A321
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Air Traffic Control Provided Assistance; Flight Crew Returned To Gate; Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
Climbing through 23;000 ft. to 37;000 ft. I was PF (Pilot Flying). As I scrolled through the systems display pages I noticed the APU LP fuel valve indication on the FUEL SD was amber indicating the valve is open with the APU MASTER SW in the off position. I then noticed the left fuel tank quantity was decreasing at a rate of about 20 lbs. every 10 seconds as the fuel was now showing a 400 lb. imbalance. The FO addressed the COM; we [requested priority handling]; and began an air return to ZZZ. During the descent; I noticed APU LP changed to a white indication indicating the valve had closed. The landing was under MLW (Maximum Landing Weight); ARFF (Airport Rescue and Fire Fighting) inspected the aircraft for leaks and found none. We taxied to the gate with no further incident.
Captain reported the APU fuel shutoff valve remained open; causing a fuel loss requiring an air turnback and precautionary landing.