steam jet air ejector asme performance...

ASMESteam jet air ejectors (SJAEs) are

utilized to create and maintain con-denser vacuum by removing free airand other non-condensible (NC) gasessaturated with water vapor from thecondenser. Following initial evacua-tion using hogging ejectors, holdingejectors are used to maintain condens-er vacuum.

The primary source of NC gasesin power plants is free air (air inleak-age). Besides these NC gases, theSJAEs have to remove large quantitiesof water vapor.

The total pressure in the condens-er is made up of the partial pressure ofthe steam (water vapor) and partialpressure of the NC gases. In a leak-tight condenser, while the total quanti-ty (and partial pressure) of NC gases tobe removed is relatively constant understeady-state base-load conditions, theamount (and partial pressure) of watervapor to be removed will increase withthe condenser pressure. For optimumperformance, it is essential that the airinleakage and partial pressure of theNC gases be kept as low as possible.

In order to remove the NC gasesfrom the condenser, provisions aremade in the condenser design to pushthe NC gases through action of thesteam to a central location designatedas the air-cooler section. From here,the NC gases saturated with watervapor are transported via air off-takepipes to the SJAEs. During its passageto the SJAEs, the mixture of NC gasesand water vapor is cooled to facilitatecondensing some of the water vapor.

Properties of Saturated NC Gases-Water Vapor Mixture

In the condenser, we are dealingwith a mixture of the NC gases andwater vapor, each exerting its ownpressure in accordance with Dalton’sLaw. The total condenser pressure Pt isthe sum of the partial pressure Pnc of

the NC gases and the partial pressurePv of the water vapor.

Then,(1) Pt = Pnc + Pv

Also,

(2)

Since air is the primary source of NCgases, equations (1) and (2) may berewritten as:(3) Pt = Pa + Pv

(4)

Substituting Mv = 18 for water vaporand Ma = 29 for air in equation (4), wehave:

(5)

or,(6) (Pa/ Pv) = 0.6207 � (Wa/ Wv)

Since the air fraction AF = Wa/(Wa +Wv), equation (6) may be rewritten as:

(7) (Pa/ Pv) = 0.6207 � (AF/1-AF)

Equation 7 indicates that for airfractions below approximately 62 per-cent, the partial pressure of air will beless than the partial pressure of watervapor. When the air fraction increasesabove 62 percent, the partial pressureof air will be greater than the partialpressure of water vapor.

Table 1 shows the data tabulatedfor an air fraction of 30 percent. It canbe noted that at a total condenser pres-sure of 1.0 in.HgA, the partial pressureof air is about 0.2 in.HgA and thevapor pressure is 0.8 in.HgA.

The vapor saturation temperatureis about 72°F, indicating a cooling ofthe mixture (Tt-Tv) by about 7.1°Fbelow the total temperature of 79°F. Asthe total condenser pressure increases,a constant air fraction can be main-tained only if the amount of coolingincreases.

Figure 1 shows the amount of cool-ing plotted for different condenser pressures and air fractions. At a totalcondenser pressure of 1.0 in.HgA, thecooling has to increase from about 4.4°F for an air fraction of 20 percent toabout 19.2 °F for an air fraction of 60percent. As we shall see later, the Heat

10 ■ ENERGY-TECH.com ASME Power & Nuclear Divisions Special Section | FEBRUARY 2004

Steam Jet Air EjectorPerformance EvaluationBy Komandur S. Sunder Raj

AF Air fraction = Wa/(Wa + Wv)DO Dissolved oxygen, parts per billion (ppb)HO2 Henry’s ConstantMa Molecular weight of air = 29Mnc Molecular weight of NC gasesMv Molecular weight of water vapor = 18Pa Partial pressure of air, in.HgAPnc Partial pressure of NC gases, in.HgAPv Partial pressure of water vapor, in.HgAPt Total pressure, in.HgAscfm Volume at 14.7 psia & 70 °F, cfmTt Total temperature, °FTv Vapor saturation temperature, °FVF Vapor fractionWa Weight of air, lbWnc Weight of NC gases, lbWv Weight of water vapor, lb

