A B727 IS DISPATCHED TO ORD ILLEGALLY; EXCEEDING THE ZERO FUEL WT; WITH THE #3 TANK AFT BOOST PUMP MEL'ED WHEN IT WAS THE #2 TANK AFT BOOST PUMP THAT WAS INOP. FLT HAD INSUFFICIENT FUEL FOR TFC DELAYS AT ORD.

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THE R AFT BOOST PUMP IN THE #2 TANK WAS INOP FOR DISPATCH. IT WAS NOT DISCOVERED UNTIL AFTER TKOF THAT MAINT HAD INOP THE WRONG BOOST PUMP. THE #3 AFT FUEL BOOST PUMP HAD BEEN DEFERRED. THIS WAS DISCOVERED BY REVIEWING OUR FLT PLAN MORE COMPLETELY THAN I HAD DONE BEFORE TKOF. FUEL LOADING AND MGMNT DIFFER WITH AN INOP BOOST PUMP IN TANK #2 VERSUS TANK #3. BECAUSE OF THIS; WE CONTACTED DISPATCH. AFTER DISCUSSING THE DISCREPANCY IT WAS DETERMINED THAT WE HAD EXCEEDED OUR ZERO FUEL WT LIMIT OF 140000 LBS BY 2600 LBS. AN OVERWT INSPECTION WOULD BE NEEDED AFTER LNDG. ABOUT 150 MI FROM ORD WE HEARD THERE WAS 40 MIN HOLDS INTO ORD DUE TO TFC. WE HAD NO ALTERNATE AND DID NOT WANT TO DIVERT THINKING THE AIRPLANE WOULD BE GNDED DUE TO THE OVERWT ZERO FUEL WT WITH AN INSPECTION REQUIRED. AFTER TALKING WITH DISPATCH AND ATC; THERE WAS MUCH CONFUSION. IT WAS MY DECISION TO GET THE AIRPLANE ON THE GND AT ORD AND RESOLVE DISCREPANCIES WITH ATC AND DISPATCH LATER. WE DECLARED AN EMER AND LANDED AT ORD UNEVENTFULLY. THE FUEL MGMNT PROC WITH AN AFT FUEL BOOST PUMP INOP IN THE #2 TANK WAS FOLLOWED CORRECTLY THE ENTIRE FLT.