It was the 14th attack on the Cano Limon Covenas pipeline this year.
The blast tore open part of the massive tube near the town of Saravena just 36 hours after pumping had resumed following another rebel blast.
Army Second Division troops were on patrol to try to stop additional guerrilla attacks.
The pro-Cuban and ultranationalist ELN, led by Spanish ex-priest Manuel Perez, has bombed the same pipeline 512 times in the past decade.