OPERATION OF CENTRIFUGAL COMPRESSORS IN CHOKE CONDITIONS

Transcription

1 Poceedings of the Fotieth Tubomachiney Symposium Septembe 12-15, 2011, Houston, Texas OPERATION OF CENTRIFUGAL COMPRESSORS IN CHOKE CONDITIONS Raine Kuz Manage, Systems Analysis Sola Tubines Incopoated San Diego, CA, USA Russell K. Maechale Aeodynamic Specialist Sola Tubines Incopoated San Diego, CA, USA Edwad J. Fowle Consulting Enginee Sola Tubines Incopoated San Diego, CA, USA Michael J. Cave Goup Manage, Gas Compesso Aeo Pefomance Sola Tubines Incopoated San Diego, CA, USA Min Ji Pincipal Enginee Sola Tubines Incopoated San Diego, CA, USA ABSTRACT Centifugal compessos ae at times equied to opeate in o nea the choke egion. Vaious limits of the degee of allowable opeation in choke have been established. Based on test data and numeical data, the behavio of centifugal compessos in the choke egion is studied. Changes in aeodynamic pefomance, thust load, volute behavio and adial loading ae consideed. The issue of excitation of impelle vanes is addessed. Paticula consideation is given to multistage machines, as well as dual compatment machines, in paticula egading the effects of impelle mismatch duing opeating conditions at flows significantly highe than the design flow. defining a choke limit o sonic limit on a compesso map, pohibiting the opeation of the compesso at low pessue atios. This can equie installation of additional thottle valves, o moe complicated ecycle and anti-suge valve selections, and thus impacts the cost of opeation. Limitations in the oveload opeating ange of a compesso not only impact the opeational flexibility, but also can equie moe complicated contol systems. The pape addesses aeodynamic, stuctual as well as otodynamic issues elated to the opeation in choke. INTRODUCTION Centifugal compessos ae fequently used in the oil and gas industy in highe pessue applications (Rasmussen and Kuz, 2009). One of the chaacteistics of applications in the oil and gas industy is the equiement to use the compessos ove a wide ange of opeating conditions (Figue 1). The machines, usually speed contolled, ae expected to opeate fom low flows nea suge to vey high flows nea o in choke. While the opeation nea suge, as well as issues to pevent compessos fom suging have dawn significant amounts of attention (Mooe et al, 2009). The potentially damaging effects of suging compessos ae widely acknowledged. Opeating compessos in the high flow egion, i.e. at highe flows than at the best efficiency point, often efeed to as oveload, choke o stonewall, has been identified by many manufactues as a egion of opeation that needs to avoided, also (Bun and Kuz, 2007).Many compesso manufactues will limit the opeation of the compessos at high flows) by Figue 1: Typical Map of a Speed Contolled Centifugal Compesso In the liteatue, we find only a few papes (Sookes, Mille and Koch, 2006, and Boe, Sookes, McMahon, and Abaham, 1997, Bun and Kuz, 2007) discussing the issue of compesso opeation in choke. Boe et al (1997) discuss the impact of aeodynamic foces on impelle vanes. They point out that downsteam flow nonunifomities, as they can be ceated by vaned diffuses o dischage volutes, can ceate fluctuating stesses in blade leading edges. Especially in high pessue compessos, the foces acting on blade leading edges can be quite lage. Sookes et al (2006) also focus on the isk of damaging the impelle leading edge o inlet egion if flow fluctuations in combination with off-optimum incidence ceate dynamic foces that exceed the allowable stesses. In paticula they point at the inteaction between non unifomties ceated by compesso

2 components such as diffuse vanes, volutes, guide vanes o etun vanes, and the flow field at the impelle inlet. The pesent pape will focus in paticula on the impact of axial thust loading of the compesso. OPERATION IN CHOKE Unfotunately, the setting of choke limits is somewhat abitay in many instances, and may be subject to questions, in paticula if it limits opeational flexibility. To be vey clea: Polonged opeation in the choke egion should be avoided, because if nothing else, the efficiency of the compesso is vey low. Whethe the compesso opeates in choke o not is not as clealy defined, and not as obvious fom an opeational standpoint as the opeation of a compesso in suge. A compesso map showing a single speed line will usually show a moe o less steep dop in head and efficiency when the compesso is opeated at flows highe than BEP. This behavio is moe ponounced at Mach numbes appoaching 1, and high mole weight gases (Figue 2). Figue 3: Inlet velocity tiangles at diffeent incidence Figue 4: Flow in a vaneless diffuse fom suge