It has been recognized that the pressure pulsation excited by rotor-stator interaction in large pumps is strongly influenced by the radial gap between impeller and volute diffusers/tongues and the geometry of impeller blade at exit. This fluid-structure interaction phenomenon, as manifested by the pressure pulsation, is the main cause of flow-induced vibrations at the blade-passing frequency. In the present investigation, the effects of the radial gap and flow rate on pressure fluctuations, vibration, and pump performance are investigated experimentally for two different impeller designs. One impeller has a V-shaped cut at the blade's exit, while the second has a straight exit (without the V-cut). The experimental findings showed that the high vibrations at the blade-passing frequency are primarily raised by high pressure pulsation due to improper gap design. The existence of V-cut at blades exit produces lower pressure fluctuations inside the pump while maintaining nearly the same performance. The selection of proper radial gap for a given impeller-volute combination results in an appreciable reduction in vibration levels.

This investigation is focused on the study of the radial gap and blade exit design in centrifugal pumps. The investigation was motivated by the need to trace the root-cause of high-level vibrations at the blade-passing frequency (BPF) and its higher harmonics, which existed in the boiler-feed pumps (BFPs) in a major power plant. The boiler feed pumps have a V-cut at impeller blades’ exit. The problem caused cracking of the connecting piping welds and the gage attachments, which required frequent replacements of such components. The pump manufacturer attempted several remedial actions; however the problem persisted. A thorough study of the vibration records obtained through various field measurements at the pump casing and bearing housing as well as trending records has led to the elimination of other possible mechanical/structural sources of vibrations and pointed out to the flow-induced nature of the problem. Accordingly, the investigation was focused on the pump design, as related to the impeller-diffuser interaction and the effect of the radial gap.

The impeller/volute radial gap is an important design and performance parameter in fluid handling machines, for example, pumps, compressors, and turbines. The proper selection of such a gap is often a best compromise between efficiency and reliability. In high-energy pumps, the minimum radial gap is a critical design consideration that controls the pressure pulsation resulting from the impeller-volute interaction. Under off-design conditions, the pressure fluctuations are excited around the impeller due to such interaction, which give rise to higher pulsating radial loads on the pump rotor, thus causing higher vibration levels.

Pressure fluctuations due to impeller-volute interaction occur mainly at blade-passing frequency and its higher harmonics. A high vibration problem at blade-passing frequencies in a double volute boiler Feed Pump operating at reduced flow rates was also addressed in [

It was also noted that the impeller-volute interaction results in circumferential unevenness of pressure fluctuations around the impeller, which becomes more significant at off-design flow rates [

In general, the optimized gap selection depends on detailed pump design, that is, impeller/volute combination, and dynamic operating conditions. Decreasing the radial gap may produce higher heads; however it may increase the pressure pulsation inside the pump and give rise to high vibrations, due to stronger impeller-volute interaction. On the other hand, reducing the impeller-volute interaction by increasing the radial gap can reduce the pressure fluctuations. However, several reported studies showed that the total pump head drops with the gap increase. The pressure pulsations arising from the interaction of the impeller and vaned diffuser were measured in the discharge pipe at twice the impeller blade-passing frequency [

The effect of radial gap between impeller and diffuser on vibration, noise, and performance was examined, under different flow rate conditions, by trimming the impeller to achieve different gaps [

The application of numerical procedures has provided a very valuable tool to pumps design and analysis. Examples of CFD studies regarding the effect of pump variables on flow field and the unsteady pressure characteristics can be found in [

In this paper, the experimental centrifugal pump model, which was developed based on similitude analysis of the actual boiler feed pump, instrumented and tested in [

The actual boiler-feed pump is a four-stage double-casing centrifugal pump. Figure

Cartridge of the original boiler feed pump.

Split Volute

Impellers with V-cut at blade exit

Model impellers.

Impeller-I: without V-cut

Impeller-II: with the V-cut

The model pump speed is 3540 rpm at 60 Hz. The flow rate is measured by an orifice meter with a discharge coefficient of 0.618. Orifice pressure taps are connected to a PDCR 4170, 700 mbar differential pressure transducer, which has

Coordinates of measuring locations.

Sensor no. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
---|---|---|---|---|---|---|---|---|---|---|---|

radius (mm) | 90 | 78 | 77 | 81 | 79 | 88 | 77 | 97 | 80 | Suction pipe | discharge pipe |

angle (deg.) | 19 | 358 | 61 | 120 | 178 | 199 | 239 | 255 | 294 |

Model pump design and measuring locations.

Measurements of pressure fluctuations: model pump.

The gap value of the original pump is 2.5% of the impeller diameter which is equivalent to 3.6 mm for the model pump. Smaller gaps were achieved by extending the volute splitters. The extension of volute vanes (splitters) was done using epoxy filled mold of wax having the same vane curvature. Larger consecutive gaps were achieved by cutting back the volute tongues. The minimum gap tested in the experiment is 2 mm, while the maximum gap is 7 mm, as shown in Figure

Schematic for gap modifications.

