PLT OF A SMA SEL RAN LOW ON FUEL DURING AN IFR APCH AND DECLARED AN EMER FOR PRIORITY HANDLING.

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FLEW A PIPER 140 FROM CDW TO RDU FILED AN IFR FLT PLAN; RTE SCHEDULE TAKE PLUS OR MINUS 3 HRS. I GOT A NEW RTE FROM ATC AND STAR INFO. RDU ATC CANCELED THIS APCH AND GAVE ME OTHER VECTORS FOR DIFFERENT APCH. THIS ONE ALSO GOT CANCELED AND ATC GAVE ME ANOTHER APCH. I WAS ALREADY UP IN THE AIR FOR APPROX 4 HRS BECAUSE OF THE NEW ASSIGNED RTE. I FILED FOR 4 HRS OF FUEL ABOARD; WHILE THE AIRPLANE HAD 4 1/2 HRS MAX. ATC GAVE ME WRONG VECTORS TO INTERCEPT THE 312 DEGS FOR THE VOR 32 APCH. THE NEEDLE WAS OUT TO THE R AND ATC GAVE ME A 190 DEG HDG TO FLY L. WHEN AFTER A COUPLE OF MINS THEY FOUND OUT THAT THEY WERE WRONG EVEN THOUGH I QUESTIONED THEM ABOUT IT. ATC WANTED ME TO DO ANOTHER APCH WHEN I INFORMED ATC THAT I HAD APPROX 10-15 MIN FUEL LEFT MAX. BY THEN I WAS ALREADY UP IN THE AIR FOR 4 1/2 HRS. ATC CLOSED RDU AND EMER TEAM WAS WAITING FOR ME AT THE END OF THE RWY. I ASKED FOR A DIRECT ASR APCH INTO RDU BECAUSE OF LOW ON FUEL. I HAD OF COURSE HAD TO FILL OUT A RPT FOR THE EMER TEAM WHICH I OF COURSE DID.