to choke Figue 2: Stage map fo M n =0.56 and M n =0.76 Fom an aeodynamic standpoint, choke efes to a situation whee flow passages become blocked eithe due to the occuence of compession shocks o due to massive flow sepaation. In centifugal compessos, these flow passages can eithe be the impelle inlet, o the inlet to diffuse vanes. Both opeation nea the suge limit and in choke leads to flow conditions that ae seveely diffeent fom the flow conditions at the compessos design point. If one uses the aifoil of an aicaft as an analogy, suge and choke would mak the stall points of the aifoil at vey high positive flow incidence (i.e. high angles of attack) and at a vey high negative flow incidence. Figue 5: Mach numbe contous (elative fame) fo impelle opeating at M n =0.56 (left) and M n =0.76 and slightly lowe flow coefficient (ight). At the lowe Mach numbe, stall due to negative incidence has developed. At the highe Mach numbe, shock has fomed at the pessue suface. Refe to Fig. 2. In the compesso, choke is elated to a flow egime at vey high flows which means that the flow channels between blade ows may expeience blockage effects, eithe fom sonic flow shocks, wake aeas, stong seconday flows, o simply by the fact that the distubed flow uses the though flow aea less

3 efficiently. In eality, one often sees a combination of all of these. If the compesso impelle is opeated at its design point, the flow will ente the impelle in the optimum diection. ( Figue 3). If the flow is educed, the diection of the flow into the impelle changes, inceasing the losses, and ultimately leading to flow sepaations on the suction side of the impelle vanes. This is often efeed to as stall. If the flow is inceased stating at the design point, the losses ae also inceased, but so ae the velocity levels enteing the impelle. Eventually, the combination of flow sepaation (this time on the pessue side of the impelle), and the occuence of compession shock waves will essentially limit the flow that can pass though the impelle (Figues 2 and 5). The shock waves themselves tend to be dynamic and fluctuating in natue. The eality is a little moe complicated, since the flow field is thee dimensional. Theefoe, compession shocks may only block pat of the flow path (typically close to the shoud side), and sepaated flow may fom athe complex flow stuctues. Vaneless diffuses offe no paticula challenge fo opeations in choke (Figue 4). In paticula, no issues with compession shocks, o a paticula dange of flow sepaation has to be consideed. -Compesso undesized fo the desied opeating conditions. -Pefomance degadation due to fouling OFF DESIGN AERODYNAMICS In multistage compessos impelles ae in geneal selected such, that at the design point, all impelles opeate at o nea thei best efficiency point (BEP) (o at about the same individual impelle suge magin. Suge magin is defined fo a compesso opeating at some flow Q, elative to the flow at suge Qs, fo a constant speed: Q Q = (1) Q s SM N = const This appoach tends to yield a good efficiency at the design point as well as a wide opeating ange. When the compesso is opeated away fom the design point, fo example at highe flows than BEP, the fist impelle will ceate less pessue atio, and theefoe achieve less volume eduction than befoe (this can be seen, fo example in Figue 5a). Theefoe, the second and subsequent impelles will incease thei espective suge magin at a faste ate than the fist impelle. In othe wods, the ea impelles will in geneal get to choke ealie than the impelles futhe in font (Figue 7). This means in paticula, that in situations whee the compesso appeas to be is nea choke, one o some of its stages may aleady be in deep choke. Figue 6: Cicumfeential Pessue Pofile of a Volute (Fiedle, 1989) Volutes, on the othe hand show a distinct change in the cicumfeential pessue distibution when opeated away fom the design point (Figue 6), which has to be consideed when the adial foces on the oto ae to be assessed. While opeation at high flow is unattactive to the use due to the associated dop in efficiency, thee ae opeational situations whee it can be encounteed: -Duing stat-up when the ecycle valve is opened too much. -Pocess upsets, fo example if two compessos opeate in paallel, and one of them has to be shut down (Ohanian et al, 2002) Figue 7: Shift in elative impelle opeating points at inceased flow fo an 8 stage centifugal compesso. THRUST LOADS Thust loads of centifugal impelles ae a esult of a pessue imbalance between the font face and the ea face of the impelle. The sum of these foces ove all impelles and the foces ceated by the balance piston ae the esulting load on the compesso thust beaing.

