By Beth Foley-Saxon
A big stride forward towards energy efficiency at your plant begins with understanding how essential the plant condenser is to the overall operation of the unit. Power engineers often overlook the performance of the condenser, but they do so at their peril. Good condenser upkeep, or the absence of it, is a “pay me now or pay me later” proposition. The condenser can make or break unit efficiency and the ability to meet power output goals.
At last count, there are more than 7,600 power generation plants in the U.S. and virtually all of them are working towards greater energy and operational efficiency. What’s more, many power engineers regard conservation of energy as a moral imperative and view process efficiency as a means to that end. Efficiency can be a moving target, though, and many Rankine-cycle engineers feel challenged to move the needle on efficiency improvements. Engineers face daily pressures to comply with emissions regulations, to operate effectively when staff and budget cuts have occurred, and to remain competitive within the marketplace of renewable energy providers and natural gas turbines, and this can leave the work of efficiency far down on the to-do list.
These challenges require a sharpened focus on the unit components in the power generation system that account for significant energy loss and-by extension-financial cost. The condenser is one of these components. Ignoring the signs of poor condenser performance is a missed opportunity to improve efficiency, especially because condenser technology is well defined, the causes of compromised condenser performance are well understood and correction is usually straightforward and effective.
Design specifications for condensers typically define a maximum effective rate of removal of the latent heat in the exhaust vapor from the turbine entering the condenser as well as its transfer into the circulating water. Some variables affecting the heat transfer can include:
- Cooling Water Flow Rate
- Inlet Cooling Water Temperature
A poorly designed or poorly maintained condenser will contribute to:
- Increased unit heat rate
- Increased losses to cooling water
- Increased CO2 emissions
- Reduced generation capacity
- Increased fuel consumption
Most utility scale steam condensers in use at electric generating stations are either single or multiple compartment condenser designs. In most single compartment condensers, the cooling water passes through the compartment one time. The single compartment condenser may also be built with two-pass flow. Regardless of the number of passes in the single compartment condenser it has one backpressure, it is much easier to monitor the performance with accuracy. There is only one cooling water inlet temperature and one outlet temperature. The turbine exhaust steam enters the condenser, is cooled, and returns to the feedwater heaters for reheating in accordance with Rankine cycle principles. Since a pound of steam takes up more volume than a pound of water, the condensation process creates a vacuum. The vacuum optimizes the performance of the turbine.
Larger fossil and nuclear plants are designed with multi compartment condensers. The multiple compartment designs relate directly to turbine design, i.e. multistage turbine, multi compartment condenser, the relationship has to do with the desire for separation of the steam flow and the pressure in the condenser. A variety of layouts and configurations exist to accommodate the need for multiple compartments. There are many inputs and outputs to monitor the performance. The performance monitoring must be done for all compartments.
In achieving the maximum effect rate of removal of the latent heat, the amount of surface area required, the number of tubes, and tube material selection must all be considered. Starting with the Total Duty (MMBtu/hr) required from the turbine exhaust and steam inlet (lbs/hr), along with known cooling water temperatures the amount of surface area necessary to perform under the most difficult environmental, thermal and mechanical conditions can then be calculated. All manufacturers in the United States currently design and build to the Heat Exchanger Institute’s (HEI) Standards for Steam Surface Condensers (2012, Cleveland, OH) currently in their Eleventh Edition specifications and requests for quotations are written to it, as well offers and designs are presented on the basis of it. HEI is the standard for the industry and the members of the Steam Surface Condenser Section are from well-established national and international condenser manufacturers. HEI have now been active for over 70 years.
The HEI standards include formula, symbols, nomenclature, performance calculations, required service connections, venting capacities for removal of non-condensable and material construction details and all the associated mechanical and thermal properties under consideration.
When a condenser is delivered to a utility, its design, though based on HEI Standards, will have to be accepted by the owner/operator and, in most cases, the test parameters for acceptance will be based on the ASME PTC 12.2-2010 Steam Surface Condenser Performance Test Code (2010 NY, NY). The Condenser Performance Test Code (PTC), once quite comprehensive and difficult to apply in the field, has been modified to be practical and useful, and the code has been updated to reflect the most current condenser technology in practice.
How well a condenser is operating is largely a function of how efficiently heat is being transferred from steam to cooling water. If heat is being transferred quickly, so, too, is the turbine exhaust quickly being condensed into liquid water. One important metric in monitoring condenser performance is condenser vacuum. When a condenser pulls a strong vacuum it has increased cycle efficiency and is likely to produce more power. Close attention to condenser monitoring will reveal developing trends in condenser function.
