Vibration Certification Case Studies Vertical Pump Machinery Controlled with Variable Frequency Drives

Transcription

1 R RUSH USH E EQUIPMENT QUIPMENT A ANALYSIS, NALYSIS, IINC. NC EAST # EASTSEVENTEENTH FLORIDA AVE.,ST., #F331 Rush@RushEngineering.com Rush@RushEngineering.com SANTAHEMET, ANA, CALIFORNIA CA FAX FAX Vibration Certification Case Studies Vertical Pump Machinery Controlled with Variable Frequency Drives Copyright REAI /07/2008

2 Rush Equipment Analysis, Inc. Page 2 of 101 Introduction Executive Summary The use of variable frequency drives, or variable frequency controllers, in the pumping industry has become very common place over the last five years. Pump manufacturers and pump machinery operators have been plagued with the operation of these machines due to vibration issues made very complex by variable speed operation. This document presents eight case histories of vertical pump machinery that have had difficult certification processes due to the variable speed operation. The objective of the document is to alert operators, machinery suppliers, contractors, and pump station designers of the potential complications and some remedies that REAI or REAI clients have implemented. Additionally, the vibration analysis technologies and the vibration test technologies have evolved such that the operation of variable speed machinery can be successfully implemented at the certification phase when proper steps are implemented early in the design phase of the pump stations and the pumping machinery and followed up at the equipment startup certification phase. Design analysis of machinery using finite element analysis has been implemented for over 30 years and has been integrated into the personal computer work station for over 15 years. By appropriate finite element models the machinery can be constructed to minimize dynamic problems associated with variable speed operation. Even so, there are situations where the technology hand off from pump station system designers to pump machinery manufacturers and pump machinery operators has resulted is severe start up vibration problems. By the examples presented herein the reader can become aware of the principle issues and some remedial actions. Machinery vibration analysis has witnessed an explosion of electronic devices designed to be used in predictive maintenance of machinery over the last two decades. These devices started out as single channel data collectors and then dual channel data collectors. These machinery vibration analyzers (MVAs) can gather tremendous volumes of data and they can manipulate the data through semi-automated reporting systems for predicative maintenance vibration surveys. As such, these instruments are especially useful for simplifying the startup vibration certification process of new machinery. For well over forty years startups of large machinery have included the use of tape recorders and signal analyzers. The instruments were quite large and expensive and were not of use in the startup of vertical pump machinery due to cost ratio of the machinery and the instrumentation. Over the last five years or so the microelectronic technology industry has provided multi-channel digital signal recorder/analyzers (DSRAs) for use in startup vibration certification. A modern 24 channel digital signal recorder/analyzer will easily fit in a shoe box. Some of these devices can be daisy chained to well over one hundred channels for aerospace and turbo machinery startup tests. No special evaluation of these analyzers is provided herein, although they are employed in all the examples presented. For vertical pump machinery over 500 HP the use of a multi-channel digital signal recorder/analyzer can dramatically reduce the data acquisition time. On very large machines in excess of 2000 HP these multichannel signal analyzers can act like medical CAT Scan devices for a complete evaluation of the machinery health at relatively low cost and time expended compared to the cost of operation of the machinery. The multi-channel digital signal analyzer can be used to perform resonance evaluations through the operating range without performing complex bump test procedures. Essentially the variable speed operation and pump system transients excite the resonances and the digital signal analyzer captures the complete event. The effectiveness of multi-channel digital signal recorder/analyzers for vertical pump machinery vibration certification is well established by the case studies presented herein.

