vibration certification case studies vertical pump ...this case has a similar problem to case ii...

RUSH EQUIPMENT ANALYSIS, INC. [email protected] 714-998-7433 2401 EAST SEVENTEENTH ST., #181 SANTA ANA, CALIFORNIA 92705 FAX 714-998-0121

Vibration Certification Case Studies Vertical Pump Machinery

Controlled with Variable Frequency Drives

Copyright REAI 2008 http://www.rushengineering.com/VFD-Pump-MultiChannelIntroduction.pdf 09/07/2008

Rush Allen

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RUSH EQUIPMENT ANALYSIS, INC. [email protected] 951-927-1316 43430 EAST FLORIDA AVE., #F331 HEMET, CA 92544-7210 FAX 951-927-0512

Rush Allen

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Rush Equipment Analysis, Inc. of 101 Introduction Vibration Certification of Vertical Pump Machinery Controlled with Variable Frequency Drives

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 multi-channel 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.


Rush Equipment Analysis, Inc. 5 of 101 CASE III: Submersible Pump Dry Pit Lateral Resonance Vibration Certification of Vertical Pump Machinery Controlled with Variable Frequency Drives

CASE III: Submersible Pump Dry Pit Lateral Resonance This case has a similar problem to Case II with regard to a 2xRPM resonance encroachment problem on the operating range. However, in this case the driver was a close coupled submersible motor. The mass of the motor and the sole plate spanner frame resulted in a resonance that produced unacceptable 2xRPM critical speed operation. The criteria for certification of submersible pumps was applied to this case by the utility and in that criteria a single required measurement location would have resulted in acceptable vibration while the pump vibration was well above the allowable value ninety degrees from the required measurement location. The case shows that the evaluation of vibration cannot be limited to a single measurement, especially since the submersible pump was operated in a dry pit with a sole plate installation. DISCUSSION Figure C1 presents excerpts from the ANSI/HI 11.6-2001 Submersible Pump Tests standard. This case is being presented primarily to demonstrate the potential problem with the standard for this application. Figure 11.6.15—Transducer Location illustrates a single location for vibration certification. It shows the location as being above the water level. However, that is not necessarily the location of maximum vibration. REAI has performed vibration tests on sewage pumps installed at depths exceeding ten feet in an environment that is hostile not only to the test personnel, but to the vibration probes themselves. In recognition of the problem the standard was undoubtedly developed to minimize the hazards of the test requirement and the instrumentation involved. Regardless of the fluid around the pump the test requires a waterproof transducer and cable assembly. These are not normal items in a vibration analyst’s tool box. In most cases the probes have to be disposed of after the test because of contamination issues. So, our concern is not in the logic of the standard, but in the potential vulnerability of using a single vibration probe for certification of the pump vibration. Note in Paragraph “11.6.8.2.1 b) Pump Support” on Figure C1 that the standard applies to dry pit submersible pumps as well as wet well submersible pumps. Also, the vibration limits for floor mounted and discharge elbow mounted pumps in Figure 11.6.16B are different. This fact indicates a clear understanding of the potential for non-hazardous installation with no adjustment of the measurement requirements when multiple locations are easily measured. Figure C2 illustrates a CSI 2120-2 two channel machinery vibration analyzer (MVA) used for measurements made on a double weir mounted submersible pump situated 12 feet below the control room. The certification specifications also called for vibration readings on the piping. Three probes were mounted as shown in Figure C3 on the top of the motor because of the double weir mounting arrangement and for compliance with ANSI/HIS 11.6. The third probe was located as close as feasible to the location identified in the standard illustrated in Figure C1. A bump test was performed and the result indicated a pair of resonances as illustrated in the mobility response function in Figure C4. The resonances were identified at 1590 CPM and at 2265 CPM. Both of these resonances were within the range of the 2xRPM rotational order for the operating speed range. The 1590 CPM resonance did not have a classic 180 degree phase shift through the resonance, indicating that it was in internal or appendage mode of the motor-pump-piping assembly. The 2265 CPM is a classic single degree of freedom resonance characteristic representing the reed mode of the assembly on the weir sole plate and spanner weldment. The 2xRPM critical speeds based upon the bump test data are 795 RPM and 1132.5 RPM.


