Abnormal Vertical Pump Suction Recirculation Problems Due to Pump-System Interaction, D.R. Smith, S.M. Price, Bruno Schiavello, Proceedings of the 21st International Pump Users Symposium, March, 2004

It is well documented that the proper operation of vertical centrifugal pumps is greatly dependent upon the entire pump/piping system, which includes the piping geometry and the system operating conditions. Oftentimes, pumps operate satisfactorily during shop tests but experience problems after they are installed in the field.
Here, three large vertical pumps that operated satisfactorily on the test stand experienced excessive vibration after installation. Additionally, pulsation in the system piping was found to be causing unexpected vibration in downstream equipment. It was discovered that the problems were the result of complex interaction between several phenomena. Improper inlet conditions caused suction recirculation which generated broadband turbulence. The turbulent energy excited acoustical resonances of the pump/piping system, resulting in pulsation at several discrete frequencies. This energy subsequently excited mechanical natural frequencies of the motor/pump/piping system causing high amplitude non-synchronous vibration of the pump and other structures far downstream from the pump.
Field data are presented. Diagnostic techniques and instrumentation needed to obtain the field data required to solve these problems are discussed. Also, additional data from pump hydraulic analysis and sump model tests are presented. Further, the solution strategy with two-step field changes (sump and pump) is shown. Following these modifications, the pumps have operated satisfactorily for more than four years.

GENERAL CONCLUSIONS
• Pump suction recirculation was clearly observed at flow rates above the BEP in a field installation with wet pit vertical turbine pumps. To the authors’ knowledge, this event was never reported in the published literature and also is apparently in contrast with pump designers’ experience.
• Pumps can operate satisfactorily on the test stand and then experience problems after they are installed at the site due to improper inlet flow conditions.
• Flow visualization and quantitative measurements performed with the sump model tests indicated a sudden change of the flow pattern in the sump causing an unsteady swirling flow inside the suction bell up to the impeller eye, which may strongly affect the onset of suction recirculation.
• The change of flow regime in the open sump was occurring at a threshold capacity, which was likely due to a fluid interaction between the intake internal flows (governed by the Reynolds-number) and the sump free-surface flows (governed by the Froude-number). This “critical” intake/sump interaction would need confirmation with further investigations.
• The intake–sump fluid interaction has strong practical importance, because it has implications regarding the key factors for model tests (fluid similarity criterion, model scale factor, and model optimization) and also the correlation with actual site flow conditions.
• The onset and intensity of suction recirculation for centrifugal pumps with radial outlet impellers are different compared to pumps with mixed-flow impellers for the same Ns. Therefore, certain criteria used to evaluate the potential for suction recirculation in centrifugal pumps, such as suction energy and suction specific speed are not applicable to vertical turbine pumps.
• Recirculation will occur on the test stand as the flow rates are reduced to measure the flow-head curve, but may not be noticed unless the pulsation generated by the recirculation excites a mechanical natural frequency of the pump or motor. For certain pumps, the head (power) curve may give “gross” qualitative indications. However, accurate direct detection of pressure pulsations with dynamic pressure transducers should always be made on the test stand and on site during the pump commissioning. Frequency spectra should be obtained to determine the amplitude and frequencies of the pulsation and vibration data.
• The magnitude of the pulsation and noise levels at the onset capacity are worse with high-energy pumps.
• High-energy pumps are sensitive to the inlet flow conditions. Uneven flow into the pump inlet can cause the flow rates to be significantly lower in various sections of the impeller.
• Recirculation occurs instantly within one pump rotation at a threshold flow rate, with a character of rotating stall having sub-synchronous frequencies. The resulting high-level pulsation can excite interaction with the entire pump/piping system from the sump, through the pump, and throughout the discharge piping.
• The frequencies of recirculation induced pulsation are determined by the acoustic natural frequencies of the entire pump/piping system. The acoustic natural frequencies of the pump/piping system are controlled by the geometry of the system, the speed of sound of the fluid, and the stiffnesses of the expansion joints.
• The pump recirculation typically generates broadband turbulence (random, low-amplitude, non-synchronous pulsation over a large frequency range from 1 to 2000 Hz). In addition to exciting the acoustic natural frequencies, the broadband turbulence can also excite the mechanical natural frequencies of the pump and motor, and the lateral natural frequencies of the pump rotor.
• The vibration levels are further increased when the acoustic natural frequencies are coincident with the mechanical natural frequencies.
• Modifications to pump inlet bell (guide vanes, larger diameter, etc.) can correct possible flow distortion of the upstream approaching flow and improve the flow uniformity/steadiness at the impeller inlet.
• Changing the acoustical natural frequencies of the piping system, changing the pump structural natural frequencies, or in principle eliminating the recirculation are all potential solutions to reduce the vibration of the pump/motor/piping system. However, generally the preferred solution is to reduce the excitation by just shifting the recirculation outside the normal operating range and also correcting in a practical way the inlet flow disturbance induced by the suction system.
• Circumferential variation of the static pressure measurements in the suction bell is indicative of unexpected inlet flow distortion.
• Recirculation was identified by:

o sudden increase in the suction static pressure as the discharge pressure was increased and flow reduced,
o sudden increase in non-synchronous pulsation in suction and discharge as the discharge pressure was increased and flow reduced,
o sudden increase in non-synchronous vibration of the pump, motor, and piping as the discharge pressure was increased and flow reduced,
o sudden increase in non-synchronous motor amps as the discharge pressure was increased and flow reduced, and
o sudden backflow observed in underwater videos of the pump inlet.