4 Fom the axial momentum equation, which takes into account the change of the axial momentum of the gas, and the foces due to the static gas pessue in axial diection: ρ C ( C d A ) = p d A F (2) + we get the esulting foces on the impelle as (Figue 8): F F impelle pessue = F ( p momentum ( c exit, p, c inlet ), p, p cavity, font cavity, ea inlet exit The font and ea cavities ae fomed between the impelle tip and the labyinth seals at the impelle inlet, and the impelle hub seals, espectively. ) (3) The foce on the thust beaing is thus (Figue 9): Fthustbeaing = (4) F F p, p ) impelle balancepiston ( disch ag e suction In the simplest appoach to calculate the foces on the impelle, one would assume the pessue in the font and ea cavities to be equal to the pessue at the impelle tip. In a shouded impelle howeve, the gas in the cavity is subject to swil, and as a esult, the static pessue at lowe adii is lowe than at the tip. The amount of swil is a function of the cavity geomety, and the leakage flows though the labyinths. The cavity static pessue distibution can be calculated by: p 1 2 tip 2 2 () = p ( q ω ) ( ) tip 2 ρ (5) accounting fo the cavity chaacteistics by intoducing a cavity swil coefficient q. A simple appoach would assume constant swil coefficients fo font and ea cavities. This appoach is fequently used in the industy, but high pessue compessos equie moe accuate estimates. Coelations and CFD analysis (Figue 10 a)ae being used fo these, along with subscale test measuements fo validation. Figue 8: Foces on the impelle Of paticula impotance fo the topic of off design opeation is the fact that the swil coefficient changes when the impelle is opeated away fom its design point (Figue 10 b). Also, the magnitude of the swil coefficient on the impelle backside changes in the opposite diection fom the swil coefficient on the font side of the impelle. This means that the thust imbalance (fo a given pessue level and a given speed) changes not just due to the pessue diffeence between the impelle eye and the coesponding backside, but also due to diffeent swil factos in the cavities in the font and back of the impelle. This imbalance, in paticula, changes when the compesso moves fom the design point to choke. In geneal, the shoud side swil is highe than the backside swil, a esult also epoted by Koenig et al, Figue 9: Balance Piston

5 Figue10 a: Swil atio in the shoud and the backside cavity 0.8 ATF C2-HP wc2-60tv Figue 11 a: Non dimensional map fo multistage compesso Cavity Swil Coefficient, q Font Cavity - Coelation Font Cavity - CFD Analysis Font Cavity - Pessue Measuement Rea Cavity - Coelation Rea Cavity - CFD Analysis Inlet Flow Coefficient, Phi Figue10b: Cavity Swil Coefficient fo a Medium Flow Stage fo diffeent opeating points Axial Thust, lbf (+: Outboad diection) Speed Incease Inlet Flow Coefficient Figue 11 b: Change of axial thust with opeating point Tempeatue F Thust Beaing Tempeatue vs. Inlet Flow Coefficient Low-speed InBoad Design-Speed InBoad High-Speed InBoad OutBoad OutBoad OutBoad Inlet Flow Coefficient Figue 11 c: Thust beaing tempeatue as a function of opeating point

6 Aveaged Axial Gap (mil) Axial Gap vs. Inlet Flow Coefficient Low Speed Design Speed High Speed Inlet Flow Coefficient Figue 11 d: Axial position of the oto as a function of the opeating point Because the thust load has a diect impact of the thust beaing tempeatue, which can be measued conveniently, Figues 11 a d establish the coelation between non dimensional opeating point (Figue 11a), thust load at diffeent speeds (Figue 11 b), the esulting beaing tempeatue of the loaded and unloaded pads of the thust beaing (Figue 11 c), as well as the axial position of the oto as a esult (Figue 11 d). The inboad beaing shows a significant incease