Pnc

Pv

Wnc

Wv

Mv

Mnc= �

Pa

Pv

Wa

Wv

Mv

Ma= �

Pa

Pv

Wa

Wv

1829

= �

NO

MEN

CLA

TUR

E

FEBRUARY 2004 | ASME Power & Nuclear Divisions Special Section ENERGY-TECH.com ■ 11

ASMEExchange Institute (HEI) Standards forSurface Condensers1 recommend thatthe venting equipment be sized based ona design suction pressure of 1.0 in.HgAand suction temperature of 71.5 °F,assuming cooling of the mixture of NCgases and water vapor by 7.5 °F. FromFigure 1, with air as the primary sourceof NC gases, this would yield an airfraction of about 31.4 percent at the suc-tion of the venting equipment.

Relationship Between PartialPressures, Air Fraction,Condensate Dissolved Oxygen& Subcooling

The solubility of oxygen in wateris a function of the partial pressure ofthe air and the temperature of the liq-uid. Henry’s Law states that theamount of dissolved oxygen in a liquidsolution is directly proportional to thepartial pressure of the gas above theliquid and inversely proportional to thetemperature of the liquid.For water, the relationship2 may beexpressed as:

(8) DO = (25.07E06 � 0.49115 �Pa) / Ho2

Henry’s Constant at various tem-peratures is shown plotted in Figure 23.For a given partial pressure of air, as thecondensate temperature increases,Henry’s Constant increases and the DOlevel decreases. Condenser performanceguarantees for power plants are normal-ly based on condensate dissolved oxy-gen levels of 7 ppb.

At a total condenser pressure of 1.0in.HgA, the partial pressures of air andwater vapor, the air fraction, and theamount of condensate subcooling maybe computed as shown in Table 2. It canbe noted that the condensate DO level of7 ppb corresponds to an air fraction of 4percent at the condenser air-cooler sec-tion and the amount of condensate sub-cooling is about 0.8 °F.

At 2.0 in.HgA, for the same DOlevel of 7 ppb, the air fraction decreasesto about 2.4 percent and the subcoolingalso decreases to 0.5°F. At 3.0 in.HgA,the air fraction decreases further to 1.7percent and the subcooling to 0.4 °F.Finally, at 4.0 in.HgA, the air fraction is1.4 percent and the subcooling 0.3 °F. Ifthere were no air present, the partial pres-sure of air would be zero and there wouldbe no subcooling of the condensate.

Figure 3 shows the partial pres-sures of air and vapor plotted for dif-ferent air fractions and DO levels.Figure 4 shows the condensate sub-cooling for different condenser pres-sures and DO levels.

Per the HEI Standards for SurfaceCondensers1, at a total condenser pres-sure of 1.0 in.HgA, in order to maintain7 ppb of DO in the condensate, the actu-al amount of NC gases removed shouldnot exceed 6 scfm, regardless of theinstalled capacity of the venting equip-ment. The value should not exceed 10scfm for a DO level of 14 ppb and, 20scfm for a DO level of 42 ppb.

The HEI values for DO levelsassume zero air inleakage directly intothe condensate below the condensatelevel in the hotwell. Furthermore, theDO levels reflect equilibrium condi-tions between the air inleakage into thecondenser and the air removed by theventing equipment.

It is clear from the foregoing that ifthe air removal rate is less than the airinleakage rate, then air that is notremoved will accumulate inside thecondenser. This will lead to an increasein the partial pressure of air and drivemore oxygen into solution.Consequently, condensate DO level isan extremely important performanceindicator of the air-removal capabilityof the venting equipment.

Sizing Venting EquipmentThe HEI Standards for Surface

Condensers1 recommend that theventing equipment be sized to handlethe mixture of NC gases and watervapor based on a design suction tem-

Table 1: Total Pressure Pt, in. HgA0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Tt 58.8 79.0 91.7 101.1 108.7 115.1 120.6 125.4 129.8 133.8AF 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%Ma 29 29 29 29 29 29 29 29 29 29Mv 18 18 18 18 18 18 18 18 18 18Pa 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1Pv 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 3.9Tv 52.3 72.0 84.3 93.4 100.7 106.9 112.2 116.9 121.1 125.0Tt–Tv 6.5 7.1 7.5 7.7 8.0 8.2 8.4 8.5 8.7 8.8

