Chandra R., Kyle R., Rakesh K.  

Today we replaced the 12"CFF-ISO adapter mounted to CP1's 10" gate valve (in closed position since 1998) with a 12"CFF x 8"CFF x 4.5"CFF x 2.75"CFF reducing cross.  

Last week, during our initial preparation for leak testing the vacuum volume bound between GV5 and GV6, i.e. CP1 plus misc. flanges, we had tightened the 10" gate valve (1/2 turn) and noticed a pressure response on PT114B. This observation, combined with the fact that this volume had been helium leak tested in the past without finding any external air leaks, caused us to suspect that the blanked-off adapter volume could be at a relatively high pressure and may have been acting as a "virtual leak" across the inadequately closed gate valve for the past 20 years.  To test this theory, we "cracked" open the ISO blank so as to allow some room air into the adapter volume.  In doing so, we did not perceive that any air back-filled into the adapter.  Rather, it seemed as if the adapter was already at atmosphere.  However, there may have been a slight pressure increase in PT114B when we did this (see attached graph).  

The pressure response of PT114B seems counter-intuitive but is not unprecedented.  The as found adapter volume may have been less than an atmosphere and back-filled in a small amount of room air (noisy work area, may not have heard this) and thus increased the leak rate(?).  This would explain the small "bump" in pressure.  Following this by then pumping the newly installed reducing cross would have significantly reduced the closing force experienced by the gate which could then have resulted in more gas making it across the closed gate(?) 

We will continue tomorrow.  For tonight, we are leaving the 10" valve closed and the reducer cross under vacuum.