Research on the Reciprocating Sealing of Fracturing Pump

Transcription

1 TELKOMNIKA, Vol.10, No.3, September 2012, pp. 499~504 ISSN: accredited by DGHE (DIKTI), Decree No: 51/Dikti/Kep/ Researc on te Reciprocating Sealing of Fracturing Pump Wei Bing Zu 1, Gang Zou 2, He Sun Wang 3 1,2,3 Scool of Mecanical Engineering and Automation, Xiua University, Cengdu, , Cina qazzwb@sou.com 1, zougang555@yea.net 2, wangesun@sou.com 3 Abstrak Kegagalan penyegelan timbal-balik di dalam keretakan pompa OPI-1800AWS tela dianalisis kegagalannya dengan teknologi pemindaian mikroskop elektron (SEMT). Mekanisme pelumasan pasangan gesekan segel plunger tela dianalisis dengan teori pelumasan idrodinamik. Selama peluaan dan stroke suction, pasangan gesekan penyegelan timbal balik tela memenui kondisi pelumasan idrodinamik, suatu ketebalan tertentu film minyak dapat dibentuk dan dipeliara diantara plunger dan bentuk V segelan antarmuka, yang dapat melumasi pasangan penyegel, mengurangi gesekan dan meningkatkan usia pakai. Distribusi tekanan aksial dari penyegelan berbentuk V dilakukan dengan analisis teoritis dan pengujian eksperimental. Kegagalan bentuk-v penyegelan dan penentuan jumla loop juga tela dibaas menurut yang dibaas menurut kurva distribusi tekanan sumbu. Kata kunci: kegagalan, keretakan pompa, pasangan gesekan segel plunger, tekanan aksial Abstract Te failure reasons of reciprocating sealing in OPI-1800AWS fracturing pump are analyzed by scanning electron microscope tecnology (SEMT). Te lubrication mecanism of plunger seal friction pair is analyzed by ydrodynamic lubrication teory. During its discarge and suction stroke, te reciprocating sealing friction pair meets te conditions of ydrodynamic lubrication, a certain tickness of oil film can be formed and maintained between te plunger and V-saped sealing interface, wic can lubricate sealing pair, reduce friction and improve lifetime. Te axial pressure distribution of te V-saped sealing is carried out by teoretical analysis and experimental testing. According to te distribution curve of axial pressure, te V-saped sealing failure and te determination of te number of loops are discussed. Keywords: axial pressure, failure, fracturing pump, plunger seal friction pair 1. Introduction At present, te proportion of increasing oil and gas production by fracturing or acidification tecnology is getting larger. A successful fracturing may increase te production of oil and gas wells several times, even more tan a dozen or dozens, te benefit is extremely considerable. According to te working principle of fracturing and acidification, large displacement and ig-pressure injection are necessary for tis tecnology, and fracturing pump is te core equipment to complete te work. In te long-term practice and researc, fracturing and acidification tecnology is getting better and better, in contrast, fracturing and acidizing equipment is not compatible. Tis seriously affects te normal implementation of te fracturing and acidizing tecnology [1], [2]. Te working conditions of plunger, V-saped seal, valve disk and valve seat are seriously bad. Tese wearing parts are te weakest link of te fracturing pump. From te current using of plunger and seals used in oil fields in Cina, teir lifetime is very low, and for annual consumption is great. Te investigation indicated, in Sicuan area, during acidizing operations, te lifetime of plunger is more tan 100 ours, te lifetime of sealing ring is ours, sometimes ours. Wit te frequent fracturing and acidizing operation, te lifetime of plunger and seal will become increasingly prominent. So, te site urgently requires te systematic study of te plunger seal friction pair in fracturing pump, in order to improve teir lifetime and ensure te smoot construction. Tis paper intends to carry out systematic researc on te plunger seal friction pair of OPI-1800AWS fracturing pump. All tese can provide a basis for design improvement and experimental researc of reciprocating sealing in ig-pressure fracturing pump. Received May 4, 2012; Revised June 3, 2012; Accepted June 13, 2012

2 500 ISSN: Te Structure of Plunger Seal Friction Pair Te structure of ydraulic end of OPI-1800AWS fracturing pump is sown in Figure 1, te plunger 1 can only ave reciprocating linear motion under constraint, te back pressure cap 2 is used to compress and adjust sealing, te seal body comprised of tree V-saped rubber sealing ring 7 and tree V-saped PTFE pad rings 8 alternately can ensure sealing ring working successively, wic means after te failure of te first sealing ring, te differential pressure will transmitted to te second one wic can also function as te same, tus te wole lifetime improved. Supporting ring 6 is te key component to support V-saped sealing ring, pressure ring 9 is used to give V-saped sealing ring initial compression and keep contacting adequately between