Regenerative Turbine Pumps & Hydraulic Resonance Solutions

Q. What is a regenerative turbine pump? A. Regenerative turbine pumps are characterized by a low flow rate and high head. They use peripheral or side channel vanes or buckets that are manufactured with a rotating impeller to impart energy to the pumped liquid. Shearing action causes the liquid to acquire a spiral rotation as it passes from the buckets of the rotating impeller into the stationary side channels. The buckets are radial channel cavities separated by teeth, also called vanes or webs. Pressure increases uniformly from the pump inlet to the discharge. For more information on rotodynamic pumps, see ANSI/HI 1.1-1.2 Rotodynamic Centrifugal Pumps for Nomenclature and Definitions.

Q. When does hydraulic resonance occur, and what are some problems associated with it? A. Hydraulic resonance occurs when forces induced by pump operation excite the resonant frequency of the piping system. When normal pump-induced pressure pulsations are reflected by the piping systems and added in phase to the source pulse, the amplitude of the pulse is magnified. Resulting high pressures can cause mechanical fatigue failures, generate unacceptable noise levels and cause vibration. While a certain level of pressure fluctuation is unavoidable and does not harm the system, excessive pressure pulsations can excite pump and pipe vibrations and cause damage. Typical problems include the following:

• Bearing housing vibrations can cause fatigue fracture of instrumentation or pipes.
• Baseplate vibrations can occur.
• Fatigue fracture of the tie bolts in multistage segmental pumps or joining elements in the stage casings of barrel pumps may result.
• The pressure pulsations generated by the pump can lead to standing waves (acoustic resonances) in the system that may upset the control system, induce pipe vibrations, cause breakage of instrument lines or affect resonances with other components.

Pressure pulsations are an important source of noise emissions from the pump. Airborne noise is radiated from the casing, piping and baseplates, and structure-borne noise is radiated into the piping and foundation. The following measures may correct hydraulic resonance:

• Alter the resonant piping.
• Change the pump speed.
• Change the internal design characteristics of the pump.
• Insert a pulsation damper on the pump/piping system.

For information on rotodynamic pump vibration, see ANSI/HI 9.6.4 Rotodynamic Pumps for Vibration Measurement and Allowable Values.