Figure 1

ASMEperature of 71.5 °F and design suc-tion pressure of 1.0 in.HgA. At 1.0in.HgA, the steam saturation temper-ature is 79.0 °F resulting in coolingof the mixture by 7.5 °F in the con-denser air-cooler section. As notedfrom Figure 1, if air is the only sourceof NC gases, then the design air frac-tion at the suction of the ventingequipment is 31.4 percent at suctionconditions of 1.0 in.HgA and 71.5 °F.This means that under these condi-tions the venting equipment has tohandle approximately 2.2 lbs. ofwater vapor for every 1 lb. of air.

Section 6 in the HEI Standards forSurface Condensers1 provides guid-ance regarding sizing of venting equip-ment. The sizing takes into account thetotal number of condenser shells, thetotal number of exhaust openings forthe main turbine including any auxil-iary turbines exhausting to the maincondenser and, the exhaust steam flowfrom each main turbine opening.Various tables in Section 6 of the HEIStandards indicate the recommendedtotal capacity of the venting equipmentin scfm of dry air as well as the equiv-alent in lb/hr and water vapor in lb/hrat suction conditions of 1.0 in.HgAand 71.5 °F necessary to saturate theair. For example, consider 100 lbs/hr offree air, which corresponds to 22 scfmof air at 14.7 psia and 70.0 °F. Theamount of water vapor necessary tosaturate the air is 220 lbs/hr (2.2 lbs of

water vapor for 1 lb of air) at 1.0in.HgA and 71.5 °F. The total mixtureof air and water vapor is 320 lb/hr. Thewater vapor requirements will increaseif other NC gases are present in addi-tion to free air. For example, if we have200 lbs/hr of oxygen and 25 lbs/hr ofhydrogen in addition to 100 lbs/hr offree air, the water vapor requirementswill be 1409 lbs/hr or, 4.33 lbs of watervapor for 1 lb of the mixture of air andNC gases.

Several plants are designed tooperate at condenser pressures inexcess of 1.0 in.HgA. Dependingupon the type of cooling (once-through, cooling towers, etc.), designcondenser pressures may vary from1.5 in.HgA to 4.0 in.HgA. Highercondenser pressures translate to high-er water vapor pressures. FromFigure 1, as the condenser pressureincreases, a constant air fraction atthe suction of the venting equipmentcan be maintained only if the amount

of cooling of the mixture of air andwater vapor increases.

If the cooling capability remainsunchanged at 7.5 °F, the air fractionhas to decrease as the condenser pres-sure increases. As can be seen fromFigure 1, at 1.5 in.HgA, the air fractiondecreases to 30.13 percent resulting inabout 2.32 lbs. of water vapor to beremoved with every 1 lb. of air; and, at4.0 in.HgA, the air fraction decreasesto 27.04 percent with 2.70 lbs. of watervapor to be removed with every 1 lb. ofair. The HEI Standards for SurfaceCondensers1, however, state that theactual amount of cooling that can berealized in practice may or may not beequal to the design value of 7.5 °F andwill depend upon the operating charac-teristics, the amount of NC gases, andthe capacity characteristics of the vent-ing equipment.

Venting equipment installed inmost plants typically have 100 percentspare capacity. As an example, if four


Table 2Total Cond. Press. Total Cond. Henry’s Constant Ho2 Partial Press. Partial Press. Vapor Sat. Air Fraction Subcooling

Pt, in. HgA Temp. Tt, ˚F Atmosphere/Mole Fraction Pa, in. HgA Pv, in. HgA Temp. Tv, ˚F AF, % Tt–Tv, ˚F1.00 79.04 44377 0.025 0.975 78.26 4.0% 0.782.00 101.10 52541 0.030 1.970 100.64 2.4% 0.503.00 115.06 56720 0.032 2.968 114.67 1.7% 0.384.00 125.42 59731 0.034 3.966 125.10 1.4% 0.31

Notes: 1. Pa = (Ho2 � DO)/(25.07E06 � 0.49115) 3. Pv = Pt – Pa 5. AF = Pa/(Pa + 0.6207 Pv)2. Tt corresponds to Pt 4. Tv corresponds to Pv

