A B737-300 FLC LOST PRESSURIZATION ON DEP FROM DEN.

Narrative

THE L AIR CONDITIONING PACK WAS DEFERRED INOP. PRIOR TO THE FLT; I CONSULTED WITH DISPATCH AND MAINT ABOUT THE DEFERRAL AND THE HISTORY ON THE R PACK WHICH HAD TRIPPED OFF PREVIOUSLY. I WAS TOLD BY MAINT THE R PACK HAD NOT FAILED IN 10 DAYS AND THAT A VALVE AND SENSOR HAD JUST BEEN REPLACED PRIOR TO MY FLT. DURING CLBOUT AT APPROX FL190; THE R PACK TRIPPED OFF; RESULTING IN A LOSS OF PRESSURIZATION. THE CABIN ALT BEGAN TO RAPIDLY INCREASE. I INSTRUCTED THE FO TO LEVEL OFF AND I ASKED ATC FOR A LOWER ALT AND A CLRNC BACK TO OUR DEP STATION. I RECEIVED BOTH. IT WAS OBVIOUS THE CABIN ALT WAS GOING TO EXCEED 10000 FT. OXYGEN MASKS WERE DONNED AND THE QUICK REF EMER CHKLIST FOR CABIN ALT WARNING WAS ACCOMPLISHED. THE MAX CABIN ALT WAS 12500 FT. THE PAX OXYGEN MASKS DID NOT DEPLOY. THE IRREGULAR PROC FOR PACK TRIP-OFF WAS ALSO ACCOMPLISHED; BUT THE R PACK REMAINED INOP. I TURNED ON THE DEFERRED INOP L PACK AND IT WORKED. SOME PRESSURIZATION WAS RESTORED. I AGAIN PUSHED THE TRIP RESET BUTTON AND THE R PACK RESET. THE L PACK FAILED A SHORT TIME LATER.