7 replies to this topic

[Mathi]

I am working on a project where we study the operation from high pressure to low pressure. The high pressure operation involved operating at 36 barg and at low pressure, it is 18 barg. The RO across the BDV was sized for the  high pressure operation.

 

Now my question is can the same RO be reused for low pressure operation or should it be replaced?

 

[fallah]

Mathi,

 

Sizing of RO after BDV (not across the BDV as you mentioned!) is being done based on peak flow rate and the corresponding delta P across the RO. Then, if in low pressure application there would be another flow rate, new RO should be sized and applied...

[shan]

If you have the identical line size, identical pressure drop, and identical actual volumetric flow, you may use the same RO for the low pressure operation because the flow area requirements should be identical.

[shvet]

RO is needed for controlling Mach number in flare header pipes during emergency depressuring. Existing RO in new service have less DP and bigger volumetric capacity, therefore you need new RO with less bore diameter. So you privide the same Mach number inside downstream pipes.

[Mathi]

Fallah, Shan, Shvet

 

 

My question is that initially the RO is sized for a certain flow to drop the pressure from 36 to 7 barg in 15 minutes. Now, the flow has reduced and it is supposed to drop the pressure from 18 to 7 barg only. The line size remains the same. 

 

The pressure drop and flow has reduced considerably and the RO was actually sized for higher flow and pressure drop. When using the same RO, shouldn't the depressurization time reduce? Also, why shouldn't the same RO be reused?

[fallah]

Mathi,

 

You should size a new RO for new conditions within 15 minutes; if the Beta ratio is higher than that of  the existing RO, by using existing RO you will face with faster depressuring rate and subsequent lower outlet temperature, and if the Beta ratio is lower than that of the existing RO the depressuring rate will be slower and the effects of this slower rate on vessel stability should be evaluated in mechanical strength view point ...

[shan]

If you view your original depressurization as two stage process, Stage 1: 36 bar>18 bar and Stage 2: 18 bar > 7 bar, your new depressurization process is about identical to the Stage 2 of original depressurization with some temperature variations that is insignificant in term of RO sizing.  Therefore, I don't see any reason that prevents you to keep using the existing RO. 

[Mathi]

Fallah and Shan 

 

When I sized the RO, the Beta ratio was higher. Therefore, I have decided to retain the existing RO.

 

Thanks everyone for your valuable comments