Rush Equipment Analysis, Inc. Page 36 of 101 CASE III: Submersible Pump Dry Pit Lateral Resonance Vibration Certification of Vertical Pump Machinery Controlled with Variable Frequency Drives

Figure C5 presents the vibration orbit acquired at 1123 RPM. The vibration was 1.19 in/sec peak (0.841 in/sec rms) in the direction parallel to the suction line (MTI) and 0.2789 in/sec peak (0.197 in/sec rms) in the direction perpendicular to the suction line. Because of ribs on the motor the ANSI/HI 11.6 probe was placed almost perpendicular to the discharge pipe and about 22 degrees off the location of the standard. Figure C6 indicates that the MTI vibration parallel to the suction pipe and the centerline of the concrete weirs was dominated by 2xRPM vibration at 2256.8 CPM with an amplitude of 0.8012 in/sec rms. Figure C7 indicates that the MTD vibration perpendicular to the discharge pipe and 22 degrees off the location for the standard was dominated by 2xRPM vibration at 2257.1 CPM with an amplitude of 0.0565 in/sec rms. According to the ANSI/HI 11.6 allowable vibration from Figure C1 for the 44 BHP operating point of the pump was 0.23 in/sec rms. Thus, the pump meets the requirements of the ANSI/HI 11.6 Submersible Pump Standard at the measurement location required by the standard. However, good engineering practice and general vibration measurement procedures would consider the 0.8012 in/sec rms vibration measured parallel to the suction pipe to be excessive. CASE III CONCLUSION In summary, the vibration standards have to be established to allow feasible measurements. However, the use of a single measurement location at the top motor bearings location for establishing allowable vibration is at considerable risk if the vibration is oriented nearly perpendicular to the measurement axis. In this case study the cause of the resonance was the means of attachment of the pump to the concrete weirs. As such, the criteria for vibration certification should have been altered from the single measurement location to a minimum of two locations orthogonal to the suction pipe, which would be normal practice. Although REAI acknowledges the difficulty for measurements on submerged pumps, the use of the same single measurement criteria for dry pit elbow mounted, floor mounted, or weir mounted pumps is not good practice. The Hydraulic Institute is sponsored by and dedicated to the proper implementation of the pump manufacturers’ products. As such, the manufacturers are quite likely to apply expedient and subjective requirements. Nevertheless, the pump owners need to apply the single measurement location requirement in a way that ensures appropriate certification of the machinery and treat the standard as a minimal condition.. For this case, REAI applied the vibration amplitude limit at the plane of the top motor bearing, regardless of orientation, and the dry pit pump could not be certified at all orientations at all speeds. The utility was advised of the situation, and the utility engineers made the final decision regarding acceptability and certification with the appropriate negotiations with the manufacturer.



Figure C1: ANSI/HI 11.6-2001 Submersible Pump Test Vibration Probe Location



Figure C2: Pumping Plant, VFD Controls Room

Figure C3: Pumping Plant, Pump Installation & Sole Plate Spanner Frame Detail



Figure C4: Pump 2; MTI Bump Test, 2xRPM Critical Speeds at 795 and 1133 RPM

Figure C5: Pump 2; MTI versus MTC, Motor Top Parallel versus Perpendicular Velocity Orbit VFC = 57 Hz, 1123 RPM (Run with auxiliary pumps on at 13:43 06-Feb-08, 0.86 in/sec rms vectored)



Figure C6: Pump 2; MTI, Motor Top Parallel to Suction, VFC = 57 Hz, 1129 RPM

Figure C7: Pump 2; MTD, Motor Top Diagonal to Suction, VFC = 57 Hz, 1129 RPM


vibration certification case studies vertical pump ...this case has a similar problem to case ii...

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