the NC gases and the partial pressure Pv of the water vapor. Then, (1) Pt = Pnc + Pv Also, M v M nc

Transcription

1 Steam Jet Air Ejector Performance Evaluation By Komandur S. Sunder Raj Steam jet air ejectors (SJAEs) are utilized to create and maintain condenser vacuum by removing free air and other non-condensible (NC) gases saturated with water vapor from the condenser. Following initial evacuation using hogging ejectors, holding ejectors are used to maintain condenser vacuum. The primary source of NC gases in power plants is free air (air inleakage). Besides these NC gases, the SJAEs have to remove large quantities of water vapor. The total pressure in the condenser is made up of the partial pressure of the steam (water vapor) and partial pressure of the NC gases. In a leaktight condenser, while the total quantity (and partial pressure) of NC gases to be removed is relatively constant under steady-state base-load conditions, the amount (and partial pressure) of water vapor to be removed will increase with the condenser pressure. For optimum performance, it is essential that the air inleakage and partial pressure of the NC gases be kept as low as possible. In order to remove the NC gases from the condenser, provisions are made in the condenser design to push the NC gases through action of the steam to a central location designated as the air-cooler section. From here, the NC gases saturated with water vapor are transported via air off-take pipes to the SJAEs. During its passage to the SJAEs, the mixture of NC gases and water vapor is cooled to facilitate condensing some of the water vapor. Properties of Saturated NC Gases-Water Vapor Mixture In the condenser, we are dealing with a mixture of the NC gases and water vapor, each exerting its own pressure in accordance with Dalton s Law. The total condenser pressure Pt is the sum of the partial pressure Pnc of the NC gases and the partial pressure Pv of the water vapor. Then, (1) Pt = Pnc + Pv Also, (2) Since air is the primary source of NC gases, equations (1) and (2) may be rewritten as: (3) Pt = Pa + Pv (4) Substituting Mv = 18 for water vapor and Ma = 29 for air in equation (4), we have: (5) P nc P v P a P v P a P v W nc W v = or, (6) (P a /P v ) = (W a /W v ) Since the air fraction AF = W a /(W a + W v ), equation (6) may be rewritten as: (7) (P a /P v ) = (AF/1-AF) NOMENCLATURE W a = W v Equation 7 indicates that for air fractions below approximately 62 percent, the partial pressure of air will be less than the partial pressure of water vapor. When the air fraction increases above 62 percent, the partial pressure of air will be greater than the partial pressure of water vapor. Table 1 shows the data tabulated for an air fraction of 30 percent. It can be noted that at a total condenser pressure of 1.0 in.hga, the partial pressure of air is about 0.2 in.hga and the vapor pressure is 0.8 in.hga. The vapor saturation temperature is about 72 F, indicating a cooling of the mixture (Tt-Tv) by about 7.1 F below the total temperature of 79 F. As the total condenser pressure increases, a constant air fraction can be maintained only if the amount of cooling increases. Figure 1 shows the amount of cooling plotted for different condenser pressures and air fractions. At a total condenser pressure of 1.0 in.hga, the cooling has to increase from about 4.4 F for an air fraction of 20 percent to about 19.2 F for an air fraction of 60 percent. As we shall see later, the Heat AF Air fraction = W a /(W a + W v ) DO Dissolved oxygen, parts per billion (ppb) HO 2 Henry s Constant M a Molecular weight of air = 29 M nc Molecular weight of NC gases M v Molecular weight of water vapor = 18 P a Partial pressure of air, in.hga P nc Partial pressure of NC gases, in.hga P v Partial pressure of water vapor, in.hga P t Total pressure, in.hga scfm Volume at 14.7 psia & 70 F, cfm T t Total temperature, F T v Vapor saturation temperature, F VF Vapor fraction W a Weight of air, lb W nc Weight of NC gases, lb Weight of water vapor, lb W v M v M nc M v M a W a 18 W v 29 = 10 ENERGY-TECH.com ASME Power & Nuclear Divisions Special Section FEBRUARY 2004

2 Figure 1 ASME Exchange Institute (HEI) Standards for Surface Condensers 1 recommend that the venting equipment be sized based on a design suction pressure of 1.0 in.hga and suction temperature of 71.5 F, assuming cooling of the mixture of NC gases and water vapor by 7.5 F. From Figure 1, with air as the primary source of NC gases, this would yield an air fraction of about 31.4 percent at the suction of the venting equipment. Relationship Between Partial Pressures, Air Fraction, Condensate Dissolved Oxygen & Subcooling The solubility of oxygen in water is a function of the partial pressure of the air and the temperature of the liquid. Henry s Law states that the amount of dissolved oxygen in a liquid solution is directly proportional to the partial pressure of the gas above the liquid and inversely proportional to the temperature of the liquid. For water, the relationship 2 may be expressed as: (8) DO = (25.07E P a ) / Ho 2 Henry s Constant at various temperatures is shown plotted in Figure 2 3. For a given partial pressure of air, as the condensate temperature