Troubleshooting crude vacuum tower overhead ejector …

1 Troubleshooting crude vacuum tower overhead ejector systemsUse these guidelines to improve performance and product qualityJ. R. LINES AND L. L. FRENS, GRAHAM MANUFACTURING CO. INC., BATAVIA, NEW YORKR outinely surveying tower overhead vacuum systems canimprove performance and product quality. These vacuum sys-tems normally provide reliable and consistent , process conditions, supplied utilities, corrosion, erosionand fouling all have an impact on ejector system vacuum distillation towers use ejector systems to main-tain tower top pressure and remove overhead gases (Fig. 1).However, as with virtually all refinery equipment, performancemay be affected by a number of variables. These variables may actindependently or concurrently. It is important to understandbasic operating principles of vacuum systems and how perform-ance is affected by: Utilities Corrosion and erosion Fouling Process vacuum -system suppliers have service engineers thatwill come to a refinery to survey the system and troubleshoot per-formance or offer suggestions for improvement.

2 A skilledvacuum-system engineer may be needed to diagnose and remedysystem a vacuum system is initially designed, utilities are estab-lished and the most extreme conditions are usually used for thedesign basis. Once operating, actual utility supply conditions canbe different than those set at the design stage and vary occasional-ly. Important utilities for ejector systems are motive steam andcooling water. Motive steam pressure, quality and temperature arecritical variables. Flowrate and inlet temperature are important forcooling steam conditions. These are very important and have adirect impact on an ejector s operation. If motive steam supplypressure falls below design, then the nozzle will pass less this happens, the ejector is not provided with enough ener-gy to compress the suction load to the design discharge same problem occurs when the supply motive steam temper-ature rises above its design value. Result: Increased specificvolume and, therefore, less steam passes through the ejector may operate unstably if it is not supplied with enoughenergy to allow compression to its design discharge pressure.

3 Ifthe actual motive steam pressure is below design or its tempera-ture above design, then, within limits, an ejector s nozzle can berebored to a larger diameter. The larger nozzle diameter allowsmore steam to flow through and expand across the nozzle. Thisincreases the energy available for motive steam supply pressure is more than 10% to 20% abovedesign, then too much steam expands across the nozzle. Thistends to choke the diffuser. When this occurs, less suction load ishandled by the ejector and vacuum tower top pressure tends torise. If an increase in tower top pressure is not desired, then ejec-tor nozzles must be replaced with ones with smaller quality is important. Wet steam can be damaging to anejector system. Moisture droplets in motive steam lines are accel-erated to supersonic velocities and become very erosive. Moisturein motive steam is noticeable when inspecting ejector accelerated moisture droplets erode nozzle internals. Theyetch a striated pattern on the nozzle s diverging section and mayactually wear out the nozzle mouth.

4 Also, the inlet diffuser tapersand throat will have signs of erosion. The exhaust elbow at theejector s discharge can erode completely through. Severe tubeimpingement in the intercondenser can also occur dependingupon ejector orientation. To solve wet steam problems, all linesup to the ejector should be well insulated. Also, a steam separatorHydrocarbon Processing , March 19951 Fig. 1. Twin-element, three-stage ejector system on crude a trap should be installed immediately before an ejector smotive steam inlet connection. In some cases, a steam superheatermay be steam can also cause performance problems. When waterdroplets pass through an ejector nozzle, they decrease the energyavailable for compression. The effect is a decrease in load han-dling ability. With extremely wet steam, the ejector may evenbreak water. ejector system intercondensers and intercon-densers are designed to condense steam and condensiblehydrocarbons, and cool non-condensible gases. This occurs at apressure corresponding to the preceding ejector s design dischargepressure and the following ejector s design suction pressure.

5 Whenthe cooling water supply temperature rises above its design value, ejector system performance is penalized. A rise in cooling watertemperature drives down a condenser s available log-mean temper-ature difference (LMTD). The condenser does not condenseenough and more vapors are carried out with the non-condensiblegases as saturation components. A pressure drop increase acrossthe condenser is noticeable. The ejector following this condensercannot handle the increased load at this pressure. Pressure risesand the preceding ejector does not have enough energy to dis-charge to the higher pressure. Result: The preceding ejectorbreaks operation and the system may become also occurs if the cooling water flowrate falls below lower-than-design cooling water flowrates, there is a greaterwater temperature rise across a condenser. This also lowersLMTD and the above situation with cooling water normally occur during summermonths. This is when the water is at its warmest and demands onrefinery equipment are highest.

