Blanket gas flow rates 1

[dreidpc]

Currently our SAGD plant's VRU is down and we are flaring. We use fuel gas to blanket our produced water tanks but we're interested in reducing the flow rates while the VRU is down so as not to contribute to flaring.

Our DBM reccomends a flow rate that is sufficient to completely purge the volume above the liquid every 8 hours. No explanation of this 8 hr requirement is given and I'm wondering if its not just an arbitrary number.

Can anyone offer some advice on appropriate purge rates? The tanks are also equipped with pressure regulator valves for protection during rapid level changes.

 

[Montemayor]

dreidpc:

I assume you are a Chemical Engineer and in British Columbia or California. Therefore, I also presume that you literally mean what the term purge signifies: to cause the evacuation of.

I'm not dwelling on semantics or proper English; I want to make sure we understand what it is that you want to do so that we can suggest a safe and operable solution. You title this thread as "blanket gas" and yet you refer to "purging" the gas. Blanket gas is theoretically a static volume of gas maintained over stored liquid. In real-world applications the blanket gas has to be vented (when liquid level rise) or replenished (when liquid levels fall) in order to maintain a stable blanket gas pressure over the liquid. Knowing this basic fact, why would anyone want to "purge" the blanket gas out on a timely basis instead of on level demand? - especially over something as basic as produced water from a gas well. It doesn't make sense without further explanation or details.

I've designed and operated a lot of tanks with either Nitrogen or Natural Gas Blanketing systems and I've never employed a "purge" of the gas blanket. Therefore, I'm curious to find out the reason for this requirement. Ask your "DBM" for the reason and let us know. Maybe we can all learn something new.

 

[dreidpc]

Not sure why you assume I'm in BC or California but I'm actually in Alberta.

I agree that it's important to get terminology correct. The term "Blanket gas" is how we refer to this system on site but it does appear to be a misnomer. We also sometimes call it "Sweep Gas".

I believe the reason for continuously moving the blanket gas is to sweep away and air that might contaminate the system causing an explosive risk. Let me quote from the Design Basis Momorandum or "DBM" (currently we have a United Way fundraiser where employees are fined $5 for use of an unexplained acronym so I guess I have to pay up).

"The vapour phase of the tank has blanket gas to reduce fugitive emissions. 345 kpag fuel gas continuously sweeps the vapour in the tank to the vapour recovery unit. Use the 1” globe valve at the sweep gas station to set the sweep gas rate at 95 sm3/h. This rate ensures that the fuel gas purges the vapour space at least once in 8 to 9 hours. When liquid level declines rapidly, the small sweep gas volume is not adequate to maintain positive pressure in the tank. In this case, fuel gas regulator 02-PCV-302, located on top of the tank, would open automatically to add fuel gas to prevent exposing the tank to vacuum risk. "

 

[Montemayor]

dreidpc:

I have to guess that you're running a gas well or a tank battery sevicing gas or gas/oil wells with associated produced water and flash gas from heater-treater(s) & separators. The produced water is stored in API tanks pending disposal methods. In the process of storage, flash gas is deposited in the produced water tank as the pressurized separators dump their separated produced water into it; plus, the produced water emits dissolved gas as well.

In the "old days", we didn't care about the flash gas and much less about the dissolved, emitted gas. We just let them vent out into the "Big Dumpster" - the atmosphere. Things have changed and VRUs (Vapor Recovery Units) are now mandated to recover (instead of emitting) the gas vapors from the tank. [at the this point I want to mention that you really owe the United Way $15 instead of $5 - you also sinned on SAGD and VRU]

The way you describe your system, you seem to have the produced water tanks connected to the suction line to the VRU - which is OK, depending on your controls and instrumentation - and all the gas going to the VRU is originating in your upstream, flowline separators. I can't understand why your DBM calls for a purge on a time basis. The way I design this type of system - and I've done a bunch in the last 14 months - is that I send my separated vent gas to a Low Pressure Surge drum where I control the suction pressure to the VRU. I employ High Pressure gas source as my produced water tank blanket source. I use a regulator to adjust the pressure to the tanks and any venting required from the tanks is sent to the suction of the VRU - whenever that occurs or is needed. This way, I don't subject my produced water tanks to a possible very low pressure in the VRU suction line caused by excess capacity in the VRU compressor.

