. The approach to corrective maintenance also varies based on the specifics of the exchanger.

Common BOP coolers include: main turbine lube oil coolers, station air compressor inter-coolers, condensate coolers, auxiliary lube oil coolers, hydrogen coolers, air ejector/gland steam condensers and others. Common BOP heaters include: fuel gas or fuel oil heaters, auxiliary boilers, reboilers, air and glycol heaters, as well as other applications.

Service water systems

Now that we have introduced some of the most common BOP heat exchangers, we need an understanding of the systems that they’re an integral part of. Since the most popular BOP’s are auxiliary coolers, we will continue our discussions with examples of equipment coolers that utilize service water as the cooling medium. Station service water systems are either “closed loop” (cooling tower) or “open” (river, lake or pond) systems. Open systems can be either fresh or brackish water consistency, and as one would expect, different environments demand different water chemical treatments, and yield different fouling and corrosion potentials.

Categories of typical problems and susceptibilities

The following outlines some of the more common problems associated with BOP heat exchangers:

  • Designs with poor/marginal performance
  • Configurations with poor maintainability
  • Corrosion susceptibility based on materials of construction
  • Lack of on-line performance monitoring
  • Propensity for fouling/clogging
  • Improper maintenance procedures
  • Lack of Preventative Maintenance plans

Each of these issues is discussed below.

Poor/marginal performance designs

Often, many smaller exchangers are bought “off the shelf” based on standard OEM designs, and are fitted into the “component package” provided by the supplier. Sometimes these exchangers are selected based on order of magnitude rating of the total amount of heat to be transferred, but are not actually sized or constructed for all the specifics of the application. Cast iron dished heads, relatively thin tubesheets, inadequately spaced baffle/support plates and overall poor quality construction usually result in premature failure of these exchangers, and in some cases the cost to repair and maintain these units is more than the cost of a new exchanger. There is typically no conservatism in these designs that allows for adequate performance if a number of tubes fail or become clogged. This usually results in higher tube velocities, which can result in tube inlet erosion in the remaining free tubes. This is particularly compounded in the summer months when the service water can be much hotter.

Due to the parallel configuration of the entire service water system, one cannot assume that the tube-side cooling water flow is as designed. The actual flow will be based on the system head characteristics, and the pressure drop that exists in each parallel cooler branch. The cooling water will always take the path of least resistance, so depending on manual valve positions, details of each piping run, changes in elevation, the degree of fouling/suspended debris/mud/silt, etc., we might have more or much less cooling water flow than we suspect. In most cases, we just don’t know how much cooling water is actually flowing through the exchanger.

Poor maintainability configurations

Often, if an exchanger is in a difficult to access location, it goes ignored. The ability to remove a head or waterbox to get full access to the tube field and conduct maintenance is paramount in extending the life of these exchangers. In some cases, changes in design to the piping runs or head configurations of the exchangers should be considered to assist and more easily facilitate maintenance.

Corrosion susceptibility - materials of construction

Obviously the materials must be compatible with the fluids on both sides of the exchanger and are typically selected based on resistance to the potential problems of the application.  BOP exchangers are typically made of lower-grade materials that are susceptible to different types of corrosion. The attack can be compounded by the effect of the type of deposits/debris that precipitate out of the cooling water flow. This might manifest into other corrosion susceptibilities specific to each exchanger/application, which must also be identified and, where possible, mitigated.

In some cases there might be a mix of different materials within a given exchanger, usually non-ferrous (brass) and ferrous (steel). Therefore the system engineer must always be concerned with the galvanic corrosion potentials. Additionally, the tube material affects the rate of heat transfer; therefore, if considering a change in tube material for a specific exchanger, it must be re-sized to ensure that the proper amount of surface is provided for the application.

Fouling/clogging

Closed cooling water loops that employ cooling towers typically suffer degradation in performance, due to fouling from calcium carbonate and other hard water deposits that accumulate in the tubes.

Open cooling water systems can result in tube fouling due to debris and biological deposits. Fouling results in degraded heat transfer performance, but with time excessive fouling can result in clogging with higher pressure drops due to grass, mud or silt blocking and reduce the normal cooling water flows. Clogging can result in increased erosion due to higher localized velocities in the less affected tubes. In addition, other types of corrosion mechanisms (under-deposit corrosion, crevice corrosion, MIC, etc.) can compound the situation.