3 Rush Equipment Analysis, Inc. Page 75 of 101 CASE VII: Tall Motor Stand VTP Test Procedures CASE VII: Tall Motor Stand VTP Test Procedures This case involved vertical can pumps with two story motor stands using 1200 RPM, 600 HP motors with Kingsbury Thrust Bearings. The added twist is that the very tall motor stands required seismic bracing per the California Building Code. The installation of the braces was simple; however, the braces did not provide uniform lateral stiffness for operating vibration dynamics. The four machines in the group exhibited substantial variance in the resonances such that they had either 1xRPM critical speeds or 0.46xRPM Oil Whip critical speeds, or both. The resolution of the critical speeds required increasing the viscosity of the lubrication to prevent oil whip and manually tuning the seismic braces for each pump before vibration certification could be achieved. DISCUSSION Seismic building codes in California require special consideration of machinery installations to analytically verify the stability of machinery during significant ground motions, which are approximately 0.4g in the populated areas of California. A tall cantilevered column anchored to a foundation will produce significant anchor pull out loads if resonance amplification exists. A damping ratio of 5% will produce an equivalent g-load amplification, Q Factor, of 10, resulting is four times the mass load at the center of gravity of the column. If the center of gravity is well above the anchor plane the leverage against the anchor pull out force limit can result in failure of the anchoring system and catastrophic damage to the pump system. Figure G1 presents the motor grade at a pump station with Figure G2 presenting the discharge head grade. The anchors are at the corners of the sole plate under the discharge head bottom flange. The distance to the motor center of gravity is approximately 12 feet above the anchors. Between the motor stand and the motor grade concrete floor beams are four seismic braces shown in Figure G3 interlaced with the motor grade grating support beams. These seismic braces were required to meet the California Building Code. The close up view of one of the seismic braces in Figure G4 reveals a potential problem. The shoe of the seismic brace is not square to the concrete floor beam. As a result, there is only a point contact between the shoe and the concrete floor beam. This interface adds flexibility to the seismic brace at the concrete. The saddle of the brace at the motor stand shown in Figure G3 also adds flexibility to the seismic brace. REAI was not involved in the seismic analysis and it is not known whether a static or dynamic analysis was performed. Assuming that the pump manufacturer performed a numerical analysis of the pump assembly with sufficient detail to accurately identify the seismic brace at the motor stand, it can be assured that accuracy at the concrete floor beam interface was compromised by the misalignment of the shoe of the seismic brace. The potential for poor correlation between analysis and installation is significant. Figure G5 and Figure G6 present parallel and perpendicular vibration Sonagrams for an initial attempt to certify the pumps. The machinery tripped on vibration before achieving full speed. The maximum vibration for the parallel MTI reading was in/sec rms, while the maximum vibration for the perpendicular MTC reading was in/sec rms. The vibration frequency for the trip was 8.20 Hz, 492 CPM at a shaft speed of approximately 1050 RPM, assuming a 47% speed ratio. The ratio of the vibration frequency to the shaft speed frequency was in the range of oil whirl excitation. At the time of the test the lubrication was ISO Grade 68. Based upon prior experience with the Kingsbury Thrust Bearings and oil film journal bearings the lubricant was changed to ISO Grade 150 to increase the viscosity and effective damping. Subsequently investigations were performed to determine the cause of the critical speed. The investigations identified the poor alignment conditions of the seismic braces and adjustments were made. Figure G7 presents an impact response measured in the plane parallel to the discharge nozzle at the top of the motor (MTI) for Pump A01 after minor adjustments. The 1st Reed resonance in the parallel plane is identifiable at

4 Rush Equipment Analysis, Inc. Page 76 of 101 CASE VII: Tall Motor Stand VTP Test Procedures CPM. Since the motors have fluid film radial journal bearings there are two critical speeds associated with the 1st Inline Reed mode, RPM and 1190 RPM (46% RPM) for the modified installation. The operating range of the pump was 600 RPM to 1200 RPM. Thus, the 1xRPM critical speed represents an encroachment margin 9% below the minimum speed and the 46% RPM critical speed is in the operating speed range within 1% of the maximum speed. Figure G8 presents an impact response measured in the plane perpendicular to the discharge nozzle at the top of the motor (MTC) for Pump A01 after preliminary adjustments. The 1st Reed resonance in the perpendicular plane is identifiable at 510 CPM. The two critical speeds associated with the 1st Crossline Reed mode are 510 RPM and 1108 RPM. Thus, the 1xRPM critical speed represents an encroachment margin 15% below the minimum speed and the 46% RPM critical speed is in the operating speed range 8% below the maximum speed. The oil whirl speeds were not critical speeds after changing the lubricant to ISO Grade 150 because the excitation was acceptable. Figure G9 and Figure G10 present the MTI parallel vibration Sonagrams for the pump near minimum speed of 612 RPM, 51% Load. The 1xRPM vibration was mils rms (1.77 mils p-p) in Figure G9. The 49%RPM vibration at CPM was mils rms (1.97 mils p-p) in Figure G10. However, the Sonagrams indicate that the 49% RPM vibration was independent of speed and could not be an oil whirl response. The most likely source of the CPM component is a column mode of the pump bowl assembly. Figure G11 and Figure G12 provide the Sonagram for the MTC perpendicular plane vibration. The 1xRPM vibration at 612 RPM in Figure G11 was mils rms (3.22 mils p-p). This spectral value exceeds the Water District 3.12 mils p-p rms specification for Allowable Operating Region (AOR) vibration. The 0.49xRPM vibration at 612 RPM in Figure G12 was mils rms (1.91 mils p-p). The RMS sum of these two components is 3.74 mils p-p rms. Figure G13 provides the full speed MTI vibration parallel to the discharge nozzle as mils rms (3.645 mils p-p). This spectral value exceeds the Water District 2.5 mils p-p rms specification for Preferred Operating Region (POR) vibration. Figure G14 provides the full speed MTC vibration perpendicular to the discharge nozzle as mils rms (2.81 mils p-p), which also exceeds POR vibration limit. Figure G15 presents the MTI parallel impact response on Pump A02 with a 1st Reed Mode resonance at CPM and critical speeds at 104% of minimum speed and 113% of maximum speed. This pump has a 1xRPM critical speed within the operating speed range, but does not have a potential oil whirl critical speed. Figure G16 shows that the only region of elevated vibration occurred at 625 RPM with a 1xRPM amplitude of mils rms (6.35 mils p-p), which is 203% of the WD vibration limit for AOR conditions. Figure G17 presents the vibration certification data summary acquired for Pump A01. As the Sonagram data indicated this pump pushes the vibration limits at the extreme ends of the speed range. The values in Figure G17 are averaged over 30 seconds and the values provided in the Sonagram evaluations represented maximum values. At the time of the certification tests the seismic braces were evaluated on an as installed basis. Figure G18 shows that Pump A02 substantially exceeded the WD vibration limit at the low speed range. No action was taken to reduce the vibration at the time of the acquisition of the certification data because the actual operating range for the pumps had not been confirmed under plant operating conditions. If the pumps could have been operated between 55% Load and 100% Load the certification of the pumps was complete. Subsequent operation of the pumps under plant load conditions indicated that the