increases, Henry s Constant increases and the DO level decreases. Condenser performance guarantees for power plants are normally based on condensate dissolved oxygen levels of 7 ppb. At a total condenser pressure of 1.0 in.hga, the partial pressures of air and water vapor, the air fraction, and the amount of condensate subcooling may be computed as shown in Table 2. It can be noted that the condensate DO level of 7 ppb corresponds to an air fraction of 4 percent at the condenser air-cooler section and the amount of condensate subcooling is about 0.8 F. At 2.0 in.hga, for the same DO level of 7 ppb, the air fraction decreases to about 2.4 percent and the subcooling also decreases to 0.5 F. At 3.0 in.hga, the air fraction decreases further to 1.7 percent and the subcooling to 0.4 F. Finally, at 4.0 in.hga, the air fraction is 1.4 percent and the subcooling 0.3 F. If there were no air present, the partial pressure of air would be zero and there would be no subcooling of the condensate. Figure 3 shows the partial pressures of air and vapor plotted for different air fractions and DO levels. Figure 4 shows the condensate subcooling for different condenser pressures and DO levels. Per the HEI Standards for Surface Condensers 1,at a total condenser pressure of 1.0 in.hga, in order to maintain 7 ppb of DO in the condensate, the actual amount of NC gases removed should not exceed 6 scfm, regardless of the installed capacity of the venting equipment. The value should not exceed 10 scfm for a DO level of 14 ppb and, 20 scfm for a DO level of 42 ppb. Table 1: Total Pressure Pt, in. HgA The HEI values for DO levels assume zero air inleakage directly into the condensate below the condensate level in the hotwell. Furthermore, the DO levels reflect equilibrium conditions between the air inleakage into the condenser and the air removed by the venting equipment. It is clear from the foregoing that if the air removal rate is less than the air inleakage rate, then air that is not removed will accumulate inside the condenser. This will lead to an increase in the partial pressure of air and drive more oxygen into solution. Consequently, condensate DO level is an extremely important performance indicator of the air-removal capability of the venting equipment. Sizing Venting Equipment The HEI Standards for Surface Condensers 1 recommend that the venting equipment be sized to handle the mixture of NC gases and water vapor based on a design suction tem T t AF 30% 30% 30% 30% 30% 30% 30% 30% 30% 30% VF 70% 70% 70% 70% 70% 70% 70% 70% 70% 70% AF/VF 43% 43% 43% 43% 43% 43% 43% 43% 43% 43% M a M v P a P v T v T t T v FEBRUARY 2004 ASME Power & Nuclear Divisions Special Section ENERGY-TECH.com 11

3 Figure 2 ASME perature of 71.5 F and design suction pressure of 1.0 in.hga. At 1.0 in.hga, the steam saturation temperature is 79.0 F resulting in cooling of the mixture by 7.5 F in the condenser air-cooler section. As noted from Figure 1, if air is the only source of NC gases, then the design air fraction at the suction of the venting equipment is 31.4 percent at suction conditions of 1.0 in.hga and 71.5 F. This means that under these conditions the venting equipment has to handle approximately 2.2 lbs. of water vapor for every 1 lb. of air. Section 6 in the HEI Standards for Surface Condensers 1 provides guidance regarding sizing of venting equipment. The sizing takes into account the total number of condenser shells, the total number of exhaust openings for the main turbine including any auxiliary turbines exhausting to the main condenser and, the exhaust steam flow from each main turbine opening. Various tables in Section 6 of the HEI Standards indicate the recommended total capacity of the venting equipment in scfm of dry air as well as the equivalent in lb/hr and water vapor in lb/hr at suction conditions of 1.0 in.hga and 71.5 F necessary to saturate the air. For example, consider 100 lbs/hr of free air, which corresponds to 22 scfm of air at 14.7 psia and 70.0 F. The amount of water vapor necessary to saturate the air is 220 lbs/hr (2.2 lbs of water vapor for 1 lb of air) at 1.0 in.hga and 71.5 F. The total mixture of air and water vapor is 320 lb/hr. The water vapor requirements will increase if other NC gases are present in addition to free air. For example, if we have 200 lbs/hr of oxygen and 25 lbs/hr of hydrogen in addition to 100 lbs/hr of free air, the water vapor requirements will be 1409 lbs/hr or, 4.33 lbs of water vapor for 1 lb of the mixture of air and NC gases. Several plants are designed to operate at condenser pressures in excess of 1.0 in.hga. Depending upon the type of cooling (oncethrough, cooling towers, etc.), design condenser pressures may vary from 1.5 in.hga to 4.0 in.hga. Higher condenser pressures translate to higher water vapor pressures. From Figure 1, as the condenser pressure increases, a constant air fraction at the suction of the venting equipment can be maintained only if the amount of cooling of the mixture of air and water vapor increases. If the cooling capability remains unchanged at 7.5 F, the air fraction has to decrease as the condenser pressure increases. As can be seen from Figure 1, at 1.5 in.hga, the air fraction decreases