30 replies to this topic

[CMA010]

Sorry mate, for blanketing gas the same principles apply. However, when converting Nm³/h or SCFH of air to Nm³/hr of SCFH of a blanketing gas the conversion indeed boils down to volumeric rate of air in Nm³/hr of SCFH * √(Mw air / Mw of blanketing gas) as the flow conditions are the same and all physical properties are assumed equal except Mw. Obviously for the majority of cases these assumptions hold, but the basis for the conversion is not Graham's.

[latexman]

Effusion requires the flow path to be about the same size as the molecules, such that molecules can only come through the flow path one at a time (i.e. there is no room for two or more at one time).  An industrial PVRV or vent unit of 2" to 24" diameter is far, far larger than that, and Graham's does not apply.

[latexman]

As established in earlier posts, API 2000 gives the relieving rate in SCFH of air.  To convert this volumetric flow rate to ACFH of blanketing gas or sat'd vapor from a boiling liquid, I use a different approach.

 

In an ideal gas world, 1 mole of air at a T and P has the same volume as 1 mole of blanketing gas or sat'd vapor from a boiling liquid at the same T and P.  This is also true for flows (moles/hr) and volumetric flows (CFH).

 

The basis in the real world is:

 

PV=ZNRT and  N=PV/ZRT

 

Keeping number of moles constant (also flow rates).

 

[PstdVstd/ZstdTstd]/[PactVact/ZactTact] = 1  (The R’s cancel out.)

 

A little algebra, rearranging, and renaming yields

 

ACFH = SCFH * (Pstd/Pact)*(Tact/Tstd)*(Zact/Zstd)

 

This is similar to http://en.wikipedia.org/wiki/SCFM but it should be more accurate by adding the compressibility factor.
 

Agree/disagree?

[Olidin]

CMA010:

I think you are stuck with converting from actual flow rate to equivalent air flow rate (at reference conditions), whereas I am talking about converting air flow rate in Nm3/h as calculated in API STD 2000 to blanketing gas rate.
 
The blanketing gas rate is indeed calculated based on Graham's law of effusion:
 
Volume flow rate of blanketing gas in Nm3/h = Volume Flow Rate of Air in Nm3/h *√(MWof Air / MW of blanketing gas)
 
This is a fair approximation of converting air flow rate at given reference conditions to blanketing gas rate at the same reference conditions and has been used in design calculations for blanketing gas flows when the air flow rate is calculated as per API STD 2000.
 
Regards,
Ankur.

Ankur, why would we ever need to convert flow rate from Air to Actual gas? I always needed to it the other way around.

Per API 2000, it stated that most vent capacities are expressed in air in certain conditions. Therefore, Annex D9 was written to convert flow rate from actual gas to air (at standard or normal cond. or any cond if you have a whacky vent). Maybe I haven't seen a vent capacity shown for my particular gas before(would be nice though).

Also, CMA010, please note that API 2000 D36 equation is essentially Graham law if assume ideal gas. Is it not? 

 

Edit: after a quick inspection, I also note that Graham's law is applicable to gases with the same conditions (temp & press.). It would be erroneous to convert say a gas flow rate at different temperature and pressure to air at standard condition. When there is a change of temperature and pressure, one must take care, perhaps ideal gas law as latexman outlined is the best option. 

Edited by Olidin, 01 May 2013 - 03:29 PM.

[latexman]

Olidin,

 

I work at a large chemical company.  We have 1 "expert" who has provided a relief design manual to all engineers.  He probably has access to API-2000 and other standards, but I do not.  I'm sure there are many engineers who do not have access to published standards.  It's a cost saving thing big companies do.  Could you be so kind to reproduce API 2000 D36 equation for us here?  Thank you.

[Olidin]

latexman, 
 
I saw your approach to converting volumetric flow rate above and I absolutely agree. That is how I deal with gas at different conditions. As a general user of API, I would not bother to get bogged down with converting using Isentropic nozzle flow assumption. What you are doing is fine. I don't know if it's the right direction to answer the original poster's question to get into details of Graham's law and such. It's just not practical.

 

Also, since you are in a big company, it's more likely that they subscribe to a standard library. I would ask around in your company for access to API standards. 

 
However, as you requested, I attached a snapshot of the relevant section. http://imgur.com/Nrw1Cat. If this violate forum's policy, please let me know. 
 
For the equations:

 

W = mass flow rate (lb/hr)

q = volumetric flow rates (cfh)

k = specific heat ratio (of air for this case) @ the condition

n = specific heat ratio (of gas) @ the condition

x = conversion factor (379.46 scf/lbmol)

 

Conversion between W and q can be deduced as: W = q*M/x. 

 

D36 is effective Graham's law if under the same temperature (Tair = Ti). Note that Graham's law is ratio of volumetric flow (q) *not mass flow (W). Do a bit of substitution and you will arrive. 

 

Feeling like an undergrad. Cheers.