Originally Posted by Jagiven
You can open the valves under pressure. That could blowout and debris that might be stuck in the o-ring.
[potential argument alert]
Yes, you can
open the pressurized system low-point drain valves while the system is pressurized. But, because of the on/off nature of the valves they POP open and closed when operated. It can be unsettling to those unfamiliar.
[back to our regularly scheduled reasonable discussions]