Alvaro Morata had broken Palace's resistance with 32 minutes on the clock, reacting quickest to convert Pedro's low centre from the right.
Palace, though looking solid at the back, had rarely threatened in the first 50 minutes, but they found themselves level eight minutes after the interval in part thanks to some sloppy Chelsea defending.
Townsend was able to receive the ball with his back to goal under no pressure whatsoever and exchange passes with James McArthur.
The return ball sent the England international through and clear and he made no mistake as he fired a low right-footed effort across Kepa and into the far corner to level the scores.
The strike was Palace's first effort on target of the game as they gave chelsea a lesson in finishing.