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Emergency depressurization Valve in Diesel and Vacuum gas oil Hydrotreater unit brings down the design operating pressure to half of it on its actuation in 15minutes as per design Philosophy of the valve. What is the importance of this 15mins, why not less than or more than 15minutes.

Feb-2023

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Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

The Emergency Depressurization System (EDS) is one of the most important process safety systems of a hydroprocessing unit and requires adequate criteria for design to avoid a worsening of the scenario of operational emergencies.

According to the literature, there is two criteria to design an EDS system:
Low Rate Depressurizing System - Capable of reducing the pressure from the operating level to close 7,0 kgf/cm2 in 60 minutes. In this case the system is designed to reduce the operating pressure into 50 % of the designed pressure in 15 minutes with the objective of cut the sequence of hydrotreating reactions and block the raising of temperature and pressure;

High Rate Depressurizing System - Capable of reducing the pressure from the operating level to 20 % of the designed pressure in 15 minutes, this system is normally applied in high severity processing units considering the emergency scenarios like temperature runaway, fire and leak in the reaction system.

The reason of 15 minutes as design parameter is to avoid the excessive mechanical stress of the processing unit through controlling the depressurizing rate which can lead to a excessive stress over critical systems like reactor internals if is much high, or can limit the capacity of the processing unit to deal with emergency scenarios if the depressuring rate is to low.
Another side effect related to high depressuring rate system is the risk of damages in the flare operating systems, the capacity of the flare system to deal with the processing unit during emergency shutdowns in hydrotreating units needs to be considered during the design step in order to minimize the risks of damages in the flare tubulation system which can lead a worsing in the emergency scenario.

The depressuring rate is controlled through a restriction orifice (one or more) downstream of the emergency depressuring valve, this system needs to be adequately tested during the start up of the processing unit to ensure that the EDS will act adequately under real emergency scenarios.

Feb-2023