Extending Emergency Vent piping on an Aboveground Petroleum Storage Tank

[TanksAlotVT]

Background:
A steel, Aboveground Storage Tank (AST) built to UL142 standards and containing diesel fuel (Class II fuel). This is an atmospheric (low pressure) tank with operating pressures <= 1.0 psig but which may experience pressures as high as 2.5 psig when exposed to a fire (emergency conditions).
The tanks are equipped with an atmospheric (Normal) vent opening that are sized to keep pressures less than 1.0 psig when product is transferred into and out of the tank. They are also equipped with an Emergency Vent opening sized to keep the pressure inside the tank less than 2.5 psig when exposed to a fire with a specified heat input and a specified calculated discharge expressed in SCFH.
The Emergency Vent openings are sized assuming a 12” long nipple extending from this vent opening. When this Emergency Vent piping needs to be extended, to terminate outside a building for example, the additional length of piping and pipe fittings (elbows, etc.), a significant reduction in vent flow can occur unless the size of the vent and connecting piping is increased. In such cases, the size of vent and vent pipe extensions need to be calculated to ensure that that tank will not experience pressures greater than 2.5 psig.
Given:
A 400 gallon UL142 AST containing diesel fuel with a 4” diameter emergency vent opening and a required emergency venting capacity of 94,800 SCFH. The Emergency Vent piping needs to be extended 28’ from the tank and will have three long radius (r/d=1.5) pipe bends. The piping will terminate outside into the atmosphere and will include an Emergency Vent cap at the termination point that is designed to remain closed until it reaches 0.5 psig when it opens and releases the vapor in the piping.
Problem:
What is the pressure inside the tank with the extended Emergency Vent piping and fittings? Or put another way, what discharge can flow through this extended piping and fittings to keep the pressure inside the tank less than 2.5 psig?

This is my first posting so hope the format is OK. Thanks for any help.

 

[LittleInch]

Tanks...

This is why you shouldn't double or triple post. They have a tendency to get deleted.

Anyway if you have CRANE 410 you should be able to use the imcomprsisble flow for your gas (vapourised diesel).

For bends I would use an equivalent length calculation and then just work out pressure drop at your rather high 300 ft/sec.
How the opening vents work will need to be established if they continue to provide a pressure drop or if this pressure drop increases due to flow.

Then just add it all up.

What is your role / experience in all of this?
Do you have someone else checking this work?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TanksAlotVT]

LittleInch

Wow, I see the other posts were deleted and I lost the comments I received and the rather detailed replies that I made to them yesterday. I won't do that again. I wish I could reconnect with the folks who replied to me on the other thread. Live and learn.

As I posted yesterday, I'm trying to use those equations (I thought it was compressible flow though) but I keep coming up with variables that I don't know. For starters, the Class II fuel has vaporized when the tank is subjected to a "pool fire" so what is the vapor going through the pipe? NFPA 30 states that Hexane is best used in models but I don't see the physical properties of that in the book. Once I do figure out the variables and add up all the losses in the pipe due to elevation, friction along the pipe, and bends/fittings how does that equate to a corresponding backpressure?

I bought the latest edition of the Crane Technical Paper No. 410 since it was mentioned in NFPA and it said there was an example but I couldn't find it in there. I later read that the calculations for this were in the 1957 edition so I bought that on ebay but still don't see an example of this situation.

I am a Civil Engineer and have been working on the permitting of underground and aboveground petroleum tanks for 2 decades. This is a new requirement and so wanted to get some insight from others who are more familiar with the flow of fluids.

 

[danschwind]

Once I do figure out the variables and add up all the losses in the pipe due to elevation, friction along the pipe, and bends/fittings how does that equate to a corresponding backpressure?

If you have a pipe connecting Point A to Point B and you are able to calculate the pressure drop of this pipe for a given flow rate, you then know the difference of pressure between Point A and Point B.

In your case, Point B is the atmosphere, therefore you know the pressure at Point B. If you calculate the pressure drop between your tank and the atmosphere, the result is equal to the pressure in your tank.


