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AdV-VAC (Towers vacuum management)
Fiori, Vacuum - 18:09 Thursday 27 July 2017 (38884) Print this report
Activities related to the fans switch off
During the switch off tests of 20 July, the fans of turbo_PR , turbo_BS-PR, turbo_BS were identified as producing seismic noise on PR B4_ghost flange (we didn't make a distinction between them) so we installed a simple mechanical damping system for each of them (= rubber feet +  6-7 kg metallic flange). The effect is shown in plot 1, purple curve=original, blue curve = damped configuration, as seen by the accelerometer on B4_g flange.
 
During the maint. break of 25/07 , at around 10h UTC, we removed the shaker device from the B4_g flange, with the effect of changing the local mechanical vibrations of the flange (around 90Hz in particular, see plot 2 ).   
Today morning, during the minitower intervention, we tried to add back a weight to the B4_g flange (about 20 kg) fixed in a provisional way (picture 3)  from approximately 9h30 UTC to 9h50UTC. In attached some spectrograms .  Finally we restored the previous situation = no added weight.  
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fiori - 18:26 Thursday 27 July 2017 (38886) Print this report

The attached figure shows that the mitigation of the turbo pump fans (blue vs purple curves) was effective in reducing the 45Hz bump noise in DARM and MICH. In fact after the mitigation (blue one) there is no evidence of residual bump noise in DARM with respect to when the fans were switched off  (green curve).

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