I suspect your system has a lot of heavies (C₃+) and you may be forming a layer of light condensate floating on the top of your produced water. This will happen if you don't have a skimming operation. If that is the case, the intermittent purge may be a tool to try to avoid too much hydrocarbon lights condensing in the produced water tank. But I'm just guessing at this stage. Normally, I would not give importantance to the need for intermittent purging as you have described it. However, I would keep my eye on the amount of hydrocarbon condensate floating on top of your water layer. This can be a potential hazard condition if left unaccounted for.

There is no way that I, or anyone else, could give you even approximate purge rates. I'm sorry if I haven't been of much help, but without being there and seeing the entire process, it's very difficult to advise on something we are unfamiliar with. I can only give you examples of what I have and continue to do here in the Texas oil patch.

 

[djack77494]

dreidpc,
I am in agreement with Art's comments. Maintain a small positive pressure in your Produced Water Tanks, and do not admit any blanket gas in excess of what's needed to maintain that pressure. There's no need to. Doing things this way, you are not doing anything to increase fugitive emissions, so I don't see any problems with this. My guess is that the blanket gas may look quite similar to any gases released from the produced water, so purging seems futile. Please correct me if I'm somehow missing the point.
Doug

 

[shahyar]

I came across this issuie before.
What I learnt from my supervisors is this:
"We need a little purge in blanket gas system to make sure H2S concentration in vapor phase in tank is less than a "safe" level."
Obviously this flowrate is a function of "dissolved H2S" in produced water, but I haven't seen any calc to correlate "dissoved H2S Conc." with blanket gas purge rate and I do not know what that "safe level" is.
I want to add that we did not put bypass around blanket gas regulator to have purging, where we believe water is not sour.
Hope this help.

 

[symd]

We are designing crude separation, produce water storage with blanket gas and re-inject to injection wells. tank size is 50'x40' fixed roof. We know blanket gas (from separator) is require for this particular design/liquid but the basis of calculation we do not know. For example, with this kind of separator gas composition how much flow ir require. and if the gas composition change (different wells), blanket gas is this much.

can anyone here explain or tell their story about gas blanket. can we dispose straight to atm without VRU.

 

[Montemayor]

symd:

You don't give enough basic data to address your issue. Besides, this is a different case and should go in it's own, different, and independent new thread.

When you start the new thread tell us where you are, what you mean by "flow". Do you mean flow rate? If so, I can tell you that the blanket gas is static and is only augmented when the tank level is decreased (which lowers the existing blanket gas pressure. There is NO purging with gas needed. If you do that, you are contaminating the atmosphere and should have an operating permit to do so (in the USA or Canada). A VRU (Vapor Recovery Unit) is required to avoid atmospheric contamination (while recovering valuable condensate).

 

[symd]

Montemayor
Thanks for your reply. I am from singapore and the project will be in mid east.
From your explanation my understanding is how much blanket gas require depending on how much blank space create because of decrease of tank level. We can calculate from there. If the blanket gas mean to mix with condensate or any liquid or gases from by produce water, that samething i require someone to explain.

We do have a lot of data. What data your require?

 

[Montemayor]

symd:

As I stated: START A NEW THREAD. This posting will probably be red-flagged and deleted within the next few days at least. I recommend you start the new thread in the PROPER FORUM where this subject belongs: Petroleum production engineering Forum.

I will gladly respond and resolve any issue or problem you have with blanketing Produced Water storage tanks with natural gas. I've done dozens of these applications - in South America and here in West Texas.