Edit: If you are looking for Hexane properties, maybe nist.gov could supply the data. I assume you want the density, viscosity and latent heat. I think you will kinda of oversize it a bit by using Hexane, but as far as I know this is common for API 2000 evaluations.

 

[TanksAlotVT]

danschwind

I'm going to start cranking out the equations based on the responses I have received so far and will update the forum after that is complete, either with my conclusion or with more questions. I do have a PipeFlo software that I have been using but wanted to understand this in more detail rather than rely on the "black box" to give me the answer.

Thank you for your help.

 

[danschwind]

TankAlotVT,

What equations are you using? Darcy-Weisbach?

 

[LittleInch]

Well first and foremost get your data straight and accurate.

Where does 94,800 SCFH come from?

Does it allow for some to exit via the normal vent or is that assumed to be blocked for some reason?

If that is form some sort of calculation or simulation then it should give you some physical properties. If not ask the process engineer who calculated them.

Pressure drop across these mysterious vents needs to be included.

If you run into trouble you maybe need to remove them and use bursting discs or maybe even sheets of plastic / blow off cap on the outlet of the emergency vent.

After all it is only for an emergency and if you've got a fire under your diesel tank then replacing some components afterwards is only a small fraction of the costs for repair.

4" at that flow is really quite small. The difference made by going to 6" would be substantial.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TanksAlotVT]

danschwind

Yes, I was thinking of using Darcy-Weisbach for lack of a better equation but my book says that is intended for liquid flow and has restrictions for using this with compressible fluids like gases (vapors). I'm going to start by seeing if the restrictions are met then go from there.

 

[TanksAlotVT]

LittleInch

I didn't get into that level of detail with my original post in the interest of brevity. There is fine line between brevity and details and was hoping there was someone out there that has done this exact thing and could help me out. Many of the questions you ask are determined through testing or using empirical formulas that have been developed over several decades. These are all discussed in the code/commentary of NFPA 30 and I've been reading those as well in trying to solve this problem but at some point it gets down the the basics of fluid flow through pipes and fittings.

A tank that is built to UL142 standards has an emergency vent opening and a corresponding discharge that is given. The emergency vent opening is properly sized by the manufacturer (by code) such that the pressure in the tank won't exceed 2.5 psi when exposed to a pool fire (in the code, a certain amount of heat is applied to the tank for a specified period of time). This is based on the testing mentioned above. The discharge is labeled on the tank by the manufacturer and/or derived from equations in NFPA30 based on the wetted surface of the tank. Referring back to my original post, everything is determined by the manufacturer based on meeting UL142 except that this only applies for a 12" long pipe extending from the Emergency Vent opening. When the piping is extended, you get into the problem I have described.

Yes, all UL142 tanks have a normal vent that keeps the pressure in the tank to approximately atmospheric (+/- 1.0 psi) when fluid is added or withdrawn from the tank and/or to account for temperature changes. The normal vent keeps the pressures in the operating range of the tank. Yes, the code states that normal vents can be used along with the emergency vent to meet the required discharge. I left that detail out of my question to simplify things. Once I get the basic understanding down, I will go on to apply this concept to consider the contribution of the normal vent capacity to the required discharge.

 

[LittleInch]

Ah,

Now I get the drift. What you probably needed to say was that this was really an NFPA 30 code issue. Some other posts which may help are below. It looks like the NFPA 30 stuff was all based on hexane as the most conservative fluid. Diesel should produce a lot less vapour.

Anyway the normal imcompresisble fluid flow calcs apply.

I've just done a quick online calc and the pressure drop looks very big. Might have got it wrong somewhere.

https://www.eng-tips.com/viewthread.cfm?qid=110799

https://www.eng-tips.com/viewthread.cfm?qid=243735

https://www.eng-tips.com/viewthread.cfm?qid=281086

https://www.eng-tips.com/viewthread.cfm?qid=84702

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.