Understanding Ejector Systems Necessary To Troubleshoot .

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TECHNOLOGYUnderstanding ejectorsystems necessaryto troubleshoot vacuum distillationJames R. Lines Graham Corp. Batavia, NY.A complete understanding of ejectorsystem performance characteristics canreduce the time and expense associatedwith troubleshooting poor crudevacuum distillation unit (CVDU)performance.Variables that may negatively impactthe ejector-system performance ofvacuum-crude distillation units includeFig 1.utilities supply, corrosion and erosion,fouling, and process conditions.Fig. 2

Fig. 5Tables 1 and 2 are troubleshooting guides toejector and condenser problems in vacuumejector systems. Fig. 1 is a photo of aninstalled ejector at a CVDU.Two actual case studies conducted by serviceengineers on CVDU-ejector systems showhow to troubleshoot ejector problems. Thefirst problem was a result of improperreplacement of an intercondenser, and thesecond was a result of underestimation ofnoncondensible loading during design, whichhas recently become a common problem.EjectorsAn ejector converts pressure energy ofmotive steam into velocity. It has no movingparts. Major components of an ejectorconsist of themotive nozzle, motive chest, suctionchamber, and diffuser (Fig. 2).High velocity is achieved through adiabaticexpansion of motive steam across aconvergent/divergent steam nozzle. Thisexpansion of steam from the motive pressureto the suction fluid operating pressure resultsin supersonic velocities at the exit of thesteam nozzle.The motive steam actually expands to apressure below the suction fluid pressure.This expansion creates a low-pressure region,which draws suction fluid into an ejector.Typically, velocity exiting a motive steamnozzle is in the range of 3,000-4,000 fps. Thishigh-velocity motive steam then entrains andmixes with the suction fluid. The resultantmixture is still supersonic. As the mixturepasses through the convergent, throat, anddivergent sections of a diffuser, high velocityis converted back to pressure.The convergent section of a diffuser reducesvelocity as cross sectional area is reduced.Intuitively, one normally thinks that as flowarea is reduced, velocity is increased. But aunique thermodynamic phenomenon occurswith gases at supersonic conditions: Ascross-sectional flow area is reduced, thevelocity is reduced.The diffuser throat is designed to create ashock wave. The shock wave produces adramatic increase in pressure as the flow goesfrom supersonic to subsonic across it. In thedivergent section of the diffuser, crosssectional flow area is increased and velocity isfurther reduced and converted to pressure. Ashock wave occurs in the diffuser throat whenthe compression ratio of an ejector is 2:l orgreater, which is the case with CVDU ejectorsystems.An ejector-performance curve gives theexpected suction pressure as a function ofwater-vapor equivalent loading (Fig. 3). HeatExchange Institute Standards for Steam JetEjectors describes the method to convert themixture (air, water vapor, and varioushydrocarbons) to a water-vapor equivalent oran air-equivalent load.Other important information noted on anejector performance curve includes theminimum motive steam pressure, themaximum motive steam temperature, and

the maximum discharge pressure.If field measurements differ from aperformance curve, then there may be aproblem with the process, utility supply,or the ejector itself.CondensersA condenser in an ejector system reducesthe amount of vapor load that adownstream ejector must handle.Condensers of an ejector system aredesigned to condense steam andcondensible hydrocarbons and coolnoncondensible gases.In many cases, the inlet load to acondenser is many times greater thanthe load to a downstream ejector.Consequently, any loss in condenserperformance will have a dramatic ef-fecton a downstream ejector.Although vacuum condensers areconstructed like process shell-and-tubeheat exchangers, their internal designsdiffer significantly due to the presenceof two-phase flow, noncon-densible gas,and vacuum operation.Vacuum condensers for crude-towerapplications have cooling water on thetube side. Condensation of water vaporand hydrocarbons takes place on theshellside. A major portion of thecondensibles contained in the inletstream (shell side) change from a vaporto liquid phase. The remainingcondensibles and the noncondensiblegases are removed from the condenserthrough a vapor-outlet connection by adownstream ejector.Intercondensers are positioned between two ejector stages. Condensationof intercondensers occurs at a pressurecorresponding to the dis-charge pressureof a preceding ejector and the suctionpressure of a downstream ejector.Steam pressure and temperatureThe temperature and pressure ofmotive-steam supply is one of the mostimportant variables affecting ejectoroperation. If the pressure falls belowdesign pressure, then the motive nozzlewill pass less steam. If this occurs, anejector does not have enough energy toentrain and compress a suction load tothe design discharge pressure.Similarly, if the motive-steam supply

