Why we need a seal oil system , the Mechanical Seal is it enough to do the sealant ?
the rpm is to high to use mechanical seal
options are gas seal or oil seal
Why we keep it above the suction pressure not the discharge pressure?
because normally a compressor is designed to have suction pressure on the seal (via compensating holes in the impeller or via external pipe)
what will happen if the difference pressure in the seal system reach 0 PSIG or the seal pump get shutdown, the gas will escape out side?
gas will accumulate inexpansion vessel of seal oil system
balance line connects seal area to suction side (hence the pressure at the seal=suction pressure)

This is quite an old thread, but seeing that I have just joined the forum I though I would add some comments.

Why we need a seal oil system , the Mechanical Seal is it enough to do the sealant ? No, this seal is designed to operate with a liquid fluid film - the seal oil. That means that the oil pressure must be greater than the gas pressure.
Why we keep it above the suction pressure not the discharge pressure? The seal chambers see suction pressure, so you keep it above this pressure to ensure that the seal runs on liquid (oil).
what will happen if the difference pressure in the seal system reach 0 PSIG or the seal pump get shutdown, the gas will escape out side? The mechanical seal will loose the liquid film, overheat and fail, or, the gas pressure may force the face open if the balance and spring pressure are overcome and you will certainly have gas escaping in either case.
Is there a relation ship between the seal oil system and the balance line ? Yes. The balance line is to keep the pressures in the seal chambers as close as possible to the suction pressure on both suction and discharge end of the compressor. Your tapping point for your PDCV may well come off this line.