Figure 2


SJAE units are installed to service twomain condenser shells, two of theSJAE units would be spares. Undernormal operating conditions, with airinleakage at normal levels, only twoSJAE units would need to be in serv-ice. If air inleakage levels increase sig-nificantly, one or both of the sparesmay need to be placed into service.However, even with low to normal airinleakage levels, it is not uncommon towitness plants operating all their SJAEunits at all times or operating them in aconfiguration other than the originalrecommended design. While there maybe several factors accounting for thisdeparture from design, the end resultsare wasted steam due to all steam jets

being in service, little or no improve-ment in condenser vacuum, inability toperform maintenance online, increasedwater vapor loadings, lack of spares,and excessive wear and tear on theSJAE system. Since the performanceof the SJAE system in many cases isnot routinely monitored, problemsmay go unnoticed for a long timebefore they surface.

Condenser/SJAE SystemInterface

The SJAE system relies upon thecondenser to provide the proper ratioof NC gases and water vapor in orderfor the SJAE system to function nor-

mally. At base-load conditions, in a leak-tight condenser with the air-removal equipment operating satisfactorily, the condenser pressure iscontrolled by the circulating waterinlet temperature. The amount of airinleakage and the amount of airremoved, the condensate DO level,condensate subcooling, and the con-denser pressure would all be in a stateof equilibrium at expected levels.Condenser pressure, however, willincrease if there is macrofouling ormicrofouling, if the circulating waterflow rates decrease with circulatingwater pumps removed from service,condenser water boxes are cycled, etc.As discussed earlier, increases in

ASME“Besides operating the SJAE units per the original

design philosophy, the steam jets should be operated atsteam pressures no less than the design steam pressuresand no greater than 10 to 15 psi above design pressures,

for operational margin.”

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ASMEcondenser pressure mean higher vaporpressures and increased water vaporloadings for the SJAE system. Theincreased water vapor loadings need tobe carefully evaluated for impact uponthe SJAE system and could becomesignificant if the SJAE system is oper-ated in a mode other than the originaldesign configuration. Cleaning thecondenser to control both macrofoul-ing and microfouling helps to improvecondenser vacuum and reduce watervapor loadings on the SJAE system.While cleaning the condenser tubes formicrofouling, the tubes in the air-cool-er section may need to be especiallytargeted where the heat transfer is thepoorest due to air and NC gases blan-keting the tubes. If, on its way to thesuction of the air-removal equipment,

the mixture of NC gases and watervapor is insufficiently cooled in the air-cooler section, the water vapor loadingwill increase at the suction of the air-removal equipment. It is important tokeep condenser pressures close toexpected levels consistent with designto keep water vapor loadings on theSJAE system at reasonable levels.

Performance IssuesCondenser Air-Cooler

SJAE systems for power plantstypically utilize two stages of ejectors.For most plants, the suction of the firststage ejector is usually connected to theair off-take piping from the condenser.The mixture of NC gases and watervapor transported from the condenserair-cooler section via the air off-take

piping is cooled during its passage toallow some of the water vapor to con-dense prior to the mixture entering thefirst stage. The air-cooler section is thusthe only source of cooling of the mix-ture before it enters the first stage. Ifthe cooling capability in the air coolersection is curtailed, the vapor fractionat the inlet of the first stage willincrease and could compromise theperformance of the SJAE system.

Plants do not have provisions orseldom monitor the pressures and tem-peratures of the mixture of NC gasesand water vapor as it leaves the condenser and enters the air-removalequipment. Clearly, installing instru-mentation will not only facilitateobtaining data over a wide range ofoperating conditions, but will also assist


“While the performance of both the intercondenserand aftercondenser is affected by cooling water flow

and temperature, tube fouling and tube leaks, a majorperformance concern is possible flooding and theresulting back pressure from excessive steam and

water vapor loadings, especially at higher condenser pressures.”

Figure 3 Figure 4


in performance monitoring and trou-bleshooting condenser/SJAE problems.

