Pressure correction for gas flowmeters

[Proton10]

I am looking at pressure corrections for gas flow meters. The vendor says that the reading should be multiplied by sqrt(Pactual/Pcal), where Pcal is the pressure the meter was calibrated at. That says if the pressure rises, the actual flow is higher than the reading.

But it also says that the correction factor for different density is 1/sqrt(specific gravity) or 1/(actual density/cal density). If the specific gravity rises, the actual flow is lower than the reading.

When I increas the pressure of a gas, the specific gravity increases. Doesn't that mean the two corrections contradict? If I pressurize air, or I substitute a denser gas, what is the difference?

 

[zdas04]

Proton,
Not quite right. When you raise the pressure on a gas the density rises, but the Specific Gravity is a function of gas composition and does not change unless you change the gas composition.

The P(cal) bothers me a lot. What you are trying to do is get the reading to standard conditions not to local atmospheric conditions. If they calibrated the gauge where I live (5,400 ft elevation) they would have to adjust the calibration flow rate by 14.73/12.06) to get to a standard condition that can be related to the volume anywhere else.

It looks like the manufacturer was being pretty sloppy for a measurement device manufacturer.



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.

The harder I work, the luckier I seem

 

[pmover]

Proton10,

To supplements David's response, if this flow measurement device is used for determining sales (cash register), perhaps further investigating this meter is warranted.

i would also urge you to review the definition of specific gravity (gas and liquids) and to learn the basis or fundamentals for measuring volumetric flows of gases. The volumetric flow measure value must be established at some datum (pressure and temperature) and correcting to that datum is a simple straightforward process/calc. there are other postings in this forum that address this matter or you may search via google.

meanwhile, good luck! oh! good observation as well! sounds like you are learning . . .
-pmover

 

[katmar]

An important factor in determining the correction factor is to know in what units you are measuring your flow. If your read-out is in mass units (eg lb/h or SCFM) then the correction factor will be different from a situation where you are measuring volumetric flow (eg m3/s or ACFM). Some gas meters are even calibrated in energy units (eg BTU/h). You cannot say categorically what your correction factor is until you know what it is that you are measuring.

In practice, the SG of a gas is just the ratio of its molecular weight to that of air. It seems a strange thing to use, but it is actually quite practical. Or rather, it was practical in the days of slide rules and charts - I believe it is better in these computerised days to do it with the correct fundamentals and use the actual density.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

American Gas Association AGA-8 report detailes an explicit method often used in custody transfer agreements. There is an online calculator which supposedly calculates to that standard here,

http://www.mysolv.com/AGA8CalcIn.asp

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[zdas04]

Guys, you are answering an mainstream industrial question, he's asking a small sidestream question. For square-edged orifice measurement (AGA-8, ASME 14.3) there is no such thing as a "pressure correction" because pressure is an explicit portion of the basic calculation. The other chapters in the ASME measurement section also have pressure as an explicit term.

This guy has a little meter that he's using to measure some auxillary flow stream that is far from mission critical. I'm guessing that it is a little neutating [I don't remember how to spell it, and I'm currently in an airplane and don't have a dictionary] disk or Roots Meter.

David

 

[LeSabre]

Proton10, the expression your vendor gave you is correct for estimating a min flow rate for a turbine meter at a pressure other than its calibration pressure, assuming the specific gravity is 0.6 and the compressibility factor is 1.0. Check out equation B on page 4 of the attached PDF file.

http://www.americanmeter.com/products/files/IM_4720.3.pdf#search

 

[Proton10]

Thanks for the responses.

I understand the specific gravity part now - it is a function of molecular weight, not actual density.

I think I have a good grasp of the ideal gas law to calculate variations in actual flow with temperature and pressure. This is really a general question about compensating for variations in pressure with a volumetric flowmeter.

I believe the general formula for flow is something like

Q=Cv*sqrt(dP/density)

For a gas at STP, if I increase the SG, I increase the density, correct? So for a given dP the Q would decrease, i.e. less flow.

