As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PDX-EN-ROUTE TO LMT AT FLIGHT LEVEL 21,000 FEET, HEARD A LOUD POP AND THEN EXPERIENCED A RAPID DEPRESSURIZATION. DESCENT TO FLIGHT LEVEL 10,000 FEET. UPON INSPECTION, FOUND RIGHT HAND DUCT BLOWN OFF OF RECIRCULATION FAN. REINSTALLED DUCT AND TIGHTENED ALL FOUR CLAMPS ON FAN INTAKE AND OUTLET. OPERATIONAL CHECK GOOD. NO PARTS REPLACED. AIRCRAFT RETURNED TO SERVICE.