in tempeatue (albeit not to a level that would cause concen) when the compesso entes the choke egion. The outboad beaing shows only a much lowe incease in tempeatue when the opeating point moves towads suge. Fo this paticula application, with the paticula selection of the balance piston size, the thust load eveses diection, which explains the behavio of the in boad and out boad beaing tempeatue. Of couse, the beaing tempeatue inceases also with speed. As a esult of the thust load changes and the changing load capacity of the thust beaing with speed, the axial gaps fo all speeds ae faily close togethe, but change significantly when the compesso is opeated fom design point to suge o into choke. If we compae the magnitude of the foces acting on the impelle (Figue 12), the pessue fom the inlet eye and the pessues in the cavities ae usually dominant, but act in opposite diection. In geneal, they geneate a esulting foce, much smalle than the pessue foces, in diection of the compesso inlet, but as can be seen in Figue 12, this is not always the case. The momentum foce, geneated by deflecting the gas fom moe o less axial to moe o less adial diection, is usually much smalle than the pessue foces. At vey high dischage pessues nea choke, when the pessue diffeential ove the impelle is athe small, the momentum foce can become dominant, and ceate a net foce towads the dischage end of the compesso. The desciptions in this section ae based on shouded impelles. As opposed to open faced impelles, that have fee standing blades, the blades in shouded impelles ae coveed. Theefoe, the pessue distibution on the font face of the impelle is govened by the impelle dischage pessue and the impact of swil flow. In an open faced impelle, the pessue distibution would be govened by the pessue build up in the impelle flow passages. MULTISTAGE MACHINES AND MACHINES WITH MULTIPLE SECTIONS Anothe issue that has to be consideed with multistage compessos opeating nea choke: The oveall compesso may still poduce head, when some individual stages ae aleady educing head, thus, acting as thottles. These impelles will consequently see a lowe pessue on thei dischage side than on thei suction side, which can in some instances alte the axial thust balance in the compesso, leading to inceased load on the thust beaings. The mechanical design of compessos with multiple sections egading the aangement of impelles can eithe be a staight tough design, o a back to back design. In a back to back design, with the impelles in the fist section facing in opposite diection of impelles in the second section, most of the axial thust balance is accomplished by the impelles themselves. Usually, a elatively small balance piston can handle the axial thust. Fo all applications, the axial thust has to be detemined fo all opeating conditions. Two section machines ae paticulaly citical in this espect if they have to handle multiple steams that change flow independently of each othe. Thust limitations can ceate limits fo the unbalance between the sections. In highly tansient conditions, such as emegency shutdowns (Mooe et al,2009), this has to be consideed. In some instances, hot gas ecycle valves have to be employed to equalize the pessue between the sections befoe the thust imbalance can cause damage. Figue 12: Contibuting Foces to the Impelle Thust in a 6 Stage Compesso. In the pevious consideations, the impact of seal leakage flows on impelle and balance piston thust foces has been mentioned. Impelle seals, as well as balance piston seal cleaances can incease ove time, although designs whee the labyinth teeth ae facing abadable mateial ae less pone to this poblem. Nevetheless the fact emains that inceased seal cleaances can change the tust balance. Howeve, monitoing

