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1 THE AMERICAN SOCIETY OF MECHANICAL,ENGINEEFtS 345 E. 47th St:, New York, N.Y The Society shall not be responsible for statements or opinions advanced Inpers or clioettssion at meetings ot the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the per is published in an ASME Journal. Authorization to photocopy - CP material for Internal or personal use under circumstance not falling within the fair use, provisions of the Copyright Act is granted by ASME to libraries and other users rogistered with the Copyright Clearance Center (CCC)Transaedonal Reporting Service provided that the bastes of $0.30 ' per page is paid direcby to the CCC, 27 Congress Street, Salem MA 01970, Requests for special permission Or bulk reproduction should be actin :wed. to the ASME Tectmleal Publishing Department Copyright by ASME MI Rights Reserved Printed in U.S.A HIGH VIBRATION PROBLEM IN COMPRESSOR PIPING SYSTEMS - A CASE STUDY G.Ifijaya Kumar, S.Raghava Chary And A.Rajamani Rp Structural Dynamics Group, Corporate Research & Development Division, Bharat Heavy Electricals Limited, Vikasnagar, Hyderabad , INDIA. ABSTRACT: High vibration problems resulting in damage to supports, instrument stubs etc. have been experienced in many compressor piping systems installed at different fertilizer plants. Investigations aimed at a solution to the problem included vibration measurements on the suction and discharge piping, and mathematical modeling of the piping. The measurements indicated presence of an excitation frequency in the range of 30-35% of the compressor running speed. Dynamic analysis of the piping system showed the presence of natural frequencies coinciding with or very near to the excitation frequencies. This has been further confirmed by impact tests. Analytical mode shapes clearly show that the antinodes match with high vibration zones observed at the site. The mathematical models were used to determine optimum configurations which would separate mechanical responses from excitation frequencies. These modifications have been implemented at site and the piping vibrations are within normal limits. compressor piping systems installed at different fertilizer plants. The high vibration problem coupled with leakage has created an unfavorable working environment ultimately leading to high noise levels and shut downs in some compressor stations. Detailed investigations which include site measurements and analysis have been carried out to solve the problem in some of the stations and two case studies (A & B ) are presented on the compressor piping as the vibration behavior is more or less similar in nature. In case A there are three units (Unit-IA, Unit-2A & Unit-3A) of carbondioxide compressors, which are driven by steam turbines, whereas in case B there is only one turbine compressor train (Unit-1B) used for manufacturing urea In all the three Units (case A) the compressor discharge piping was vibrating heavily and the noise levels at the compressor operating floors were high, especially in Unit-1A. In Unit-113, vibrations in the carbondioxide (CO, ) final discharge line carrying hot compressed CO, from BCL compressor to receiver tank were also high. This paper deals with the analytical studies carried out to overcome the vibration problem of compressor suction and discharge piping at both the stations. 1.0 INTRODUCTION: Bharat Heavy Electricals Ltd., (BHEL) a leading manufacturing and engineering organization in India, supplies power plant equipment, auxiliaries and broad range of centrifugal compressors to various fertilizer, power, steel and petro chemical installations. It has been observed at few sites that centrifugal compressor piping systems have high vibrations and the source of vibrations appear to be the result of an unusual problem with the compressor. The excitation generated from the source is further transmitted to the associated interconnecting piping and structures leading to severe vibration problems. The high vibration problems result in damage to supports / instrument stubs and leakage of fluid from the flanged joints in 2.0 COMPRESSOR AND THE PIPING ARRANGEMENT: The general schematic arrangement of the high pressure (HP) compressor (2BCL), medium pressure (MP) compressor (2MCL), with associated piping, heat exchangers and drive turbine is shown in Fig. I. The design parameters and running speeds of the compressors and turbines for both races A and B are furnished in Table-I. Figs.2 and 3 show some of the details of the suction and discharge piping interconnecting various compressors and after coolers of the compressor station A to give an overall idea of the arrangement. Presented at the ASME ASIA '97 Congress & Exhibition Singapore September 30 October 2, 1997