te ring and te surface, te end face widt of pressure ring is smaller tan te widt of te cavity. Te gap between te pressure ring and te plunger is about mm, so te pressure can act on sealing lip and make it fully open [3], [4]. Te small ole 5 on te ousing is te entrance of forced lubrication oil. It is evident tat dirty pumping liquid will damage te seal. Te solution to tis problem at ome and abroad is injecting lubrication oil troug te small ole, and te injecting pressure of lubrication oil is sligtly iger tan te perfusion pressure of te fracturing pump. Te purpose of tis metod is forming a ig-pressure oil film between te plunger and seals to prevent dirty fracturing fluid (medium) from entering sealing interval and protect seals. 1 plunger, 2 back pressure cap, 3 O-sealing ring, 4 scraping ring, 5 lubrication oil entrance, 6 supporting ring, 7 rubber seal ring, 8 PTFE seal ring, 9 pressure ring, 10 front supporting ring, 11 testing pressure ole, 12 pressure sensor Figure 1. Te structure of plunger seal friction pair 3. Failure Reasons of Plunger Seal Friction Pair 3.1. Reciprocating Speed and Surface Quality of Plunger Wen reciprocating speed of te plunger is ig, more frictional eat will produce between te plunger and sealing rings. Tis will accelerate aging of te rubber pieces, losing teir elasticity and reducing te radial acting force on te plunger. Along wit well dept increasing, and fracturing acidizing radius expansion, it is urgent to strengten fracturing acidification tecnology, wic is means te injection of fracturing liquid wit ig pressure, ig gravel ratio, ig acidity is needed, so tat te working condition of te plunger is arser, and its lifetime reduces. Te Crome-plated tecnology is applied to te plunger surface of OPI- 1800AWS fracturing pump. Teoretically, te surface as ig wear resistance, ig eat resistance and ig corrosion resistance, but actually te lifetime of te plunger is sort. Te surface quality of te plunger is poor, so te plunger will be worn, pulling injury, erosion and corrosion frequently [5]. Wen te plunger is damaged, te fracturing and acidizing fluid will enter te crankcase, wic will cause lubricating oil in te crankcase serious deterioration and losing lubrication. So te key components of te crankcase, suc as, crossead, gear, tension rod, bearing and so on, will ave serious wear, erosion and corrosion, so tat a major accident will occur. We use termal spray tecnology to strengten te plunger surface, te materials of spray welding are PHNi60A and WC, te tickness of spray welding is 0.5 mm, te surface ardness is more tan HRC 60. After te spray welding tecnology, te plunger surface sould be grinding processed. After tese processing, te wear resistance and corrosion resistance of te plunger are improved evidently Wen Crossead, Plunger, Tension Rod and Plunger Head Reciprocate teir Concentricity wit Cylinder Crossead and tension rod are connected by tread, plunger ead and plunger are connected by tread, and plunger ead and tension rod are connected by sperical inge. All TELKOMNIKA Vol. 10, No. 3, September 2012 :

3 TELKOMNIKA ISSN: tese parts as a wole ave a reciprocating motion. Teir public axis sould be coincidence wit te public axis of crossead and cylinder, oterwise, te sealing rings will be worn partially, and lose efficacy Materials of Sealing Ring Te material performance of sealing ring is te key condition of application scope and important factor of lifetime. Te failure modes of sealing ring of OPI-1800AWS fracturing pump are permanent deformation, partial wear, burn, abrasive wear and fatigue wear, etc [5]. Burns caused by overeat are main failure reasons of te sealing rings, and burns will make te seal immediately lose working ability, wile oter failure modes resulting in loss of sealing capability is a gradual accumulation process. So we must coose te rubber wit good eat resistance as te material of sealing ring. Te original material of sealing ring is single clot rubber, its wear resistance, pressure resistance, eat resistance and corrosion resistance are poor, so te lifetime of sealing rings was sort. Te new sealing rings are made of polyuretane rubber and filled PTFE (filled wit bronze powder), wic are used troug interactive permutation and combination. Polyuretane rubber as good wear resistance, pressure resistance, corrosion resistance, eat resistance and ageing resistance, as well as as ig breaking strengt, good air permeability and elasticity. Filled PTFE as te caracteristic of strong bearing capacity, good resistance of ig and low temperature, strong resistance of oil and corrosion, anti-aging, small friction coefficient and good self-lubricating property. Combination use of tese two sealing rings will ave better performance and longer lifetime [6] Abrasive Wear of Sealing Ring Te abrasive wear of sealing ring is also an important factor