Table 1temperature is appreciably above thedesign value, insufficient steam passesthrough the motive nozzle. Both lowerthan-design steam pressure and higherthan-design steam temperature increasethe specific volume of the motive steamand reduces the amount of steamthrough a motive nozzle.I n certain cases, it is possible to re-borean ejector-motive nozzle to permit thepassage of more steam through thenozzle, thereby increasing the energyavailable to entrain and compress thesuction load.If motive-steam pressure is more than20% above design, too much steamexpands across the nozzle. This oftenchokes the diffuser throat of an ejector.When this occurs, less suction load ishandled by an ejector, and the CVDcolumn pressure rises. If an increase incolumn pressure is undesirable, thenTable 3new ejector nozzles with smaller throatdiameters are required.Steam qualityWet steam is very damaging to anejector system because high-velocitymoisture droplets are erosive. Thesedroplets are rapidly accelerated as steamexpands across a motive nozzle.Erosion of nozzle internals caused bywet motive-steam is noticeable wheninspecting ejector nozzles or diffuserinternals. There is an etched striatedpattern on the diverging section of amotive nozzle, and the nozzle mouthmay actually wear out. Also, the inletdiffuser section of an ejector will showsigns of erosion as a result of directimpingement of moisture droplets (Fig.4a).Fig. 4b depicts an ejector cutawayshowing severe damage caused by wetsteam. The inlet diffuser shows

