Internal Floating Roof for Ethanol Storage 1

[NickVil]

Hello,
We have an internal floating roof tank of capacity 3500 barrels and we need to use it for storage of Anhydrous Ethanol.
This tank already has air circulation vents provided on the fixed roof which is above the internal floating roof for breathing in and out during product movement.Also there is a vent on top of the fixed roof.
Does any code specify about the need of nitrogen blanketing in between internal floating roof and fixed roof? It is so, we need to close the circulation vents? What is the normal practice being followed regarding nitrogen blanketing of internal floating roof tanks used for Ethanol storage?

Regards

 

[Chance17]

One point you failed to mention is the pressure -vacuum devices on the actual floating roof.
These items must react to atmospheric pressure in order to protect the floatroof and tankshell.
If the space above the floating roof is closed and padded, then the floating P_V devices may not work properly.

The P-V devices on an external floating roof has direct communication with atmospheric pressure.
The purpose of the fixed roof is to keep rain, snow, and other contamination out of the tank fluids.
The internal roof circulation vents are liberally sized to maintain normal atmosphere underneath the fixed roof.

If roof leakage is sufficient to generate flammable air mixtures, I would view this as an equipment breakdown.
The roof seal should be replaced.

 

[IFRs]

In the USA, internal floaters are used quite often for Ethanol service, both food and fuel grade. No special details are needed. A good IFR with good double seals should do the trick. Make sure your floating roof metal, gaskets and seals are compatible with your particular ethanol (they can vary in corrosivity with feed stock). Some ethanol induced stress corrosion cracking has been experienced on steel floating roofs, I think aluminum ones have seen good service. If you do decide to Nitrogen blanket then you have to close up the shell and roof vents and put in PV vents to protect the tank. The IFR vents will work just fine since they are not active unless the IFR is on its legs (or cables) and there is product movement beneath the IFR. The IFR will otherwise be just fine with the internal pressure.

 

[NickVil]

Thanks for the comments.
So, we can say that:
1. If the tank has peripheral circulation vents (that are in the top of the cilinder or shell), and has the vent on the fixed roof, then there is no need of P-V valve or N2 blanketing.
2. If we decide to close the peripheral circulation vents and the fixed roof vent, then we can install a P-V valve or to put a N2 blanket.

I know that Ethanol is liquid IB Class (according to NFPA, Flash point<22.8°C and boiling point > 37.8°C.
If we close all the circulation vents, we would need to install a flame arrester on the fixed roof vent?

Regards

 

[IFRs]

To clarify:

1) The existence of circulation vents does not determine the need for N2 blanketing. That determination is made by others. Because the circulation vents are there, there is no need for other PV vents or valves to protect the tank, assuming the vents meet API 650 Appendix H. Because the circulation vents are there, putting a N2 system on the tank would be foolish.

2) If you close the circulation vents then according to API 650 blanketing or another acceptable method to prevent the development of a combustible gas mixture within the tank is required. Additionally, the tank shall be protected by
pressure-vacuum vents in accordance with 5.8.5, based on information provided by the Purchaser. It is not a either/or it is both blanketing and PV.

3) Since the flash point is less than 100 DegF, flame arrestors are required on open vants. PV vents that vent to the atmosphere do not require flame arrestors.

 

[Chance17]

Per my comments above, I think you are just asking for trouble with the floating roof P-V's if the air circulation is closed.
If you really want to close the air circulation, the floating roof should be removed and the tank operated as fixed roof equipment.

 

[IFRs]

Yitbos - I hate to disagree with you but I have put many aluminum IFRs in blanketed tanks and they work just fine. There is no excess pressure above the IFR - it equalizes just fine wihtout the PV vent. Think about a cookie floating in a glass of milk. If you increase the pressure above the cookie, it does not sink (try it!). The PV vent does not open unless the IFR is landed on it's legs (by rule and code). So, there is no adverse effect on the IFR by the blanket. The IFR will reduce evaporation in to the vapor space and the vapor space will be inerted by the N2. Not that I'm advocating doing so, just that it works.

 

[NickVil]

The API STD 2000 indicates:
"4.4.1.2 ...A flame arrester is not consider necessary for use in conjunction with a PV valve venting to atmosphere because flame speeds are less than vapor velocitiesacross the seats of PV valves."
"4.4.1.3 Open vents with a flame-arresting device may be used in place of PV valves on tanks in which petroleum or petroleum products with a flash point below 100°F (37.8°C) are stored and on tanks containing petroleum and petroleum products where the fluid storage temperature may exceed the flash point."

Considering the item 4.4.1.3, can we consider the circulation vents as open vents? In that case, shall the flame arrester be installed in each circulation vent?

Regards

 

[IFRs]

You are mixing metaphors. API 2000 does not apply to your tank.

Read the forward: "This standard does not apply to external floating roof tanks or free vented internal floating roof tanks."

You have a free vented internal floating roof tank. The circlation vents are open vents. They do not have flame arresters.

 

[NickVil]

Thanks IFRs,
Just one more question:
According your post of 17 Jul 09 10:46, you said:
"3) Since the flash point is less than 100 DegF, flame arrestors are required on open vents...."

And, in your last post you stated that "the circulation vents are open vents".

So, in order to have a clear understanding of this issue: Do we need to put a flame arrestor on each circulation (pheripheral) vent? In addition, do we need to put a flame arrestor on the vent located on fixed roof?

Thanks & Regards

 

[IFRs]

Here's how I see it (and I may not be 100% correct):

1) API 650 Apendix H circulation vents may be on the shell or fixed roof.

2) API 650 Appendix H also requires a vent in the center of the fixed roof

3) Since API 2000 does not apply, the question of flame arrrestors does not come up. Flame arrestors are not required on API 650 Appendix H circulation vents.

4) API 650 Appendix H does not refer to the circulation vents as "open" vents. I should not have used the word "open" as it clearly confused matters. That was my mistake and I apologize. What I meant was that the API 650 Appendix H vents are "free" or "not closed" or "not obstructed" or some description to differentiate them from any other style of vent that has some cover, obstruction or device between it and the atmosphere on either side.