A subsea fluid processing system, comprising: - a rotatable machine configured for subsea deployment where a process fluid is processed, the rotatable machine including one or more mechanical seals to separate a barrier fluid and the process fluid; - a barrier fluid supply source configured to be located at a surface location; and - a mechanical barrier fluid pressure regulation system, which is configured to maintain a pressure differential between the barrier fluid and the process fluid between a predetermined upper limit and lower limit and comprises: - a valve mechanism which is configured to provide the barrier fluid to the rotatable machine when the pressure differential reaches the lower limit and to dump the barrier fluid when the pressure differential reaches the upper limit, wherein the valve mechanism is further configured to balance hydraulic forces and to use the hydraulic forces to generate hysteresis in the opening and closing of the valve mechanism; and wherein the valve mechanism provides a pressure balanced supply valve having: - a valve housing; - a valve end sealably engageable with a valve seat to separate a high-pressure supply chamber and a discharge chamber cooperating with a discharge nozzle; - a pilot piston mechanically coupled with the valve end and slidably positioned within a pilot chamber within the valve housing to separate a process fluid side of the pilot chamber from a barrier fluid side of the pilot chamber; - a spring acting on the pilot piston to bias the valve end to an open flow position and to bias the pilot piston toward the barrier fluid side of the pilot chamber, the discharge chamber being in fluid communication with the barrier fluid side of the pilot chamber via a barrier fluid flow passage; and - a return seal that isolates the pressure in an area of the pilot piston to delay valve actuation when the barrier fluid pressure increases rapidly. - a rotatable machine configured for subsea deployment where a process fluid is processed, the rotatable machine including one or more mechanical seals to separate a barrier fluid and the process fluid; - a barrier fluid supply source configured to be located at a surface location; and - a mechanical barrier fluid pressure regulation system, which is configured to maintain a pressure differential between the barrier fluid and the process fluid between a predetermined upper limit and lower limit and comprises: - a valve mechanism which is configured to provide the barrier fluid to the rotatable machine when the pressure differential reaches the lower limit and to dump the barrier fluid when the pressure differential reaches the upper limit, wherein the valve mechanism is further configured to balance hydraulic forces and to use the hydraulic forces to generate hysteresis in the opening and closing of the valve mechanism; and wherein the valve mechanism provides a pressure balanced supply valve having: - a valve housing; - a valve end sealably engageable with a valve seat to separate a high-pressure supply chamber and a discharge chamber cooperating with a discharge nozzle; - a pilot piston mechanically coupled with the valve end and slidably positioned within a pilot chamber within the valve housing to separate a process fluid side of the pilot chamber from a barrier fluid side of the pilot chamber; - a spring acting on the pilot piston to bias the valve end to an open flow position and to bias the pilot piston toward the barrier fluid side of the pilot chamber, the discharge chamber being in fluid communication with the barrier fluid side of the pilot chamber via a barrier fluid flow passage; and - a return seal that isolates the pressure in an area of the pilot piston to delay valve actuation when the barrier fluid pressure increases rapidly. - a valve housing; - a valve end sealably engageable with a valve seat to separate a high-pressure supply chamber and a discharge chamber cooperating with a discharge nozzle; - a pilot piston mechanically coupled with the valve end and slidably positioned within a pilot chamber within the valve housing to separate a process fluid side of the pilot chamber from a barrier fluid side of the pilot chamber; - a spring acting on the pilot piston to bias the valve end to an open flow position and to bias the pilot piston toward the barrier fluid side of the pilot chamber, the discharge chamber being in fluid communication with the barrier fluid side of the pilot chamber via a barrier fluid flow passage; and - a return seal that isolates the pressure in an area of the pilot piston to delay valve actuation when the barrier fluid pressure increases rapidly.