First & Second Stage Steam JetsMotive steam is used to remove

the NC gases and uncondensed watervapor that enter the first stage from thecondenser air-cooler section. For thesecond stage, motive steam is used toremove the NC gases and uncondensedwater vapors that enter the secondstage from the intercondensers. Whilethe design pressure for the motivesteam depends upon the specific appli-cation, pressures may range from 125psig to 250 psig.

The throat diameter of the steamnozzle of an ejector is sized for criticalflow. From the Heat Exchange InstituteStandards for Steam Jet VacuumSystems5, the critical flow of steamthrough the nozzle may be computedusing the following:

(9) Ws = 892.4 � C � Dn2 � (P/v)

where:Ws = Critical steam flow, lbs/hrC = Nozzle coefficientDn = Nozzle throat diameter, inchesP = Upstream steam pressure, psiav = Upstream specific volume, cft/lb

From equation (9), it can be seenthat for a given steam pressure the steamflow varies as the square of the throatdiameter. For example, a 10 percentincrease in the nozzle throat diameterwill increase the steam flow by 21 per-cent. This could occur due to nozzle ero-sion, if a different size nozzle is used,etc. Also, for a given throat diameter, thesteam flow through the nozzle varies asthe square root of the motive steam pres-sure. For example, an increase in steampressure by 25 psi, from 125 psig to 150psig, would increase the steam flow byabout 17.5 percent. This would be thecase if the operating steam pressurewere higher than the recommendeddesign steam pressure.

For various reasons, plants tend tooperate the steam jets at steam pressureshigher than design. It is not unusual toencounter operating pressures 25 to 50psi higher than the recommended mini-mum design pressures. In addition, if allthe SJAE units are operating with nospares, the steam flows and water vaporloadings are now well over double theflows that would otherwise be encoun-tered when operating at design steampressures per the design configuration.This combination of all SJAE units inservice with steam pressures well inexcess of design, results in excessive aswell as wasted steam flows andincreased water vapor loading, whichcan seriously jeopardize SJAE systemperformance, especially at higher con-denser pressures. Besides operating theSJAE units per the original design phi-losophy, the steam jets should be operat-ed at steam pressures no less than thedesign steam pressures and no greaterthan 10 to 15 psi above design pressures,for operational margin. This would helpto reduce motive steam consumptionand relieve the system of excessivewater vapor loadings, especially at high-er condenser pressures.

Steam nozzles can erode resultingin excessive steam consumption andincreased water vapor loadings. Thethroat sizes should be checked againstvendor design and the nozzles replacedif there is evidence of erosion.

Intercondensers &Aftercondensers

The intercondenser is used to con-dense water vapor from the mixture ofsteam, NC gases, and water vaporentering the intercondenser from thefirst stage steam jets. The aftercon-denser is used to condense essentiallyall the water vapor from the mixture ofsteam, NC gases, and water vaporentering the aftercondenser from thesecond stage steam jets.

While the performance of both theintercondenser and aftercondenser isaffected by cooling water flow andtemperature, tube fouling and tubeleaks, a major performance concern ispossible flooding and the resultingback pressure from excessive steamand water vapor loadings, especially at higher condenser pressures.Maintaining operating steam pressuresclose to design, and operating only therecommended number of SJAE unitsshould help alleviate excessive steamand water vapor loading.

PerformanceMonitoring/Troubleshooting

Since the performance of the con-denser and the performance of theSJAE system are closely intertwined, itis important to establish a performancemonitoring program that encompassesboth. The ability to effectively monitorthe performance and troubleshoot con-denser/SJAE systems will dependupon the availability of requisiteinstrumentation, manpower resources,and the frequency of the program.Recognizing these limitations, the fol-lowing listings are provided merely asguides.

For the condenser, the performancedata/parameters to be monitored are:

• Condenser pressure• Circulating water inlet and outlet

temperatures• Circulating water flow rate

(determined through heat bal-ance techniques or other means)

• Condensate DO level• Condensate hotwell temperature• Pressure and temperature at air

off-take piping

For the SJAE system, the performancedata/parameters to be monitored are:

• Pressure and temperature at suc-tion and discharge of first stageand second stage ejectors

ASME

ASME• 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 onSJAE systems, the HEI Standards forSteam Jet Vacuum Systems5 may beconsulted. The methods provided inthe standards reflect many years ofaccumulated experience within theindustry and are considered to be reli-able and accurate. The Standards pro-vide guidelines on location of testinstrumentation for different testarrangements.

