Pressure Safety

• Pressure safety valves are specifically designed keep people safe and have no other purpose. These valves typically drain the excess liquid or gas into a bilge or onto the floor.
  
  

Pressure Relief

• The pressure relief valve is also designed to keep people safe, but typically drains into another system or even into itself. The relief valve just leads the excess back to a holding tank for later use.
  
  

Comparison

• These two valves are essentially the same. They serve the same purpose but it's labeled safety valve if it does not interact with another system and pressure relief if it dumps into another system.