in failure. Under te action of working pressure, te V-saped rubber sealing rings ave deviation effect [7], [8]. Tat is, te lip of te V-saped sealing rings will depart from te plunger surface, so abrasive will enter te plunger sealing friction pair, especially in te suction stroke of fracturing pump. Because, during te discarge process, te plunger puses to cylinder, and te contacting stress between te plunger and seals is very large, abrasive is not easy to enter. But in te suction process, te contacting stress is small, abrasive is easy to enter. In order to reduce abrasive wear and improve te lifetime of plunger and seals, te abrasive media sould be prevented from entering te gap of reciprocating sealing pairs as far as possible. 4. Lubrication Mecanism of Plunger Seal Friction Pair Site investigation indicates tat, wen tere is no lubricating oil, te V-saped sealing rings will fail due to overeating in a few minutes, but under good forced lubrication condition, te lifetime of te sealing rings will reac ours. Obviously, good forced lubrication is good for improving te lifetime of te rings. According to te working property, plunger seal friction pair is reciprocating sealing, its working medium as certain viscosity, for example, working medium is acid liquid, lubricating medium is lubricating oil. If convergent oil wedge can be formed, te conditions of ydrodynamic lubrication will be formed. Finite Element Analysis [7] as sown tat, during te suction stroke of fracturing pump, under te action of working pressure, te V-saped rubber sealing ring as deviation effect, as sown in Figure 2. Tat is, because of stretcing, te lip of te V-saped sealing ring will depart from te plunger surface, so convergent wedge is formed, working medium is brougt into te sealing surface, and conditions of ydrodynamic lubrication are satisfied, so oil film is formed, wic is called lubrication of medium. At te back of te sealing, because of te velocity direction of te plunger, convergent wedge can not be formed. During te discarge stroke of fracturing pump, as sown in Figure 3, depending on te pre-design of round corners or deformation at te seal back, convergent wedge is formed, lubrication oil is brougt into te sealing surface, and conditions of ydrodynamic lubrication are satisfied, so oil film between te plunger and te seals is formed, wic is called forced lubrication. So, during te discarge and suction strokes of fracturing pump, te reciprocating sealing friction pair can meet te conditions of ydrodynamic lubrication. A certain tickness of oil film can be formed and maintained between te plunger and V-saped sealing interface, wic can lubricate sealing pair, reduce friction and improve lifetime. Researc on te Reciprocating Sealing of Fracturing Pump (Wei Bing Zu)

4 502 ISSN: Seal back Seal lip Lubrication oil Seal back Seal lip Working medium u Plunger Figure 2. Suction stroke of fracturing pump u Plunger Figure 3. Discarge stroke of fracturing pump 5. Axial Pressure Distribution of Plunger Seal Pair 5.1. Teoretical Analysis on Axial Pressure Distribution of V-Saped Seal Assume tat V-saped seals ave sufficient rigidity, te deformation of sealing ring caused by liquid pressure between te plunger and sealing rings is very small. Assume tat te circumferential pressure distribution is uniform, so te gap between te plunger and te inner surface of sealing rings is concentric cylinder ring, according to literature [9], te axial pressure can be expressed wit a formula below: dp dx 1 Q 12η u 2 (1) 2 u = 3 were: p : η : u : axial pressure liquid viscosity relative speed of reciprocating motion : Q : x : tickness of liquid film flow of per unit circumference distance along te seals According to Equation 1, wit designed equipment, certain pressure and medium, te axial pressure distribution is linear. Actually, te assumptions above are far way from te actual working conditions, on one and from te view of material, V-saped seals ave low rigidity and te large deformation will emerged from liquid pressure, and ig pressure generate large deformation, so te gap between te inner surface of sealing ring and plunger can not be parallel, and te tickness of ydrodynamic film between sealing ring and plunger is canging in axial direction. On te oter and, te plunger is produced by macining, turning or grinding will leave marks on te surface of te plunger. Simultaneously, te difference of rotational speed, feeding and granularity of grinding weel will generate different rougness on te surface of plunger. From te view of microscopic point, different sape wave ridge and troug are generated on te surface of plunger, and te distribution of trougs is irregular, te cannel of trougs will be formed on te place were trougs contact. Wen installing sealing rings, initial contacting stress produced by extrusion and interference press te sealing