RecommendationsCondenser performance monitor-

ing programs do not always take intoaccount the interface with the SJAEsystem. By incorporating the interfaceinto the condenser performance-moni-toring program, it is possible to ensureproper and optimum performance.Beside performance gains that couldbe significant, other benefits includebetter maintenance, less wear and tear,increased availability, etc.

Here are some recommendations:• Incorporate the SJAE system

into the condenser performancemonitoring program.

• As a minimum, monitor on aroutine basis the condenser pres-sure, circulating water inlet andoutlet temperatures, condensateDO level, hotwell condensatetemperature (and subcooling),and air removal rate. If condi-tions are normal, these parame-ters should be at expected levelsand will correlate with eachother.

• Consider operating the SJAEsystem in accordance with the

original design philosophy(motive steam pressures, numberof SJAE units in service, line-ups, etc.). If the system does notperform properly when operatedper design, the causes should beinvestigated and correctiveaction taken to permit operatingper design. This area is ripe forsignificant gains in performance,maintainability, and availability.

• Operating more than the recom-mended number of SJAE units ona continuous basis can lead tolong-term degradation in perform-ance. As discussed earlier, thispractice does not necessarilyimprove condenser performanceand has several negative ramifica-tions for the SJAE system.

• Consider installing instrumenta-tion in the condenser/SJAE sys-tem for performance monitoringand troubleshooting problemsrequiring detailed evaluation.

• If the SJAE system arrangementoffers the flexibility, considerswapping the air ejectors to opti-mize the performance.

• Check the condition of the com-ponents in the SJAE system.These include the ejectors, dif-fusers, strainers, precoolers,intercondensers, and aftercon-densers. The nozzle sizes shouldbe checked for conformancewith design.

• Maintain a set of spare nozzlesand diffusers in case the origi-nals need to be replaced.

References & Footnotes1. Heat Exchange Institute – Standards for

Steam Surface Condensers, 9th Edition.2. EPRI-2294 – Guide to the Design of

Secondary Systems and TheirComponents to Minimize Oxygen-Induced Corrosion. Bechtel Group(S.W. S. Shor et al) – March 1982.

3. Chemical Engineer’s Handbook, Table14-27, page 14-6.

4. Spencer, E., and Impagliazzo, A. M.,1984, "Enhanced Condenser Ventingfor Condensate Oxygen Control,"ASME Paper 84-JPGC-Pwr-32.

5. Heat Exchange Institute – Standards forSteam Jet Vacuum Systems.

This article was edited down from apaper (#40003) that was originallypublished in the Proceedings of the2003 International Joint PowerGeneration Conference, June 16-19,2003, Atlanta. These proceedings areavailable from ASME in both digitaland print formats. For more detailscontact infocentral @asme.org. PDFsof individual papers can also be pur-chased at the ASME Digital Storehttp://store.asme.org

FOR MORE INFORMATION, ENTER 03890ON INFOLINK AT ENERGY-TECH.COM OR EMAIL

[email protected]

Komandur S. Sunder Raj is the founderand owner of Power & Energy SystemsServices providing extensive training,software development/applications,and consulting/troubleshooting servic-es to the power industry. He has 35years of experience in the power indus-try and has specialized in power plantdesign, performance, economic studies,analyses, and project engineering/management activities. He has heldresponsible positions with major engi-neering companies such as Stone &Webster Engineering Corporation,Burns & Roe, Inc. and RaytheonEngineers & Constructors, Inc. Heserved as Director of ProjectEngineering at the New York PowerAuthority and was responsible for proj-ect engineering/management activitieson the Indian Point 3 Nuclear PowerPlant. He is the author/developer of thePERFORM™ software which wasdeveloped for power plant design, per-formance analysis, troubleshooting, andtraining.


“The ability to effectively monitor the performanceand troubleshoot condenser/SJAE systems

will depend upon the availability of requisite instrumentation, manpower resources, and

the frequency of the program.”

steam jet air ejector asme performance...

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