April 1, 2011
Condenser Performance Improvement Through Innovative Detection Technologies By Eric H. Fayard Condensers are one part of the power plant where the technology is well defined and the benefits of keeping a clean condenser maintenance is a "pay me now or pay me later" proposition. The theory of condenser operation is reasonably straightforward. Condenser design specifications define a maximum effective r exhaust vapor entering the condenser, as well as its heat transfer into the circulating water, determined by the condenser backp temperature. Variations in the latter two parameters will change the backpressure and also affect t he heat rate for a given load. I subcooling, caused by variations in inlet water temperature, some control over backpressure (and heat rate) may be achieved b However, the reduced tube velocities can cause silt to become deposited on the tube surfaces and, thus, negatively affect heat r To avoid these effects, it may be possible to allow some of the circulating water returned from the condenser to bypass the cooli temperature to maintain the backpressure, but without reducing the total water flow rate or tube water velocity. These are the na for a clean condenser. Unfortunately, condensers seldom operate under clean conditions for very long, and the ills to which they are prone during norm
Fouling of the tube surfaces Tube or tube sheet fouling due to shellfish or debris Circulating water in-leakage Excess ambient air in-leakage Deficient air/noncondensable removal capability
The first two categories are related to fouling and tend to be cyclical in nature. They may, therefore, be regarded as predictable, vary from plant to plant, and even between units at the same plant. The last three categories of problems, concerning water or ai Both categories of leaks are likely to occur at some point, and a correction strategy can be prepared. When a problem will occur
Tube Surface Fouling It's only a matter of time until almost every condenser experiences some kind of tube or tubesheet fouling. Most condenser circul solids that can precipitate and become deposited on the inner surfaces of the tubes, adversely affecting the unit heat rate and/or deposits can also contribute to various types of corrosion and, if not removed periodically, the corrosion may eventually penetrat to leak into and contaminate the condensate. Fouling can affect not only unit heat rate but also the ability of the turbine to generate its design load capacity. In fossil-fired plan higher fuel costs for a given load, and increases of 2% are not uncommon. In both fossil and nuclear plants, if the fouling becom to rise to its upper limit, forcing a reduction in generated power. There are reports of up to 20 MW having been recovered by the deposits.
Tube Fouling Characteristics To effectively combat tube fouling, one must better understand the characteristics of tube fouling and how it applies to site-speci which is mostly a tube interior concern, generally falls into one of the following five categories. Microbiological Fouling. Microbiological fouling routinely occurs in natural waters, as many bacterial species will naturally colo temperature at the interior wall of condenser tubes is ideal for growth of some bacteria. The resulting mass is frequently low in o and the majority of constituents are inorganic particulates from the cooling water that have become incorporated into the microbi microbiological fouling can be particularly detrimental to heat transfer, as much of the slime mass consists of water, which condu Scale Fouling. Scale (mineral crystallization) occurs on heat transfer surfaces under a combination of dissolved mineral concen of certain common constituents in natural waters (such as calcium carbonate and calcium phosphate) is promoted by elevated t
wall of condenser tubes, particularly toward the outlet end. Other scale-forming minerals (including calcium sulfate) are more like can drastically reduce heat transfer, depending on the specific mineral formed and its thickness. Although scaling water chemist corrosion, it is very possible that crevice corrosion will occur beneath scale. Particle Deposition Fouling. Particle deposition generally occurs in condenser tubes when the flow rate is inadequate to keep through condenser tubes is often 7 to 10 ft/s, but this is an average (bulk flow), and some tubes may experience much lower flo the waterbox is not full and upper tubes receive intermittent flow. Commonly, areas of low flow result from partial blockage on th lodged within a tube. It is unlikely that particle deposition will cause a significant loss of heat transfer for the condenser, but it ma corrosion. Common types of fouling particulates in condensers include sediment/silt, diatoms, coal dust, and minerals precipitate sulfate, calcium phosphate, silicates, etc.). Corrosion Products Fouling. Corrosion products can grow relatively thick on the surfaces of certain tubes, primarily copper all copper oxide growth, and in some cases a thin surface scale will enhance the growth of a thick underlying copper oxide layer, w establish sites for crevice corrosion. Tubesheet Pluggage. Macro fouling can include blockage at the tubesheet and w ithin the tubes by various materials and debris cooling tower materials (plastic fill/wood), chunks of ash or coal, pieces of rust, paper trash, leaves and other vegetation, and aq The primary effect is reduced flow to certain tubes, which results in particulate deposition and increased opportunity for microbiol high velocities and tube wall erosion can occur as a result. If major tube sheet blockage occurs, the condenser vacuum can be s
