New engineer here, first year out of college. I'm designing a blanket gas system for a Central Tank Battery.
There are Blanket gas regulators for the headers of each set of tanks, Brine and Oil, keeping 2 oz/in2 pressure on the header above the tanks.
There are in line dead weight loaded vent valves for the header s of each set of tanks, Brine and Oil, set to open at 8 oz/in2 of pressure in the case of an overpressure in the system. A DFA and Flare scrubber are also going to be downline form the vent valve. During routine operation the excess vapor will flow from the header to the vapor recovery compressor.
Tank MAWP = 16 oz/in2
Vent Valve Set Pressure = 8 oz/in2
300' equivalent line loss from the vent valve to the Flare
I have 5 Oil Tanks totaling 5,000 BBL Storage; 850 gpm max fill rate
I have 6 Brine Tanks totaling 6,0000 BBL Storage; 700 gpm max fill rate
I estimated the relieving capacity needed for the in line vent valve in SCFH of air according to API 2000 7th ed.
9,600 SCFH air equivalent - Brine Tanks
15,100 SCFH air equivalent - Oil Tanks
I used data from the vent valve vendor to estimate pressure at the required flowrates across the vent valve and compiled a spreadsheet to analyze the system as a whole. See attached.
I'm trying to choose the correct vent valve connection size as to not oversize the valve and cause chattering.
I used this formula to gauge sizing: Flow at Set Pressure/Flow at MAWP Pressure = % Open
Is this a valid assumption?? I don't have any information on stem travel.
A 4"x4" connection gives me 40% Open at the required flowrate for Oil Tank Vapor and 25% Open at the required flowrate for Brine Tank Vapor.
I believe any connection larger than that will be oversized and cause chattering.
However with a 4" line size and the equivalent line length present, I have too much frictional loss to make it to the flare with the gas. I know I could expand the line to 6" after the Valve but I just want to make sure I'm thinking about this correctly?
Thanks,
KG