The chillers at all stations produce seismic noise in the 5 to 25 Hz band because of turbulence in the pipes. The chillers can be the dominant source of seismic noise in their band. The chiller signal at EY tends to be peaked at 9 Hz (e.g. here), while the chiller signal at the LVEA and EX tend to be broader band, between 10 and 25 Hz (e.g. here). Studies have shown that the seismic level can be reduced by reducing the chiller pump speed (same links). For this purpose we have installed variable frequency drives at several pumps so that we can run them at reduced frequencies (typically around 35-45 Hz instead of 60 Hz). The chiller turbulence frequencies overlap with one of the most troubling resonances in the seismic isolation system, at about 10 Hz, so it is likely that we will want these VFDs on both pumps at the end stations.
The chiller turbulence does not bear primary responsibility for the recent unusually sharp peak at just above 10 Hz that is troubling SEI commissioners at Y-end, but it makes sense to try and reduce the background for them. With this in mind John and I have upped the pressure in the chilled water lines at both end stations and I have set the VFDs properly. The Y-end VFD pump is not yet being used because I could not get the chiller to start remotely, but I hope John and Ski can get this started Monday.
The figure shows the results of the changes we made at the other end station, X-end. A week of minute trends for the 10-30 Hz band is shown. Notice the regular spikes in quiet periods prior to the 4th day. These have a 40 minute period, correlating with the cycling of the temperature control valve. We attempted to reduce them by increasing coolant pressure at the beginning of the 4th day. This seems to have worked, but there is a short resurgence at the beginning of the 5th day. My switch to VFD on the 6th day is likely responsible for the significant drop in the quiet-time background after that.
I record the keying instructions for the VFD changes I made because the manual, like at least some other Mitsubishi manuals, is extremely poor (e.g. some keys are labeled differently on the controller than in the manual).
1) Set Pr. 79 operation mode to 3 (combined operation, external and hand). This mode allows external on/off and hand unit setting of frequency.
Switch below controller OFF, PrSET, 79, READ, 3, WRITE
2) Set the minimum frequency to 30 Hz
HAND, PrSET, down arrow to Pr.LIST, READ, down arrow to Min.F1, READ, 30, WRITE
3) Set the run frequency to 35 Hz
HAND, 35, WRITE
Robert S. John W.