ring on te surface of plunger, ten, after aving enoug contacting stress, te material can be pressed into troug to stop leakage from te cannel of trougs. Wen sealing ring works, to getting te purpose of sealing and guaranteeing lubrication and dropping temperature to extending lifetime of seals, te general leakage rate sould be controlled in 5-8 drops per minute. So, in normal working condition, te contacting between seals and te plunger can not fill all trougs, te cannels of trougs are always existed. Wen te plunger is stationary and te cylinder as pressure, te liquid will permeate from tese cannels. As for multiple sealing rings, first of all liquid will permeate into te gap on te ig pressure side between te first and te second sealing ring, at te beginning, tis gap as no pressure. But wen te permeating liquid fills tis place, te pressure will form, and te pressure will be increased because of te continuing permeating caused by pressure difference, so te pressure in te cannel of trougs is ig, ten te filling from seals to trougs is decreased and te cannel is increased. Wen contacting stress is decreased, te pressure in cylinder can be almost completely transferred to te gap between te first and te second sealing ring on te ig pressure side, and tis pressure is almost equal to sealing pressure. Te permeating liquid from te last sealing ring (near te air side) can not be collected, because te out side of tis seal is air, and te pressure in cannel of trougs can not be increased, so te cannel will not be increased, te pressure difference produced by resistance will not be eliminated. Te biggest pressure difference exists in last sealing ring. Wen te plunger moves, during te discarge TELKOMNIKA Vol. 10, No. 3, September 2012 :

5 TELKOMNIKA ISSN: stroke, te trougs sometimes communicate, and sometimes separate. Due to te dynamic pressure oil film, communicating becomes te majority. Te axial pressure distribution of inward V-saped sealing rings almost as no pressure reduction, te biggest pressure difference exists in last sealing ring. So, wen te working pressure is ig, multitude sealing rings can not witstand te pressure difference equally Axial Pressure Testing of V-Saped Seal In order to test te axial pressure distribution between te plunger and V-saped sealing rings, a testing system was designed, as sown in Figure 1. Troug te testing pressure ole 11 in te plunger, te pressure is elicited, and connected wit BP4-600 semiconductor strain pressure sensor. In order to ensure testing accuracy, te diameter of te testing pressure ole is 0.2 mm. Performance indexes of te sensor are, nonlinearity error is less tan 0.35%, ysteresis error is less tan 0.04%, non-repeating error is less tan 0.15%, output sensitivity is bigger tan 39 mv/v. Wit no amplifying, te testing pressure is recorded by SC-16 ligt oscilloscope [10]. If tere is air in testing pressure ole, dynamic pressure response will vary greatly and distortion will be serious. Terefore, before installing te pressure sensor, te air in te testing pressure ole sould be drained. Because te V-saped seal is stationary, wen te testing pressure ole in te plunger moves a stroke, te pressure sensor will get te axial pressure distribution of te plunger seal friction pair. In te back and fort strokes, te pressure-rise spot in te cylinder and te point were plunger leaving te sealing surface to atmosperic side can be determined troug analysis. Te preload caused by back pressure cap can not be too large, oterwise, altoug te leakage is reduced, but te pre-radial deformation of seals will accelerate te wear of seals and reduce te lifetime of seals. Te materials of sealing rings are V-saped clot rubber and V-saped PTFE. Te internal diameter of tese two kinds of sealing rings is φ mm, te outer diameter is φ 127 mm, te working liquid is water, te reciprocating speed of plunger is 2.21 m/s. Adjust te trottle valve, so tat te discarge pressure respectively is 30 MPa, 40 MPa, 60 MPa, 80 MPa, and we can get te axial pressure distribution of sealing rings, as sown in Figure 4 and Figure 5. P (MPa) X (mm) P (MPa) X (mm) Figure 4. Axial pressure distribution of PTFE seal Figure 5. Axial pressure distribution of rubber 5.3. Influence of Axial Pressure on Failure and Number of Seal Ring Wear and fatigue are te main reasons of sealing failure. Wear is related to te rougness of te plunger surface, lubrication and cooling. Wen te ydrodynamic oil film is existed and tick, wear can not easily appen. Fatigue damage is te result of ig and low pressure periodic forcing, and te alternate contacting of wave ridge and troug between sealing ring and plunger surface. Te plastic deformation is produced in te space were sealing ring contacts wit wave ridge, and sifted to adjacent trougs. Because of te alternate existence of wave ridge and troug and te motion of plunger, te material