API 2000 - Conversion from Nm3/h of air when tank is blanketed with fuel gas 1

OP,
You are getting tripped up by Normal, instead of me typing a bunch, the MVPs do a good job of explaining in this post.


API2000 - Section 3, gives you the conditions of Normal.

In essence yes, normal air = normal gas in a volumetric sense. Your actual volume will be different by 8-10% or so due to the 0.1 barg, 1.1 bara pressure in the tank, but keep everything at normal flowrate - that's why its so useful as its independent of pressure.

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Thanks for responding.

Why do they state "of air" for all the rates in API2000 if it is a perfect gas, reasonable assumption at tank conditions, then stating Nm3/h makes the MW irrelevant?

There are though differences in viscosity and other physical properties between gases which can impact the volumetric flow through nozzles and relief valves.

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

Justice100,

I suspect that API 2000 also uses "of air" specifically because low pressure conservation vents are tested and certified by manufacturers using air only, and they provide pressure drop curves as a function of normal air flow rate. It makes some things easier, as you don't need to work out pressure drop correlations for each specific gas since API gives you a required "air" flow rate and the manufacturers specify pressure drop as a function of that same "air" flow rate.

It has been a long time since I did an API 2000 calc and I don't have an API 2000 with me (and I don't recall doing a tank gas blanketing calc) but I think it may come down to equivalent mass of a SCFM of air versus the gas. How much mass you need to replace in the tank to keep the pressure constant for a given flowrate in/out and a given evaporation/condensation of liquid in the tank is what is given in API2000 expressed in SCFM air (Nm3/hr)?

So for one SCFM on a mass bases of air it takes a gas SCFMg = (MWa/MWg)*(SCFMa)to equal the same mass of air. There should be something in API2000 that discusses this.

Hi,
Probably what you are looking for:

Pierre

After a lot of head scratching I have determined that the equivaloent SCFM of air is the same amount of air that will give you the same pressure drop as the SCFM or CFM of the actual gas being vented. I believe it is expressed as such since all manufactured tank vents are tested based on air flow so if it passes the air flow per API SCFM of air then that is your pressure drop/resulting pressure in tank during venting regardless of gas being vented.

I found an equation in API2000 Appendix B that converted mass flow of gas being vaporized to SCFM of air. That made me think of what could the conversion be that they were using from mass of gas to a SCFM of air.

In the attached I first wrote down the API2000 equation which for this equation the pressure and temperature of the vaporized is considered at standard conditions per API discussion.

Then I determine an equation to express the SCFM of a gas at STP using the ideal gas equation.

Then I determed for same piping pressure drop what quantity of air flow is equivalent to a quantity of gas flow using the spitzglass equation

Then I substituted the SCFM of gas per the ideal gas equation into the spitzglass equation.

The result was the same equation of API2000.

Great work snickster!

As you say the point for the "of air" is that the basis for the relief rate is based on air through a nozzle. This depends on composition for an ideal gas so I need to use equation D.35 to convert. I saw someone suggest Grahams law, which also makes sense to me now.

The conversion is the equation shown in the bottom right corner of my calculation of sheet 1.

Qa = Qg (MWg)[sup]0.5[/sup]/(29)[sup]0.5[/sup]

Note that the conversion gives you the amount of gas flow to produce the same amount of pressure drop when relieving throuh a vent rated in pressure drop at SCFH of air.

Now for a blanketed tank with gas then on filling with gas outflow, this would be used to determine pressure build-up in tank due to pressure drop across vent device with gas flowing out of it. However for in breathing due to liquid movement out then this conversion I don't think applies since inbreating is not throuh the vent. In this case you would just need to replace an equal SCFM of air with gas to keep pressure in tank from reducing, since a cu ft of volume is same regardless of gas.