Need for on-line performance monitoring

To ensure that the compounding effect of excessive fouling/clogging is minimized, we recommend on-line performance monitoring. Trending tube-side thermal rise and pressure drop parameters across the exchanger will help identify the most optimum cleaning schedule necessary to prevent fouling from manifesting into clogging, and various other problems as a result.

Tube failures and plugging

Often, tube leaks that develop in BOP exchangers go unnoticed until significant damage to the exchanger has already occurred. The exception is usually lube oil coolers, where a cross-contamination of the systems occurs, and the leak must be identified and corrected promptly.

Sometimes these failures cause environmental issues when oil leaks into the river. When choosing a plugging device, consideration needs to be given to tube-side pressure, shell-side pressure, tube surface irregularities and the potential to loosen with time, due to either deterioration or thermal cycling. Another consideration is the additional stresses imparted to the exchanger based on how the plug is installed. Plug materials should be compatible with the parent heat exchanger tube material.

Lack of preventative maintenance plan

Many plants do not have a proactive PM plan for these BOP exchangers, typically waiting until there are obvious signs of damage before repairing the exchanger. This lack of planned, scheduled cleaning and routine maintenance is almost always a contributing factor when major repairs to an exchanger are required. It is imperative to develop a program that works. The schedule can be arbitrary at first, but should be adjusted with time, based on the trended timeframe to reach unacceptable loss in thermal performance or excessive pressure drop across the exchanger. It can be further biased by the as-found conditions between inspections and the propensity for failures/problems with the specific exchanger.

In this regard, a full lifecycle management plan offers the best approach to long-term, reliable operation for those critical BOP exchangers. Based on the results of exchanger performance monitoring trends, a Preventative Maintenance Schedule can be developed that includes the following activities:

  • Routine cleaning
  • Inspections, testing and required repairs
  • Failure cause analysis
  • Repair options

Despite the best efforts in establishing preventative maintenance programs to extend the lives of BOP exchangers, the exchangers will inevitably fail or become damaged at some point. A number of repair options exist for various problems experienced by BOP exchangers, from plugging leaking tubes, to retubing or even complete replacement tube bundles. In many cases, custom repairs to heads/waterboxes and tube sheets might be required independent of tube bundle failures. In cases where inlet erosion damage is prevalent, tube sleeving or epoxy coating of tubesheets and waterboxes might help extend the life of the exchanger until a replacement can be found. For smaller exchangers, especially when the plant has several of the same type, it is often advantageous to have a spare exchanger that can be swapped in place with the existing exchanger.

The removed exchanger can then be cleaned and prepared for reuse in a shop environment and ready for re-installation during the next cycle.

Summary

For the engineer responsible for maintaining BOP exchangers, an understanding of the failure potentials and other major problems associated with the equipment is paramount in ensuring long-term reliability. Preventative maintenance schedules should be optimized based on the results of on-line performance monitoring trends, where degradation in thermal performance and increased pressure drop with time provides the best indicators of when to schedule cleaning and routine maintenance. Many times, using good, practical logic in repair vs. replace decisions will result in lower costs for the long term. Improvements to exchanger designs, construction materials, operation modes and operational flexibility in the specific and allied systems should be considered when replacing any heat exchanger. Retaining documentation of all maintenance and inspections is paramount to conducting failure cause analysis, which should be the engineer’s primary objective in an effort to establish remedial actions.


Michael C. Catapano has more than 35 years experience in the operation, design, procurement and maintenance of feedwater heaters, condensers and other shell and tube heat exchangers, including 7 years with PSE&G and 28 years as president of Powerfect Inc. His current work at Powerfect is primarily devoted to consulting, troubleshooting problems and assisting utilities with feedwater heater replacement and operating and maintenance activities. Catapano is an ASME fellow and has assisted ASME and EPRI in numerous feedwater heater projects, seminars and publications. He also holds three patents pertaining to feedwater heater testing and repair. Catapano has a bachelor’s degree in Mechanical Engineering from Newark College of Engineering.

Eric Svensson graduated from the Georgia Institute of Technology in 1993 with a bachelor’s in Chemical Engineering. He joined the Naval Nuclear Propulsion program shortly after graduation, where he received training in Nuclear Power Theory and Operations. In 2000, he received a master’s degree in Operations Management from University of Arkansas. His current role as vice president of Engineering at Powerfect is devoted to consulting, troubleshooting problems, as well as operations and maintenance activities. Since joining Powerfect, he has been involved in writing the specification and conducting quality control checks for more than 20 replacement feedwater heaters. He also is a member of the ASME Heat Exchanger Committee and has co-authored several technical papers.

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