5 Rush Equipment Analysis, Inc. Page 77 of 101 CASE VII: Tall Motor Stand VTP Test Procedures full operating speed range was required and it was necessary to fine tune the seismic braces to assure compliance with the WD vibration limits. TUNING SEISMIC BRACES Tuning the seismic braces required knowledge of the minimum and maximum 1st Reed Mode frequencies of each pump. For the sake of example, Figure G19 presents the parallel plane reed mode response functions with parallel plane 1st Reed Modes at 435 CPM and 630 CPM. Figure G20 presents the perpendicular plane reed mode response functions with 1st Reed Modes at 420 CPM and CPM. These conditions represent the resonances with the seismic braces backed away from the concrete floor beams and with the seismic braces in direct contact with the concrete floor beams per their design. The tuning strategy involved insertion of Oil Resistant Nitrile Vibration-Damping Pads, McMaster-Carr P/N 4159K64 (Green Square) or Oil Resistant Neoprene Vibration-Damping Pads, McMaster-Carr P/N 4159K64 (Black Round) as shown in Figure G21. The use of a single design vibration pad was preferred, but the maximum clearances between the seismic brace shoes and the concrete floor beams required thinner pads in some locations. The thinner pads were also stiffer pads and they compromised the tuning objective. After the vibration damping pads were inserted bump tests were performed to determine the 1st Reed Mode resonance frequencies. Figure G22 demonstrates the tuning procedure. The resonance frequency was sensitive to compression of the vibration damping pads. The seismic brace shoes were installed to initial contact with sufficient compression to hold the vibration damper pads in place and then the angular position of the shoe as shown in Figure G21 was modified by partial rotation. The objective in the tuning process was to achieve a 7.5% encroachment margin of the 600 RPM minimum speed, or a resonance frequency near 555 CPM. The data in Figure G22 shows the results of tightening the shoes of the seismic braces. At 3/4 turn (3Q) the resonances were at 555 CPM and 570 CPM. The two parallel (MTI) braces were backed off 1/6 turn and the MTI resonance decreased from 570 CPM to 495 CPM. This value was then increased by adding 1/6 turn to the East seismic brace of Figure G21. The result of the second adjustment included a cross plane effect based upon spectral examination of the impact acceleration responses. Figure G23 provides the MTI parallel plane impact response functions with the 555 CPM, g/lb tuned vibration damper pads condition and the CPM, g/lb direct contact condition. The vibration response reduction provided by the damper pads is marked on the impact response plot with the peak ratio reduction by 77%. Figure G24 provides the MTC perpendicular plane impact response functions with the 555 CPM, g/lb tuned vibration damper pads condition and the CPM, g/lb direct contact condition. The vibration response reduction provided by the damper pads is marked on the impact response plot with the peak ratio reduction of 72%. By tuning the 1st Reed Mode resonance frequencies the encroachment margins were set. The use of vibration damper pads for the tuning process has the additional advantage of reducing the peak response of the machinery to lateral forces by a factor of four based upon damping added to the resonances. This, coupled with the change in lubricant viscosity, provides the operating conditions that certify the compliance of the pumps with the WD specifications for vibration limits.