Condenser Cleaning Methods Regardless of the tube material, the most effective way to ensure that tubes achieve their full life expectancy and heat transfer e each time tube deposits, sedimentation, biofouling, and obstructions are effectively removed, the tube surfaces are returned to b heat transfer. The cleaned tube is given a new life as a result of the rebuilding of the protective oxide coating. Data obtained from an independent survey of 100 plants (two per state) found that the majority of c leaning procedures are perfo being mechanical cleaning. Mechanical cleaning incorporates the use of metal scrapers or brushes being shot through the tubes two. Among other off-line methods is the use of v ery high pressure water. Because the jet can only be moved along the tube slowly, t be become extended. Great care must be taken to avoid damaging any tubesheet or tube coatings that may be present; otherwi deposits maybe become associated with new tube leaks or increased tube sheet corrosion that are only observable after the uni Chemicals are also used for off-line cleaning of condenser tubes. Several mildly acidic products are available and will remove m this option is expensive, takes longer, and the subsequent disposal of the chemicals-an environmental hazard-creates its own s quite frequently that some residual material still needs to be removed by mechanical cleaning methods. Very few online methods are available to clean condenser tubes, but the best known is the online ball-cleaning system, which us the cleaning vehicle. These systems often operate for only a part of each day and, rather than maintaining absolutely clean tube of tube fouling. Unfortunately, although the tubes may become cleaner if abrasive balls are used, tube wear can become a probl There have been, on occasion, problems with the distribution of online sponge balls and their ability to clean all of the condense that numerous sponge balls have become stuck in condenser tubes, and these appear among the material removed during off-li these reasons, the tubes of condensers equipped with online systems still have to be cleaned off-line periodically, especially if lo concern.
Tube Cleaning Innovations Off-line mechanical cleaning is especially useful where fouling problems exist or are too severe to be handled by any of the othe has to be the most appropriate for removing a particular type of deposit. Molded plastic cleaners (pigs) are quite popular for som be used to remove these soft deposits as well as some microbiological deposition. Brushes are also useful for cleaning tubes wi those that are spirally indented or finned) or tubes with thin wall metal inserts or epoxy-type coatings. Spring-loaded metal clean types of deposits, particularly from tubes that are fouled with hardened deposits such as mineral scale. All of the aforementioned tubes by means of a water pump at approximately 300 psig. An advantage of using 300 psig water pressures for tube cleaner propulsion is that the fouling material can be safely collected in
weighing, to establish the deposit density. That is followed in many cases by X-ray fluorescent analysis of the deposit cake. This mechanical cleaning. Water pressure of 300 psig is very effective for propelling cleaning tools through the tubes while preventing their exit velocity fro cleaning systems use air or a mixture of air and water to propel the cleaner, but air pressure is compressible and dangerous to u
Case Study No. 1: South Texas Project As a result of an innovative research program organized to resolve problems encountered in the field and to develop new produ to be inadequate, new tube cleaners were developed. One notable example involves South Texas Project (STP) nuclear power buildup of calcium carbonate scale in more than 96,000 of its condensers' titanium tubes. Conventional cleaning methods prove from the inner diameter of the tubes. Upon further examination, tests revealed that scale thickness ranged from 7.6 mils to 31.2 Approximately 50% of the top 20 rows in waterbox four were completely blocked with scale. An innovative scale removal tube cleaner was utilized to restore the tubes to operational cleanliness. The tube cleaner known as Teflon body on which are mounted a number of carbide rotary cutting wheels. These are placed at different angles around the b Cal-Buster fracture the crystalline structure of the calcium carbonate, effectively breaking the bond to the tube wall as the cleane pass of a standard C4S cleaner forces the loosened calcium out in long strips.
