15 replies to this topic

[wpeete]

Hello everyone,

 

I have a question about the API 2000 7th ed. but first I will explain the situation.

 

Situation:

We have several storage tanks that contain flammable liquids. Some tanks contain pure solvents and others contain mixtures.  The venting systems of all these tanks are connected to a common vapor collecting system. Each tank is provided with nitrogen-blanketing.

1)      Tank example containing pure solvents

·         Volume:                                                           ± 72 m³ (tank is max filled up to 63 m³)

·         In- and outflow:                                                 ±30 m³/h in and 7-14 m³/h out

·         Design-pressure in/out:                                   4,18 bar/ 0,333 bar

·         The nitrogen-blanketing on these tanks is a 'pressure-controlled' system. When the pressure inside the tank drops to a certain level, a valve will open and let nitrogen in until low over-pressure is reached again. When the pressure rises to high, another valve will open and the mixture of nitrogen and vapors is released in the vapor-collecting system until the low-overpressure is reached again. The low-overpressure in the tank is maintained between 30 mbarg and 200 mbarg.

 

2)      Tank example containing mixture solvents

·         Volume:                                                         ±  10 m³

·         In- and outflow:                                               ±30 m³/h in en 7-14 m³/h uit

·         Design-pressure in/out:                                3 bar / full vacuum

The nitrogen-blanketing on these tanks is a ‘constant flow’ system, a constant flow of nitrogen is introduced into the tank so low over-pressure of 15 mbarg is maintained inside the tank.

 

Solvents: ethyl acetate, acetone, tetrahydrofurane, isopropanol, toluene, ethanol, methanol,…

 

Question:

Are flame arresters required on these tanks according to the API 2000 7^th ed. or not? Or is our inert-gas blanketing good enough?

First the API 2000 states

“3.5.2 Design Options for Explosion Prevention
…
Inert-gas Blanketing—An effective means of reducing the likelihood of a flammable atmosphere
inside a tank, when engineered and maintained properly."

 

But then in annex F

 

“This annex describes three tank inert-gas-blanketing design levels. All three levels provide comparable

flashback protection. Level 1 has minimum inert-gas-blanketing requirements in combination with a

specific flame-arrester classification. Level 2 has more stringent inert-gas-blanketing requirements with a

different flame-arrester classification. Level 3 has the highest inert-gas-blanketing requirements with no

flame arrester.”

 

But does this annex F applies to the ‘pressure-controlled’ blanketing-system and the ‘constant-flow’ blanketing-system?

 

Kind regards

 

wpeete

[Ankit_Kumar]

Hopefully below link will be useful for u

 

http://www.cheresour...l-tank-venting/

[wpeete]

Ankit_Kumar

 

Thank you for your response!

 

I checked the link but I still don't understand if i need a flame arrester or not, when the storagetanks already have inert-gasblanketing installed on them?

The tanks do not vent to atmosphere but to a vapor-collecting system. Only the emergency venting (rupture disk) vents to atmosphere

Edited by wpeete, 10 March 2016 - 08:09 AM.

[fallah]

 

I still don't understand if i need a flame arrester or not, when the storagetanks already have inert-gasblanketing installed on them?

The tanks do not vent to atmosphere but to a vapor-collecting system. Only the emergency venting (rupture disk) vents to atmosphere

 

wpeete,

 

Then no need to FA, because the tanks vent to a vapor-collecting system...
 

[Art Montemayor]

In accordance with my field experience, you do not need a flame arrester.  And I would caution you about considering its application.

 

I have written on this subject within our Forums in the past and I continue to counsel against the application of flame arresters because of personal field experiences, the reported experiences of other engineers, and API publications in the past that have warned of potential bad results with flame arresters.  I specifically make reference to previous API documents #2210 and #2028 - some excerpts that I attach herein.  The API, to my knowledge, has never recommended the use of flame arresters.  In fact, in almost all their past publications they have avoided this topic.  If there has been a recent reversal of thought, I would like to know about it.

 

There are some definite down-side, negative effects that you must carefully evaluate before thinking of applying a flame arrester as a safety device.

 

Additionally, I would never use a rupture disk on a storage tank as a safety relief device.  Why don't you use what everyone else employs universally - a PSV?

 

 API 2028 - Piping Flame Arresters.docx   20.73KB   107 downloads

 API 2210 -Flame Arresters for Petroleum Storage Tank Vents.docx   10.5KB   98 downloads

[fallah]

Dear Art,

 

API 2000, even though states that a FA isn't necessary in conjunction with a PV valve venting to the atmosphere, some companies have a combination of PV valve and FA in their products. But in atmospheric storage tanks containing liquid with flash point equal to or lower than around 40 C, API 2000 recommends using FA at the free vent outlet to avoid sucking the flame inside the tank through the vent...

[wpeete]

Dear Fallah and Art,

 

Thank you for your responses!

 

Art,

 

The rupture disk was already installed a long time ago. I will mention to my boss if it is possible to consider replacing the rupture disk with a PSV.

[fallah]

 

The rupture disk was already installed a long time ago. I will mention to my boss if it is possible to consider replacing the rupture disk with a PSV.

 

wpeete,

 

Rupture disk as an "Emergency Vent" cannot be replaced with a conventional PSV. Indeed, it's not uncommon to consider a rupture disk as a last diffence against overpressure in atmospheric tanks besides the conservation vent and PSV. It would be activated very rarely when all previous protection layers might be failed to activate...
 

[wpeete]

Oh okay, thank you for the info!

[wpeete]

Two more questions

 

1) once the rupture disk breaks we have an open end. Isn't this a problem?

 

2) The vapor-collecting system ends into a scrubber-unit where the air is prurified before release. Inside op the vapor-collecting piping is almost certain an explosive atomsphere. Is it necessairy to install a flame/detonation arrester in front of the scrubber-unit?

[fallah]

wpeete,

 

1- Anyway it should be replaced with a new one at the first occasion...

 

2-Depending on the degree of explosivity, it might need to a FA at the outlet...

[wpeete]

Fallah,

 

Could you maybe tell me where I can find documentation or a guidline to determine if a FA is needed or not, depending on the degree of explosivity?

And shouldn't the FA be installed right in front of the inlet of the scrubber-unit? 

Edited by wpeete, 11 March 2016 - 03:17 AM.

[fallah]

wpeete,

 

Per API 2000 the main parameter affecting to decide considering FA is flash point based on which for storing the liquid with flash point of equal to or below 100 F it should be considered at the outlet of the free vent...

 

The location of FA, if really needed, at the scrubber system you described appears to be at the outlet...but if a simple sketch  provided it can be specified accurately... 

Edited by fallah, 11 March 2016 - 04:04 AM.

[wpeete]

Okay I added a drawing. I crossed out the arresters that shouldn't be necessary if I understand correct.

Attached Files

•  schets.PNG   47.67KB   13 downloads

[fallah]

wpeete,

 

Yes, you are correct about the crossed out FA's, but i think the FA at the inlet of the scrubber should be at the scrubber outet if the the vapor inside the scrubber is within relevant explosive range...

[wpeete]

Fallah,

 

Thank you very much for your time and effort!

I think I understand now.