As the name suggests, start & stop buttons are to start & stop a centrifugal pump. The color scheme for these buttons is green for start & red for stop taking a clue perhaps from traffic light color coding which is universally recognized. Start & stop buttons are installed in the field typically next to the pump and not everybody is allowed to push start & stop buttons as they wish.
In the event things go horribly wrong, a centrifugal pump can be killed off by hitting the red button. Power supply to motor will be cut off and pump will come to a complete stop. However, this does not necessarily mean a pump can only be energized on location. Remote start & stop of a centrifugal pump is possible. Start & stop button is part of plant control systems designed to safe guard plant integrity. Interlocks and trip systems are common for oil & gas processing facilities. It is designed and well thought off to ensure that certain response will be taken in the event of specific failure.
Unlike positive displacement pump, centrifugal pump typically does not need a relief valve. Relief valve is a mechanical device installed to protect the system against the damage from excessive pressure. A relief valve will be triggered at certain set pressure to prevent pipe or vessel from rupture due to pressure exceeding its designed values.
All these layers of protection either electronic or mechanical provide the necessary line of defense for safe & reliable plant operations. In complex oil & gas processing plants, there is no way humans can reliably detect all the abnormalities all the times regardless how vigilance they are. This issue of human reliability is addressed by built in these safety features into the systems. Plant operators need all these bells & whistles to help them in day to day operations.