CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE by H. M. Davis Manager, Ct•ntrifugal Compressor Engineering Department N. DELA VAL Turbine Division, Tr<�nton, J. Centrifugal compressors can he designed and built llugh iVI. Davis is the Manager of with operation and ease of maintenance practices in Centrifugal Compressor Engineering mind. Specific item;; such as off-design ;•pera!.ion, bear­ Department, DELAVAL Turbine Di­ ings and seals, ca;;in;_; design and mounting, am! lubri­ ui.sion of DELAVAL Turbine cation systems are discussed. Trenton, Neu1 Jersey. He is respon­ sible for the organization and direl;. lNTROl)UCTJON tion of all engineering and drafting The process industry has experienced tremendous functions related to the product. growth during the past two decades. The growth of Mr. Davis received a B.S. in ME single line m1d continuous process plants and the in­ degree from Rose Polytechnic lnst£­ creasing u,;;e of automation have demonstrated tbe im­ tute, Terre Haute, Ind. in 1956 mul portcmee of component reliability. lVIachinery user� are a8snmed his present position 1:n ]aruwry, 1970. now demanding dependahle performance, simplicity of op1'ration and case of maintenance. The suppliers of ABSTRACT centrifugal compressors have heen forced to review their The operali(Jn and rnaintenance of centrifugal com­ designs and sometimes to design new equipment io satis­ pre:;;;ors varie� considerably depending upon the fy these users' demands. This paper discusses the ways speed, horsepower, and type of driver. in which a centrifugal compressor can be built and usf:d Typical centrifu�al compressors vary from high t.o satisfy a customer's operating and maintenance speed, low flow applications to very large flow, low requirements. speed machines. Each has its own operating and main· TYPICAL l\WLTJSTAGE CO.MPHESSOR DESIGN tenance problems. A typical multistage centrifugal compressor, de­ (>ntrifugal compre:;sors are normally driven hy signed to meet a particular customer's needs, is shown electric motors and speed increasing gears for the lo in Figure l. This machine consists of () impellers flow, low horsepower applicationc;. The middle sizew m ranges arc usually rnotor-gear, steam turbine or gas APPLICATION CHART turbine direct connected combinations. The very large FOR CENTRIFUGAL flow, large horsepower machines are practically all steam COMPRESSORS turbine driven with a few being driven hy ga" turbine where natural gas fuel is available. i 100.000 ::; "- '-' 31: 0 I -' ... 1- w ..J STEAM ;!!; TURBINE 15 ORIVER Vi 10,000 :z MOTOR GEAR DRIVER �0 � () STEAM TUR61NE GAS TURBINE MOTOR GEAR DRIVER _ __ _ _ _ L.__L _L_____L .J____j_ _L___[ _L__j _t___J�__L___\___j 1,000 0 2 4 6 s 10 12 14 16 18 20 22 24 26 28 COMPRESSOR SPEED, RPM x 10-3 Compressor. Figure 1. jl,'fulti-stage Fabricated Case Figure 2. Application Chart for Centrifugal Compressors. 10 CENTRIFUGAL COMPRESSOR OPERATION AND MAINTENANCE 11 to the gas being compressed. This velocity energy is converted into increased pressure in the diffuser passage. The cross-over passage and the return guide vanes lead the gas to the next impeller where the compression is continued. The volume of the gas stream is reduced as it is compressed and each stage is designed to accept a successively smaller flow. RANGE OF APPLICATION Figure 2 is a curve which shows the limits of appli· cations for centrifugal compressors in terms of flow and speed. The speed is limited by the stresses in the im­ pellers. The small flow, high speed compressors have the same working stress levels as the large flow, low speed machines. The compressor applications in the low flow range are almost entirely driven by motors and speed increasing gears. The compressors in the mid­ range of flows are driven by motor-gears, steam turbines, and some gas turbines. The large, high flow compressors are practically all driven by steam turbines. The size and operating speed of a centrifugal com­ Figure 3. Four-Stage Barrel Compressor. pressor have a direct effect on the operation and the maintaining of the compressor. series and is designed to compress 4000 cfm of gas from A small machine such as the one shown in Figure 3, an inlet pressure of 25 psi to a discharge pressure of besides having high rotating speeds, is usually high pres­ 425 psi. Each impeller imparts velocity (kinetic) energy sure as well. Shaft alignment is more critical since the Figure 4. Turbine Driven Blast Furnace Blower. 