What is difference between design pressure and working pressure?

The working pressure is also referred to as the Maximum Operating Pressure (MOP). The MOP is the highest pressure expected during operation. The Maximum Design Pressure (MDP) includes the maximum pressure that is protected by relief valves in case of failure.

For example, if the MOP is 100 psi and their are relief valves set at 10% above the MOP the MDP would be 110 psi.