7 beaing tempeatues seems a good way of potecting the compesso fom thust load issues, especially when opeating at exteme points of the map. IMPELLER VANES Boe et al (1997) and Sookes et al(2006) have specifically discussed in moe detail the issue of damage to the impelle vanes due to high cycle fatigue. With espect to the opeation in choke, the fequency of the exiting foces on the impelle vanes can be captued in a Campbell diagam (an example is given in Figue 13). Compaing the exiting fequencies (in this case the vane passing fequency fom the inlet vane) with natual fequencies of the impelle and its vanes eveals the sepaation magin between excitation and esponse. If the sepaation magin is deemed too small, impelle modifications such as inceases in vane thickness can educe stess levels and modify impelle natual fequencies. Figue 14 shows the esult of such a edesign, whee it was possible to achieve the stess eduction and a lage sepaation magin, while etaining the impelle pefomance. It should be noted that in some axial compessos, but also in centifugal compessos with fee standing blades, a phenomenon called choke flutte can be obseved when they ae opeated in the choke egion (Fottne, 1989). The mechanism is diffeent fom the mechanism mentioned above, as it is the flow aound the blade that causes the excitation. Howeve, the fequencies that ae excited ae, just as in the case of distubances in the inlet o exit flow, the blade natual fequencies. The aeodynamic excitation of the blade is usually caused by bounday laye sepaations, tansonic shock pattens, and the elated votex fequencies. This poblem has, to ou knowledge, not been encounteed in centifugal compessos with shouded impelles. Figue 14: Pincipal stess distibution nea the leading edge of an impelle vane. The ight pictue shows the impoved vesion of the same impelle with educed stess in the hub and shoud egion. CONCLUSIONS Besides the pefomance penalties, the study indicates that opeating in choke is often not a poblem fo the compesso povided: -The balance piston is sized coectly to povide adequate thust load balance ove the entie opeating ange -Issues like blade stength to deal with altenating stesses ae consideed, o the occuence o stength of altenating stesses is educed. Figue 13: Campbell Diagam fo an Impelle. NOMENCLATURE ρ = Density p = P essue C = Velocity A = Aea v F = Foce Q = Flow M n = Machine Mach SM = Suge M agin Numbe

8 = p = tip = q = ρ = ω = adius pessue impelle cavity density impelle REFERENCES tip swil coefficient otational speed Boe, C., Sookes, J., McMahon, T., and Abaham, E., 1997, An Assessment of the Foces Acting Upon a Centifugal Impelle using Full Load, Full Pessue Hydocabon Testing, 26 th Tubomachiney Symposium, Houston, Tx. Bun,K., Kuz, R.,2007, My Compesso can (cannot) un in Choke o Stonewall, Tubomachiney Intenational, Nov/Dec Fiedle, K., Tubinen und Tubovedichte, Univesitaet de Bundesweh Hambug, Fottne,L., 1989, Review of Tubomachiney Blading Design Poblems, AGARD LS 167,Toonto, Canada. Koenig,S., Pety, N., Wagne, N.G., 2009, Aeoacoustic Phenomena in High pessue Centifugal Compessos A possible Root Cause fo Impelle Failues 38 th Tubomachiney Symposim, Houston, Tx. Mooe, J.J., Gacia-Henandez, A., Blieske, M., Kuz, R., Bun, K., 2009, Tansient Suge Measuements of a Centifugal Compesso Station duing Emegency Shutdowns, 38 th Tubomachiney Symposium, Houston, Tx. Ohanian,S., Kuz, R.,2002, Seies o paallel Aangement in an Two-Unit Compesso Station, ASME J Eng fo GT and Powe Vol. 124, pp Rasmussen, P.C., Kuz, R., 2009, Centifugal Compesso Applications-Upsteam and Midsteam, 38 th Tubomachiney Symposium, Houston, Tx. Sookes, J., Mille, H., Koch, J., 2006, The Consequences of Compesso Opeation in Oveload, 35 th Tubomachiney Symposium, Houston, Tx.