deforms, transfers, and finally fall soff from te body due to te wear. During its discarge stroke, te wole pressure difference occurs on te last ring, tis indicates tat troug cannel is only squeezed by te outmost V-saped sealing ring to prevent leakage, at te same time, te ydrodynamic oil film on V-saped sealing lip is very tin, te plastic deformation produced by extrusion is large. During its suction stroke, te pressure in te cylinder is low, te ydrodynamic oil film between te sealing ring and plunger is near zero, so te portion of eac lip is squeezed into troug cannel. But te pressure caused by te back pressing cap can not be evenly transferred to eac ring, te compressive stress of te outmost Researc on te Reciprocating Sealing of Fracturing Pump (Wei Bing Zu)

6 504 ISSN: ring is biggest. Terefore te outmost ring is te most one seriously squeezed into te troug cannel tan oters, te outmost one as more fatigue and wear in working process and more easily fails tan middle ones. Actually, te same situation is proved troug te information collected from te users. Wen middle ones discarge liquid, because te oil film as large pressure and ig tickness, teir filling content in trougs are small as same as teir wear. During suction stroke, due to te tickness of oil film is close to zero, te portion tat squeezed into trougs is large, so te wear on te middle rings appen in te suction stroke. From te distribution curve of sealing axial pressure, we can get tat, te pressure difference in every V-saped sealing ring is different, in te outmost sealing ring is largest. Increasing or decreasing te number of rings can not cange te pressure distribution wic te outmost ring bears, te pressure is iger, te lifetime of te most outside ring is sorter. Just from te effective sealing, fewer sealing rings can acieve, but in actual applications, wen te sealing pressure is ig, appropriately increasing te number of te sealing ring as a certain effect to extend te wole lifetime of seal. Te autor tink tat wen te outmost fails, te second outmost one bears te igest pressure difference, so for te same working condition and same sealing rings, te wole lifetime will be improved wen te number of rings increase. 6. Conclusion Performance and service lifetime of reciprocating sealing of fracturing pump depends on surface quality of plunger, te material and structure size of sealing ring, and manufacturing precision of box and related parts. Te design of reciprocating sealing sould adapt to complex conditions and also ave good performance and long lifetime. During te discarge and suction strokes of fracturing pump, te reciprocating sealing friction pair meets te conditions of ydrodynamic lubrication, a certain tickness of oil film can be formed and maintained between te plunger and V-saped sealing interface, wic can lubricate sealing pair, reduce friction and improve lifetime. Te axial pressure distribution of te V-saped sealing as been gained by experimental testing. Teoretical analysis and experimental studies ave sown tat, wen te plunger of fracturing pump as reciprocating motion, in te working scope of sealing ring, te biggest pressure difference appens in te outmost seal ring, and as a sarp decline in te small section near te air side. Tis means tat, tis kind of sealing structure of fracturing pump is suitable for iger pressure sealing, and not suitable for low pressure seal. Acknowledgments Tis work was supported in part by NSFC under Grant Nos , Training Fund of Sicuan Academic and Tecnical Leader under Grant Nos and te Key Scientific Researc Fund of Sicuan Province Education Department Nos. 12ZA168. References [1] H X Wang. Principles of ydraulic fracturing. Beijing: Petroleum Industry Press [2] H W Wang, D G Xu. Oil drilling equipment, tool and accessories. Beijing: Petroleum Industry Press [3] T H Guang, D T Wang. Seal user's manual. Beijing: Mecanical Industry Press [4] Z L Zang, Y T Zang. Development of model RST200 plunger/rod sealing member test stand. Field Equipment. 2000; 4: [5] Z L Zang, W B Zu. Failure analysis of te plunger of plunger pump OPI-1800AWS. Journal of XiHua University (Natural Science Edition). 2005; 5: [6] J.F li, S W Zang. A review of reciprocal packing material and its forecast. Petroleum Macinery. 1992; 12: [7] W B Zu. Finite element analysis of sealing ring in plunger pumps. Sicuan University of Science and Tecnology. 2004; 1: [8] Alimuddin, Kudang Boro Seminar, I Dewa Made Subrata, Nakao Nomura, Sumiati. Temperature Control System in Closed House for Broilers Based on ANFIS. TELKOMNIKA. 2012; 10 (1): [9] X Dong. Lubrication teory. Sangai: Sangai Jiaotong University Press [10] Herry Z Kotta, Kalvein Rantelobo, Silvester Tena, Gregorius Klau. Wireless Sensor Network for Landslide Monitoring in Nusa Tenggara Timur. TELKOMNIKA. 2011; 9 (1): TELKOMNIKA Vol. 10, No. 3, September 2012 :