12 PROCEEDINGS OF THE FIRST TU HBOMACHINERY SYMPOSIUM shaft ends are small and external pipe forces must re­ puter analysis allows the engineer to predict these bear­ ceive proper attention to ensure good operation of the ing loadings and to design a dependable maintenance equipment. Maintenance is easier due to the light weight free machine. of the compressor components, hut the clearances of the rotor internal seals and hearings must be watched more closely Figure 5 shows the calculated and measured due to their small physical size. response curves for an eight-stage compressor rotor. The measured values were obtained first during the A large compressor, such as the one shown in Figure mechanical test of the compressor. Although the vibra­ 4, is more difficu It to vwrk on due to its size and weight. tion level was less than 0.7 mils and the bearing forces The clearances between rotating and stationary parts is were below the design dynamic load limit, the steepness less critical hut the shaft alignment i� more difficult to of the vibration curve near the maximum operating obtain, although more liberal tolerances are acceptable, speed was understandably cauiie for concern. The rotor because the components are not easy to move and special was modified and retested. lifting facilities are required. The foundation for these These results are shown large machines is also of special concern because unless in 6. The test the supports are de,.igned, constructed, and maintained shows quite low vibration amplitudes and low bearing properly the machinery may never achieve trouble-free loading. The compressor, once in actual service, will become unbalanced rotor. The operation. due to build-up on the difference between the calculated and measured response curves shows that this compressor will be tolerant !o ROTOR DYNAMICS con:oiderable rotor deposits before it will have to be Modern process compressors are buill in accordance cleaned. ! ightly loaded journal bearings, such as those with the API specification 617. One important item used in compressors, can he unstable at high speeds, and defined by this specification is the natural frequencies a number of "'olutions to this problem have been used. of the rotor. These natural frequencies Inusl not occur The tiltinf!; pad hearing, like the one shown in Figure 7, in the variable speed nwg:e of the compressor. The dy­ is widely usNl in compressors. Each shoe tilts inde­ namics of a rotor can be �turlied with the help of the pendently to maintain its load carrying hydrodynamic computer and the effect on the rolr•r of operative pressure wt.�dgf:. Extensive service in many types of till · balance due to build-up misalig:nnu,•n!. can be evaluated. cornprr:ssors have proven the dependability of this These rotor unbalm1ces will load the bearings. Com- hearing. ROTOR RESPONSE CURV FOR EIGHT STAGE ROTOR 2 2 AT ANTI·NODE OF CRITICAL CALCULATED WITH UNBALANCE 2nd 300% + MEASURED WITH NON·CONTACT PICKUP 20 -+ 1.8 200% 1.6 w <t u Cl 0:: 14 200% U) f2 _j (!) z :E 1.2 0: <t � 150% w z co w 1.0 � w w > u <t .a _j 100% �_j 0... w U') a:: a .6 4 50% + 2 / 0 II 0 2 3 4 5 6 7 8 9 10 12 13 14 15 16 17 SPEED, RPM Figure 5. Eight-Stage Rotor Response Curves. CENTHIFt:GAL COMPHESSOH OI'EHATION AND MAINTENANCE 2.2 300% - CALC ULATED WITH UNBALANCE AT ANTI-NODE OF 2nd CRITICAL + - + MEASURED WITH NON-CONTACT PICK-UP 1.8 250% w <( (.) c:i1.6 a::: 0 (/) LL. 0 1 4 w 200% (.!) � w z I a. a: (/) <( !z1.2 w w :IE III :IE :::> �1.0 :::!1 150% <( � ...J � g, .8 .... 0 ALLOWABLE DYNAMIC LOAD 100% � .6 .4 50% .2 Figure 6. Rotor Response Cun1es for Modified Eigh t Stage Rotor. OFF-DESIGN OPERATIO� are se1·ere enough, can melt the soft lahHinth seals "hieh Most compressor users take trouhle-free mechanical control the internal leakage. The co1�1pressor perform­ operation for granted, hut they are concerned with com­ ance suffer� once the seal,; are damaged and the machine must he opened and the st>als repl ced to restore it to pressor performance. Figure a shOI\S a typical com­ � pressor performance curve. l"ncomplicatt>d and trouble­ the ori;.dnal condition. Excessi1e temperatures in a com· free operation can be expected in the stable performance pressor ha1ing a balance drum lain rinth seal made of region to the right of the surge line. Surging, or un­ a soft material 11ith a low melting l�mperalure can melt stable operation, can occur in an� centrifugal compres»or the seal. This will upset the rotor thrust balance and 1rhen the inlet flow is reduced to approximately 60�; of 01·erloacl the thrust hearing:. When the thru�l hearin!! the design inlet flow or lower. Compressors that pro· fails the rotor 11ill shift axially and the impellers wiil duce large pressure ratios. ratio of inlet pressure to dis­ ruh ai!ainst the stationary paris causini! further damai!e. charge pressure lend to have more 1 iolent surp:es. \\"hen To help a1·oid these operalinl! problems the compressor the compressor is operated repeatedly or for prolonp:ed can he JH"ol·icled "ith a hii!h temperature balance drum periods of time in surge the pressure forces can damage seal made from compressed metal fihers that \1 ill \1ith­ the internals of the machine.