What is the function of reefer compressor head spring?

There is a spring just underneath the cylinder cover and on top of the suction valve seat. This is to prevent the liquid refrigerant to enter the compressor and causing damage, or oil entering the compressor. So when liquid is returned to the compressor, then this spring will force the VALVE SEAT to be raised preventing any damage to the head. Further, when the difference between the internal cylinder pressure and high pressure increases more than 3 Kg/cm2, this spring lifts up the VALVE SEAT.