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Thermal Outbreathing Rate for Tanks in Parallel?

Thermal Outbreathing Rate for Tanks in Parallel?

Thermal Outbreathing Rate for Tanks in Parallel?

Hi guys. I'm working on designing a vapor recovery system for five API 650 crude oil tanks: (4) 80,0000 bbl and (1) 37,000 bbl. The tanks are being converted from floating roof to fixed roof. The tanks will be manifolded together via a vapor recovery header which will go to a vapor recovery compressor. The crude is stabilized ("dead") so any flashing may be safely neglected at the temperatures in question. SCADA history shows the crude never gets above about 85° even in summer. The design requirement is for the VR system to take all vent vapor (except fire case) because release of gas to atmosphere is not permitted via environmental regs, except in emergency.

The question is what to use for the design vapor recovery gas rate for sizing of the tank vapor collection lines and the compressor. We are using the API 2000 methods to establish venting rates. The worst case gas rate comes from the API 2000 thermal outbreathing (atmospheric heating) with all five tanks online plus the displacement volume due to liquid pump-in. This makes for a huge gas rate and the owner is questioning our methods because of course this leads to large vapor recovery pipe and a large compressor. Is that a realistic design basis?

Tank you! (heh)

RE: Thermal Outbreathing Rate for Tanks in Parallel?

Well it sounds ok to me on the basis that all five tanks are in service. Is that 800,000 bbls or 80,000?

If you can post the calculation or a sample tank then we might be able to see an error, but there is a reason most crude storage tanks are EFR. Vent systems to avoid overpressure of API 650 tanks can be huge.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Thermal Outbreathing Rate for Tanks in Parallel?

Thanks LI and apologies for the late reply. The tanks are 80,000 bbl; sorry about the typo.

After a bit more research it turns out there is an answer by API to a technical inquiry made in 2004 regarding this very topic, i.e. should the API 2000 thermal outbreathing calcs be used for sizing the vapor recovery system. Their opinion is that designing the vapor recovery system to handle the API2000 vent capacities will result in excessive capacity that will rarely, if ever, be used (their terms). So we are going back to run some more numbers. So based on this the owner has more or less directed us not to use the API2000 numbers for the vapor recovery system design. I think that is probably reasonable but it still leaves open the question as to what to use for the design vapor recovery gas rate. We will probably end up using a combination of displaced volume due to liquid influent plus predicted flash gas, such flash gas being created as the crude is heated over a summer day, then add a margin of say 25%. Not sure what else we can do at this point. Thanks!

RE: Thermal Outbreathing Rate for Tanks in Parallel?

Protego has software on their website that will do the calculations.


RE: Thermal Outbreathing Rate for Tanks in Parallel?

This is why you get the big bucks. Your approach looks good to me, but the magnitude of your fudge factor is simply engineering judgement and there ain't no rule of thumb in the world that will do a better job than an experienced engineer applying his experience and judgement.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

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