6 replies to this topic

[Vimalesh Agnihotri]

Hello,

 

In CDU/VDU unit Desalter is provided downstream of crude charge pump.  What should be the design pressure of this vessel?

Is it should be crude charge pump-shutoff pressure

or

it should be maximum operating pressure of vessel + 2 kg/cm2g similar to design pressure of other pressure vessel.

 

It may be noted that this vessel is protected with PSV.

 

The conservative approach is to design this vessel for pump shut-off however this will lead downstream piping and equipment to be in higher class which has cost impact.

 

Please suggest what is the practical approach and why?

 

Many Thanks 

[Sharma Varun]

I have seen both approaches, more common and acceptable is to design for the shutoff. it is advisable to optimize the design pressure restricting it to 300# rating. 

Special consideration is required in case of revamp.

If you choose lower design pressure than shutoff pressure, routing of PSV liquid relief becomes a concern.

Further you have to detail out the downstream system if a flash drum is envisaged you can provide spec break at it's inlet.

[Chemitofreak]

What is the difference in the design pressure based on the aforementioned cases. That will help people to show you the path forward.

[Vimalesh Agnihotri]

Thanks.  Actually we don't have flash drum in the current system that's why i would like to know how to limit design rating of down stream to desalter to 300#. 

[Vimalesh Agnihotri]

Option 1: If Desalter vessel is designed for feed pump shut-off, 

                Design Pressure of desalter = 34 kg/Cm2g (300 # Rating)

                considering this as maximum suction pressure for desalted crude pump, the design pressure of discharge piping and associated                      equipment would be approx 75 Kg/cm2 g (600# rating)

 

Option 2: If Desalter vessel is designed for Max operating pressure and design margin, 

                Design Pressure of desalter = 15 kg/Cm2g (300 # Rating)

                considering this as maximum suction pressure for desalted crude pump, the design pressure of discharge piping and associated                      equipment would be approx 38 Kg/cm2 g (300# rating)

[Sharma Varun]

Option 1: If Desalter vessel is designed for feed pump shut-off, 

                Design Pressure of desalter = 34 kg/Cm2g (300 # Rating)

                considering this as maximum suction pressure for desalted crude pump, the design pressure of discharge piping and associated                      equipment would be approx 75 Kg/cm2 g (600# rating)

 

Agreed, you need to design the downstream preheat train for higher rating & break the rating upstream of heater. I have seen precedence for same.

 

Option 2: If Desalter vessel is designed for Max operating pressure and design margin, 

                Design Pressure of desalter = 15 kg/Cm2g (300 # Rating)

                considering this as maximum suction pressure for desalted crude pump, the design pressure of discharge piping and associated                      equipment would be approx 38 Kg/cm2 g (300# rating)

 

In my opinion, if this case is chosen due thought process is required to design the PSV relief scenario. Its a trade off between liquid relief consideration & higher rating exchangers.

Edited by Sharma Varun, 05 February 2020 - 11:02 PM.

[Chemitofreak]

My two cents

 

The option I will choose also depends on the type of crude and salt content in it. 

 

With option 2, one should take care that the flare MOC is selected accordingly.