Extension of volute splitters to get smaller gaps

Cutting back volute splitters to get larger gaps

The experimental matrix associated with testing the two impellers (with and without the V-cut designs) is established for different gaps of 2, 3, 3.6 (original gap), 4.85, 6, and 7 mm. Each impeller-gap combination was tested at different flow ratios (

Best efficiency conditions with uncertainty limits.

Parameter | Impeller-I | Impeller-II |
---|---|---|

Flow rate, | ||

Total head, | ||

Efficiency, |

A typical pressure fluctuation history (for two shaft revolutions time, 0.034 second) and the frequency spectrum, FFT, at location 3 are shown in Figure

Waveform and frequency spectrum of pressure fluctuation at location 3, gap 3.6 mm for Impeller-I, at

Effect of flow rate and measuring location on pressure fluctuations: impeller-I.

Pressure fluctuations (peak-to-peak)

FFT magnitudes at 1st BPF

The variation of pressure fluctuations with radial gap for the two impellers at location 4 is given in Figure

Variation of pressure fluctuations with radial gap at different flow rates.

Impeller-I

Impeller-II

A clearer picture of the effect of radial gap on pressure fluctuations is depicted by the FFT magnitudes, as represented by Figure

Variation of FFT magnitudes of pressure fluctuations with radial gap at different flow rates at location 4.

Impeller-I

Impeller-II

The experimental findings attest to the fact that increasing the gap reduces the impeller-volute interaction and consequently the strength of pressure pulsation resulting from this interaction. Pumps, in general, are designed to operate at the best efficiency conditions. Changing the gap design shifts the operating point; usually to a lower head when the gap is increased. Therefore, the practical selection of the optimum gap should be based on the evaluation of performance loss in view of the reduction in pump vibration. It is a trade-off to select the most effective gap, which minimizes the pump vibrations while maintaining an acceptable performance according to pump application requirements.

A typical vibration signal (vertical direction) of the pump casing is shown in Figure

Typical vibration signal of model pump casing at

Now, let us further examine the effect of changing the gap on pump vibrations. Increasing the gap reduces the pump vibration due to the reduction of the energy of pressure pulsation inside the pump. Figure

Effect of doubling the radial gap on vibration of pump casing.

The effect of different gaps on pump performance for impeller-I is shown in Figure

Effect of gap on pump performance,

Gap (mm) | Impeller-I | Impeller-II | ||

% change in head | % change in efficiency | % change in head | % change in efficiency | |

2 | −1.30 | −1.88 | 0.74 | 0.74 |

3 | 0.55 | 0.11 | −0.42 | 0.27 |

— | — | — | — | |

4.85 | −0.15 | −1.36 | −2.92 | −0.88 |

6 | −0.93 | −0.50 | −4.99 | −2.40 |

7 | −2.41 | −1.43 | −5.51 | −1.53 |

Effect of gap on pump performance: impeller-I.

Decreasing the gap

Increasing the gap

Impeller-volute interaction is an important design factor in developing high-energy pumps like the double-volute boiler feed pumps. Different designs and combinations of impellers and gaps result in different characteristics of flow field inside the pump leading to different pump vibration behaviors. The flow-induced vibrations at the blade passing frequency stem from the pressure pulsation at the impeller exit. It was found that for the present pump, smaller gaps increased the pressure fluctuation and did not improve the performance except at very high capacities. The effect of the V-cut with different gaps on the pressure pulsation and the resulting pump vibrations at the blade-passing frequency was addressed for the first time. Different designs of impeller blade exit attained minimum pressure fluctuations inside the pump for different gaps. In general, it was concluded that increasing the gap reduces the pressure fluctuations particularly at part-load conditions. Another important conclusion, which is of particular interest to vibration analysts, is that severe vibrations at the BPF are most likely due to a design problem and can be corrected by the proper gap adjustment. Taking into account the similarity relationships, the results obtained from the experimental model pump were utilized to modify the gap in one of the actual BFP in the power plant. An average reduction of nearly 50% in pump vibrations at the BPF was obtained, as measured at both the inboard and outboard bearing housing. This was an excellent achievement, wherein the vibration levels were brought down below the severity limit. The results also serve as a testimony to the similitude accuracy of the constructed single-stage model pump, the proper selection and positioning of instrumentations, the data acquisition, and the test loop setup.

This research work was funded by the Saudi Electricity Company (SEC), Project no. CER-2289. The authors greatly appreciate the support provided by SEC and King Fahd University of Petroleum & Minerals (KFUPM) during this research. The valuable discussions and remarks made by Dr. R. Ben-Mansour and Dr. F. Al-Sulaiman are highly appreciated.