Seal Plan 53A and 53B

Hi Kanif77, for Plan 53A, the seal pot is needed to be consistently pressurized by Nitrogen. If the nitrogen pressure drops, the sealing system would be converted into Plan 52, of which the process fluid will flow into the sealing system and inboard seal faces now become lubricated by the process fluid.(original intention is to use barrier fluid to lubricate the inboard seal faces) This will also cause the LSH(level switch high) transmitter in the seal pot to hit and trigger the signal to DCS.
Generally, Plan 53A is to prevent process fluid from flowing into the seal supply system, and thus atmosphere. Normally, these process fluid could be abrasive, corrosive, polymerizing which might cause failure.

For Plan 53B, seal supply system pressure is maintained by Bladder Accumulator, of which the accumulator will maintain the seal system pressure. Although Plan 53B does not require consistently pressurized by Nitrogen source, but you will have to refill the Accumulator when the accumulator pressure drops.

I will recommend that you go for API Plan 53 C . This does not require Nitrogen for pressurization.

Also it maintains almost constant pressure on Inboard Seal.

What about explosion fear if friction gives higher temperature and gas and oxygen mix occurs?

In Plan 53 C no gas is being used, hence there is no problem of gas mixing with Nitrogen

For both plan 53A and B, it is necessary to trip on low N2 pressure.

If it is not tripped, process liquid will get into to seal oil system - if process liquid has grit / solids in it, it will destroy the seal rubbing surfaces in no time. If process liquid has high volatility, the seal could be dry running, resulting in high localised temp at the rubbing surfaces, destroying the seal. Seal oil must also have a suitable viscosity to provide the necessary lubrication on the seal surfaces.