There are two different valves. One is a safety vale and one is a relief valve.

Safety Valve: An automatic pressure relieving device actuated by
the static pressure upstream of the valve and characterized by full-opening
pop action. It is used for gas or vapor service.

Relief Valve: An automatic pressure relieving device actuated by the
static pressure upstream of the valve which opens further with the increase
in pressure over the opening pressure. It is used primarily for liquid
service.