to percent resulting in about 2.32 lbs. of water vapor to be removed with every 1 lb. of air; and, at 4.0 in.hga, the air fraction decreases to percent with 2.70 lbs. of water vapor to be removed with every 1 lb. of air. The HEI Standards for Surface Condensers 1, however, state that the actual amount of cooling that can be realized in practice may or may not be equal to the design value of 7.5 F and will depend upon the operating characteristics, the amount of NC gases, and the capacity characteristics of the venting equipment. Venting equipment installed in most plants typically have 100 percent spare capacity. As an example, if four Table 2 Total Cond. Press. Total Cond. Henry s Constant Ho 2 Partial Press. Partial Press. Vapor Sat. Air Fraction Subcooling P t, in. HgA Temp. T t, F Atmosphere/Mole Fraction P a, in. HgA P v, in. HgA Temp. T v, F AF, % T t T v, F % % % % 0.31 Notes: 1. Pa = (Ho2 DO)/(25.07E ) 3. Pv = Pt Pa 5. AF = Pa/(Pa Pv) 2. Tt corresponds to Pt 4. Tv corresponds to Pv 12 ENERGY-TECH.com ASME Power & Nuclear Divisions Special Section FEBRUARY 2004

4 Besides operating the SJAE units per the original design philosophy, the steam jets should be operated at steam pressures no less than the design steam pressures and no greater than 10 to 15 psi above design pressures, for operational margin. SJAE units are installed to service two main condenser shells, two of the SJAE units would be spares. Under normal operating conditions, with air inleakage at normal levels, only two SJAE units would need to be in service. If air inleakage levels increase significantly, one or both of the spares may need to be placed into service. However, even with low to normal air inleakage levels, it is not uncommon to witness plants operating all their SJAE units at all times or operating them in a configuration other than the original recommended design. While there may be several factors accounting for this departure from design, the end results are wasted steam due to all steam jets being in service, little or no improvement in condenser vacuum, inability to perform maintenance online, increased water vapor loadings, lack of spares, and excessive wear and tear on the SJAE system. Since the performance of the SJAE system in many cases is not routinely monitored, problems may go unnoticed for a long time before they surface. Condenser/SJAE System Interface The SJAE system relies upon the condenser to provide the proper ratio of NC gases and water vapor in order for the SJAE system to function normally. At base-load conditions, in a leak-tight condenser with the air-removal equipment operating satisfactorily, the condenser pressure is controlled by the circulating water inlet temperature. The amount of air inleakage and the amount of air removed, the condensate DO level, condensate subcooling, and the condenser pressure would all be in a state of equilibrium at expected levels. Condenser pressure, however, will increase if there is macrofouling or microfouling, if the circulating water flow rates decrease with circulating water pumps removed from service, condenser water boxes are cycled, etc. As discussed earlier, increases in For more information, enter on infolink at energy-tech.com or see the AD INDEX page 34 FEBRUARY 2004 ASME Power & Nuclear Divisions Special Section ENERGY-TECH.com 13

5 While the performance of both the intercondenser and aftercondenser is affected by cooling water flow and temperature, tube fouling and tube leaks, a major performance concern is possible flooding and the resulting back pressure from excessive steam and water vapor loadings, especially at higher condenser pressures. condenser pressure mean higher vapor pressures and increased water vapor loadings for the SJAE system. The increased water vapor loadings need to be carefully evaluated for impact upon the SJAE system and could become significant if the SJAE system is operated in a mode other than the original design configuration. Cleaning the condenser to control both macrofouling and microfouling helps to improve condenser vacuum and reduce water vapor loadings on the SJAE system. While cleaning the condenser tubes for microfouling, the tubes in the air-cooler section may need to be especially targeted where the heat transfer is the poorest due to air and NC gases blanketing the tubes. If, on its way to the suction of the air-removal equipment, the mixture of NC gases and water vapor is insufficiently cooled in the aircooler section, the water vapor loading will increase at the suction of the airremoval equipment. It is important to keep condenser pressures close to expected levels consistent with design to keep water vapor loadings on the SJAE system at reasonable levels. Performance Issues Condenser Air-Cooler SJAE systems for power plants typically utilize two stages of ejectors. For most plants, the suction of the first stage ejector is usually connected to the air off-take piping from the condenser. The mixture of NC gases and water vapor transported from the condenser air-cooler section via the air off-take piping is cooled during its passage to allow some of the water vapor to condense prior to the mixture entering the first stage. The air-cooler section is thus the only source of cooling of the mixture before it enters the first stage. If the cooling capability in the air cooler section is curtailed, the vapor fraction at the inlet of the first stage will increase and could compromise the performance of the SJAE system. Plants do not have provisions or seldom monitor the pressures and temperatures of the mixture of NC gases and water vapor as it leaves the condenser and enters the air-removal equipment. Clearly, installing instrumentation will not only facilitate obtaining data over a wide range of operating conditions, but will also assist Figure 3 Figure 4 14 ENERGY-TECH.com ASME Power & Nuclear Divisions Special Section FEBRUARY 2004

