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Research Papers: Gas Turbines: Turbomachinery

An Experimental Investigation of the Influence of Flash-Back Flow on Last Three Stages of Low Pressure Steam Turbines

[+] Author and Article Information
Naoki Shibukawa

Rotating Machine Technology R&D Department,
Power and Industrial Systems Research
and Development Center,
Toshiba Corporation Power Systems Company,
2-4, Suehiro-Cho, Tsurumi-ku,
Yokohama 230-0045, Japan
e-mail: naoki.shibukawa@toshiba.co.jp

Takao Fukushima

International Project Engineering Department,
Nuclear Energy Systems & Services Division,
Toshiba Corporation Power Systems Company,
72-34, Horikawa-Cho, Saiwai-ku,
Kawasaki 212-8585, Japan
e-mail: takao1.fukushima@toshiba.co.jp

Yoshifumi Iwasaki, Yoshiaki Takada, Itaru Murakami, Takashi Suzuki

Turbine Design and Assembling Department,
Keihin Product Operations,
Toshiba Corporation Power Systems Company,
2-4, Suehiro-Cho, Tsurumi-ku,
Yokohama 230-0045, Japan

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 14, 2014; final manuscript received August 30, 2014; published online November 18, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(5), 052601 (May 01, 2015) (10 pages) Paper No: GTP-14-1394; doi: 10.1115/1.4028674 History: Received July 14, 2014; Revised August 30, 2014; Online November 18, 2014

A shutdown operation of a large size steam turbine could possibly cause flashing phenomena of the pooled drain water in low-pressure heaters. The boiled steam is sometimes in the same amount as the main flow in the case where shutdown is executed during low load conditions, and returns to the steam flow path through the extraction lines. A series of experimental work with a subscale model turbine facility has been carried out to investigate the vibration stress behavior, and the steady and unsteady pressures under the flashing back (FB) conditions. It was observed that the blades of the two stages before the last stage (L-2) and a stage before the last stage (L-1) presented their peak vibration stresses immediately after the flash-back flow reached the turbine. In the meantime, the vibration stresses of the last stage (L-0) blades were reduced for a few tens of seconds. It can be thought that the flash-back flow pushed out the reverse flow region around the L-0 blades and allow the blades to be more stable. A detailed examination with measured data of the L-2 blade explained that, as long as the flash-back flow has small wetness, the blade is excited in its specific vibration modes in larger than eighth harmonic of rotational speed, but once the flash-back flow carries water droplets, the fluid force in random frequencies remarkably increases and excites the blade in less than seventh harmonic range.

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References

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Shibukawa, N., Tejima, T., Iwasaki, Y., Murakami, I., and Saito, I., 2011, “A Correlation Between Vibration Stresses and Flow Features of Steam Turbine Long Blades in Low Load Conditions,” ASME Paper No. GT2011-46368. [CrossRef]

Figures

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Fig. 1

Typical extraction system and flash-back routes of a large scale LP turbine

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Fig. 2

Configuration of the model turbine

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Fig. 3

Bird's-eye view of the model turbine with the flash-back system

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Fig. 4

Schematic diagram of the flash-back simulation facility

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Fig. 5

Flash-back steam paths of the model turbine

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Fig. 6

A pressure tap and unsteady sensors for pressure measurement

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Fig. 7

Layout of the steam path measurement

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Fig. 8

Strain gauge on the L-0 blade

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Fig. 9

Block diagram of FM telemetric system

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Fig. 10

Tested main steam conditions

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Fig. 11

Heat and mass flow model of the flash-back estimation

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Fig. 12

Estimated flash-back mass flow ratios to the main steam design flow rate

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Fig. 13

Vibration stresses of the L-0 blade against Vax (before FB)

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Fig. 14

Comparison of the blade vibration stresses of the last three stages (before FB)

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Fig. 15

Variation of vibration stresses by flash-back steam injection (L-2)

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Fig. 16

Trend of the vibration stress after flash-back steam injection (L-2)

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Fig. 17

Comparison of vibration stresses with/without flash-back steam (L-2)

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Fig. 18

Comparison of pressure fluctuations with/without flash-back steam (L-2)

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Fig. 19

Growth ratios (with FB/without FB) of vibration stress and pressure fluctuation by flash-back steam (L-2)

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Fig. 20

Variation of vibration stresses by flash-back steam injection (L-1)

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Fig. 21

Variation of vibration stresses by flash-back steam injection (L-0)

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Fig. 22

Trend of the vibration stress after flash-back steam injection (L-0)

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Fig. 23

Vibration stress against mass flow rate of flash-back steam injection (L-0)

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Fig. 24

Flash-back flows with/without water droplets at the sight glass (FB_A)

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Fig. 25

Comparison of flash-back flow trend with/without water droplets

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Fig. 26

Trend of vibration stress after flash-back steam injection with water droplets (L-2)

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Fig. 27

Comparison of vibration stress with/without water droplets (L-2)

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Fig. 28

Comparison of pressure fluctuation with/without water droplets (L-2)

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Fig. 29

Growth ratios (with FB/without FB) of vibration stress and pressure fluctuation by flash-back steam with water droplets (L-2)

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