Condenser Tube Fouling
No matter the configuration of the steam condenser, they all share a propensity towards tube fouling. The cause of deteriorating condenser performance is often progressive fouling of internal tube surfaces, and is frequently found within a condenser when reduced heat transfer capability is observed. Condenser tubes are fouled when unwanted material has accumulated on the tube wall and fouling almost always interferes with the efficient operation of the condenser. Fouling results in higher backpressure in the condenser and less efficient turbine performance, requiring increased fuel and even limiting generation capacity. Tube fouling is a perpetual problem with condensers, but careful management of condenser maintenance can significantly contribute to improving a unit’s economic performance.
The types of condenser tube fouling fall into five categories: microbiological, scale, deposition, corrosion products and tubesheet pluggage.
- Microbiological fouling routinely occurs at plants that use seawater or river water in their circulating water system, and can consist of marine plants and animals, mud and organic slime.
- Scale deposits occur when there are high temperature conditions and dissolved mineral content, such as calcium carbonate and calcium phosphate. Scale fouling can drastically reduce heat transfer in the condenser, and crevice corrosion can form beneath the hard scale coating.
- Deposition of particles onto the interior of the tube wall generally occurs when water flow rates are not adequate to keep particles in suspension. Common deposits include sediment, silt, diatoms, coal dust and minerals. Areas of low water flow in the condenser often result from partial blockage on the tubesheet or a tube obstruction.
- The formation of corrosion products within condenser tubes is a potentially serious problem that is more likely to occur when source water is corrosive. Corrosion products can become relatively thick on the surface of some tubes, particularly tubes made of copper alloys. Tubes that contain hard scale fouling are prone to copper oxide growth and, in some cases, a thin surface scale will inhibit heat transfer and promote crevice corrosion.
The inlet of the condenser tubesheet is vulnerable to blockage by a variety of material and debris, including rocks, concrete, broken pipes, cooling tower materials like plastic fill and wood, chunks of ash and coal, rusted metal, leaves and other vegetation, and aquatic animals like fish, clams and crawfish. The reduced water flow to some of the tubes results in particulate deposition and increased likelihood of microbiological growth. If the tubesheet blockage is severe, the condenser vacuum can be significantly degraded.
On-line Condenser Maintenance
Preventing or minimizing fouling in the first place is the best medicine for the condenser. Online foulant removal measures can be enacted to minimize accumulation, and are more successful when the probable foulant is known. Water chemistry modifications, such as reducing the pH of the circulating water by injecting additives, have been used to reduce calcium carbonate and calcium phosphate fouling. Online mechanical cleaning systems, such as sponge balls or recirculating cleaning tools, can be effective with very soft deposits and some microbiological fouling. On-line systems are less effective when there is hard scale fouling or corrosion in the tubes. Some plants have opted to use high doses of biocides for a short period to remove biofilms from condenser tube walls, although many microbiological growths are resistant to biocides.
Off-line Condenser Maintenance
For many plants, fouling processes have been underway for some time, and more aggressive foulant removal measures should be taken. Removal of fouling from the condenser tubes when a unit is off-line is usually the most effective approach. When the unit is off-line, condenser tubes can be directly evaluated for fouling and overall condition, and this allows for an accurate diagnosis of the problem and precise mitigation strategy. The direct approach to fouling assessment by deposit sampling is one of the later and more practical aspects of performance diagnostics included in the ASME’s Steam Surface Condenser Performance Test Code, designed specifically to confirm the necessity of mechanical cleaning.
Chemical removal of corrosion and scale can be successful, provided the process is correctly designed and implemented. There are drawbacks to chemical cleaning, including: safety, cost, waste disposal, duration of cleaning, incomplete foulant removal and damage to the base metal of the condenser tube.
There are several off-line mechanical cleaning techniques that are commonly used to remove foulants. Metal and plastic tube cleaners (scrapers) have been developed to remove virtually all types of foulants, even hard mineral scale such as calcium carbonate. Typically, mechanical tube cleaners are propelled through the length of the condenser tube with pressurized water at approximately10-20 feet per second and loosened debris is rinsed from the tube with the scraping process. The advantage of metal scrapers is that they are effective at removing a variety of common foulants, and careful evaluation over many years has determined that there is virtually no risk of base metal damage when well-designed cleaners are used properly.