2 The 813 mm outer diameter (OD) suction piping (Fig.2) interconnects knockout drum or vessel exhaust which is at m elevation and 2MCL compressor inlet. There is an antisurge line of mm size to the antisurge valve branching from the main line before 2MCL inlet. Another mm OD leg interconnects compressor discharge and suction piping with reducers and a valve. The thickness of the 8I3mm and mm OD pipes are 7.92 & 6.35 mm respectively. The piping material is stainless steel. Carbondioxide is supplied to all the unit suction knockout vessels through a common header running on the pipe vessels. The main discharge piping arrangement is shown in Fig.3. It starts at the exhaust of HP compressor and goes to the respective processing units and carries carbondioxide at high pressure of KPa. There is one by pass line connecting the compressor suction Fig.1 Schematic arrangement TABLE-1: DESIGN PARAMETERS DESIGN DATA CASE-A CASE-B 2MCL SPEED rpm rpm 2BCL SPEED rpm rpm TURBINE SPEED 6787 rpm rpm SUCTION PRESSURE 40 KPa 57 KPa DISCHARGE PRESSURE KPa KPa SUCTION TEMP. 31 C 40 C DISCHARGE TEMP. 110 C 101 C DISCHARGE FLOW 32,500 NM3/11r 32,000 NM3/hr NUMBER OF STAGES 2MCL 2BCL Fig.3 Discharge Piping (Case -A) line. Two safety relief valves of and mm OD with steam tracing arrangement are also existing on the main discharge line. The discharge pipe is made of stainless steel material having min op and 18.3 nun thickness. The complete discharge piping only has been shown in Fig.4 for Unit 1 B. 3.0 SITE OBSERVATIONS: The following are the salient observations during site inspection of various units and associated piping systems. a) The Suction piping supporting arrangement is different for each unit even though the suction piping layout is same. The 813trun suction pipe of Unit-3A is supported at I10.7m elevation level at two points where as these supports are not existing in Units IA & 2A. The mm antisurge line is supported additionally at two places for the Unit 3A. Fig.2 Suction Piping (Case-A) 2

3 11:LaiLl; 6310N92 de VAIN! DV IS RECEIVING VESSEL cts ASV VALVE PICV ISO (100 MP ( 2) ( 3) ) ERRING HANGER GUIDS/RESTING APIC DO : d ads I At At ' Fig.5 Suction Piping Spectrum (Case-A) (Horizontal range: 0-500Hz, Hz, Hz, Hz Vertical : velocity range mm/sec) Fig.4 Discharge Piping (Case-B) b) The supporting arrangement for the compressor main discharge piping is also different for all the three units even though the piping layout is same. Unit-IA discharge piping is firmly clamped to the operating floor structure at as many as eight locations in the horizontal leg parallel to the compressor axis, where as three clamped supports in Unit-2A and two clamped supports in Unit-3A are existing on this leg. ID Kt 5011 ICCORXID 03/04/111 it: At OS c) The compressor discharge piping immediately after the first vertical leg is having additional springs provided in the horizontal direction for Unit IA. d) An angle piece of about 40mm length has been welded to the structure which is in contact with the 813nun OD vertical pipe at the operating floor level for the Unit IA. e) Vibrations on CO2 discharge piping are so severe that many instrument stubs and supports have given way. Most supports on the piping are adequate for static loads but inadequate under dynamic loading in both the case studies. :(4 ) 13) - (2) Thass 7 DO 04Alifil 000 WO.O ,060-6 Fig.6 Discharge Piping Spectrum (Case-A) (Horizontal range: 0-500Hz, I Hz, Hz, 3-I49.4Hz, Hz: Vertical : velocity range mm/sec) 5C; VIBRATION MEASUREMENTS: The vibration spectra [2) at few locations on suction and discharge piping for case A are shown in Figures 5 and 6 respectively. Table-2 TABLE-2: DOMINANT FREQUENCIES OF VIBRATIONS LOCATION 2A 3A 2BCL DISCHARGE PIPING 68.0 Hz Hz DISCHARGE VALVE Hz Hz SUCTION PIPING - HOR 67.5 Hz Hz SUCTION PIPING - V ER 67.5 Hz Hz NAG MS o 1 1 (a) a PIR W 0 LIN 110Iz Yr reel 1 ON Vs Fig.? Discharge Piping Spectrum (Case-B) (Horizontal range: 0-11cHz, I-77.0 Hz, 2- I55Hz Vertical : velocity range E-1 nun/see) 4 3

4 SLICTION PPE (Mode no 17 Frequency Hz.) Fig.8 Suction Piping Mode Shape (Case-3A) COMPRESSOR OPERATING FLOOR TOP VIEW Dam at operving!leer Angle piece welded to barn 0 Fig.11 Angle piece in contact with suction pipe gives various dominant frequencies and its vibration values are given in spectra (Figs. 5&6) for Unit IA and Unit 3A at these locations. It is clear from the spectra that the discharge piping of Unit 2A and Unit 3A are vibrating with a frequency of Hz and Hz respectively. (Mode no. 17 Frequency Hz.) Fig.9 Discharge Piping Mode Shape (Case-1A) The vibration measurements have been carried out using B&1C/ Endevco accelerometers, charge amplifiers, velocity probes, diagnostic instruments (PL-2I) portable analyzer, and Ono-Solcki two channel (Model 920) analyzer. Similar studies made on Unit 1B [3] discharge piping indicated the presence of a forcing frequency of about 77 Hz. The