1. The Cal-Buster removes scale on the inside of condenser tubes. Courtesy: Conco Systems Inc.
Working in teams of eight operators per 12-hour shift, around the clock, it took approximately five and a half days to clean each portable water pump systems that shot the tube cleaners with 200 to 300 psi water. Three pumps were used on the inlet side an back-and-forth shooting was required to free the occasionally stuck cleaner. In all, seven and a half cubic yards of calcium carbonate, weighing more than 5,000 pounds, were removed from the 96,234 con thereafter, the same number of condenser tubes in Unit 2 were cleaned using the same method, and more t han four cubic yards more than 3,000 pounds, was removed. STP personnel believe the tubes in Unit 2 were less fouled because it had been cleane cleaners in 2001, whereas the tubes in Unit 1 had been cleaned with nylon brushes that left ridges inside the tubes, promoting f As a result of the condenser tube cleaning and removal of the calcium carbonate scale, STP plans to establish a preventive mai pass of standard metal-bladed tube cleaners to prevent the severe fouling that was observed and slow the buildup of scale.
Case Study No. 2: Ratcliffe-on-Soar Another innovation was the development of the stainless steel tube-cleaning brush. The brush is made from stainless steel and cleaner (Figure 2). The stainless brush was successfully utilized at the 2,034-MW Ratcliffe-on-Soar power station in the United fouling that consisted mostly of stones that had entered the cooling water intake due to a breach in a retaining wall. The stones turn rendered the online ball-cleaning system ineffective. In addition to macro fouling, the condenser tubes had been fitted with t and outlet ends and were in varying degrees of disrepair.
2. Another tube cleaning option is the stainless steel brush. Courtesy: Conco Systems Inc.
The stainless brush performed exceptionally well due to the inherent flexibility of its design. The cleaning contractor utilized clea than-normal trailing end caps that allowed the cleaner to pass through the inserts and open fully to provide a full 360 degree cle Following the four-day cleaning at Ratcliffe, performance engineers realized a 3- to 4-mbar improvement in backpressure at f ull l target vacuum for the first time in many years. A 3-mbar improvement equates to approximately $200,000 a year savings in effici utilization), which is extraordinary, as the unit cleaned hadn't been considered a bad performer.
Determining Circulating Water In-leakage In addition to innovations in tube cleaning, an awareness of best available technology for the resolution of circulating water and Circulating water in-leakage can result from penetrations through the tube walls, from joints between the t ubes and tube sheet th penetrations between the waterbox and condenser shell that have lost their integrity. The contaminants in the circulating water c tending to increase boiler or steam generator corrosion. They may also result in increased consumption of water treatment chem the change in water chemistry. Poor water chemistry can also cause stress corrosion cracking of steam turbine components. Even a small circulating water in-leakage into the condensate can be damaging to the unit as a whole and is often the cause of that outage will depend on the means adopted to locate the source of the leak quickly; the online and off-line use of tracer gas ( The Electric Power Research Institute (EPRI) Condenser In-Leakage Guideline (Condenser In-Leakage Guideline, EPRI Techni these problems in detail and shows how tracer gas can be used to rapidly locate the source of either water or air in-leakage, allo quickly.
Methods of In-leakage Detection The condenser is designed to form a barrier between the cooling water, which flows between the waterboxes through the conde condenser, in which the exhaust vapor is collected as condensate. However, even small circulating water leaks will quickly find t contaminating it with undesirable dissolved solids, which tend to cause corrosion in the feedwater heaters, boilers, or steam gen instruments are used to indicate the presence of a leak, and steps should be taken to rectify the problem as soon as possible. U the unit out of service, which translates into a loss of revenue, which increases with the length of the outage. Thus, the time take be economically significant. This time can be reduced dramatically if the waterbox associated with the leak can be identified whil Among the leak detection methods commonly employed in the past were smoke generators, foam or plastic wrap applied to the membrane-type rubber stoppers. These earlier techniques could not confirm whether the leak was confined to only one tube, so well (often unnecessarily) as a form of "insurance plugging." All these methods require that the shell side of the condenser be un removal system or, if the waterbox is divided, by running the unit at low load, taking each waterbox out of service in turn and che Original investigations incorporated the use of helium as a tracer gas. This not only reduced the time required to locate a leak bu uncertainty about whether the actual source of the leak had been found. However, the lowest detectible concentration of helium i background level, and helium was often unable to detect small water in-leaks. Thus, a tracer gas with greater sensitivity was sou developed. It was found that SF6 in concentrations as low as one part per 10 billion (0.1 ppb) can be detected, so small leaks no The tracer gas method is illustrated in Figure 3, in which a tracer gas monitor, the Fluorotracer Analyzer (Figure 4), is connected removal system. A technician is stationed at the monitor to observe the shape of the trace on the strip chart recorder. A typical r
(Figure 5). Another technician is stationed in the waterbox and dispenses the tracer. The two technicians communicate through t to avoid RF interference with other equipment.