6 If the cooling water flowrate ortemperature is off design then new ejectors or condensers may berequired to provide satisfactory AND EROSIONC orrosion may occur in ejectors, condensers or vacuum corrosion can cause holes and air leaks into the destroys vacuum system may occur within the ejectors. Poor steam quality andhigh velocities erode diffuser and motive nozzle internals. Anejector manufacturer will provide certified information that givesthe motive nozzle and diffuser throat design diameters. If a rou-tine inspection of these parts indicates an increase incross-sectional area over 7%, then performance may be compro-mised and replacement parts will be Processing , March 19952 Fig. 2. ejector components and pressure FUNDAMENTALSThe basic operating principle of an ejector is to convert pressureenergy into velocity. This occurs with adiabatic expansion ofmotive steam across a converging/diverging nozzle from motivepressure to suction load operating pressure. Supersonic velocityfrom the nozzle mouth results.

7 Typically, velocities of mach 3 to 4are operation, motive steam expands to a pressure below the suctionpressure. This creates a driving force to bring the suction load intothe ejector . High-velocity motive steam entrains and mixes withthe suction load gas. The resulting mixture is still supersonic. Asthis mixture enters the converging/diverging diffuser, high velocityis reconverted into pressure. A diffuser s converging section reducesvelocity as crossflow area is reduced. The diffuser s throat isdesigned to create a normal shock wave. A dramatic increase inpressure occurs as the flow across the shock wave goes from super-sonic to sonic to subsonic after the shock wave. In the diffuser sdiverging section, cross-sectional flow area is increased and velocityis further converted to pressure. Fig. 2 details ejector componentsand a pressure profile for an ejector having a compression ratio inexcess of 2 systems are required to operate over a wide range of condi-tions from very light loads to loads above design.

8 An ejectorsystem must stably adapt to all anticipated operating the design non-condensable and light-end hydrocar-bon loading is essential for stable operation. Furthermore, anaccurate understanding of system back pressure is systems may be configured a number of different ways tooffer flexibility in handling various feedstocks and differing refin-ery operations. A single vacuum train with one set of ejectors andcondensers has the lowest initial capital cost, but flexibility is limit-ed. Often, parallel ejector trains are installed for each stage. Eachparallel ejector will handle a percentage of the total loading. Forexample: Twin element ejectors, each designed for 50% of total load Triple element ejectors, each designed for 40% of total load-ing for 120% capacity Twin element, 1 3:2 3ejector trains Other ejector trains allow one train to be shut down for mainte-nance while the column operates at reduced conditions. Also, atlight loadings, a train may be shut down to conserve refinery oper-ating costs.

9 Fig. 3 shows a typical vacuum tower ejector systemwith a triple element ejector and first intercondenser. The secondintercondenser and aftercondenser are a single is a result of improperly selected metallurgy. Ensure thatthe most appropriate materials are used before replacing parts. Acommon corrosion problem occurs when carbon steel tubing isused in condensers. Although carbon steel may be suitable for thecrude feedstock handled, it is not always the best practical does offer the initial advantage of lower capital cost. However,operating problems far outweigh modest up-front towers undergo periods of extended shutdown for routinemaintenance, revamp or other reasons. During this period, a con-denser with carbon steel tubing will be exposed to air and will rustand develop a scale buildup. When the system starts up, the con-densers are severely fouled. They will not operate as designed andvacuum system operation is compromised. Modest savings in ini-tial investment for steel tubing is quickly lost with less-than-optimal tower operations due to rusted and scaled tubing.

10 Vacuumsystem manufacturers often caution against using carbon steel Intercondensers and aftercondensers are subject to foul-ing like all other refinery heat exchangers. This may occur on thetubeside, shellside or both. Fouling deters heat transfer and, atsome point, may compromise system tower water is most often used as the cooling fluid for vac-uum condensers. This water is normally on the tubeside. Typicalfouling deposits on tubing internals cause a resistance to heat trans-fer. Over a prolonged period of time, actual fouling may exceed thedesign value and condenser performance falls short of tower overhead gases, vapors and motive steam are nor-mally on the condenser s shellside. Depending on towerfractionation and the type of crude processed, a hydrocarbon filmmay develop on the tube s outside surface. This film is a resistanceto heat transfer, and over time, this fouling will exceed this occurs, condenser performance falls refinery procedures should include periodic cleaning ofcondenser bundles.