spectrum at a typical location is given in Fig ANALYSIS OF SUCTION & DISCHARGE PIPING: A detailed analysis on the compressor suction piping for case A and discharge piping for both the cases as existing has been performed. As the problem is concerned with vibrations, analysis is limited to natural frequencies and mode shapes computations. Straight and curved pipe elements are employed while modeling the piping systems. The spring elements for simulating the boundary conditions and mass elements for lumping the concentrated masses of various valves are appropriately utilized during modeling. (Mode no. 21 Frequency Hz.) Fig.10 Discharge Piping Mode Shape (Case-1B) In order to check possible resonance with any compressor turbine generated excitations, the first thirty natural frequencies of each of the system are obtained. Table-3 contains the first twenty two natural frequencies of the suction and discharge piping systems of all the 4

5 TABLE-3: SUCTION & DISCHARGE PIPING FREQUENCIES (Hz) TABLE-4: DISCHARGE PIPING (1B) FREQUENCIES (Hz) ---S UCT ION DISCH ARGE SUC. ORIG INAL MODI FED MODE IA 2A 3A IA 2A 3A 1A* I MODE 1B MODE 1B MODE 1B MODE II I ' (* modified suction ) TABLE-5: MAXIMUM STRESS (KPa) three units of A. Some of the natural frequencies of Unit 18 are given in Table-4. Various modifications have been tried in supporting arrangement and the modified systems finally arrived at for both cases A & B (A suction piping and B discharge piping). Natural frequencies of the modified systems (i.e suction for Unit-1A and discharge piping for Unit-IS) are also furnished in Tables 3 and 4 respectively. EXISTING MODIFIED ALLOWABLE SUSTAINED 56,050 45, ,657 OPERATING 146, ,961 - EXPANSION 121, , ,014 Maximum stress in the system in existing as well as modified system with final modifications are listed in Table -5 for Unit-1B. 5

6 6.0 DISCUSSION OF RESULTS Based on the results of various analysis carried out and site data, the following points are of interest for solving the vibration problem. (a) It is seen from the vibration measurement data of casea, that 68 Hz and 70 Hz frequencies are more or less appearing in all the components in suction and discharge piping of Units-2A & 3A respectively (data on Unit IA is not available as the unit was shut down). The velocity levels are maximum on discharge piping both in vertical and horizontal directions. Similar trend has also been observed in compressor suction piping in the vertical leg. The frequencies 68 Hz and 70 Hz also appear in the vibration spectra taken on 2BCL compressor during site measurements and do not coincide with the compressors running speed or its harmonics. As pointed out, the exciting frequencies about 68 Hz and 70 Hz for Units 2A&3A respectively are aerodynamic in origin caused by high density gas pressure pulsations due to rotating stalls [41 (b) A look at the free vibration analysis results of all the Units for case A indicated that the following frequencies are in the possible resonance range. SUCTION PIPING 1A 2A 3A Mode Hz Mode Hz Mode Hz A typical mode shape of relevant frequencies is shown in Fig.8. These three legs are clamped or supported from the structures at compressor operating As the suction pipe size is large, resonance in any one of these legs can also excite the supporting structures on the compressor operating floor from where the compressor discharge pipe is supported. DISCHARGE PIPING 1A 2A 3A Mode Hz Mode Hz Mode Hz Discharge pipe mode shapes for mode 17 is shown in Fig.9. These mode shapes clearly indicate the piping legs mode of vibration. The reasons for large jump in frequencies of Unit-IA discharge piping in Table-3 are due to 1) additional spring elements at location 'L l' and 2) additional clamped supports in Unit-IA (shown in Fig.9). In Units '2A' and '3A', there are only three and two clamped supports respectively. c) The excess vibration in Unit-IA compared to the other two Units is due to the presence of a 40 mm angle piece, which is in contact with the 813 mm size vertical pipe. The location of the angle piece is shown in Fig. II which is welded during erection for maintaining the clearances and should have been removed afterwards. This short stiff member can induce additional excitation in the compressor operating floor structures and aggravate the vibration problem for Unit 1A. d) Based on the results of analysis of the existing system for Unit-IB it is observed that there are four frequencies 70.0, 74.9, 75.5 and 84.0 Hz which are very close to the excitation frequency of 77 Hz, for Unit 1B. Mode shapes of the system pertaining to 74.9 Hz is shown in Fig. 10. It can be observed that there is local resonance at a particular frequency. For example, 74.9 Hz frequency corresponds to a local resonance at the manually operated valve connecting PV 15. Similarly 75.5 Hz and 84 Hz correspond