Smooth running
The term smooth running of centrifugal pumps as viewed from a technical perspective refers to the evaluation of mechanical vibrations generated by a pump.
DIN ISO 10816 for the "Evaluation of machine vibration by measurements on non-rotating parts" assesses vibration class as a function of vibration velocity for three machine groups used in centrifugal pump applications.
ISO 7919 provides general guidelines about how to measure and evaluate machine vibrations if measurements are performed directly on rotating shafts. In this way, shaft vibrations can be determined with a view to changes in vibration behaviour, such as unusually high dynamic loads, or monitoring of radial play.
The vibration displacement amplitudes ascertained in this context correspond to the recommended limit values as a function of rotational speed.
The vibration velocity and amplitude are measured both on the rotating pump shaft and at various locations on the pump casing and foundation, as agreed between the customer and contractor, using electrodynamic vibration acceleration, vibration velocity, and vibration displacement sensors. Pump shaft measurements must be performed in the direction of the pump shaft and in several defined directions normal to the pump shaft.
The smooth-running characteristics of the drive, the tuning of the pump foundation, the vibration behaviour of the piping connected to the pump, and ambient noise (including from surrounding persons) should also be factored into the evaluation.
A centrifugal pump exhibits satisfactory smooth-running characteristics when its balanced rotating internals (see Unbalance) enable a rotationally-symmetric flow. This is typically the case at optimal flow rate (Qopt) and with turbulence-free inflow (see inlet conditions) if the NPSH available is sufficient. If the NPSH available is insufficient, cavitation occurs in the impellers that is frequently not rotationally symmetric and can thus lead to transverse forces of varying intensity (see Radial thrust). Operation in a significantly low-flow range is definitely possible for centrifugal pumps with radial impellers but should be avoided or at least minimised for larger pump units.
Mixed-flow centrifugal pumps and axial flow pumps in particular cannot be operated freely in the low-flow range (e. g. propeller pumps with Qmin/Qopt ≈ 0,8). When the flow breakaway limit is reached, flow breaks away from the impeller vanes in an irregular pulsating pattern and the pump no longer runs smoothly due to the eccentric effect caused by the resulting force. This limit, which is generally very pronounced for propeller pumps, can be counteracted by impeller blade pitch control so as to reduce the incidence angles and the flow rates. The Qopt value then declines, however, and the Qmin/Qopt ratio of approximately 0.8 is reinstated.
Measures for correcting rough running
- Rectify any unbalance.
- Optimise inlet conditions.
- Avoid unnecessary elbows upstream of the pump.
- Install devices that straighten flow, minimise turbulence, and equalise conditions in the suction line.
- Increase the NPSH available by selecting sufficiently sized piping diameters and fitting low-loss valves, elbows, and other fittings to achieve a low-loss flow upstream of the pump.
- Reduce the NPSH required by the pump by using an inducer, enlarging the suction eye of the impeller, specially shaping the vanes, and introducing a light co-swirl effect (see Vortex flow))
- Avoid low-flow operation through closed-loop control measures such as adjusting the impeller blade pitch and bypassing or by using additional, smaller pumps (half-load pumps).
