Puma 80

I recently changed the pressure relief valve on my potterton puma 80.

Behind the PRV and to the left, there is a small white dial (see pictures below) and next to this is a small outlet in the pipe. This outlet is letting out a drip every 3-4 minutes.





It appears to have been leaking for a while because there is a small rusty spot on the boiler cover directly underneath it.

My questions are: What is this part? Is it normal for it to drip after refilling the boiler?

this is the drain off for the boiler try turning the white dial as you call it to tighten it to shut the drip off as toy probably knocked it whilst changing the prv

I had already turned it as tight as I could, so I took a pair of pliers to it, and now no drips!
Thanks-a-million for your help!