3 replies to this topic

[junior1]

Hi there. I have a several questions regarding the blow down system. The general arrangement is: an actuated blow down valve (ball vale usually), RO (restricted orifice), isolation valve followed by the expansion piece. In this arrangement the flow is dependent on the size of the RO (sized to depressurise down to 50% pressure in 15mins). Hence most of the pressure drop occurs across the RO. Now comes the confusion part for me. Why is there an expansion piece downstream of the isolation valve; to further drop the pressure (by increasing the volume with larger diameter pipe) so that the flare system is not pressurized? If so why is the spec break at the inlet of the expansion piece? Also I’ve noticed t hat in some system there isn’t an expansion piece. Why is this? And is there a rule of thumb that if the pressure is dropped by so much then an expansion piece is required.


I know that I’ve raised many questions here but your help would be much appreciated.

[fallah]

Now comes the confusion part for me. Why is there an expansion piece downstream of the isolation valve; to further drop the pressure (by increasing the volume with larger diameter pipe) so that the flare system is not pressurized? If so why is the spec break at the inlet of the expansion piece? Also I’ve noticed t hat in some system there isn’t an expansion piece. Why is this? And is there a rule of thumb that if the pressure is dropped by so much then an expansion piece is required.

The expansion piece would be located just downstream of the RO not isolation valve.The main concern in this situation is gas/vapor velocity (mach number) which would be greatly increased after RO due to expantion if expansion piece not being considered.
It is obvious that by considering larger pipe diameter after RO,not only pressure drop doesn't increase but also decreases due to higher pressure recovery.

Spec break normally would be considered at isolation valve located after RO for maintenance purpose.

Hope above help you out.

[Qalander (Chem)]

Dear junior1 Hello/Good Afternoon,

Although fallah has guided you fairly well.

I add my little explanation in very simple words;as linked to the possible point pressure/ temperature conditions of discharged (i.e. de-pressurizing stream.)

Most usually any system being safegurded is physically pressurized to MAWP condition just prior to 'physical blow down' or 'pressure relieving' and

localized sudden change of pressure to a much lower value may enhance its volume tremendously(subject to the characterstics of stream in question)

Now in order to protect against this increase of volume impacting the pipelines/ structures mechanically;it becomes imperative to have a properly designed expansion piece in-place accordingly ;where such tremendous volume increase is not foreseeable such expansion loops are not mandated.

Hope this guides somewhat on the conceptuality.

[junior1]

Thank you very much fallah and Qalander (Chem) for your explanation. It does all makes sense and is easier to understand as well.

Regards,
Junior1