The Saints now face midlands rivals Leicester in next Sunday's final at Worcester.
The Saints had the stronger pack, with lively performances from Alex Waller and Scott Armstrong and they scored the only two tries of the game, but they showed little attacking flair behind the scrum and had to rely on 14 points from the impressive Stephen Myler to emphasise their superiority.
In contrast, Scarlets were the more creative but they threw away two early chances which may have changed the course of the game and after that were their own worst enemies, making too many unforced errors at crucial times and could only manage four penalties from Stephen Jones.
Scarlets nearly took the lead after only eight minutes. The visitors won a lineout in the Northampton 22, a strong burst from Gareth Maule took them over the gain line before Dominic Day surged over the try line only for the TMO to rule no try as the second row had been held up by some desperate home defence.
Scarlets declined to take a kickable penalty in favour of more attacking options and they almost earned their reward when Andy Fenby stormed clear only to be hauled down 10 metres short by an excellent cover tackle.
Scarlets were made to pay for those missed opportunities when Northampton took a 15th-minute lead. The Saints won a lineout 10m out from the visitors' line and had little problem in driving over for a Waller try, which was converted by Myler with the outside half then adding a penalty.
Scarlets collected their first points when Jones kicked a 45m penalty after the Saints infringed at the lineout but this was nullified with a second successful penalty from Myler.
Straight from the restart, Northampton foolishly gave away a penalty and Jones made no mistake with his kick to leave his side 13-6 behind at the interval.
Within three minutes of the restart Jones had further reduced the arrears with his third penalty, this time a kick from 40m, but Northampton soon responded with an excellent try.
The Saints produced their best passage of play, pressurising the Scarlets' defence in their own 22. There were key drives from James Craig and Waller before a missed tackle on George Pisi allowed the centre to send Armstrong in at the corner for the try which Myler was unable to convert.
Back came Scarlets and a further Jones penalty made it 18-12 to the hosts with half an hour remaining, but two well-struck penalties from Myler gave the Saints a healthy 12-point advantage.
Scarlets continued to give away too many needless penalties which prevented them building up any real momentum. Replacement Ryan Lamb hit a post with a long-distance penalty but then succeeded with another, after Day was sin-binned for offside, to give the Saints an ultimately comfortable victory.
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