Feng, Quanke

Shizhao, S.

Yang, Shaokan

English

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Purdue UniversityPurdue e-PubsInternational Compressor Engineering Conference School of Mechanical Engineering1990Research on the Lubrication Conditions of thePacking/Rod System in CompressorsQuanke FengXi'an Jiaotong UniversityShaokan YangXi'an Jiaotong UniversityS. ShizhaoXi'an Jiaotong UniversityFollow this and additional works at: https://docs.lib.purdue.edu/icecThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlFeng, Quanke; Yang, Shaokan; and Shizhao, S., "Research on the Lubrication Conditions of the Packing/Rod System in Compressors"(1990). International Compressor Engineering Conference. Paper 782.https://docs.lib.purdue.edu/icec/782RESEARCH ON THE LUBRICATION CONDITIONS OF THE PACKING/ROD S'I'STEM IN COMPRESSORS Quanke Feng, Shaokan Yang, Smzhao Department of Power Machinery Engineer:l.I1g Xi'an Jiaotong Univers~ty, Ch:l.I1a A s:unulating test for packing-rod systems :li1 reciprocating cc:rtpressors is con-ducted to measure the pressure distribut~on in the packing , the frictional res~­tance, the teroperature on the sliding surface of sealing elanents emd the gas leak-age through the packing. MathE!llatical simulations for t~ systan is alsO achieved. The results shaw that :li1 order to mawtain an excellent lubricating condition the seal:l.I1g pressure drop in one set of sealing elanents should not excee:l. a certain magrntude, which can be determined according to the red speed, the tanperature of o~l film and the surface profile of the sealing elanent, etc. INTRODUCriON In reciprocating c~essors the cylinder packing is liable to lose their seal-ing function because of rapid wear. For a ~ressor lubricated with oil this means that the oil film between the piston rod and sealing elements often splits. In '"'";>arisen with piston rwgs the sealing elements in a packing often bear much larger sealing pressure drop and wo:r:-k wj.th pocrer coolmg. On the other hand, the gas leakage through the packing elanents will cause a loss of a valuable gas and environmental pollutions. Therefore to achieve a better cc:rtprehens~on for the sealong mechanJ.Sm and lubricatmg cond;.tions of packing/red systems is very i.rrpor-tant. There are two aspects in the research of the paCking/red systems. One is the wvestigatlon of the gas leakage and influences of related factors. 'Ire oti2: is tlJ: analys~ of the lubricatwg conMtions J.n the packings. In recent years few papers concernwg the sealmg and lubrication of cc:rtpressor packings have been found. How-ever, the publ;.shed papers dealmg with that of p;.ston r:li1gS ~ qu;.te ccmnon. In pr:l.I1Ciple, the function of sealmg elements in packing is similar to that of p;.ston r:l.I1gs, though the rings slide aga;.nst the llner surface with their outer surface, and the sealing elements slide with their lnner surface aga;.nst the pj.ston rod. Under certain assumpUons the effechve gas P,essure governing the loading of the rmgs can be calculated according to the sealmg condition of the rings. For a pack-ing the gap clearances of tangential sealing rings are concealed by a three-piece front rwg(Fig.l). In thls way, not a fixed passage cross-section is left free. M:>rever, a greater pressure drop across a certain set of sea!mg elements tends to minimize the leakage clearance rroce effectively. which would further increase the pressure drop accross this set of sealmg elanents. Hence this gives rise to another sealing mechanJ.Sm, and the pressure drop can no longer be calculated because the geanetry of the leakage cross-secbon can not be determined now. It may h<ippen, for exarrple, that a certain set of sealmg elanents provides the entire sealmg/1/. Consequently, only measurements can reveal the actual pressure distribution in the packi!lg. If the pressure distribution m the packing is obtained, the loading on a seal:l.I1g rmg can be determined, and then the lubrlcatmg conMtion can be simulat-ed with mathematical equations. EXPERIMENTAL TECHNIQUES In order: to measure the main parameters which charactecize the sealing and lubricatiilg condition :li1 a packing, a rig is constructed by the authors. The r~g consists of a ~gh-pressure ccmpressor, a crank-link systan, a packing/rod systan and a measurwg systan. It is used to simulate the working precess of a packmg/ 872 :-od sysc:B'I\ in compressors and to test the frlctional resl.stances, the sealmg pres-sure dl.St:ributlon, the surface temperature and the leakage lll this systm~. The com-pressed au from the compressor enters the packing frcm the middle box. Then l.t leaks along the pl.ston-rod towards the top and boctcm of the paclUng in other words, the sealing system consists of two packings-~ and lower one. The PlSton-rod is connected to the crank-link systEm, whlch lS driven by a motor whose rotational speed l.S adJustable. In order to test the fLl.C'CiOnal resistance the