Dear all

Can someone say what should the BDV status be, after depressurizing - from operational point of view? I mean, after reaching the isolated vessel pressure of 50% of design pressure or 100 psi (based on API 521) BDV should be closed or would be opened. That in the latter case, the mentioned vessel pressure will be equalized with the flare system pressure and we need purging for next start-up.

Regards

In my opinion, BDV would be in open position (Fail safe position) to ensure evacuation of HC inventory...

To equalize the vessel pressure with the flare system pressure would take very long. Normally, an operator would take action to initiate the shutdown of the BDV.

Dear Fallah,

I fully endorse Joe's concept,with an exception that it may vary on the actual valve's design basis and performance.

Most usually we observe that these valves do have a pop-pressure and re-seat pressure values while calibrating and testing at bench is carried out.

Thus it is fair and justified to assume for normal cases that when the re-seat pressure is attained the re-seating(shutting closure) should occur.

Additionally the system of vessel under depressurizing can only flow towards flare piping as long as there remains pressure differential;which, I believe, will not remain available infinitely to keep the valve open/depressurization continued till equalization to flare system pressure as many frictional etc. resistances are to be over-comed.

Hope this helps.
Qalander

Thanks for you endorsement...

You mean BDV pop and re-sit ?

Yes,
Qalander

My first reply - hope it works.

The BDV remains open throughout the depressurisation - API 521 is quite clear on that, I believe. Normally (in my experience), BDV's are supplied and configured in the control system (high integrity shutdown system) such that they are "manual reset". This means that the activating solenoid has a physial pushbutton that must be pressed to re-close the valve. Such a reset should only be possible once the cause of the ESD/blowdown has been rectified and cleared through the shutdown system. If the cause is not "cleared" then pressing the button won't close the solenoid valve and the BDV itself. Consult your Control systems/instrument engineer.

Another point is that blowdown should only really be initiated on a confirmed fire (some offshore facilities also do so on confirmed gas leak as well), in which case, operating philosophy should be to allow the blowdown to continue to "zero" or steady state flare header pressure hich should be very low in a shutdown case. I have operated offshore and if designed properly, it doesn't take that long to completely blowdown. Your simulation should show this.

Hope this helps.

Dear bob4161;thanks for this rectification update info and I do stand corrected on BDV control aspect.Which I had missed-out from the memory while responding to last post by 'Joe'

But Fallah's query about back flow from flare system and purging needs seems still un-answered.

I assume that on a good adminitrative system and operator intervention such a situation should not occur.As BDV is operator cotrolled;as discussed/referred indeed!
Best regards
Qalander

Thanks for your reply,but as far as i know after relevant vessel/drum... and associated pipings (e.g. in the case of fire) automatically isolated by SDVs,then BDVs activated only manually by operator from CCR.Therefore there may be the possibility of re-closing them manually without any pre-defined conditions (correct me if i am wrong).
Regards

Dear Fallah
It depends on level of shut down. in offshore there is shut down level 0 which acts if toxic gas detector be activated therefore BDV's will be activated atomatically , But in onshore beside of ESD1 there is another push botton in CCR called EDP (Emergency Depressurization Plant) for opening BDV's
Regards