BE1900 First Officer experiences a Right Firewall Fuel Valve illumination at FL240. Upon shutting down and feathering the engine; the Firewall Fuel Valve remained illuminated. The extinguishing agent is discharged with no effect and the crew declares an emergency and returns to departure airport.

Narrative

We were just leveling off at our final cruising altitude of FL240 when we got a Master Warning flasher and an associated Right Firewall Fuel Valve illumination. The Captain called for the memory items; which were performed. Upon shutting down and feathering the engine; the Firewall Fuel Valve remained illuminated. The fire could not be confirmed visually; but because of the continued illumination of the warning system; the Captain and I made the decision to activate the extinguisher. Even after the discharge; the T-Handle remained illuminated. We then followed by completing the QRH Emergency checklist for an engine fire or failure in flight. While completing this checklist; it was noticed that the operative engine's propeller was not full forward; so it was accomplished at that time. We declared an emergency with Center; and decided to return for landing. We received priority handling for a visual approach. On approach; we briefed the passengers; completed the descent and approach checklists; and then the one engine inoperative approach and landing abnormal checklist. Approximately 2 minutes prior to landing; the fire indication finally disappeared. We made a successful landing; and were able to roll clear of the runway; where we shut down the aircraft and were met by emergency crews. Once the fire crews confirmed that there was no fire occurring; the passengers were returned to the terminal by bus and the Captain and I stayed with the aircraft while it was towed into the ramp.