packings connect only to a wheel, on whose spoke several strain sensors are rrounted. The outer edge of the wheel lS fixed to the fLame of the ng (Fig.2). Each sealing cha!llber is provided wl.th a pressure sensor to measure the pressure distribution in the packing. Several therm:>couples are mounted on the slidlng surface of the sealing elenents to measure the temperature there. The air leaked out rrom the top of the packmg box enters a gas collector, and then lS measured wlth a float-type flow meter. At first, two sets of sealmg elErnents type T tMde of cast-iron are tested on the rJ-g. One set is in the upper packing, and the other ln the lower one. Under the condition of a sufficient oil supply the measured rrictional reslStance between the rod and sealing elB!lents is shOWn in Fig. 3. In order to ccmpare the cl:laracte-rl.stl.cs of the reslStance, the same experiment as mentioned above is carrl.ed out without the ol.l supply. The measured resistance is also given in Fig. 3. In all the above experiments the total pressure drop across the packl.ng lS 6 MPa. The diameter of the plSton rod lS 45 mm. and the rod stroke is 120 mm. Because the loadwg acted on the above-tested sealing elements is constant m a working cycle, the factors affecting the frl.ctional resistance are expected to be the speed of the rod, the lubricating conditl.On m the paclung and the tem-perature of the sliding surfaces. The frictional resistance in Fig.3a varies with the rod speed. ReslStance in Fig.3b lS not closely related to the rod speed. In the case of dry fLlction the reslStance does not vary with the rod speed. On 1:he principle of tribology/2/ a rric1:ional state is divided into three types as shown in Fl.g.4: 1. hydrodynamic lubrl.cation: in this case two sliding surfaces are separa'Cecl by a hydrodynamic oil film, and the resistance increases directly with the sllding speed; 2. Inl.Xed lubrication, in which the loading is supported by both the hydrodynSffil.c film and ml.c:roprotuberances on the sliding sur faces, and the resistance partly relates to the slidmg speed1 3. boundary lubrication, in which the microprotuberances on the sliding surface apparently contact each other. and the resistance is nearly independant of the sliding speed. Obvl.ously, the resistance =ve shown in Fig.3a characterizes a condition of hydrodynamic lubrication, and the curve shown in Fig.3b indl.cates a mixed luhricat-l.ng conditl.on. In order to achieve a hydrodynamic lubricating condition for packing/ rod systmlS, mathemat~cal s:unulations for this lubricated systan are necessary. THEOREtiCAL ANALYSIS Many reseachers have carried out mathematical simulations of the lubricating conditl.ons in compressor cylinders. Consl.dering the si!nilarity of the lubricating mechanism of packing elements to that of piston rings, Reynolds equation is expec:t:ed to be available to the packing/rod system, though these are sane differences between these two systems, sane of whlch will be discussed later. For a segrrent of a three-pl.ece sealing ring the forces acting on it in radial direction are gas pressure, the strain force of the sprwg, the supportwg force of the oil film and the frictional resistance on the flank of "the sealwg ring ( Fl.g. 5) . According to the equalibrlum condl.tion of these forces, the force acting on the ol.l film can be determined. Under the condition of hydrodynamic lubrication, the simulating equatlon for a seall.ng ring is described as follows' z(J{;;p) +_g_c4'.H) -w&-r- 12 U a:x: AJ. ax :>J ..... 4J - <Y-1: at ( 1) where p is pressure m the film, h is film thlckness, u is oil viscosity, x and z are axial and c:~.rcumferentl.al coordmates on the sll.ding surface. For analysl.S of the pressure dlStrlbUtion of the oil film, the neglect of the "saueeze" term in equac:l.on(l) will not cause a remarkable error /3/. Consequently, equatlon(l) becomes (2) The results calculated frcm this equation show that the regions at the ends of the segment in which the pressure drop of the oil film oc=s are very SIM.ll (Fig. 6) . 873 On the o1:her hand, the Clicumferent:~.al length of the segment is, in general, over 4 ti-nes larger 1:han :1.1:s axial height. Obviously, on the ossurnption of a whole ring, che- si:nulating calculat:I.on wlll be still vali.d. That is to say, the second_ toerm a to tohe left side of equatl.on( 1) can be neglected. The equatJ.on applied to Slffiulating che lubricating condition of packing elements l.S (3) The frictional resistance between the packing and rod l.S calculated fran equation( 3) and the following formula (4) where 't. lS shear1.ng stress, u is velocity of 0:1.1 film, and y is redial coordinate on the sliding surface. The calculated resistance is shown in Fig.3. In carparison with the measured results the difference between than lS small. Therefore the vali-dl.ty of the simulation can be confllmed. ANALYSIS OF RESULTS Agaw expenmen1:s are carried out Wl.th a constant rotating speed. For different-sealing pressures a series of curves of the frictional resistance VS. the crank angle are obtained. The results show that with the increase of the sealing pressure the resistance curve chomges fran a sine wave to a rectangular one. Therefore a cirtical sealing pressure under which the shape of the resistance curve begins to vary can be determined by the experiments. A group of the critical sealing pressure drops can be measured for different rod speeds (as shown in Fig. 7). According to reference/4/, the contact of the microprotuberances on sliding sur faces occurs when the oil film between than lS less than 1. 