If I increase the pressure, I also increase the density, but the correction factor says that for the same dP, I get MORE flow. How can this be?

 

[Proton10]

Never mind, I have it now.

I am looking at STANDARD cfm, so changing SG does not change the moles of gas, but changing P does. If I pressurize, I should decrease flow by sqrt(density) but increase directly proportional to Pabs, with the net being an increase by sqrt(Pabs), since density is directly proportional to Pabs.

Q'=Q*Pabs/P0*sqrt(density0/density)

where sqrt(density0/density)=sqrt(P0/Pabs)

so Q'=Q*sqrt(Pabs/P0)

Thanks

 

[katmar]

Proton10,

I agree with you understanding. If the density increases then for a constant dP the volumetric flow decreases.

The way your vendor has worded the pressure effect it is wrong, for exactly the reason you have described - as pressure increases so does density. I can see two alternatives that could make the vendor correct. If he was talking of differential pressure across the meter and not upstream pressure then his relationship would hold true. Or if he was talking mass flow then the flow would increase with upstream pressure (exactly as the mass flow would increase for an increase in density).

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Proton10]

Katmar,

No, I think the vendor is right, in that he is referring to STANDARD cfm. The pressure decreases the flow as the square root, because of density, but it increases the flow in direct proportion because the standard volume is getting larger as the actual volume stays the same.

 

[katmar]

Exactly. That is because SCFM is a unit of mass, and not volume.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[zdas04]

Katmar,
That is a really subtle point that needs some clarification. SCFM has units of volume, but it is actually a surrogate for mass flow.

The way that this is possible is that the "S" in SCFM stands for "at standard conditions". We have a lot of discussions on eng-tips.com about what "standard" really is, and the only good answer is that it is stated in some specific reference (often a sales contract) and that is what you use. If I know the pressure, temperature, gas composition, and gas compressibility factor then I know the density and it is an easy task to convert SCFM to lb/min by multiplying density times SCFM regardless of actual flowing pressure and temperature. If I'm using actual CFM then I have to calculate the density at flowing conditions. The arithmetic is just a lot less error-prone to use standard.

David

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.

The harder I work, the luckier I seem

 

[Ashereng]

Proton10,

SG is not really a function of molecular weight. It is a function of the fluid (gas or liquid). There may be two things with the same molecular eight, but different SG.

For example.

N2O (Nitrous Oxide) has a mol wt of 44, and a SG of 1.97 @ 25°C.

CO2 (Carobn Dioxide) has a mol wt of 44 also, but a SG of 1.53 @ 21 °C.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[zdas04]

Ashereng,
Just curious, what parameter would you extract from the physical characteristics of gases to calculate SG if not MW?

I think you are the victum of a long standing typo. If you go to

http://www.osha.gov/SLTC/healthguidelines/nitrousoxide/recognition.html

OSHA quotes the SG of Nitrous Oxide as 1.97, but every other reference I found quote it as 1.53 just like CO2. For example see:

http://www.general-anaesthesia.com/images/nitrous-oxide.htm

http://www.praxair.com/praxair.nsf/...61edf97c25fa17e98525655500407165?OpenDocument

http://www.engineeringtoolbox.com/specific-gravities-gases-d_334.html

There are hundreds more.

The OSHA reference comes up first on a Google Search and is often quoted, but wrong. SG(air) is simply the ratio of the MW of the gas divided by the MW of air. Who you going to believe, Praxair or OSHA?

David

 

[Proton10]

Based on the ideal gas law, it should be a function of MW. 1 mole of gas at STP is a constant volume, and the weight is equal to MW, so the SG is proportional to MW, at least when the gases are behaving close to the ideal gas law.

 

[Ashereng]

Oh. Okay. I stand corrected.

I didn't think that SG would be tied to MW, but perhaps to molecular structure also.

Sorry about that.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?