What was found
CLIMBING THROUGH FL180, CABIN ALTITUDE PRESS RAPIDLY INCREASED TO 15,000 FEET. CABIN ALTITUDE WARNING LIGHT ILLUMINATED & HORN SOUNDED. PAX OXYGEN MASKS DROPPED. UNABLE TO ESTABLISH PRESS IN AUTO OR STANDBY, AN EMERGENCY WAS DECLARED. FLIGHT RETURNED TO FIELD AT CLT & LANDED WITHOUT INCIDENT. FOUND THE AIR CONDITIONING GND SERVICE VLV REMAINING OPEN, REPLACED VLV IAW AMM 21-21-15. OPERATIONS CHECKED GOOD, ACFT PRESSURIZATION TESTED GOOD AT THIS TIME. REPLACED OXYGEN GENERATORS & REPACKED MASKS.