Flow Conversion (what is 36.732) 3

[jbrady5594]

There is a constant of 36.732 in an equation that we are unable to figure out. It is to convert raw natural gas flow in KACFH to a pressure /temp compensated KSCFH.


36.732*raw flow*(pressure+14.7)/(temp F+459.7)

Thanks for your help.

 

[Zoobie]

It looks like the ratio of T to P at the standard conditions except the T works out to 80F which is a 'standard' I have never heard of.

 

[25362]

It seems Zoobie is right. The formula may contain a correction to account for the "actual" compressibility factor of the gas.

 

[katmar]

This number works out to be too close to a temperature of 80F for it to be a coincidence. I'm sure that Zoobie is correct and the original reference conditions were 80F and 14.7 PSIA. I don't think compressibility comes into it because that would vary with temperature and pressure.

The most important lesson to learn from this exercise is that there is nothing standard about "standard" conditions. The reference conditions must always be specified, whether you call them "standard" or "normal".

JBrady - I hope you can come back and confirm whether the reference temperature (probably specified in your supply contract) was indeed 80F.

 

[25362]

A conversion from actual (a) volume to standard (s) volume would be as follows:

SCF = ACF x Pa/Ps x Ts/Ta x Zs/Za​

Where,

P = pressure
T = absolute temperature
Z = compressibility factor

For ideal gases Zs/Za = 1

For real gases Zs/Za [≠] 1

 

[quark]

More precisely,

SCF = ACF x [Pa-(Psata x RHa)]/Ps x Ts/Ta x Zs/Za

PS: SCF corresponds to 0%RH

Happy Independence Day to all fellow Indians

 

[25362]

I thought the original query referred to natural gas.

 

[quark]

I was just stating the conditions. If the natural gas is totally dry before compression, then RHa becomes zero and your equation comes into picture.

 

[Zoobie]

I don't want to get away from the original post, but I have never heard of relative humidity used for natural gas calculations. Not only that, I was not aware that determining a standard cubic foot of natural gas had any moisture correction in its calculation.

 

[quark]

No problems, per se. What I mean is whenever the conversions are made from ACFM to SCFM or vice versa, that is the standard equation. I have been observing in the past threads, omission of RH even in compressed air application. So, I thought to put it here.

 

[Zoobie]

Quark, I understand how this applies to compressed air. I just question if its valid for natural gas. In fact I think it may not be.

As an example: natural gas is sold per unit of energy (GJ). Natural gas is usually metered in standard cubic feet or cubic meters. To get from one to the other (volume to energy) the gas heating value per unit volume is used. As far as I know all of the constituents of the gas go into the calculation of the heating value. This would seem to create a disconnect with the equation you provided if applied to natural gas. Perhaps the affect of moisture is small but I wonder what it would be applied to a billion cubic feet of gas sold at $9/GJ. Again maybe its negligible....oh well I suppose I should get back to work.

 

[vzeos]

Typically, the characteristics of pipeline quality natural gas as well as standard conditions are established in pipeline sales contracts. It is normal to specify that the custody transfer be reckoned on a dry heating value basis. It is also normal for the contract to limit the water content of the gas to about 5lb - 10lb per million cubic feet. The pipeline companies condition the gas to meet the contract terms. Thus, it is not necessary to consider water content in calculations involving pipeline quality natural gas. On the other hand, if we are dealing with unconditioned natural gas, then the water content (as well as other liquids) should be considered.

 

[Zoobie]

I think the right person popped by this thread. RGasEng, thanks for the clarification on the moisture issues. I did a couple of simple calcs. At 5-10 lb per MMCF the heating value of methane changes by about 0.07% according to HYSYS. I suppose my example is not relevant. That's OK.

The point that what I was trying to get at was that I believe its correct that when quoting a 'standard cubic whatever' of natural gas that this represents all of the moles in that gas expressed as a volume at standard T&P. Perhaps a better example is this: If you have a certain flow of gas expressed at standard conditions entering a dehydrator (glycol for arguements sake) then you would expect that the dry gas out would have a lower gas flow expressed at standard conditions (due to the moles of water absorbed by the glycol). Is this wrong? If so then I need to make some adjustments in how I do my calculations.