A B737-500 AT LEVELOFF FL260 DECLARED AN EMER AND DIVERTED DUE TO MASSIVE LOSS OF FUEL FROM THE R MAIN TANK CAUSED BY #2 ENG FUEL FILTER HOUSING COVER SEPARATION.

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ACR FLT XYZ DFW-DEN. DEP AND CLBOUT WERE NORMAL. JUST PRIOR TO LEVELOFF AT FL260; THE CTR BOOST PUMP LIGHTS CAME ON INDICATING THE CTR FUEL TANK WAS CLOSE TO EMPTY. AT THAT TIME; I TURNED THE CTR TANK PUMPS OFF AND NOTICED THE R WING TANK QUANTITY WAS 1500 LBS LOWER THAN THE L -- THEY SHOULD HAVE BOTH INDICATED APPROX 10000 LBS (FULL). I XFED TO BAL THE WINGS. EACH QUANTITY WAS 8000 LBS. JUST A FEW MINS LATER; WHILE THE FO AND I WERE DISCUSSING THE FUEL SIT; 'THE USING RESERVE FUEL' PROMPT APPEARED ON OUR CDU. THIS HELPED CONFIRM WE WERE LOSING FUEL AND NEEDED TO LAND ASAP. OKLAHOMA CITY WAS OUR CLOSEST ARPT. I NOTIFIED ARTCC WE NEEDED TO LAND IMMEDIATELY AT OKC AND THE FO ADVISED ACR. I BRIEFED THE CABIN CREW AND MADE AN ANNOUNCEMENT TO THE PAX. CONTINUING OUR EVALUATION OF OUR SIT. WE CHKED CIRCUIT BREAKERS AND NOTED NORMAL ENG INDICATIONS. THE ONLY ANSWER WAS A LARGE FUEL LEAK. WE CONTINUED OUR DSCNT FOR A VFR LNDG ON RWY 31; 10-15 MINS AFTER TURNING TOWARDS OKC. THE LNDG WAS NORMAL. AFTER TURNING OFF THE RWY; I SHUT DOWN THE R ENG AND ASKED THE TWR IF THE LEAK HAD STOPPED. WE WERE TOLD ON ROLLOUT TWR PERSONNEL COULD SEE LEAKING FUEL. I WAS TOLD BY TWR THE LEAK HAD CEASED THEN CALLED BACK AND TOLD THE LEAK AND STARTED AGAIN. I SHUT DOWN THE L ENG AND ADVISED THE CABIN CREW AND PAX I WANTED TO EVACUATE THE ACFT USING THE L DOOR SLIDES ONLY. THE EVACUATION WAS CALM AND ORDERLY WITH NO PAX INJURIES. ONE FLT ATTENDANT INJURED HER ANKLE. I WAS SHOWN LATER THE RUPTURED FUEL FILTER IN THE R ENG THAT HAD CAUSED THE LOSS OF FUEL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A B737-500 AND WHEN ON THE GND AND MAINT OPENED THE #2 ENG; IT WAS DISCOVERED THE ENG FUEL FILTER COVER INCLUDING THE FASTENERS AND HELICOILS HAD PULLED COMPLETELY OUT OF THE FILTER HOUSING AND EXPOSED A GAP OF 1 INCH. CALLBACK CONVERSATION WITH RPTR ACN 433576 REVEALED THE FOLLOWING INFO: THE RPTR STATED ALL THE SIGNS OF A LARGE FUEL LOSS WERE PRESENT WITH THE CTR TANK BOOST PUMPS BLINKING; THE R TANK QUANTITY INDICATION LOWER THAN THE L AND THEN THE FMS PROMPT 'USING RESERVE FUEL.'