A B727-200 IN CRUISE DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CABIN PRESSURIZATION CAUSED BY A FAILED OUTFLOW VALVE.

Narrative

ACR FLT DTW-LAS DEPARTED 2 HRS LATE DUE TO PREVIOUS WRITE-UP OF THE PRESSURE CTLR OF ACFT XYZ. MAINT REPAIRED THE PRESSURE CTLR -- AND SIGNED THE ACFT FOR RETURN TO SVC. AFTER DEPARTING DTW ENRTE TO LAS AND JUST AFTER LEVELOFF AT OUR CRUISE ALT OF FL310; THE CAPT NOTICED THE CABIN PRESSURE EXCEEDING 10000 FT UPON WHICH THE CABIN ALT WARNING ACTIVATED. THE CAPT AND CREW IMMEDIATELY DONNED OUR OXYGEN MASKS AND BEGIN WORKING WITH THE RED BORDERED EMER CHKLIST. AT THAT POINT IN TIME; THE CAPT RELINQUISHED CTL OF THE ACFT TO ME (FO) AND BEGAN WORKING WITH THE FE AS I DSNDED TO 10000 FT IN COORD WITH ATC AFTER DECLARING AN EMER. UPON REACHING 10000 FT WITH ATC CLRNC WE BEGAN DUMPING FUEL TO OUR REQUIRED LNDG WT. THE CAPT AND ENGINEER WERE UNABLE TO CTL THE CABIN WITH ANY OF OUR PROCS. WHEN THE CABIN WAS COMPLETELY DEPRESSURIZED WE RETURNED TO CHICAGO O'HARE FOR LNDG. UPON ARR MAINT FOUND THE AIR-CONDITIONER CTLR COMPLETED FROZEN. NO PERSONAL INJURY OR DAMAGE OCCURRED DURING THIS PROC; TO CREW; PAX OR THE ACFT.