Q & A > Question Details
We are having Feed Surge Drum in Diesel Hydrotreating Unit, for maintaining pressure of FSD we provided Blanketing Hydrogen and relief to LP Flare. Fail safe positions for the Control Valves in Hydrogen is Fail Open, LP Flare is Fail Close (Where as it was reverse in previous company where I worked last). If in case of Air failure Hydrogen to FSD CV gets open and may get pressurise as there will be no any relief
What may be the basis of selecting the fail safe position of both CVs?
07/11/2014 A: Prabhakar Reddy Seerapu, SATORP, redi_nfcl@yahoo.co.in
I initially had the same opinion that Make up valve should be Fail close and Vent valve should be Fail open. But in our company we had both supply as well as vent valve as Fail close valves. In our case we have Nitrogen as supply gas and Nitrogen header pressure is well above the design pressure of the Feed surge drum. So in fact there is no specific design criteria followed everywhere.
19/06/2014 A: Kumar Kurapaty, KNPC, kumar83che@gmail.com
Sh. Ravindra Chib:
This is the case of CFP in RIL, Jamnagar
18/06/2014 A: rvidner chib, RIl, rschib@rediff.com
Hydrogen has to fail close and Flare has to be fail close as per normal operating philosophy.
There is just no justified reason with the reverse scheme...
26/04/2014 A: chandrasekhar kurcharlapati, toyo engineering, k_chandra_sekhar@yahoo.com
Hydrogen Cv shall be fail close and LP flare Cv shall be fail open so that over pressure is avoided.
09/04/2014 A: Narendra Naidu, Bapco, nbpnnaidu@gmail.com
You have look at
1. What is the FSD operating pressure, design pressure?
2. What is the Feed pump min required NPSH?
3. What is the Blanketing Hydrogen supply pressure?.
if the Hydrogen Blanketing supply pressure is well below the design pressure of the vessel the to protect the Feed pumps and supplying the NPSH, having a Fails open valve on Hydrogen is right approach. if the hydrogen pressure is higher than design, the safety valves would lift and this is not the right approach.
If the min required NPSH of the feed pump is not a concern then fail close on Hydrogen and fail open on flare valve is the right approach.
04/04/2014 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Neither choice is ideal. However, since the upset should include stopping the feed pump, a low suction pressure is of no concern. Therefore, I would stop the incoming hydrogen blanket gas and open the vent CV to flare. Having both CVs fail in position is sometimes the best choice, but would not help in this case.