I created this workout about 6 years ago and it caused my arms to EXPLODE. They're currently around 20" (natural) and this is what I did:
Preacher Curls-3 sets, 10, 8, 6 reps
Superset with
Close Grip Bench Press-3 sets, 10 reps each set
Alternate Bicep Curls-3 sets, 6 reps each set
Superset with
Overhead Tricep Extensions-3 sets, 10 reps a set
Dumbbell Hammer curls-3 sets, 6 reps per set
Superset with
Tricep Pushdowns-3 sets, 10 reps per set
Increase weight every week. Make sure you pyramid all weights. This routine calls for some heavy weight so it will be rough on your elbows. Keep that in mind and listen to your body, warmup good, eat properly, rest sufficiently and take a week off every 6-8 weeks. You will grow like a madman. I promise.