to local resonance at U bend near compressor and ASV branch respectively (Fig.4). The analysis results clearly show that the pipe line will vibrate under operating conditions resulting in severe vibration of not only the main line but also the attached branch lines. e) It was observed that the forcing function has the first harmonic at 77 Hz and the second harmonic at 155 Hz. The response of the piping system at the first frequency was of the order of 600 microns on 100. NB line connected to PSV 17 and 110 microns on the main line of Unit-1B. f). Impact tests also indicated that the U-bend near compressor has a bending frequency of 84 Hz, the main line after ASV branching i.e., the vertical run has a frequency of 75 Hz and the 100 NB line (Fig.4) connected to PSV 17 has a frequency of 72 Hz. These frequencies are very close to the excitation frequency and therefore the pipe line vibrations are high in the operating conditions. g) It can be observed from Table-5 that the sustained stresses i.e weight pressure loading have come down to 45,775 Kpa from 56,050 Kpa for the modified arrangement due to incorporation of additional supports where as expansion stresses under thermal have gone up to 171,356 KO from Kpa. This is an important feature of making flexible piping to more stiff piping systems. Even under this situation, it has been ensured that the stresses in the modified systems under thermal as well as sustained conditions are well within limits. 6

7 7. CONCLUSIONS: Following are the conclusions from various vibration analyses carried out and measurements made. Case A: a) A phenomenon of forced vibration has been observed on all the piping as well as on the 2BCL compressor, excitation frequencies being 68 Hz and 70 Hz b) The main discharge piping high vibrations are due to resonance in some of the large size suction piping legs with the aerodynamically induced frequencies in the 2BCL compressors. c) The excess vibrations in Unit-1A compared to the other two Units is mainly due to i) the presence of 40 mm angle piece firmly welded to the main structure on the compressor operating floor, which is in contact with the 813 mm OD vertical suction pipe under resonance and ii) too many clamped supports (as many as eight) from the operating floor structures. d) After canying out modifications at site, the discharge piping high vibrations have come down significantly in all the three Units in Case Case B: a) A phenomenon of forced vibration is observed on the discharge piping at frequency of 77 Hz. 8. RECOMMENDATIONS: In order to avoid high vibration problem in the compressor piping, the following points are to be taken care of. d) In view of the fact that the source of vibration could not be isolated, the only alternative was to shift the frequencies of system away by a minimum of % to 20% of the forcing frequency to avoid resonance by suitably modifying supporting arrangement of the piping systems by providing additional guides and restraints. It has been ensured that the stresses in the modified systems under operating as well as sustained conditions are within limits. e) The high energy compressor piping system design should consider the additional stiffness requirement aspects from vibration point of view while duly allowing for thermal expansions during the design stage itself. ACKNOWLEDGMENTS: The authors thank BHEL Management for giving permission to present the paper at the conference. The authors gratefully acknowledge the help given by Mr Malay Ghosh and Mr. IC Ramalcrishna in the measurements. REFERENCES: I. G.Vijaya Kumar and Malay Ghosh " Investigations on High Vibration & Noise Problem of Compressor Units" BHEL, Internal report,may K.Ramalcrislina " CO2 Compressor Discharge Piping vibration Analysis" BHEL, Internal report July S.Raghava Chary, G.Vijaya Kumar and A.Rajamani " Investigation on High Vibrations in a compressor discharge piping" BHEL, Internal report,dec Angelo Maretti et.al., " Shop Full-Load Testing of Centrifugal Compressors" Proceedings of the llth Turbomachinery Symposium Dec , 1982, pp a) The large suction piping supports should be isolated from the compressor operating floor. Independent supports may be erected at appropriate support locations so that the suction pipe resonance affect is not transmitted to the discharge piping through the common support structures for suction and discharge piping. b) The compressor main discharge pipe may be supported from the operating floor structures eliminating clamping at locations wherever it is not essential from the vibration point of view, avoiding clamps at short intervals. c) The column type clamped support from the ground floor, existing between the two valves in the compressor discharge piping can effect the natural frequencies of the whole operating floor structures, and hence may be eliminated. 7