Van der Merwe double gets Scarlets out the blocks
The Welsh region lost all six pool games in this competition last term and this was their first win in nine European outings.
Canadian international Van der Merwe led the way with his brace, with scrumhalf Gareth Davies also dotting down and Rhys Patchell kicking 13 points.
Neil Briggs got Sale's only try with flyhalf Dan Mugford kicking two penalties but the Sharks were made to pay for a first quarter where they gave up a 13-point lead to their hosts.
Mugford had kicked them into an early lead but Patchell levelled matters in the ninth minute before the first Scarlets try arrived four minutes later.
The Sale No.10 took too long to get his kick away and Davies pounced to charge down and regather to score, with Patchell converting.
Sale's struggles continued as a lost line-out and a ruck infringement gave Patchell two more sights of goal, both of which he put straight between the uprights to make it 16-3.
It looked a long way back for the visitors but they got back into the contest with a well-worked drive. The Scarlets did well to repel one driving maul, but Briggs dotted down when the next effort found the weak spot in the home defence.
However, they were put on the back foot again by Van der Merwe's first score. Davies made the break from the base of the scrum, and when the ball was recycled Patchell's cross-kick allowed the Canada winger to pirouette through a tackle to go over in the corner.
Mugford kicked his second penalty shortly before the break but, when Sale lost prop Ross Harrison to the bin for a high tackle on Ken Owens, the hosts killed the game off.
Van der Merwe was again the finisher, doing well to pick up Patchell's long pass on the tips of his fingertips to dive over in the corner.
Patchell converted and the hosts' attentions turned to capturing a bonus point but it was not to be during a scrappy final quarter.
The scorers:
For Scarlets
Tries:Davies, Van der Merwe 2
Cons: Patchell 2
Pens: Patchell 3
For Sale Sharks
Try:Briggs
Pens: Mugford 2
Yellow card: Ross Harrison (Sale Sharks, 47), Ross Harrison (Sale Sharks, 62)
Teams:
Scarlets: 15 Liam Williams, 14 DTH van der Merwe, 13 Jonathan Davies, 12 Scott Williams, 11 Steff Evans, 10 Rhys Patchell, 9 Gareth Davies, 8 John Barclay, 7 James Davies, 6 Aaron Shingler, 5 David Bulbring, 4 Jake Ball, 3 Samson Lee, 2 Ken Owens (captain), 1 Wyn Jones.
Replacements: 16 Ryan Elias, 17 Luke Garrett, 18 Werner Kruger, 19 Lewis Rawlins, 20 Will Boyde, 21 Jonathan Evans, 22 Dan Jones, 23 Hadleigh Parkes.
Sale Sharks: 15 Mike Haley, 14 Byron McGuigan, 13 Will Addison, 12 Sam James, 11 Paolo Odogwu, 10 Dan Mugford, 9 Mike Phillips, 8 Josh Beaumont (captain), 7 David Seymour, 6 Cameron Neild, 5 Jonathan Mills, 4 Bryn Evans, 3 Brian Mujati, 2 Neil Briggs, 1 Ross Harrison.
Replacements: 16 James Flynn, 17 Halani Aulika, 18 Andrei Ostrikov, 19 Magnus Lund, 20 Tom Curry, 21 James Mitchell, 22 Sam Bedlow, 23 Nev Edwards.
Referee: Romain Poite (France)
Assistant referees: Pierre Brousset (France), Mathieu Noirot (France)
TMO: Eric Gauzins (France)
Source: @ChampionsCup
