B737-500 FLT CREW HAS PRESSURIZATION MALFUNCTION; DECLARES EMER AND DIVERTS FOR LNDG.

Narrative

B737 LOSS OF PRESSURIZATION. DURING CLBOUT FROM ZZZ; WE EXPERIENCED A SLOW LOSS OF PRESSURIZATION WITH THE CABIN ALT REACHING 10000 FT AT FL250. THE PRESSURIZATION SYS WAS CHKED BY ME DURING THE AFTER TKOF CHKLIST AS WELL AS PASSING THROUGH 10000 FT ON DEP FROM ZZZ. IN BOTH CASES; THE ACFT WAS PRESSURIZING NORMALLY WITH A SLIGHT CLB (200 FPM) ON THE CABIN RATE VSI. AT FL250; THE CABIN ALT WARNING HORN SOUNDED. CHKING THE CABIN ALT; IT READ 10000 FT AND WAS CLBING AT 500 FPM. I ALSO CHKED TO MAKE SURE THAT BOTH ENG BLEED SWITCHES AND PACK SWITCHES WERE IN THE NORMAL POS AND THAT THE PRESSURIZATION PANEL WAS CORRECTLY SET. AFTER DONNING MY OXYGEN MASK AND ESTABLISHING COM WITH THE FO; I SELECTED MANUAL AND ATTEMPTED TO CLOSE THE OUTFLOW VALVE AS DIRECTED BY THE QRC. I NOTICED NO CHANGE IN THE CLB ON THE CABIN RATE VSI. I THEN COMMANDED THE FO TO INITIATE THE EMER DSCNT AND FOLLOWED THE QRC ACCORDINGLY IN ADDITION TO DECLARING AN EMER WITH ZZZ CTR. THE CABIN CONTINUED TO CLB AND REACHED A MAX ALT OF 13300 FT. DURING THE DSCNT; I SELECTED STANDBY ON THE PRESSURIZATION PANEL; AND NOTICED THAT THE CABIN RATE VSI SHOWED A SLIGHT DSCNT WITH THE ACFT PASSING THROUGH 15000 FT. ONCE THE ACFT WAS LEVELED AT 10000 FT; THE CABIN CONTINUED TO DSND TO THE ALT SET IN THE STANDBY WINDOW (APPROX 5600 FT). WE RETURNED TO ZZZ AND ACCOMPLISHED AN UNEVENTFUL LNDG. IT SEEMED TO ME THAT THE ACFT COULD NOT HOLD PRESSURIZATION ABOVE ABOUT 15000 FT IN ANY SELECTED MODE. IT WAS ONLY WHEN WE DSNDED TO 10000 FT THAT THE CABIN BEGAN TO PRESSURIZE NORMALLY. ADDITIONALLY; WE RECEIVED NO AUTO-FAIL LIGHT; NOR EXPERIENCED ANY ABNORMAL PRESSURE SENSATIONS.