6 in performance monitoring and troubleshooting condenser/sjae problems. First & Second Stage Steam Jets Motive steam is used to remove the NC gases and uncondensed water vapor that enter the first stage from the condenser air-cooler section. For the second stage, motive steam is used to remove the NC gases and uncondensed water vapors that enter the second stage from the intercondensers. While the design pressure for the motive steam depends upon the specific application, pressures may range from 125 psig to 250 psig. The throat diameter of the steam nozzle of an ejector is sized for critical flow. From the Heat Exchange Institute Standards for Steam Jet Vacuum Systems 5, the critical flow of steam through the nozzle may be computed using the following: (9) W s = C D n 2 (P/v) where: Ws = Critical steam flow, lbs/hr C = Nozzle coefficient Dn = Nozzle throat diameter, inches P = Upstream steam pressure, psia v = Upstream specific volume, cft/lb From equation (9), it can be seen that for a given steam pressure the steam flow varies as the square of the throat diameter. For example, a 10 percent increase in the nozzle throat diameter will increase the steam flow by 21 percent. This could occur due to nozzle erosion, if a different size nozzle is used, etc. Also, for a given throat diameter, the steam flow through the nozzle varies as the square root of the motive steam pressure. For example, an increase in steam pressure by 25 psi, from 125 psig to 150 psig, would increase the steam flow by about 17.5 percent. This would be the case if the operating steam pressure were higher than the recommended design steam pressure. For various reasons, plants tend to operate the steam jets at steam pressures higher than design. It is not unusual to encounter operating pressures 25 to 50 psi higher than the recommended minimum design pressures. In addition, if all the SJAE units are operating with no spares, the steam flows and water vapor loadings are now well over double the flows that would otherwise be encountered when operating at design steam pressures per the design configuration. This combination of all SJAE units in service with steam pressures well in excess of design, results in excessive as well as wasted steam flows and increased water vapor loading, which can seriously jeopardize SJAE system performance, especially at higher condenser pressures. Besides operating the SJAE units per the original design philosophy, the steam jets should be operated at steam pressures no less than the design steam pressures and no greater than 10 to 15 psi above design pressures, for operational margin. This would help to reduce motive steam consumption and relieve the system of excessive water vapor loadings, especially at higher condenser pressures. Steam nozzles can erode resulting in excessive steam consumption and increased water vapor loadings. The throat sizes should be checked against vendor design and the nozzles replaced if there is evidence of erosion. Intercondensers & Aftercondensers The intercondenser is used to condense water vapor from the mixture of steam, NC gases, and water vapor entering the intercondenser from the first stage steam jets. The aftercondenser is used to condense essentially all the water vapor from the mixture of steam, NC gases, and water vapor entering the aftercondenser from the second stage steam jets. While the performance of both the intercondenser and aftercondenser is affected by cooling water flow and temperature, tube fouling and tube leaks, a major performance concern is possible flooding and the resulting back pressure from excessive steam and water vapor loadings, especially at higher condenser pressures. Maintaining operating steam pressures close to design, and operating only the recommended number of SJAE units should help alleviate excessive steam and water vapor loading. Performance Monitoring/Troubleshooting Since the performance of the condenser and the performance of the SJAE system are closely intertwined, it is important to establish a performance monitoring program that encompasses both. The ability to effectively monitor the performance and troubleshoot condenser/sjae systems will depend upon the availability of requisite instrumentation, manpower resources, and the frequency of the program. Recognizing these limitations, the following listings are provided merely as guides. For the condenser, the performance data/parameters to be monitored are: Condenser pressure Circulating water inlet and outlet temperatures Circulating water flow rate (determined through heat balance techniques or other means) Condensate DO level Condensate hotwell temperature Pressure and temperature at air off-take piping For the SJAE system, the performance data/parameters to be monitored are: Pressure and temperature at suction and discharge of first stage and second stage ejectors FEBRUARY 2004 ASME Power & Nuclear Divisions Special Section ENERGY-TECH.com 15

