d. patterson-cavitation in vfd driven mainline pump rev2
Post on 26-Feb-2022
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SULZER CONFIDENTIAL
Author: Donnie Patterson
Donnie Patterson has been with Sulzer since 2007. He has held various positions within Sulzer including Design Engineer and Rotordynamic Engineer. He currently serves as Lead Field Engineer with Sulzer Pump Services Rocky Mountain Service Center in Denver, CO. He received his BSME from John Brown University and is a licensed PE in the State of Texas.
SULZER CONFIDENTIAL
ABSTRACT
After a recent pump overhaul, an end‐user reported cavitation type noise when starting up their pump station. There are two, BB3 style pumps operating in series as crude oil mainline pumps. A single VFD is used for both units. Site review of the units in operation indicated cavitation‐like noise. After a review of the pump suction hydraulics, system piping, and vibration, it was determined that the startup sequence should be changed. The change in startup sequence eliminated the noise and reduced cavitation. The units are now operating well. This case study will present the hydraulic review of these units, why the startup sequence was changed, and Key Takeaways for operating mainline pumps in series.
SULZER CONFIDENTIAL
Background Information
• Two BB3 style pumps operating in series
• Rated for 2120 ft @ 1550 GPM at 3560 rpm
• Operated on a single VFD• Excessive noise at Unit 1 pump during startup
SULZER CONFIDENTIAL
Definition of Terms
• NPSHa• Net Positive Suction Head Available
• Total suction head, in feet, available to the first stage impeller
• NPSHr• Net Positive Suction Head Required
• Total suction head, in feet, required by a pump to operate free of cavitation
• Series operation• Two or more pumps operated such that the discharge of one pump feeds the suction of the next pump. The flow rate remains constant between the pumps, while the differential head is added.
SULZER CONFIDENTIAL
Station Layout• Station may be run with a single unit, or two units in series
• Single VFD for both units• Unit 2 brought to full speed, then switched across the line
• VFD then brings Unit 1 up and controls on speed (~70‐100% of Unit 2)
SULZER CONFIDENTIAL
Preliminary Review• Review at site indicated loud, cavitation‐like noise; “Pumping Gravel”• Vibration spectra or sound measurements not available • Vibration readings (handheld probe) on bearing housings reported at <0.1 in/s peak
• Suction piping arrangement marginal: two 90 degree bends at approximately 5D from suction flange
SULZER CONFIDENTIAL
Detail Review• Review of site data indicated the following:
• Net Positive Suction Head available (NPSHa) for Unit 1 suction of 150 ft• Unit 2 operating across the line, ~ 3560 rpm• Unit 1 operating at 70% speed, ~2492 rpm
• Initial thought was that Unit 1 was running off the end of the curve in order to keep up with Unit 2
• Pump curves plotted to review how Unit 1 was operating compared to Unit 2
SULZER CONFIDENTIAL
Multi‐Unit Pump CurveBEP
OPERATING POINT: 1830 ft@ 1850 GPM
UNIT 2: 3560 RPM
UNIT 1: 2490 RPMBEP
UNIT 2 NPSHr
UNIT 1 NPSHr
SULZER CONFIDENTIAL
Detail Review
• System curve not available, but operating point obtained with both units running
• Plotted curves show Unit 1 running well off the end of curve• Additional curve made to extend out Unit 1 H‐Q and NPSHr to operating point
• NPSHr “stonewalling” common at high flows past BEP; review predicted curves to see how NPSHr curve is behaving
SULZER CONFIDENTIAL
Detail Review
• Extended H‐Q curve shows Unit 1 at approximately 164% of BEP• NPSHr curve is predicted from trend line, as test data not available that far out on the curve
• NPSHr prediction varies between polynomial extrapolation, but shows rapid increase at operating flowrate
SULZER CONFIDENTIAL
Solution
• Start Unit 1 up first on VFD• Take Unit 1 across the line• Start Unit 2 up and control on speed• Unit 2 will still runout on curve, but now has Unit 1 suction pressure• End user reported this solved the cavitation noise issue and adjusted standard start‐up sequence accordingly
• Some amount of cavitation possible when Unit 2 running at lower speed, but much less severe
SULZER CONFIDENTIAL
Take‐Aways
• If operating multiple pumps in series with a single VFD, control the last unit in the series on VFD. While the last unit may run out on the curve, it has suction pressure approximately equal to the output pressure of the pump immediately upstream to avoid cavitation.
• Be aware of where the VFD controlled unit is operating on its curve. This can change depending on system curve shifts and changes in product (e.g. change in drag reduction agent percentage (DRA), different product batch viscosity, etc.).
• If converting to VFD, consider a dedicated VFD for each unit. This allows for better speed matching to keep each unit running in the same position on the curve
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