Orifice Meter - Flow Measurement 1

[wood5896]

Can someone explain what is meant by the "the flow measurement was converted to a base pressure of 14.7 psia?" (when the outlet pressure was in fact much higher than 14.7 psia)

Does this mean that if the inlet pressure is 200 psia and the outlet is 50 psia and you covert to base pressure of 14.7 psia it's as if you did the experiment from 200 psia inlet to 14.7 psia outlet.

If so, how easy is that conversion to make, it seems as if all the field equations I see skip straight to the conversion and you never see the derivation of Bernoulli.

Thank you for the help!!!

 

[MintJulep]

What units is the flow reported in?

 

[wood5896]

Mcf/d

 

[MintJulep]

Then I would guess that the measurement has been corrected to standard cubic feet.

 

[wood5896]

Because there are different pressure bases for every state (mid continent typically around 14.7 psia and the Rockies around 15.025 psia) due to contractual agreements. They do this I assume to guarantee all gas is truly on a standardized basis. They do these calculations at the wellhead and they aren't venting it to atmosphere its going from 200+psia to 50 psia or higher so the actual volume they are seeing is much smaller than what is reported because they change the volume (pressure adjust to 14.7) so it's standardized.

I am not sure on any of this cause I can't find the answer anywhere, someone jump it and correct me please.

 

[zdas04]

Gas volume flow rate is pressure and temperature dependent. For a given mass flow rate, the volume flow rate will vary widely between 0 psig and 200 psig (it is a factor of 15). To be able to aggregate gas that was flowing at 25 psig with gas flowing at 400 psig you need a common basis. Now if you calculate the density of the gas at a contractual or regulatory pressure and temperature base then you can get an imaginary number that means "the imaginary volume of gas that would be flowing if this station was at 14.73 psia and 60F" which is mass flow rate divided by density at standard conditions. That imaginary volume has a given number of molecules. If the line is at 50 psig or 10,000 psig, an imaginary cubic foot has the same number of molecules.

In short "Standard" conditions are a very effective surrogate for mass flow rate and allow for commerce to take place. It applies to equations that are adapted to volume flow rate at standard conditions (e.g., the AGA 3 measurement equations or the Panhandle A flow equation), but doesn't work at all for equations adapted to actual conditions (e.g., most compressor hp calculations).

One important warning--NEVER use STANDARD volume flow rate to calculate velocity, it is imaginary and does not mean anything at all.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[wood5896]

Thank you very much Zdas04, that was exactly the physical interpretation I needed to see this.

So I can just take the final Mcf standard when I see it and x (14.73/ P(flowing actual)) and I will get the volume at the Pactual.

One other question, when I am looking at the orifice equation and I see Pflowing, is that inlet, outlet, at the vena cont...?

Sorry for the dumb question, and thank goodness for people like you that will help with simple things some of us don't know or can't visualize without additional help.

 

[wood5896]

One other question (I think you can explain better than before), what effect does working at high altitudes (6000-9000feet) in the rockies have on formation volume factors, if instead of standard 15.025 psia in Wyoming I use the actual 11.0-11.8 psia that atmosphere is my Bg (gas expansion factor) goes up nearly 20% which then causes drainage calculations to go down 20% for all other wells. Should I just use the standard conditions every other engineer is using cause this just doesn't matter, or does the elevation at which my reservoir exists at affect that, I guess I am confused, it seems that a well at Sea level and the rockies can have the exact same energy in the reservoir but in the rockies it will just expand much more going to surface (the volume), but the energy/volume will decrease. Thanks!

 

[zdas04]

This isn't simple. In a graduate fluid mechanics class I took once, the professor tried to explain SCF to an aerospace type--at the end of the discussion almost everyone was totally lost, I suggested that SCF was simply a surrogate for mass flow rate and you could have heard a pin drop in the room. After a few minutes of explaining myself everyone got it, but the professor with 50 years in Fluid Mechanics had never gotten it until that day.

To get from standard flow rate to actual flow rate requires more than just the ratio of pressures. I like to calculate density for both the imaginary stream and for the actual stream (which includes temperature and compressibility differences) then:

q(act)=q(std)* [rho(std)/rho(act)]

That way I don't have to remember if I'm dividing actual pressure by std or vice versa. Once you get q(act) then you can calculate bulk average velocity by dividing by area (in consistent units) and getting the time frame right.

AGA 3 has equations for either upstream or downstream pressure taps. A good rule is that P(flowing) for a square-edged orifice station is the pressure at the pressure tap wherever it is (upstream or downstream, never at the vena contracta).

David