7 The ability to effectively monitor the performance and troubleshoot condenser/sjae systems will depend upon the availability of requisite instrumentation, manpower resources, and the frequency of the program. Motive steam pressure for first and second stage ejectors Intercondenser/aftercondenser cooling water flows and temperatures, drain flows and temperatures Air removal rates As an aid to conducting tests on SJAE systems, the HEI Standards for Steam Jet Vacuum Systems 5 may be consulted. The methods provided in the standards reflect many years of accumulated experience within the industry and are considered to be reliable and accurate. The Standards provide guidelines on location of test instrumentation for different test arrangements. Recommendations Condenser performance monitoring programs do not always take into account the interface with the SJAE system. By incorporating the interface into the condenser performance-monitoring program, it is possible to ensure proper and optimum performance. Beside performance gains that could be significant, other benefits include better maintenance, less wear and tear, increased availability, etc. Here are some recommendations: Incorporate the SJAE system into the condenser performance monitoring program. As a minimum, monitor on a routine basis the condenser pressure, circulating water inlet and outlet temperatures, condensate DO level, hotwell condensate temperature (and subcooling), and air removal rate. If conditions are normal, these parameters should be at expected levels and will correlate with each other. Consider operating the SJAE system in accordance with the original design philosophy (motive steam pressures, number of SJAE units in service, lineups, etc.). If the system does not perform properly when operated per design, the causes should be investigated and corrective action taken to permit operating per design. This area is ripe for significant gains in performance, maintainability, and availability. Operating more than the recommended number of SJAE units on a continuous basis can lead to long-term degradation in performance. As discussed earlier, this practice does not necessarily improve condenser performance and has several negative ramifications for the SJAE system. Consider installing instrumentation in the condenser/sjae system for performance monitoring and troubleshooting problems requiring detailed evaluation. If the SJAE system arrangement offers the flexibility, consider swapping the air ejectors to optimize the performance. Check the condition of the components in the SJAE system. These include the ejectors, diffusers, strainers, precoolers, intercondensers, and aftercondensers. The nozzle sizes should be checked for conformance with design. Maintain a set of spare nozzles and diffusers in case the originals need to be replaced. References & Footnotes 1. Heat Exchange Institute Standards for Steam Surface Condensers, 9th Edition. 2. EPRI-2294 Guide to the Design of Secondary Systems and Their Components to Minimize Oxygen- Induced Corrosion. Bechtel Group (S.W. S. Shor et al) March Chemical Engineer s Handbook, Table 14-27, page Spencer, E., and Impagliazzo, A. M., 1984, "Enhanced Condenser Venting for Condensate Oxygen Control," ASME Paper 84-JPGC-Pwr Heat Exchange Institute Standards for Steam Jet Vacuum Systems. This article was edited down from a paper (#40003) that was originally published in the Proceedings of the 2003 International Joint Power Generation Conference, June 16-19, 2003, Atlanta. These proceedings are available from ASME in both digital and print formats. For more details contact PDFs of individual papers can also be purchased at the ASME Digital Store FOR MORE INFORMATION, ENTER ON INFOLINK AT ENERGY-TECH.COM OR EDITORIAL@MAGELLANPUBS.COM Komandur S. Sunder Raj is the founder and owner of Power & Energy Systems Services providing extensive training, software development/applications, and consulting/troubleshooting services to the power industry. He has 35 years of experience in the power industry and has specialized in power plant design, performance, economic studies, analyses, and project engineering/ management activities. He has held responsible positions with major engineering companies such as Stone & Webster Engineering Corporation, Burns & Roe, Inc. and Raytheon Engineers & Constructors, Inc. He served as Director of Project Engineering at the New York Power Authority and was responsible for project engineering/management activities on the Indian Point 3 Nuclear Power Plant. He is the author/developer of the PERFORM software which was developed for power plant design, performance analysis, troubleshooting, and training. 16 ENERGY-TECH.com ASME Power & Nuclear Divisions Special Section FEBRUARY 2004