A B737-400 WITH A MEL'D PACK RECEIVED A CABIN ALT WARNING HORN 55 MIN AFTER TKOF. AN EMER WAS DECLARED FOLLOWED BY AN EMER DSCNT WITH FLT DIVERTING AT 10000 FT.

Narrative

WHILE ON THE RAMP; WE HAD THE R HAND PACK LIGHT ILLUMINATE AND WERE ISSUED MEL 21-1A. ENRTE AT FL240; WE HEARD THE CABIN ALT HORN. WE PUT ON OUR OXYGEN MASKS; DECLARED AN EMER; CONDUCTED OUR EMER CHKLISTS AND DSNDED TO 10000 FT. AT 10000 FT; WE FINISHED OUR CHKLISTS AND COMMUNICATED WITH DISPATCH AND MAINT CTL WHAT COURSE OF ACTION THEY WOULD LIKE US TO TAKE. SINCE THE CHKLIST DOES NOT CALL FOR A DIVERT TO THE NEAREST ARPT; ALL PARTIES AGREED TO DIVERT. WE CHKED UP ON THE WELFARE OF OUR PAX AND CREW AS WORKLOAD PERMITTED AND ALL WERE RPTED AS OK. WE LANDED UNEVENTFULLY AND ALL PAX AND CREW MOVED TO A DIFFERENT ACFT AND PROCEEDED TO ZZZ1. SUPPLEMENTAL INFO FROM ACN 700465: AT FL240 APPROX 55 MINS AFTER TKOF; THE CABIN WARNING HORN CAME ON AND THE CABIN ALT INDICATED 10000 FT WITH A SMALL (1.0) PSID. CABIN ALT WAS STEADY. AT 10000 FT; THE CABIN ALT WAS 8500 FT WITH NO CLB OR DSCNT ON THE CABIN RATE INDICATOR AND THE OUTFLOW VALVE WAS FULL CLOSED. NORMAL 2 PACK INDICATIONS WOULD BE A SEA LEVEL CABIN AND APPROX 4.0 PSID AS VERIFIED ON OUR LATER FLT.