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Truck Vent Connection To Flare


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#1 Amin Va

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Posted 10 March 2015 - 11:03 AM

Hello,

There is sour emulsion storage tank system that will be trucked out every day. The idea is to tie the truck vent to low low pressure flare system so as not to vent the sour gas from the truck vapour space (due to out breathing) to the atmosphere. 

It is reported by the client that vapor space in the truck has 20% of the LEL. Once connected to LLP flare system, the truck vapour stream is mixed with the flashed gas from the storage tank as well as flare purge gas. I would like to confirm if this mixture can create a hazardous condition by 1) making an explosive mixture 2) causing flash back from the flare flame. Following are the criteria that i believe applies:

1) In order to prevent the formation of explosive mixture, I believe the hydrocarbon concentration should be above UEL (rich in hydrocarbon), which is about 15% for methane and less for other hydrocarbons (i.e. air concentration should be below 85% ). 

2) However, there is criteria in API-521 that the oxygen level should be less than 1% below the buoyancy seal (translating to less than 5% air) and 4 to 8% for velocity seal (less than 20 to 40% air)

These two criteria has a significant difference in terms of air concentration to be achieved. If I stick with the second criteria, I should inject a huge volume of fuel gas to reduce the air concentration.

I would like to know where the second criteria come from and which criteria I should use in my calculations.

Thanks for your help in advance,
Amin



#2 Zauberberg

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Posted 10 March 2015 - 12:06 PM

How did Oxygen get into the truck in the first place? It should be purged and kept under N2 once emptied, so during the loading operation it will discharge mixture of Hydrocarbon vapors and Nitrogen, without O2.



#3 Amin Va

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Posted 10 March 2015 - 12:35 PM

That's the information from the client. Nitrogen is probably not available. Since in the truck the hydrocarbon is way below LEL (20% of LEL) it doesn't make any problem.


Edited by Amin Va, 10 March 2015 - 12:35 PM.


#4 fallah

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Posted 13 March 2015 - 02:08 PM

Amin,

 

Is there a balance line between the truck and the tank?

 

Please upload a simple sketch of the system you described...



#5 Amin Va

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Posted 13 March 2015 - 04:52 PM

Hi Mr. Fallah,

 

Please see the attached sketch.

 

We can either tie the truck vent  to the tank (path 2 shown as pink) or to the flare header (path 1 shown as red). I think it is preferred to tie the vent to flare header (downstream of back pressure control valve) because if you tie it to the tank the air might be trapped in the tank and stays there; but the disadvantage is that we get some gas flashed in the truck vapor space if you tie the truck vent to flare (because of the pressure differential btw tank and truck, 3.5 kPag, flare back pressure is negligible).

 

Kind regards,

Amin 

 

 

(correction to the sketch: the flow rates are per hour not day, tank volume=1000 bbl)

Attached Files


Edited by Amin Va, 13 March 2015 - 04:55 PM.


#6 fallah

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Posted 14 March 2015 - 06:08 AM

Amin,

 

It's not specified the LLP flare tip equipped with which seal type: Buoyancy Seal or Velocity Seal...

 

Anyway, the main point is keeping the oxygen concentration well below explosive limit at the point under the seal...

 

Both paths you mentioned to vent the truck will forward the oxygen to LLP flare network but path 2 will add the oxygen to LLP flare slower than path 1...

 

Considering the complexity of evaluation if the oxygen concentration of the mixture of purge gas plus truck vent gas would be below explosive limit, it might be better, if N2 for truck purging isn't available, relieving the truck vent  to the safe location...


Edited by fallah, 14 March 2015 - 06:09 AM.


#7 Zauberberg

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Posted 14 March 2015 - 07:17 AM

I still believe that purchasing modularized N2 generation skid and purging the truck to LLP Flare prior to loading, is the best alternative. The cost is quite affordable and this will eliminate all safety concerns. After inertization of the truck, you can go for either of the two options proposed in your sketch.

 

http://www.linde-ind...tion by PSA.pdf

http://www.noxerior....et-skidded.html

http://www.ggssi.com...iteNitrogen.pdf



#8 fallah

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Posted 14 March 2015 - 07:36 AM   Best Answer

Zauberberg,

 

If the oxygen concentration in the empty truck space is so low that could be purged to LLP flare, the truck vent could be connected to LLP flare instead once the loading getting started without prior purging. If the oxygen concentration is high, purging to LLP flare can't be a proper operation in safety standpoint...



#9 Zauberberg

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Posted 14 March 2015 - 07:47 AM

In such case, purge with N2 to atmosphere. As simple as that.


Edited by Zauberberg, 14 March 2015 - 07:49 AM.


#10 fallah

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Posted 14 March 2015 - 07:52 AM

I still believe that purchasing modularized N2 generation skid and purging the truck to LLP Flare prior to loading, is the best alternative. The cost is quite affordable and this will eliminate all safety concerns. After inertization of the truck, you can go for either of the two options proposed in your sketch.

 

http://www.linde-ind...tion by PSA.pdf

http://www.noxerior....et-skidded.html

http://www.ggssi.com...iteNitrogen.pdf

 

I am also saying why to flare?...Yes it's better purging with N2 to safe location...


Edited by fallah, 14 March 2015 - 07:57 AM.


#11 Zauberberg

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Posted 14 March 2015 - 07:59 AM

I assumed (perhaps incorrectly) that the truck may contain some toxic gases/vapors, apart from Oxygen. In such case it could be risky to purge it and vent locally. If only Oxygen is present in the truck, purging to atmosphere is the way to go.

