SDV/BDV/PSV mechanism

[zacharialamsyah]

Dear All

I'm having difficulties to understand mechanism about SDV/BDV/PSV

1. Are the SDV and BDV working simultaneously, in other means when PAHH activated, SDV Closed and BDV open in same time?

2. What is setting pressure of BDV, is it equal of SDV's set pressure (PAHH) or other machanism triggered (such as manual intervention)?

3. What the safety devices working earlier when pressure high-high occur? BDV open or PSV popping ?

Sorry if my English is bad..

Thank you for response and answer

Regards
Zachari Alamsyah

 

[e43u8]

Hi,

1- SDV, in relation to BDV, will be closed by F&G signal in fire case and then BDV will be opened manually or automatically for depressuring...
2- There is no set pressure for BDV and it's opened manually or automatically following to vessel isolation by SDVs when engulfed in a pool fire...
3- BDV not to be opened per sensing pressure and will be opened maually or automatically following to a fire ocuurence for depressuring; and PSV will be opened per sensing the set pressure. If the vessel would sustain after BDV opening at operating pressure, then the PSV will be opened later at the set pressure normally equal to vessel design pressure...

 

[LittleInch]

1), No SDV and BDV operate independently, though a BDV doesn't normally open until that particular section of plant or pipe is isolated. A SDV can have many reasons to close - check the Cause and Effects chart for what causes the effect (closure) of any particular valve. PAHH might well be one reason why it closes, but that cause may not be enough to require the BDV to open.
2) BDVs also have their own cause and effects and each will vary depending on where it is, e.g. it is quite normal for a compressor skid to blowdown when it is shutdown as part of a normal operation, but other BDVs would remain closed unless there was a fire or an ESD0 trip. You can't generalise these things.
3) BDVs are not normally used to correct overpressure - that is the purpose of a PSV and shutdown valves / trips. Usually a PSV is the last line of defence, i.e. you have already exceeded the high and high high trips.

I think you need to read some operating manuals and look at some specific cause and effect charts to get a better understanding.

PS I hope this isn't college work....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zacharialamsyah]

Ok thanks for e43u8 and little inch

This is not college homework..i just having misinterpretation of that mechanism

Your answer enlightening me...

 

[amustha1407]

Hi,

Just to add,
1. The SDV actually have something to do with the BDV opening. Blowdown are normally devided by sections. Both SDV downstream and upstream the section to be depressurized shall be closed first before BDV open to avoid overwhelming the flare system with the full flow of the process.

This is done usually by using a timer.The timer on BDV opening is set to allow the SDV to close first.

Some may use limit switch for permissive logic (Logic sets so that The BDV can only open after the limit switch detects the closure of SDV). In my opinion, this is not wise though, as limit switches are prone to fail. The BDV may not be opened as demanded when that happen, and we will face the risk of explosion.

As mentioned above, BDV rarely open on PSHH (most of the time they are activated on F&G detection).

2. -
3. for fire case, BDV will open upon confirmed fire. PSV will open when the boiling liquid and vapor expansion pressurized the vessel up to its set point

Thanks!