7 replies to this topic

[furio]

Hi all, I'm a process engineer new in oil & gas engineering and this is my first stock tank design.

I'm working on a crude oil test separation train (capacity 20000 bpd) constituted by a first stage separator operating at 500 psig, a second stage separator operating at 70 psig and an atmospheric tank that constitutes the final stage of separation. The flashing gas in the tank is about 1000 Nm3/h and the operating temperature ranges between 25°C and 80°C. The oil that feeds the tank has a flash point of 44-54 °C.

1) The Client asks to vent to atmosphere the gas released from the tank. I think that I have to manage this tank in the same way as the other separators for which the gas released is flared. What’s your opinion about this point?

2) I think that the test bank working is discontinuous: when the system is in operation, the released gas flows to the flare and the tank is filling with liquid. When the system stops, the pump empties the tank and the oil is sent to the production bank. I’m not sure about the correct arrangement of the tank. I think that the better choice, in my case, is the arrangement that Art Montemayor in his Workbook calls “Standard Arrangement B” where it'is foreseen a blanketing with N2 (but I would use fuel gas, the only gas available, I think!), a back pressure regulator on the outlet of the gas and a pressure relief (for emergency). What’s your opinion?

Thank you very much.

[furio]

Why no reply? The question is not clear enough? If yes, you can find in attachment the simplified tank configuration I refer to.

Thank you for your time.

 Tank_Arrangement.pdf   15.16KB   109 downloads

[djack77494]

Why no reply? The question is not clear enough?

No furio, the question really is not clear enough. Much depends on your circumstances. I envision that you are referring to the third separator in a three vessel separator train. Let's call them the HP, MP, and LP Separators for high, medium, and low pressure. Prior to your LP Separator the oil stream is often heated for more effective water separation. Hydrocarbon gases are typically generated at each stage as the pressure drops, so venting of the gas is usually the issue. I've seen an extremely low pressure gas compressor (Recovery Gas Compressor) used to feed these nearly atmospheric gases into the main Process Gas Compressor. So really you must better define your system, indicating whether you recover any gas from this tank. Calling it a stock tank also confused me since you describe it more as an active separator. Stock Tanks are more for storage than processing. Furthermore, you say your temperature varies over a pretty wide range that brackets your flash point. Are you refering to temperature variations over the life of the field or some unknown shorter term temperature variations? All in all, however, the most important factor would be what you are doing with your produced vapors.

[furio]

Hi djack77494, thank you for your reply!

Yes, the “stock tank” (I have found this definition in some book like “Surface Production Operations” Vol.1 – Artur et alt. and “Petroleum And Gas Field Processing” - H Abdel-Aal et alt.) is really a further stage of separation.
The temperatures I refer are the minimum and maximum operating temperature during the year (winter and summer cases). The gas released during operation from each stage of separation is not recovered but flared. During normal operation, I would send also the gas flashed in the tank to a low pressure flare (and not to atmosphere as requested by the Client).
During operation there’s no problem with the tank: the gas flashed for effect of the pressure loss flows to a flare. When the test bank stops, the tank can be considered a storage tank, I think, that, for example, could in-breathes for effect of the emptying with the pump. The configuration of the tank should protect it also at this condition I think. Isn'it?

Thank you for your time.

[djack77494]

Thanks for the clarification, furio. Location also plays a very important role in what processing is employed for a facility. What may be common practice in one locale could be absolutely prohibited in another. I find your description strange, but that is probably due to size and/or location.

The situation where vacuum protection of the vessel is needed seems to be strictly when it is temporarily removed from operation. At that time you seek to maintain a positive pressure in the vessel in order to prevent the ingress of air and to avoid possible vacuum formation. This is a common situation, and the schematic you provided would be effective for meeting your objectives. When you speak of the discontinuous nature of the operation, however, it leaves me wondering as to the reliability of the fuel gas source. If there is any chance of a disruption in the fuel gas supply, I would have an alternative/supplemental source of gas available to ensure that vacuum conditions are avoided. Ideally, a very reliable, independent source of inert gas, such as nitrogen, would be available for this purpose. If that is not the case, you may even be reduced to having to use air and an emergency source of gas to relieve vacuum. If that is necessary, you will need to reexamine the system's safety, and perhaps decide if an atmospheric vent might, in fact, be safer. We're in territory I am not very familiar with. The high flash point of your material is a concern, and I wouldn't know if a flame arrestor(s) would be appropriate. Whatever happens, please be generous in setting design pressure & vacuum conditions for this vessel. (Note that I have assumed that this vessel is like an atmospheric storage tank in that it cannot withstand anything other than a very small pressure or vacuum. If the vessel is designed for vacuum, these comments would not apply.)

[furio]

Hi and thanks!
The test train is located near the production separators train and I thought to use the "production gas" for the blanketing but...I don't know...I'll think it over...
Thank you very much for your help.

[djack77494]

Use of production gas or fuel gas is not a problem. The only caveat I have is concerning the reliability of the blanket gas. You are depending on it to prevent collapse of your vessel(s) due to vacuum formation. What happens during a shutdown? If this scenario is properly addressed, then all should be well. If not, ???

[furio]

Yes, this is a scenario that needs to be taken into account...I'll think it over...
Thanks