Hydraulic Accumulator Pressure

Equalization.

Even though your bladder/Nitrogen pressure was put at 600 PSI, when the pump is running, it will equalise to 1700 PSI, same as the Hydraulic circuit.

When shut down, there is most probably a non return valve or a shut off valve that will keep the pressure in the system (hence the emergency shut down topic). This pressure will be appearing at the bladder, both sides of the membrane.

The 600 psig pre-charge inflates the bladder so that if fills the whole volume of the accumulator. When you start the hydraulic power pack, oil at 1700 psig flows into the accumulator outside the bladder through a non-return valve. Because the oil is at a higher pressure than the air within the bladder, the bladder deflates compressing the fixed volume of air until it reaches 1700psig. Now the outside and inside of the bladder are at the same balanced pressure and both the oil and the air are blocked in place by their respective valves. On emergency shut-down a second oil valve piped in parallel to the non-return valve opens and allows oil to flow out of the accumulator shutting off critical valves on your plant. The pressure gauge on the air side shows that the accumulator is fully charged and the emergency shut-down system is functioning correctly. Always check this gauge when the power pack has been shut off for about a hour. If the pressure is still 1700 psig the non-return valve is not leaking. All hydraulic oil valves left unused for a long period suffer from a condition called "silting" that makes there operation unreliable. Test the operation of the non-return valve and it's bypass periodically to check that the system will function correctly when it is required in a real emergency.

It is working as designed.

The reason for charging the accumulator at 600 PSIG while the system is down is because the hydraulic high pressure side is at very low pressure or zero PSIG.

After the accumulator is charged and the system is started, the applied hydraulic pressure generated by the pump is transferred into/onto the accumulator bladder and therefore the N2 pressure is equalized at 1700PSIG.

When the hydraulic system stops or generated hydraulic pressure is decreased for any reason the accumulator will maintain pressure on the system through the bladder at 1700 PSIG for a period of time based on the accumulator size and the hydraulic system design.

If the system hydraulic pressure is not bled off (reduced) then the N2 will remain at whatever pressure was generated by the pump.