 

Nitrogen skids come with small footprint and fairly low cost. If it was my project, I would go for it and avoid all safety studies, operational concerns, audits, etc. etc.

 

By the way, trucking 60 cubes per day seems quite large quantity (probably 3 trucks per day?). This is a lot of Opex and HSSE exposure involved. If the destination point is not far way, maybe it is worthwhile considering 2" or 3" liquid pipeline instead of trucking. Just a thought.



#12 fallah

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Posted 14 March 2015 - 08:14 AM

Zauberberg,

 

As per OP correction trucking is 60 cubes per hour....

 

Without availability of N2 appears: even though venting the truck, may contain toxic/flammable gases, to safe location isn't most reasonable solution but might be preferred to venting to LLP flare...



#13 Zauberberg

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Posted 14 March 2015 - 08:29 AM

Trucking 60 cubes per hour is an insane figure! A pure waste of money and resources. It is even better to install a treatment plant and use the treated water for industrial/agricultural purposes. Liquid pipeline should also be looked at.



#14 fallah

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Posted 14 March 2015 - 12:42 PM

Amin,

 

As i mentioned if N2 isn't available for purging the tanker before loading, i think conducting the tanker vent to safe location during loading is more reasonable solution than connecting the vent to LLP flare in which there might be the risk of explosive mixture creation. Please let's know your final decision in this regard...



#15 Amin Va

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Posted 14 March 2015 - 02:20 PM

Mr. Fallah and Mr Ivanovic,

 

Thanks very much for your comments.

 

The type of purge reducer is velocity purge reducer. There is also no liquid seal or anything like that.

 

The site is a remote small well site with minimum utilities and it should be built with lowest cost and even electricity is  not available at site (only an small 100W generator working with fuel gas). There is no pipeline at the moment. I don't think nitrogen generator is a feasible option regarding the type of the site (maybe the cost of nitrogen generator is higher than all the other equipment!).

 

I also called the trucking company yesterday and they confirmed they never purge the trucks with nitrogen (possibly due to high cost and number of trucks, etc). I asked his input he said he has seen some client use incinerator/vapour scrubber. Any thought you have on this? If they are an option I am not sure if the client approves the cost or will push toward connecting the vent to flare.

 

Based on the regulatory standard H2S must never be vented to atmosphere. The vent flow rate is also quite high (~60 m3/hr). So I am not sure if venting to safe location could be an option.

 

Best,

Amin


Edited by Amin Va, 14 March 2015 - 02:25 PM.


#16 fallah

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Posted 14 March 2015 - 02:33 PM

Amin,

 

Then it might a caustic wash drum through which the truck vent would be passed before going to the atmosphere is an option to solve the matter...


Edited by fallah, 14 March 2015 - 02:34 PM.


#17 Zauberberg

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Posted 14 March 2015 - 03:37 PM

Amin,

 

Let's confirm a few items before going deeper into solution finding:

 

1) What is the "content" of empty trucks arriving at site? Is Oxygen the only concern, or the truck contains toxic and/or flammable gases as well? Anything else that should be known?

 

2) How far away from the wellsite is the trucking destination point?

 

As Harvey Wilson says, an undefined problem has an infinite number of solutions.



#18 Zauberberg

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Posted 17 March 2015 - 11:53 AM

This morning I have got another idea - which may or may not work - but it could save you from trouble of procuring N2 skids, caustic wash system, or adsorbent for sour gas components. What if:

 

1) You pressurize the truck up to e.g. 10 psig with hydrocarbon gas. This would reduce % mol of Oxygen inside the truck to almost half of the initial value.

2) Install a restriction orifice on the truck purge line to LLP Flare.

3) Set the blanket gas PCV to 5 kPag to increase the maximum purge flow.

 

Then size the orifice in such way to give you less than 1% Oxygen in combined flow of (max purge gas + vent stream from the truck). Then calculate how much time it would take to perform 3 pressure purges of the truck which should be sufficient to reduce Oxygen below 20% LEL.

 

If you get that too much time is required, try to do it backwards - start with acceptable time period, then back-calculate the required purge gas flow. Perhaps it could come out that it is sufficient to replace the PCV only.



#19 Amin Va

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Posted 18 March 2015 - 12:00 AM

Hi Mr. Ivanovic,

 

This sound like a good idea because it eliminates the explosive mixture in the truck as well. The pressure purging might take a a little time I will discuss with the truck company.

 

Thanks for you help again.

 

Best,

Amin



#20 fallah

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Posted 18 March 2015 - 02:52 AM

Blanket gas PCV can't be set to 5 kPag because the depad valve already set to 3.5 kPag...



#21 Zauberberg

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Posted 18 March 2015 - 05:23 AM

Do the calculations first - see how much more purge flow you will need. If the entire concept is feasible and you cannot change the set point of the mechanical regulator and get the required incremental flow, you can always change the PCV.



#22 fallah

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Posted 18 March 2015 - 07:00 AM

The matter can't be solved by PCV change... In fact it's related to tank's mechanical integrity and setting the relevant safety devices such that there would be no interfering between set points with adequate margin between every two adjacent set points...



#23 Zauberberg

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Posted 18 March 2015 - 07:16 AM

I am not disagreeing with you. There is always a way to increase the purge flow - perhaps by using a valve with larger Cv, not necessarily by increasing the set point.

 

I am not even sure if this solution is feasible. Maybe it will result in excessive purge time for the truck, or excessive backpressure in the header. But if it works out, it would be an elegant and cheap way to get around the problem. One would say 'the wolf's belly is full and none of the sheep are missing'.






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