3. How to use a tracer gas to inspect a condenser for water in-leakage. Source: Conco Systems Inc.
4. A Fluorotracer Analyzer can be used to determine the concentration of the tracer gas. Courtesy: Conco Systems Inc.
5. The response time for sensing a tracer gas in the cooling water is usually 30 to 45 seconds. Source: Conco Systems Inc.
Once the waterbox is open and the tube sheet is exposed, a series of plenums is placed over a section of the tube sheet, each s tubes. The technician in the waterbox injects the tracer gas into the plenum using a portable dispenser (Figure 6). The vacuum gas to pass through any leaks that may be present and eventually appear in the off-gas stream leaving the air removal system. detector monitor warns the other technician when the presence of the gas is observed. A smaller plenum is t hen used, and so o
elimination, the problem tube can be rapidly identified.
6. A technician injects SF6 into the condenser tube bundle prior to a tube leak test. Courtesy: Conco Systems Inc.
Excess Ambient Air In-leakage The design of condensers routinely allows for a normally acceptable level of air in-leakage, often considered to be 1 scfm (2.13 standard (ASME Performance Test Code on Steam Surface Condensers, PTC.12.2-1988) shows the limit to vary with the numb exhaust flow rate. The sources of such leaks can be labyrinth glands on steam turbine shafts, as well as packing and seals that As with fouling, air in-leakage affects the concentration of dissolved oxygen in the hotwell, which can cause corrosion damage to dissolved oxygen levels can also be caused by a change in the performance of the air removal equipment, and this should be ch leaks. In many cases, the increased reliance on deaeration taking place within the condenser makes minimizing air in-leakage e
Air In-Leakage Detection Methods As mentioned, condensers are designed to perform correctly with the unavoidable and low level of air in-leakage that is always p than this low normal value will increase the concentration of noncondensables in the shell side of the condenser and c ause the t increase. An increase in backpressure and unit heat rate will result. Air in-leakage may even rise to the point where the backpre forcing a reduction in load. Another effect of high air in-leakage is often an increase in the concentration of dissolved oxygen in the condensate, a concentr condensate temperatures. The consequences are increased corrosion of feedheaters, boilers, and steam generators and/or an i treatment chemicals. Such consequences can create very complex issues involving the transport of corrosion products. Someti carbon steel high-pressure feedwater heaters and copper alloy feedwater heaters and excessive deposits in boiler waterwall tub as in the steam turbine path (published work in this area is extensive). All of these consequences have a negative impact on uni Using the tracer gas technique, the source of most air in-leaks can be located with the unit online. Once again, a tracer gas moni air removal system and the technician utilizing a handheld tracer gas dispenser roams around the unit in a methodical manner u observes a response. The leak detection survey starts at the turbine deck level and proceeds from top to bottom of the unit, one when dispensing the tracer gas so that only one potential source is sprayed at a time; otherwise, the ability to associate a respo impaired.
Enfield Power Station Air Ingress Survey An excellent example of how systematic leak detection principles can return significant performance gains was demonstrated at 400-MW combined-cycle plant with an air-cooled condenser located just north of London. Enfield had experienced problems rel 2000. An increase in steam turbine air ejector discharge rates since November 2005 as well as a gradual loss in vacuum resulte across the affected plant areas. Enfield performance engineers conducted vacuum drop tests and noted a leakage rate greater t increased leakage rate, feedwater dissolved oxygen levels were averaging 60 µg/kg. Enfield retained an experienced leak detection contractor, who systematically tested the vacuum boundary utilizing helium as th testing, the contractor located numerous leaks, including a sizeable 670-mm crack in an intermediate-pressure bypass pipe. Thi amount of lagging and, once exposed, was corrected that day. Upon sealing the crack, considerable improvement in vacuum w
7. Vacuum improvement occurred almost instantly once a crack was repaired. Source: Conco Systems Inc.
Following the leak survey, vacuum loss was reduced from 1.55 mbar/min to 0.43 mbar/min, a 72% improvement. Feedwater dis 5 µg/kg, an improvement of 91%. As a result of the efficiency improvements, Enfield estimates that the cost of the leak detection weeks, and the overall yearly savings would be significant. —Eric H. Fayard is technical marketing manager for Conco Systems Inc.
