Operating behaviour
A centrifugal pump's operating behaviour is the collective term for all pump characteristics (e.g. hydraulic, mechanical, acoustic) at a given operating point. The position of the operating point in relation to the design point has a major influence on centrifugal pumps' operating behaviour.
When selecting pumps, the operating point should coincide with or be in the proximity of the design point to ensure very low energy and maintenance costs and minimal hydraulic excitation forces.
In practice, application processes may require that the pump is operated under low flow or overload conditions, i.e. in the off-design range. As the difference between the operating point and the design point increases, unfavourable flow conditions develop at the impeller or diffuser vanes because of an unfavourable approach flow, frequently leading to flow separation, mechanical vibrations, noise and cavitation. See Fig. 1 Operating behaviour
Under low flow conditions, for example, the meridian component (v1m) of the absolute velocity at the design point decreases to the value v1mT and the relative velocity w1 to w1T.
Depending on the orientation of w1T the result is a highly unfavourable flow to the vane cascade (see also Flow profile). This prevents the relative flow from following the vane contour along the suction side, and flow separation takes place. Similar conditions are encountered in the overload range on the discharge side (see Boundary layer).
Any form of flow separation represents a transient or non-steady flow phenomenon. These transient conditions significantly disturb flow deflection at the vane cascade profile (deflection required to generate the head) and lead to pulsations (noise) in the fluid handled, in hydraulic pump components or in components connected to the pump.
The unfavourable operating behaviour exhibited when centrifugal pumps are continuously operated under low flow conditions is caused not only by flow separation, but also by instability as a result of suction or discharge recirculation. This occurs outside the impeller inlet and inside the impeller outlet if there is a significant discrepancy between flow rate and design point. Suction recirculation (S) and discharge recirculation (D) are transient flow phenomena which may occur independently of each other. If the flow rate is further reduced, they often occur simultaneously.
See Fig. 2 Operating behaviour
Suction recirculation (S) can be detected over a distance corresponding to several suction pipe diameters in the opposite direction of the incoming flow. To prevent it from extending axially, it is possible to incorporate vanes, elbows or changes in pipe cross-sections.
The higher the specific speed of a centrifugal pump, the more intense the recirculation phenomena relative to the pump's power output. This means that the low flow operating range must be further limited in the case of high specific speed centrifugal pumps as the critical operating limit is reached at an earlier stage (this operating limit is also referred to as the "rumour limit"). See Fig. 3 Operating behaviour
This limit also exists in the overload range and should not be exceeded by low and high specific speed pumps. It is primarily determined by the pump's suction behaviour and discharge-side flow separations. High specific speed centrifugal pumps such as mixed flow and axial flow pumps are characterised by a more or less pronounced saddle in their H/Q curve due to suction recirculation (S) in the low flow range. This point on the H/Q curve should be passed through as quickly as possible to avoid vibration and possible cavitation. Continuous operation is not permitted in the range from zero flow rate Q=0 to the operating limit. See Fig. 3 Operating behaviour
Depending on the system characteristic curve unstable H/Q curves of low and high specific speed pumps can lead to problems during start-up and shutdown, undefined operating points or pump vibrations.
If a centrifugal pump with a high head and motor power is operated at the lowest limit of the low-flow operating range, or even against a closed shut-off element the drive's high output power is transferred to the fluid handled leading to a rapid temperature increase. This in turn can lead to evaporation and pump damage (due to seizure in the clearance gaps) or even cause the pump to burst (due to vapour pressure increase in the case of a closed lift check valve). The unfavourable operating behaviour associated with operation in the low flow range can be improved by increasing the flow rates (via a bypass) and by impeller blade pitch adjustment.
The definition of distinct operating ranges is a necessary measure if the problems encountered due to differences between operating points and design points are to be prevented and damage and trouble-free operation ensured. The four ranges defined are continuous, short-time, minimum flow and impermissible operation.
Definition of operating ranges
- Continuous operation:
To prevent damage and unnecessary wear, only the operating points in the region of the design point are permitted. - Short-time operation:
If pump operation at operating points which would lead to damage in the long run is intended, it is necessary to limit the time of operation. Time limits vary considerably depending on multiple parameters. - Minimum flow operation:
A range which is permissible for a very short period of time only and must also be defined on a case-by-case basis.
Irrespective of the various parameters (e.g. economic efficiency, application, absolute motor power, pump size, pump type, low flow and overload) which determine individual operating ranges, the ranges of operation become narrower for pumps with higher specific speeds.
