API 2000 tank vent calcs question 5

[KernOily]

OK another dumb question here for you smart fellers. Need some opinions... I have a 2000 bbl API 650 atmospheric storage tank that has water in it with a thin oil pad on top. So, the stored fluid is >98% water. The calc in API 2000 to estimate the required tank venting flowrate due to atmospheric heating of tank and contents is based on the flash point temperature of the stored liquid, i.e. whether it is > or < 100° F. Since the stored liquid is water, what would I use as the flash point temperature? Since water does not off-gas per se, like a HC liquid does, I am thinking this calc does not apply. Opinions? Thanks! Pete

Thanks!
Pete

 

[Montemayor]

Pete:

Good question. I did this one about 2 years ago. My water was hotter than 150 oF, so I had an easier time to logically vent the tanks. However, we had to logically look at the problem and at API 2000 as well.

The answer, in my opinion lies in the operations you expect to put the tank through and the credible scenarios. You don't want to build up a pressure beyond your MAWP (nor can you tolerate going close to your MAWV), so you allow for the maximum flash gas present and the highest pump-in available. The heatup of the water due to radiation from the sun is negligible in my opinion and the maximum pump-in rates dominate for out-breathing. But be sure you account for all possible out-breathing contributions - especially any flash gas or vapors. Then calculate your tank vent nozzles appropriately.

API 2000 is a fine standard and go-by. It is not a code, of course. If you're interested, I developed an Excel Workbook to work out tank venting problems and document the results. I could send you a copy if it would help.

I hope this experience helps out.

Art Montemayor
Spring, TX

 

[KernOily]

Art thanks for your comments. I agree with what you are saying and that is the tack I am taking. The venting rate required due to atmospheric heating is far less than that required due to max pump-in rate or tank fill rate. But - as you said - I have to document all cases.

As you know API 2000 is a standard, like API 650, so it represents the 'state of the art' or the best practice and as such ourt insurance companies and (sometimes) local authorities requires us to comply.

I would love to see your spreadsheet. Please send it to me at pchandler@prou.com. Thanks Art! Pete

Thanks!
Pete

 

[JoeTank]

Folks,
While this is an interesting subject and obviously fun to learn about and debate, the bottom line is that one must eventually select a vent size. Is it really worth an extended engineering effort to justify say a 3-inch instead of a 4-inch vent? The cost difference is minimal; whereas, safety is enhanced with the larger vent size. I have come to believe that sharpening your pencil on vent sizing determination may not be in everyone's best interest.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[MMcLean]

I agree with Steve. Consider the worst-case pump in flow rate, and emergency and thermal venting requirements, considering the most volatile component in the tank. The difference in vent size may be minimal between worst and best cases.

Cheers

Mark

 

[KernOily]

Amen brothers. The reason I am calc'ing these vent rates is because I am not primarily sizing tank vents, although that is one of the deliverables; rather, I am sizing a tank vapor recovery piping system, a cooler/condensor at the end of the vapor header, and a compressor. This system has to handle all tank vent cases except the fire case. This project is in the People's Republik of Kalifornia, so all tank vapors must be collected and disposed of, be they flammable, hazardous, or not. Venting is only allowed under emergency circumstances. Preach on my brethren. - Pete

Thanks!
Pete

 

[Montemayor]

Steve:

I think Pete is doing the correct thing by his client's requirements or needs. This is not a case of trying to calculate for calculation's sake nor trying to save a buck with a smaller nozzle. I just got through doing the same thing some months back and I had to be prepared as to identifying what vent size was expected and why. In other words, the client demanded that we explain to him in detail just how we arrived at the recommended vent size and what scenarios we considered to reach that decision.

I can't fault a smart client for demanding 100% effort for the bucks he was paying us. He deserves to know, in detail, just exactly what he is buying, what capacity it will handle, and under what conditions.