substantial metal loss. Metal-scale buildupcan be seen in the outlet diffuser section.The exhaust temperature from the ejector candetermine if the steam conditions are present.Typical ejector exhaust temperatures are inthe range of 250 to 300 F. If moisture ispresent, a substantially lower exhausttemperature will exist.To solve wet-steam problems, all lines up toan ejector should be well insulated. A steamseparator and trap should be installedimmediately before the motive-steam inletconnection of each ejector. In some instances,a steam superheater may be required.Wet steam can also cause performanceproblems. Moisture droplets through anejector nozzle decrease the energy availablefor compression. This reduces the suctionload handling capacity of an ejector.Also, the moisture droplets may vaporizewithin the diffuser section of the ejector.Upon vaporization, the volumetric flow ratewithin the ejector increases. Here again, thisreduces the suction-load capacity of anejector.Cooling water conditionsA rise in cooling-water temperature lowersthe available log mean temperature difference(LMTD) of a condenser. Should this occur,the condenser will not condense enough steamand condensible hydrocarbons. This willincrease the vapor load to the downstreamejector.As a result of inadequate condensation, therealso is an increase in pressure drop across thecondenser. If an ejector following thiscondenser cannot handle an increased vaporload at the operating pressure of a condenser,the operating pressure of the condenser willrise and the system will break performance.Broken ejector system performance ischaracterized by a higher-than-design CVDUtower-top pressure. The tower-top pressuremay become unstable.This may also occur if the cooling-water flowrate is below design. At lower-than-designflow rates, there is a greater watertemperature rise across a condenser. Hereagain, this will lower the available LMTD.Poor performance is further exacerbated as aresult of a lower heat transfer coefficientresulting from low-water flow rate.Problems with cooling water normally occurduring summer months. During the summer,the water is at its warmest, and demands onrefinery equipment are highest. If coolingwater flow rate or temperature are off design,new ejectors or condensers may be requiredto provide satisfactory operation.Corrosion and erosionCorrosion may occur in ejectors, condensers,or Vacuum piping. Extreme corrosion maycause holes and allow a system. Air leakageinto the vacuum system. Air leakage into avacuum system will deteriorate performanceand can result in broken ejector operation.A common corrosion problem occurs whencarbon-steel tubing is used in condensers.Although carbon steel may be suitable for thecrude feed-stock, it is not always the bestchoice for an ejector system. Although carbonsteel has a lower capital cost, operatingproblems can outweigh modest up-frontsavings.During extended periods of shutdowns formaintenance or revamps, a condenser withcarbon-steel tubing will be exposed to air,oxidize, and develop a scale buildup. When anejector system starts up, this buildup canseverely foul the condensers and preventproper operation of the vacuum system.Poor steam quality and high velocities mayalso cause erosion of the diffuser and motivenozzle internals. Ejector manufacturers willprovide certified information that defines themotive nozzle and diffuser throat diameters.If a routine inspection of these parts indicatesan increase in cross sectional area over 7%,then performance may be compromised, andreplacement parts are necessary.Threaded steam connections may experience aphenomenon termed wire drawing, or wirecutting. Loose threads provide a leak path forthe steam. Over time, the steam will destroythe threaded joint or even put a hole in thepiece. A hole leads to a steam leak within theejector, which will act like a suction load,thereby reducing the system’s performance.FoulingIntercondensers and aftercondensers aresubject to fouling on both the tube side andthe shell side. Fouling deters heat transfer.Cooling-tower water, often used as thecooling fluid for vacuum condensers, isnormally on the tube side. Over a prolongedperiod of time, actual fouling may exceed thedesign value, and condenser performancebecomes inadequate.Vacuum-tower overhead gases, vapors, andmotive steam are normally on the shell side ofa condenser. Depending on fractionation andthe type of crude processed, a hydrocarbonfilm may develop on the outside surface ofthe tubing. This film deters heat transfer.Fig. 5 illustrates how severely a condensermay be fouled. In this example, not only didthe tubing have a hydrocarbon film, butsolidified hydrocarbon product adhered to thetubing. The solidified material blocked theflow, resulting in poor performance and anelevated pressure drop.When actual unit fouling exceeds designvalues, a condenser performs inadequately.Once fouled, a condenser is unable tocondense sufficient quantities of hydrocarbonvapors and motive steam. The result ofcondenser fouling is an increase in vapor loadto a downstream ejector and an increase incondenser-operating pressure. Ultimately, apreceding ejector will break operation.Routine refinery procedures should includeperiodic cleaning of the tube side and the shellside of condenser bundles.Process conditionsVacuum system performance may be affectedby several process variables: non-condensiblegas loading, condensible hydrocarbons, andvacuum system back pressure.Ejector systems are susceptible to poorperformance when noncondensible loadingincreases above design. Noncondensibleloading to an ejector system can be caused byair leakage into the system, the presence oflight hydrocarbons, or the existence ofcracked gases from a fired heater.Theimpactofhigher-than-designnoncondensible loading is severe. Asnoncondensible loading increases, the amountof saturated vapors discharging from acondenser increases proportionately.

