WHILE CLBING THROUGH HIGH ALT; THE CABIN ALT WARNING HORN ADVISED THAT THE CABIN WAS CLBING THROUGH 10000 FT. THE FLC DECLARED AN EMER TO ATC AND DSNDED TO FL180 BEFORE THE ACFT PRESSURIZATION WAS STABILIZED.

Narrative

DURING PREFLT; WHEN THE RECALL LIGHT WAS PRESSED; THE L PACK LIGHT ILLUMINATED. THE DISCREPANCY WAS ENTERED IN THE LOGBOOK. CORRECTIVE ACTION WAS TAKEN BY MAINT. TKOF AND CLB WERE NORMAL UNTIL APPROX FL310. AT THAT POINT; THE CABIN ALT WARNING HORN BEGAN. THE CABIN ALT WAS APCHING 10000 FT AND CLBING. BOTH PACKS WERE CHKED IN AUTO; BOTH BLEEDS WERE CHKED OPEN; AND THE OUTFLOW VALVE WAS SELECTED MANUAL AND CLOSED. I DECLARED AN EMER AND RECEIVED CLRNC TO 14000 FT. THE EMER CHKLISTS WERE COMPLETED DURING THE DSCNT. DSNDING THROUGH APPROX FL200; THE CABIN BEGAN TO DSND. WE REQUESTED LEVEL AT FL180 WHERE THE CABIN DSNDED TO 4000 FT AND STABILIZED. PRESSURIZATION WAS SWITCHED TO STANDBY WITH NORMAL OP CONTINUED IN THAT MODE. WHEN THE AUTO PRESSURIZATION MODE SELECTED; THE CABIN BEGAN CLBING. WITH STANDBY MODE SELECTED AND OP NORMAL; WE RPTED THE SIT UNDER CTL AND CLBED TO FL240; PROCEEDING ON RTE TO IND; OUR DEST. WE DISCOVERED THAT SWITCHING THE R PACK OFF CAUSED THE CABIN TO CLB AT 2000 FPM AND GREATER. WITH L PACK OFF; PRESSURIZATION REMAINED STABLE; INDICATING L PACK WAS WEAK OR INOP. IT WAS DETERMINED BY MAINT; THAT A DUCT SEAL RUPTURED IN THE L AIR CONDITIONING SYS BTWN THE AIR CYCLE MACHINE AND THE WATER SEPARATOR. THE L PACK WAS UNABLE TO SUPPLY TO THE PRESSURIZATION SYS. THE R PACK WAS ABLE TO WORK AT MAX AT LOWER ALTS TO MAINTAIN ADEQUATE PRESSURIZATION.