Other common mechanical foulant removal techniques are metal wire brushes and high-pressure water. Much like mechanical cleaners, wire brushes are propelled down the length of the condenser tube with pressurized water. Brushes are particularly useful with tubes that have inlet-end metal inserts or inlet epoxy coating, because these can reduce the internal diameter of the tube. High-pressure water cleaning of condenser tubes, commonly called hydroblasting, is a useful strategy with particularly soft foulants like particulate deposits and microbiological films. Using hydroblasting for harder and more adherent fouling conditions will result in an incomplete cleaning. With use of high-pressure water, caution and care must be taken: if the sharp stream of water is allowed to pause for too long, it can quickly cut through softer condenser tubing like copper alloys. What’s more, hydroblasting is not recommended for use with condenser tubes that contain inlet-end metal inserts or epoxy coatings, as the blast of water can severely deform the inserts. Utilizing high-pressure water consumes eight times more water than mechanical cleaning.
LEAK DETECTION TESTING
Condenser Air Inleakage
Condenser air in-leakage negatively impacts plant performance and tube leaks can lead to costly forced outages. Condensers are designed with air removal systems to allow for a certain amount of air in-leakage that will support peak operating efficiency. Sometimes, though, air leaks exceed the capability of the air removal system and the condenser’s efficiency is compromised. One of the indications of air in-leakage is climbing condenser backpressure. There are a number of root causes for excessive air in-leakage. The problem can be related to the shell, rupture disks, shaft seals, man ways, vacuum pumps, flanges and one or more of the numerous holes made by bolts throughout the equipment. Tube fouling can contribute to the rise in backpressure, it’s true, but an air in-leakage inspection can be done while the unit is online and at minimal expense, and is a prudent first response in diagnosing a problem in the condenser. Today, reputable service providers are now using helium and SF6 tracer gas technology to effectively detect condenser air in-leakage, condenser tube leaks, sources of dissolved oxygen, stator water system leakage and main generator leakage. Once leaks are detected and repaired an immediate improvement in condenser performance is achieved.
Condenser Water Leaks
Circulating water leaks in main condensers can result from penetrations through the tube walls, from joints between the tubes and tubesheet, or from penetrations between the water box and condenser shell. Contaminants in the circulating water can change condensate chemistry, which can cause boiler or steam generator corrosion, and caustic water chemistry can cause stress corrosion fractures of turbine components. As with air in-leakage, water leaks are successfully detected with helium and ultra-sensitive SF6 tracer gas. Leaking tubes are then plugged, or repairs are made, depending on the type of leak discovered.
Eddy Current Testing
The best medicine for your condenser is preventative, and the best way to monitor a condenser unit’s tube integrity, detect patterns of tube wear and damage, and determine the specific wear and damage to a particular tube is with Eddy Current non-destructive testing.
Depending on the tube material, the best non-destructive testing method to employ would be either eddy current testing or other electromagnetic techniques including Remote Field Testing (RFT), eddy current testing being the most common and effective for non-ferromagnetic tubing.
Electromagnetic testing techniques have been proven effective for many years and continue to provide viable inspection data for heat exchanger tubing condition assessment.
Armed with the detailed information on a unit’s tube integrity provided by Eddy Current Testing, power engineers can take proactive steps to either repair, replace or plug damages tubes before they fail, preventing a forced outage.
Eddy Current Testing uses an electromagnetic field to identify defects in the tubing. An electron flow (eddy current) is induced in electrically conductive material and an electromagnetic field is generated with use of a probe inside the condenser tube. Once a baseline or standard is established, variations in the eddy currents are recorded and compared to those produced by the standard.
Any defect or anomaly in the tubing that disrupts the flow of the eddy currents can be detected and graphically output to a tubesheet map. Depending on the number of frequencies and channels used, defects with unique characteristics can be discovered.
Small defects such as pitting and cracking can be detected in the differential mode, and wall thinning defects such as steam erosion or inlet-end erosion are detected in the absolute mode. Additionally, higher frequencies are more sensitive to near surface flaws, and lower frequencies are more sensitive to subsurface flaws and conditions on the outer diameter (OD) tube surface.
The complex and varied nature of anomalies and defects necessitate the use of multiple frequencies for accurate identification. Eddy Current Testing is one more important tool in the condenser maintenance toolbox.
Together with condenser tube cleaning and leak detection testing, Eddy Current Testing enables power engineers to maintain optimal condenser performance, which improves energy output and decreases downtime. Condenser availability and reliability are improved.
The performance of your condenser impacts the performance of the turbine and the performance of the feedwater systems.
Put another way, condenser performance impacts the entire power plant. It is wise for plants to invest in combustion controls and turbine upgrades, but earmark a little in the maintenance budget to take care of this important component.
Proper condenser maintenance will improve condenser performance, extend the life of the condenser, and return significant improvements to the plant heat rate.
Reputable condenser maintenance firms have refined diagnostic and cleaning technologies to make them fast, safe and effective. In other words, there’s never been a better time to take good care of your condenser.
Beth Foley-Saxon is a staff writer in the Marketing Department at Conco Services Corporation.