The ejector following a condenser maynot be able to handle increased loadingat that operating pressure of thecondenser. The ejector preceding thatcondenser is unable to compress to ahigherdischargepressure.Thisdiscontinuity in pressure causes thepreceding ejector to break operation.When actual noncondensible loading isconsistently above design, new ejectorsare required. Depending on the severityof noncondensible overloading, newcondensers may be required as well.Recently, several CVDU revamps in theU.S. Gulf Coast experienced startupdifficulties due to inaccurate estimates ofactual noncondensible loading.As different crude oils are processed, oras refinery operations change, thecomposition and amount of condensiblehydrocarbons handled by an ejectorsystem vary. Condensable hydrocarbonloading may become so much greaterthan design that condenser or ejectorperformance is adversely affected.Another possible affect of increasedcondensible hydrocarbon loading is anincreased oil-condensate film on thetubing, and consequently, a reduction inthe heat transfer rate. This situation mayresult in increased vapor discharge froma condenser. Unstable operation of theentire ejector system may result. Toovercome this type of performancelimitation, new condensers or ejectorsmay be required.Vacuum system back pressure may haveanoverwhelminginfluenceonsatisfactory performance. If the actualdischarge pressure rises above design,an ejector will not have enough energyto reach that higher pressure. When thisoccurs, the ejector breaks operation, andthere is an increase in CVDU tower-toppressure.When back pressure is above design,possible corrective actions includelowering the system back pressure,reboring the steam nozzle to permit theuse of more motive steam, or installingnew ejectors.ICase 1:Improper intercondenserA West Coast refiner experienced erraticsystem performance after replacing anintercondenser supplied by the ejectorsystem manufacturer with one designedand built by a local heat exchangerfabrication shop. The ejector systemvendor dispatched a service engineer toinvestigate the cause of the problemwithout knowing about the replacementintercondenser.The actual performance of the systemdiffered from the “as sold” system (Fig.6). The first-stage ejector was operatingin a broken mode with both suction anddischarge pressure remaining unstable.Pressuredropacrossthefirstintercondenser was excessive -at 8.5 mmHg instead of 3 mm Hg.Broken first-stage ejector performanceand high-pressure drop across the firstintercondenser suggested one of thefollowing problems: fouling, coolingwater flow rate limitation, high inletwatertemperature,orexcessivehydrocarbon loading.Prior to detailing a method to determinethe actual cause, the service engineerdiscussedgeneralperformancecharacteristics with unit operators. Atthat time, he discovered that the firstintercondenser had been replaced byanother vendor.The vendor had matched the originalunit’stubecountandexternaldimensions, but failed to properlydesign the shellside side baffling toeffectively manage hydraulic andthermal requirements.Vacuum condensers have specialshellside baffling to ensure minimalpressure drop, noncondensible gascooling,andseparationofnoncondensibles and condensate. It istypical to have different baffle spacing atstrategic locations within the shell.The vendor of the replacementcondenser used conventional software tomodel the performance. The newcondenser design had a fully baffledflow, and consequently a high-pressuredrop.In this instance, the high-pressure dropacross the intercondenser caused thesystem to break performance. The firststage ejector could not overcome theadded pressure drop and reach adischarge pressure in which the secondstage ejector would operate.Once the replacement unit was pulledout and a properly designed condenserput in, system performance wassatisfactory.Case 2:Underestimated loadingA U.S. Gulf Coast refiner grosslyunderestimateditsnoncondensibleloading when it modernized a CVDU toprocess sour South American crude. Themodernization effort involved anentirely new ejector system.Upon startup of the CVDU, the ejectorsystem was not performing properly.Tower-top pressure was significantlyabove design, and it was unstable.Initial investigation verified utilityconditions. The ejector system wasdesigned for 140 psig motive steam, andthe actual supply pressure variedbetween 138 and 144 psig.Next, the cooling water was evaluated.Design inlet temperature was 88 F., andthe actual supply temperature was at72.3 F. Temperature rise and pressuredrop across each condenser did notsuggest an abnormality. The equipmentwas new, so fouling was ruled out.A detailed analysis of the sour SouthAmerican crude oil was in order.The design and actual vacuum toweroverhead compositions are shown inTable 3.The actual simulation was too differentfrom design conditions. Significantequipment modifications were neededto achieve the desired charge rate andvacuum level.The steam equivalent loads werecalculated to be about 17,500 lb/hr and23,000 lb/hr for design and actualloading, respectively. According to theperformance curve, at the higher load,the first-stage ejector would maintainabout 19 mm Hg absolute pressure inlieu of the design 14 mm Hg. The refineragreed to accept the higher pressure.Because the noncondensible loadingvalues were drastically different (morethan twice as much as design) newequipment was necessary.The refiner added redundant ejectorsandcondensersafterthefirstintercondensers to handle the additionalnoncondensible load. The systemstabilized after two parallel trains ofsecondary equipment were installed.Tower-top pressure was still abovedesign but within an acceptable range.Figs. 7a and 7b depict the “as sold”performanceandtherevampedoperation.

motive steam into velocity. It has no moving parts. Major components of an ejector consist of the motive nozzle, motive chest, suction chamber, and diffuser (Fig. 2). High velocity is achieved through adiabatic expansion of motive steam across a convergent/divergent steam nozzle. This expansion of steam from the motive pressure

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