What was found
AT FL250 PRESSURIZATION FLOW LIGHT CAME ON. UNABLE TO REGAIN CONTROL OF CABIN, LEFT PACK ZERO FLOW. THE FLIGHT WAS OPERATING SINGLE PACK WITH THE RIGHT HAND PACK PREVIOUSLY PLACARDED. CREW DECLARED EMERGENCY AND INITIATED AN EMERGENCY DESCENT. THE CABIN PRESSURE REACHED 12,500. THE MASKS DID NOT DEPLOY. THE FLIGHT DIVERTED TO BNA AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE WATER SEPARATOR TEMP CONTROL VALVE IAW AMM 21-20-03. ALSO REPLACED BOTH COALESCER BAGS IAW 21-20-02. THE RIGHT PACK CONTINUED ON MEL. THE AIRCRAFT WAS GROUNDED.