9 replies to this topic

[Raj Mehta]

In case of Normal venting due to operational changes or weather changes, we consider air for inbreathing and outbreathing, which we get directly from the equations mentions in the API 2000.

 

The units for venting formulas are always Nm3/hr or SCFH of air. Do we need to calculate the equivalent Nm3/h or SCFH for the stored liquid? If yes, how ?

 

For eg :  

If nitrogen blanketing is given, then nitrogen would be going out in case of outbreathing, so it should be Nm3/h or SCFH of nitrogen, right ? 

 

Kindly correct my concepts here. I am very confused.  

 

Also in the foot notes of table 5B (table of wetted surface area & CFH for hexane), in the last point it is written that the latent heat is used at 15.6 deg C, instead, what I feel is that it should have been the latent heat of heaxane at the boiling point corresponding to the relieving pressure, because in case of fire exposure / emergency, it would be hexane vapors that would be leaving through the venting system. 

 

thank you.

 

 

 

 

[shan]

N2 Volume (Nm3/hr or SCFD) = Air Volume (Nm3/hr or SCFD)

[Raj Mehta]

Anyone please give me some explaination regarding my queries. I am really confused. 

[shan]

Why do you think air volume at standard conditions is different with N2 volume at standard conditions?

[Steve Hall]

Raj,

 

Don't get wrapped up in the minute details of your query. Think about the big picture.

 

For breathing, the Standard is saying that due to temperature changes from day to night to day, the gas (air or nitrogen, doesn't matter) contracts and expands and your vent needs to accommodate the gas flow into then out of the tank's head space. The actual calculation given is only an approximation - there can be so many factors affecting the actual temperature change and resulting volumetric flow such as liquid level in the tank, insulation, solar radiation, etc. In practice your vent will be many times oversized.

 

For exposure to fire, the key thing the Standard gives you is Q, the amount of heat introduced into the tank. The bit about hexane is just to give them a way to graph and tabularize the results since, as you say, the physical properties of the tank contents are important. You can use Q to draw your own conclusions about heatup time and vaporization energy, and ignore the hexane results (except to use them as a reality check).

 

To your point, NFPA-30, the Flammable and Combustible Liquids Code used in the U.S., has an Annex B that presents nearly the same analysis as API 2000. In it, they give a table of Heat of Vaporizations at the Boiling Point. They give the vent rate as CFH = 70.5 Q / (L M^(1/2)) where Q = heat input, Btu/h; L = heat of vaporization , Btu/lb; and M = molecular weight. But, again, the key input is Q and the purpose of the Code is to guide its estimation.

 

Steve

[Raj Mehta]

So to wrap it all sir, I am laying down the method to determine the emergency venting rate for 'X' liquid. 

1. Decide the alignment of the tank. (horizantal / Vertical)
2. Calculate the wetted surface area (A) accordingly based on the horizantal (75% of Wetted area) or vertical alignment (equal to wetted area upto 30 ft. high) 
3. Calculate Q, using the correlation in terms of A.
4. Find the heat of vaporization (L) of the 'X' liquid at its boiling point 
5. Now put all the vlues in the equation: CFH = 70.5 Q / L*(M^0.5). This will give me the emergency venting rate of the 'X' liquid stored.

In step no. 5, do I need to refer to the tables for the value of CFH for hexane in the API 2000 or NFPA 30 (table of wetted surface area against CFH) and multiply with 1337 / [L*(M^0.5) for the X liquid]. I mean i can also directly calcuate the venting rate using the equation in step # 5, since now I know all the variables corresponding to the X liquid i.e. its molecular weight (M), heat of vaporization (L) and Q as calculated in step # 3.

 

Thank you very much. 

[Raj Mehta]

Steve sir, Kindly verify my above post. 

 

Thank you. 

[Steve Hall]

Raj,

It looks right to me. I think the 1337 and 70.5 are just conversion factors - try it both ways and I think you'll get the same result. If not, you've done something wrong. You can also take credit for water sprays, drainage, and insulation as directed in the standard.

Steve

[breizh]

Raj ,

This resource may help you .

Hope this helps

 

Breizh

[Raj Mehta]

Thank you Steve sir & Breizh sir for your valuable inputs on clearing my concepts on this topic.  

Edited by Raj Mehta, 26 April 2013 - 02:36 AM.