*Periodic Instabilities in a Specific Low-Speed Pump Working as a Turbine DOI: http://dx.doi.org/10.5772/intechopen.109210*

**Figure 8.**

*Estimation of periodic or quasi-periodic instabilities for DYT2. Above, estimation in the time domain. Below, estimation in the frequency domain.*

• At all operating points, blade excitation is present due to rotor-stator interaction, with fn = 6. The magnitude of this disturbance gradually increases from the middle zone of the turbine-brake mode (OP 23) to the last OP in the reverse pump zone (OP 60). In the upper part of the turbine mode, there is a rapid increase and decay of this disturbance at OP 2–6. Between OP 18 and 22 the magnitude of the disturbance is practically negligible.

#### **Figure 9.**

*Contribution of periodic or quasi-periodic instabilities in the total instability for DYT2. Above, estimation in the time domain. Below, estimation in the frequency domain.*

• In G1, the evidence suggests the existence of a subsynchronous phenomenon that moves inside the volute in the opposite direction to the flow direction, with phase shift angles close to the physical separation of the sensors. In G2, a supersynchronous phenomenon is present and something like G1 occurs, only that the movement of the phenomenon is in the same direction of the flow.

In terms of the contribution of periodic instabilities to total instability, three zones can be identified. In the first one, constituted by OPs 1–31 (turbine, runaway, and

*Periodic Instabilities in a Specific Low-Speed Pump Working as a Turbine DOI: http://dx.doi.org/10.5772/intechopen.109210*


### **Table 4.**

*Operation zones in the PAT according to periodic or quasi-periodic instabilities contribution.*

upper part of turbine-brake), periodic instabilities represent approximately 1–42% of the total instability. In the second zone, between OP 32 and 48 (middle and lower turbine-brake mode), there are at least three hydrodynamic phenomena whose contribution to the total instability is between 20 and 60% approximately. The third zone covers OP 49–60 (last OPs in the turbine brake and all OPs of the reverse pump) where blade excitation predominates and presents contributions to the total instability ranging from 7 to 28%.
