ACR MLG ALTDEV DURING RAPID DSCNT BECAUSE OF ACFT PRESSURIZATION PROBLEM.

Narrative

NORMAL PROCS DURING TKOF AND CLB UNTIL PASSING THROUGH 26000. WE WERE ALERTED TO A PRESSURIZATION PROBLEM BY THE CABIN ALT ALERT (CABIN PASSING THROUGH 10000 FT). WE STOPPED OUR CLB BUT THE CABIN ALT INCREASED AT NEARLY 2000 FPM. ALL INDICATIONS ON THE AIR CONDITIONING AND PRESSURIZATION PANEL WERE NORMAL EXCEPT THE OUTFLOW VALVE WAS CLOSED (THEN MANUALLY CONFIRMED SHUT). THE CABIN WAS STILL IN AN UNCTLED CLB. THE FO ALERTED ATC (DENVER CENTER) THAT WE HAD LOST OUR PRESSURIZATION AND REQUIRED AN IMMEDIATE RETURN AND DSCNT BACK TO DENVER. AS THE PF I INITIATED A DSCNT TO MAINTAIN A CABIN ALT BELOW 14000 FT. ATC STEPPED OUR DSCNT SCHEDULE SOMEWHAT AS THEY REVERSED OUR COURSE CLRNC BACK TO DENVER THEN CLRED US TO FL240. FL240 WAS TOO HIGH AND I REQUESTED THE FO TO CONTACT ATC AND REITERATE THE NEED FOR AN IMMEDIATE DSCNT. MY ALT WAS THROUGH 23500 BEFORE WE RECEIVED A LOWER ALT FROM CENTER. WITH A PRESSURIZATION LEAK THAT SEVERE WE DID NOT KNOW OUR STATUS OF STRUCTURAL INTEGRITY. WE FELT AN IMMEDIATE DSCNT WAS NECESSARY FOR THE SAFETY OF ALL ON BOARD. AFTER LNDG; MAINT SAID THE ACFT HAD JUST EMERGED THE NIGHT BEFORE FROM A 'D' CHK AND MAY BE LEAKING AIR FROM MULTIPLE SOURCES. ALSO WE WILL BE SURE TO USE THE WORD EMER TO THE CTLING AGENCY.