6 urn. At the same time the resistance increases shatply. Therefore a critical siZe can be taken as 1. 6 um. The film thichness between the rod and sealing elements is calculated for different sealing pressure drops in a given rotating speed. Of course, the larger the seal=g pressure drop, the thinner the film. If the sliding surface of the rod on which the calculated film thickness is less than 1.6 um exceeds one third of the whole sliding surface, its lubricating condition can't be accepted. Thus a group of criti.cal sealing pressure drop can be calculated for different rotating speeds (Fig. 7). In the calculation all the gecrnetric parameters of the sur face pro-files of the sealing nng cc:me fran the measurement of the used seali.ng elements. Though there are sane differences between these two results, which probably cane fran some assuq>ti.ons, the results can give designers important information in the desJ.gn of c<:<npressor packl.ngs, for exanple, under the condition of an- average rod speed em - 3 m/s the sealing pressure drop accross a sec of sealing elements should not exceed abou1: 7 MJ?a to ma1.ntam a better lubricating condition. Of course, there are several sets of seali.ng elements J.n a packing. In the light of the results mea-sured on the rig the pressure dlStributJ.on m the packing, which consists of five sets of sealing elements, is quite uneven -('l'albe 1). This will cause sets of the sealing elements fail one after another. It should be pointed out that in the above calculations the tarperatures ot the oil fiJm cane fran the measurements on the rig. The flow of lower-tarperature gas and good heat conduction w- the packing of the rig allow the sliding surfaces to mamta= a lower temperature. The effect of the frJ.Ctionel heat on the lubricat-mg cond:I.t:~.on is not remarkable. ln practical ccmpressors the temperature of the shdwg elements in the packing is much heigher than that in the rig. The oil film will thin out, or the loading-capacity of the film wJ.ll go down with the increase of speed withlll the range of high speeds. CONCLUSIONS On the basl.S of the measured and calculated results the following conclusions can be drawn: 1. Many factors affecting the sealing and lubrJ.cat:iJ:n of the packing/rod systan can be conveniently clanfied by sli!1Ulating tes-r.s. The fr:~.ctional process in ttus systan satl.sfies hydrodynamic lubr:~.catmg condl-tions. MathE!!'atical simulation of the lubr;~.catwg a:rciom frr Uu5 system can be conducted Wl-th Reynolds equatl.on. 874 2. The sealing pressure distrJ.butJ.on m packmg l.S quite uneven, which Wl.ll cause sets of the sealmg elements to fail one after another. Therefore the effec-tJ.ve method to solve thl.S problern is to acru.eve a packing whose pressure distribu-Ll.On is even. 3. A =l.tJ.cal sealmg pressure drop under whl.ch the lubrl.catmg condl.tion rapo.dly gets poor can be detSLmmed by exper:iments or mathematical simulat:ion. Thl.S pressure drop varJ.es Wl.th the rod speed, the tempera-r:ure m the packmg and the surface profile of the sealing elements, etc. REFERENCES Ill K.Scheuber, Sulzer Technical Revl.ew 3/1980 121 H.Czl.chcs, Pubhshed by Elsevier Publishing COlli> in Netherlands 1978 /3/ U. Todsen, 1982 Purdue Com-Tech. Con. Proc pp313 141 (Ell 5~1l-.t .~.t1fr~t«Jtf.l€-#J:jM{,JSf~Jf>>.f.i¥;,~i!l_ll:~± 197'1 75 so (b) 25 0 20 15 10 so 0 36 Fig.3 Frictl.Onal resJ.Stances between the packing and rod air collector Fig.2 Installatl.on of the tested packing 875 R--Rouohness h----F~~ ttud.rw.ss .uu w Fig.4 Stribeck curve and lubricat~g state r ,, I I 1: I it ~ J -~- ~- -·- --L ~ I I Xd/3 I Fi.q. 6 Circumferential distr"Lhn~.' on of the oil film pressure uf ring segment Tablc.1 Pressure drop across each set of the sealing elanents s~g Temp of set number pressure sliding drop (MPa surface :S 1 0.15 75 2 0.23 73 3 4.64 110 4 4.7 105 5 0.28 65 P=10 MPa) 876 Fig.S Forces acted on a segment of the seal tng ri_ng P*(MPa) 8 4 2 2 3 Cm(m/s) l'~g. 7 Cirtical pressure dr"9> at different rod apeeds 

Research on the Lubrication Conditions of the Packing/Rod System in Compressors

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Research on the Lubrication Conditions of the Packing/Rod System in Compressors

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