AN ACR B737-200 FLC; OPERATING WITH 1 AIR-CONDITIONING AND PRESSURIZATION SYS; HAS THE AFT CARGO DOOR SEAL FAIL AT FL240 LOSING PRESSURIZATION CAPABILITY.

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FLT WAS CONDUCTED AND DISPATCHED FROM CVG-JAN WITH THE FOLLOWING ITEMS -- INOP ACCORDING TO THE LOGBOOK AND MEL: L INBOARD LNDG LIGHT; AUTOMATIC MODE ON THE PRESSURIZATION CTLR AND THE R AIR-CONDITIONING PACK OTS THEREBY LIMITING OUR MAX FLT CRUISE ALT TO FL250. PROPER FUEL AND PAX LOADS WERE BOARDED AND WE WERE FLT PLANNED UP TO FL240. THE PRESSURIZATION CTLR WAS OPERATED IN STANDBY MODE WITH A MANUAL MODE AS A BACKUP ACCORDING TO COMPANY ABNORMAL PROCS. APPROX 75 NM S OF BNA VORTAC; AT NIGHT IN SMOOTH AIR; A LOUD BANG NOISE EMANATED THROUGHOUT THE CABIN ALONG WITH POPPING EAR SENSATIONS HAD OCCURRED. AT THAT MOMENT; WE CHKED THE CABIN RATE/ALT/DIFFERENTIAL INDICATIONS AND DETERMINED WE WERE LOSING THE CABIN PRESSURIZATION SYS. WE TRIED TO FORCE THE OUTFLOW VALVE CLOSE IN STANDBY MODE AND THEN SWITCHED TO MANUAL; THE OUTFLOW VALVE POS INDICATED FULL CLOSE; NO OTHER OVERTEMP/BLEED VALVE FAILURE OR DUCT RUPTURE INDICATIONS WERE DISPLAYED. THE CABIN ALT CONTINUED CLBING AND WE DONNED OUR OXYGEN MASKS. BY ESTABLISHING COM WITH ZMP WE ASKED FOR A LOWER ALT DOWN TO 10000 FT MSL. THEY CLRED US TO DSND AT OUR DISCRETION AND WANTED TO KNOW IF WE WOULD DECLARE AN EMER. AFTER NOTIFYING THE PAX; FLT ATTENDANTS AND THE COMPANY WE DECIDED TO DECLARE AN EMER IN ORDER TO KEEP JAN TWR OPEN AFTER HRS OF XX00 CST. OUR ARR OCCURRED APPROX XX10 CST; WITH NO FURTHER INCIDENT AND THE EMER EQUIP STANDING BY JUST IN CASE. CABIN ALT REACHED A MAX OF 11500 FT AND PAX OXYGEN MASKS WERE NOT DEPLOYED. WE ARRIVED IN JAN AND DOWNED THE ACFT FOR CONTRACT MAINT TO INSPECT AND REMEDY WHATEVER CAUSED THIS SIT TO OCCUR IN THE FIRST PLACE. AFTER HAVING REMAINED OVERNIGHT; WE CONSULTED WITH CONTRACT MAINT AND THE COMPANY IN ATL AND DECIDED TO FERRY THE ACFT BACK TO ATL. WE ALSO RESEARCHED AND FOUND THE AFT CARGO DOOR HAD BEEN CHANGED SEVERAL MONTHS EARLIER AND FOUND THE CARGO DOOR SEAL BLOWN OUT. WE FERRIED THE ACFT UNPRESSURIZED BACK TO ATL WITH ALL DOCUMENTATION AND APPROVAL OF OUR FERRY PERMIT. WE SAFELY LANDED IN ATL.