6 Rush Equipment Analysis, Inc. Page 78 of 101 CASE VII: Tall Motor Stand VTP Test Procedures CASE VII CONCLUSION The vertical turbine pumps in Case VII demonstrate the value of a multi-channel recorder/analyzer for vibration certification of compliance to operator specifications. The pumps had oil whirl conditions that could not be predicted because of variable resonance conditions and the ability to record the complete speed range assured that problem speeds were identified. Once identified a dual channel machinery analyzer was employed to tune the resonance frequencies of the machinery. The test data provided sufficient knowledge of the vibration behavior to conclude that the pumps would meet the operator s vibration specifications based upon the improvement in the encroachment conditions and the increase in damping.

7 Rush Equipment Analysis, Inc. Page 79 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G1: Pump A03 Vibration Test Setup, 2/19/08 Figure G2: Pump A04 Vibration Test Setup, 2/19/08, Lower Microphone

8 Rush Equipment Analysis, Inc. Page 80 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G3: View of A01 Motor Stand Braces (4 each) to Shop Floor Elevation 5Nov07 Figure G4: Composite View of A01 Motor Stand West Brace Interface at Concrete Viewed From Above 5Nov07

9 Rush Equipment Analysis, Inc. Page 81 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G5: Sonagram for A01 MTI During Run Up to Vibration Trip at 4m seconds 5Nov07 (Top Left = Spectrum [Hz, IPS RMS], Top Right = Sonagram [Minutes, Hz, IPS RMS], Bottom Right = Hz [Minutes, IPS RMS]) Figure G6: Sonagram for A01 MTC During Run Up to Vibration Trip at 4m seconds 5Nov07 (Top Left = Spectrum [Hz, IPS RMS], Top Right = Sonagram [Minutes, Hz, IPS RMS], Bottom Right = Hz [Minutes, IPS RMS])

10 Rush Equipment Analysis, Inc. Page 82 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G7: Motor A01 Reed Modes Mobility Function 5Nov07 (MTI 1st Reed at CPM, 91% Minimum Speed, Oil Whip 46% Critical Speed ~ 1190 RPM) Figure G8: Motor A01 Reed Modes Mobility Function 5Nov07 (MTC 1st Reed at CPM, 85% Minimum Speed, Oil Whip 46% Critical Speed ~1108 RPM)

11 Rush Equipment Analysis, Inc. Page 83 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G9: Pump A01 Sonagram, Motor Top Inline at 51% Load, 1xRPM Figure G10: Pump A01 Sonagram, Motor Top Inline at 51% Load, 0.49xRPM Copyright REAI /07/2008

12 Rush Equipment Analysis, Inc. Page 84 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G11: Pump A01 Sonagram, Motor Top Crossline at 51% Load, 1xRPM Figure G12: Pump A01 Sonagram, Motor Top Crossline at 51% Load, 0.49xRPM Copyright REAI /07/2008

13 Rush Equipment Analysis, Inc. Page 85 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G13: Pump A01 Sonagram, Motor Top Inline at 100% Load, 0.47xRPM Figure G14: Pump A01 Sonagram, Motor Top Crossline at 100% Load, 0.47xRPM Copyright REAI /07/2008

14 Rush Equipment Analysis, Inc. Page 86 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G15: Motor A02 Reed Modes Mobility Function 5Nov07 (MTI 1st Reed at CPM, 104% Minimum Speed, Oil Whip 46% Critical Speed ~1353 RPM) Figure G16: Pump A02 Sonagram, Motor Top Crossline at 52% Load, 1xRPM

15 Rush Equipment Analysis, Inc. Page 87 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G17: Pump A01 Certification Test Displacement Summary, February 19, 2008 Figure G18: Pump A02 Certification Test Displacement Summary, February 19, 2008

16 Rush Equipment Analysis, Inc. Page 88 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G19: Pump A02 Inline Response Comparisons, No Contact & Direct Concrete Contact Figure G20: Pump A02 Crossline Response Comparisons, No Contact & Direct Concrete Contact

17 Rush Equipment Analysis, Inc. Page 89 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G21: Pump A02 Initial Vibration Pad Installation, One Quarter Turn after Contact Figure G22: Barrier Water Pump A02 Summary of Rubber Pad Compression Adjustments

18 Rush Equipment Analysis, Inc. Page 90 of 101 CASE VII: Tall Motor Stand VTP Test Procedures Figure G23: Pump A02 Inline Response Comparison, No Pads and Pads with 3/4 Turn Compression Figure G24: Pump A02 Crossline Response Comparison, No Pads and Pads with 3/4 Turn Compression