9 Poceedings of the Fotieth Tubomachiney Symposium Septembe 12-15, 2011, Houston, Texas MODELING AND PREDICTION OF SIDESTREAM INLET PRESSURE FOR MULTISTAGE CENTRIFUGAL COMPRESSORS Jay Koch Pincipal Enginee Desse-Rand Olean, NY, USA Jim Sookes Pincipal Enginee Desse-Rand Olean, NY, USA Moulay Belhassan Pincipal Enginee Desse-Rand Olean, NY, USA. ABSTRACT It is common fo some compessos in cetain applications to have one o moe incoming sidesteams that intoduce flow othe than at the main inlet to mix with the coe flow. In most cases, the pessue levels at these sidesteams must be accuately pedicted to meet contactual pefomance guaantees. The focus of this pape is the pediction of sidesteam flange pessue when the etun channel outlet conditions ae povided. A model to pedict the impact of local cuvatue in the mixing section is pesented and compaed with both Computational Fluid Dynamics wok and measued test data. INTRODUCTION Despite ecent advances in analytical tools, developes and uses of state-of-the-at centifugal compesso equipment continue to ely heavily on testing to ultimately confim the pefomance of new components o stages. This is especially tue in heavy hydocabon applications such as compessos used in liquefied natual gas (LNG), ethylene, o gas-to-liquid facilities. Heavy hydocabon gases have vey low gas sonic velocities that poduce high Mach numbes in the aeodynamic flowpath. By thei natue, such high Mach numbe, high flow coefficient stages have vey naow flow maps chaacteized by limited choke and suge magin. In addition, many of these applications equie the machines with sidesteams (o sideloads) to accommodate the flows enteing and/o exiting the compesso as detemined by the paticula pocess fo which they ae intended. The sidesteams and associated mixing futhe complicate the pefomance pediction pocess because the pessue, tempeatue and flow conditions at each one of these sidesteams as well as at the exit of the machine must be met within stingent toleances to optimize the amount of end poduct deliveed by the pocess. Theefoe, it is impeative that the compesso OEM and end use have a fim gasp of the pefomance chaacteistics of any impelles, diffuses, etun channels, and/o othe flow path components used in such equipment. The high cost associated with delays in the poject schedule incease the attactiveness of design and testing methods that ensue these machines meet the contactual opeating equiements the fist time; without the need fo epeated modifications and test iteations to coect pefomance shotfalls. Given the size, constuction style, and in-house test piping aangements fo many lage centifugals, one modification / e-test iteation could take seveal weeks. Repeated iteations on the OEM s test facility could delay the stat-up of a new plant by months, leading to lost poduction / evenue fo the end use as well as lost evenue and eputation fo the compesso OEM. Consequently, OEMs spend continually wok to enhance thei pefomance pediction methods. This pape details wok done towad that end. This pape is the latest in a continuing seies of publications descibing wok completed in association with a novel test vehicle that was designed to eplicate the pefomance of a fullscale, lage fame-size, multi-stage centifugal compesso fo high flow coefficient, high Mach numbe stages. The test vehicle, descibed by Sookes et al (2009), is equipped with a vast aay of intenal instumentation that make it possible to evaluate the aeo/themodynamic and mechanical behavio of a compesso. Sufficient instumentation is installed to allow assessment of the entie compesso as well as individual components o combinations of components. Of paticula inteest to this study, the test ig included instumentation at citical locations within the sidesteam components to pemit an assessment of the losses though each sidesteam element. BACKGROUND As pat of the compesso selection and design pocess, the OEM must be able to pedict the pefomance of the oveall