As an example, in one case I had to handle flashing crude with steam into atmospheric vented API tanks and I had to determine what size nozzle(s) were needed in the event a sudden monsoon shower hit the storage tank and condensed the water vapor inside the tank - causing a partial vacuum. This is a real life situation and a credible scenario in our case because the tanks were not insulated and the site was the Amazon jungle. The required nozzle was 14" diameter. This surprised a lot of people who insisted on seeing the calculations. No one doubted the answer when they walked through the calculations. What at first seemed to be a routine nozzle requirement, turned out to be a serious need for vacuum protection.


Art Montemayor
Spring, TX

 

[EGT01]

74Elsinore,

Since you are talking about sizing vapor recovery equipment, I thought I would point out an API 2000 Technical Inquiry that you should be aware of, if you aren't already.

http://committees.api.org/standards/tech/docs/2000ti.xls

[ignore]

http://committees.api.org/standards/tech/docs/2000ti.xls

[/ignore]

Look at the response to the question:
"Are the values in Table 2 intended for use in sizing a vapor recovery system?"

In your case, it sounds like pump-in is more of a controlling factor so using the API thermal venting tables for vapor recovery equipment sizing likely will not make a difference but it is something to keep in mind.

 

[JoeTank]

Montemayor
I think we probably agree in principle about this issue.

"I can't fault a smart client for demanding 100% effort for the bucks he was paying us. He deserves to know, in detail, just exactly what he is buying, what capacity it will handle, and under what conditions."

I don't disagree, but how do you think your client would react if you spent an additional $5000 of detailed engineering efforts in order to save him $500 on a vent and nozzle that was one or two sizes smaller. It's just a practical matter of being a good steward of the clients funds.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[jay165]

Art:

I would like a copy of your spreadsheet, if you don't mind.

jspeaser@audubon-engineering.com

Thanks,

Jay

 

[quark]

Just some thoughts.

Outbreathing rates specified by API2000 for flash points below 100F seems to be about 1.6 times that of above 100F and this may require morethan one higher size venting. I think this is redundant in this case.

How about calculating the 98% volume with flash point above 100F and 2% volume with flash point below 100F? or calculating 98% volume with a flash point above 100F plus actual evaporation rate(average)?

Both these calculations may give you a higher value than actual as Mr Montemayor rightly suggested about outbreathing from water.

PS: I have breather valve design experience with homogenous(or I think so) fuel oil tanks that are less than 100KL capacity only and I am absolutely unaware of any regulatory requirements with respect to the above system.

Regards,

 

[maddocks]

Art: Are you able to send me the spreadsheet as well? I'm working on a oil tank VRU system.

Thanks for your help

Jim Maddocks, P.Eng.
Process Engineering Manager
jmaddocks@gasliquids.com
403-250-2950

 

[KernOily]

Great discussion guys. The whole issue revolves around the operating/disposal/handling strategy of the plant's tank vapors. Fundamentally, vapors must be removed to prevent tank overpressure using either (1) a vent hatch/PVRV device or (2) a vapor recovery system. In my case, the local environmental restrictions and the conditions of the owner's APCD operating permit preclude venting to atmosphere except in the case of emergency, e.g. a fire.

Tank vapors arise from several sources: fire, liquid influent, atmospheric heating, operational upset, etc. API 2000 only requires that the vapors be removed from the tank and so, to that end, provides some guidance on how to estimate the vapor flowrates. It is up to the owner as to what he wants to do with the vapors once they are past the tank roof.

My tanks are several hundred feet from the vapor recovery compressor suction. The owner has said he wants to operate his tanks at 0" w.c. gauge and his compressor suction at -6" wcg, no matter what happens vapor-flowrate-wise, so that fixed my allowable dP for the system. Also, the piping material is 316L because the gas is corrosive to CS. Thus, the onus is on me to reduce construction cost, and nowadays there are real cost savings to be had in the difference between 14" and 16" in 316L, hence the engineering effort to minimize pipe size. $5000 in engineering effort will pay out very quickly when measured against the cost of big-inch SS pipe...

Thanks!
Pete