Upon being made aware that the Corner Station instrument air compressor(s) were creating a measureable noise in the IFO (see https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=53248), we have made various attempts to reduce or eliminate this.

Recall that 50 -100 psi is required to hold the 44" pneumatic gate valves GV5, GV6, GV7 and GV8 open.  To do this, we use air compressors and dryers remotely located in the chiller yard and then route this to the LVEA via piping.  The compressors are isolated from the ground via spring mounts and are isolated from the building via a flexible connection to the piping.  Our first attempt at noise reduction was to shut-down the mechanical compressors and dryer, isolate them from the building piping and to then substitute regulated compressed bottled gas (N2) on the building side of the isolation.  This is the preffered solution as it is the quietest but the numerous small leaks-to-atmosphere currently present in the building piping rendered this a maintenance nuisance - requiring frequent bottle changes (see https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=53271). 

Next, we realized that the piping leaks which proved a significant loss when using the bottle-supplied gas were quite insignificant when compared to the constant loss-to-atmosphere associated with the continuous regeneration of the "swing style" desiccant dryer when using the compressor system.  For this system, one desiccant tower dries the a majority fraction of the air supplied from the compressors and then routes this dry air the LVEA.  Simultaneously, a minority fraction of the air supplied from the compressors gets routed through the 2nd tower during its regeneration phase.  This "regeneration" air exhausts to atmosphere.  This system works excellent as a drier but is wasteful and requires that a constant supply of compressed air be exhausted to atmosphere.  

The recent configuration changes incude the addition of check valves downstream of the dryer (prevents building air from backflowing to atmospher via regenerating drying tower when the compressor is off), moving the pressure switch sampling from the compressor receiver tank to downstream of the newly installed check valves (prevents compressors from starting if the auto drain depressurizes the compressor receiver tank), the addition of a NC solenoid valve between the compressors and the dryer (prevents compressor receiver tank from supplying regerating drying tower when the compressor is off) and the rewiring of the auto drain solenoid valve such that it only becomes energized when the compressor is running (prevents unnecessarily depressurization of the compressor receiver tank during off periods).