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Thread: Gas flow measurement

  1. #1
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    Gas flow measurement

    Im wanting find a universal pitot I can insert into a gas line for flow measurement.

    I think I can use a Dwyer 160F for this purpose and traverse the line for an average.

    Using compression fitting around the shaft to seal in the gas. Insert into the pipe via a 3/8 or 1/2” thread o let.


    Anyone ever pitot a gas line for a flow read out.

    I want to be able to get a fairly accurate read of gas flow for power burners to be more certain of low and high fire gas flow settings.

    Relying on stack temp and fuel pressure alone works on a new boiler when everything is clean, not as well on older equipment.

    Also, would I need to correct for specific gravity of the gas?

    For air I use CFM= duct area x velocity

    Velocity = 4005 x (square root of velocity pressure)

    What formula would I want for application to natural gas, and for CFH?


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    Following.

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  3. #3
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    I found this on Omega.com - they make Pitot tubes. But I cant figure out what to use as a dimensional Constant anywhere.







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    Really wish this covid crap was not going on, cause I usually take a test and ballance refresher coarse. And the guy that teaches the class would know this off the top of his head.

    Im pretty sure Wayne would too, but Im guessing this thread has not come across.


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    Hey heatingman I saw this post a few minutes ago. Since you would be dealing with a gas correcting for SPGR is the right approach. Where I think the confusion is that the reference you gave is talking about a single plane traverse which in that case isn't really a traverse if I understand what he said. The source referred to calculating the point of velocity which isn't done with a pitot traverse. A pitot as you know measures several velocities and they are then averaged for flow calculation.
    A pitot traverse under the best conditions is assumed to have a max error of +/- 5%. For me that would rule it out if I wanted better than that. There are sensors that are guaranteed to have an overall accuracy of +/- 0.3%. I would opt for that approach unless money is a big factor. The pitot also requires two accesses to the pipe.
    I hope this answers your question. You know a lot about TAB so I know you'll understand if I didn't say it well and if I didn't you will know how to come back.
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    Gas flow measurement

    Wayne, thanks for posting.

    The pitot in the referenced page is an averaging pitot - purpose built for a specific pipe size.

    My approach would be to use a single point pitot tool, and traverse the pipe taking several readings - like we do for duct measurement.

    What Im having trouble with is the base math formula I need to use once I have readings to give meaning to.

    Im getting lost in the math. I never went to engineering school. So Im getting lost in the formulas - or conversions.

    Mainly what I need is how to get a useful velocity number from velocity pressure as measured on the manometer.

    For standard air I use 4005 x (square root of the Velocity pressure) I measured from the gauge.

    For not standard (high temp etc...)

    I use 1096 x ( the square root of (Velocity pressure/density))

    But those formulas Im not sure work with all gases. I know they work for air, but the 1096 number and the 4005 number are based on something.

    Im determined to figure this out, even if its not feasible to do in the field.

    The answer is right in front of me, I know it.


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    Heres a link with some formulas that may be of use to my purpose.

    They have K factors in the formulas which may not apply to a single point pitot.

    But other then that, these formulas are the closest to logical I can find.

    https://www.dwyer-inst.com/PDF_files/DS-300_iom.pdf


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    Glad you explained because I thought you were going to use a micro-anemometer or maybe an inclined manometer. I have all the formulas you need and will look them up in the morning to make sure I get them right. I'm pooped right now so please forgive me. You are approaching this right and your formulas are right I'm just going to double check myself because I haven't done it this way in several years. Believe it or not I was ask one time to measure the flow in a water stream. I didn't do it but did design them a weir system that would.
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    Thanks Wayne. I will be using a Micro-Manometer for getting the pressure readings from the Pitot.


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    Sorry about last night heatingman. I had insomnia for two days and was afraid I might mislead you. I have only had that maybe three times in my life.

    The formula you want for SPGR'S other than air is

    V=1096Xsquare root (Vp/d)

    Vp is velocity pressure and d is density (lbs/cu ft).


    The 4005 equation is for air only.

    There is a formula to solve for density at different temperatures but it doesn't sound like you need it.
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    Yes/No/Maybe
    Attached Images Attached Images  

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    Quote Originally Posted by VTP99 View Post
    Yes/No/Maybe
    Those would work, but the problem for me is I would need one pitot per pipe size.

    Would be easier in the long run, but alot more money.

    Likely almost 2 K by the time I get all the pipe sizes.


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    Maybe I've missed something, but why don't you use a Differential Pressure Flowmeter? Temporarily pipe it into your supply line, perform your diagnostics, then remove it.

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    Quote Originally Posted by Artrose View Post
    Maybe I've missed something, but why don't you use a Differential Pressure Flowmeter? Temporarily pipe it into your supply line, perform your diagnostics, then remove it.
    Thats the plan, but I needed to confirm the calculation to find gas cfm, so I can convert to cfh to find btu content. And also a special pitot tube.

    If I just used the standard velocity readout from the meter, the calculation is based on air which has a different specific gravity then natural gas and therefore would not be correct.

    An Airfoil type pitot will be needed. Like this one:
    https://www.dwyer-inst.com/Product/T...bes/Series160F


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    Artrose the micro-anemometer is a differential meter. It will accurately measure pressures a lot lower than a standard diff press meter.
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    If your pitot tube has a hole in the end of the tip and you point it upstream then you will measure the total pressure. Subtract the static pressure from the total pressure and what is left is the velocity pressure (Vp) and is equal to Density * V^2 /2/gravity/144 where Density is lb/ft3 and gravity is 32.2 ft/sec^2 and velocity pressure is in PSI.

    But we measure pressure (head) in inches water, “WC and velocity in ft/min. So anyway

    V = 60*SQRT(Vp*Water_Density/12*2*32.174/Gas_Density)

    Where V is ft/min, Vp is velocity pressure or total minus static p in “WC, Water_Density = 62.4 lb/cuft so

    Finally
    V= 1097 * SQRT(Vp/Gas_Density) where gas density is measured in lb/cuft and Velocity in ft/min

    Check – air has density of about 0.07 lb/cubic foot so velocity =1097*sqrt(Vp/.07) = 4148*sqrt(Vp)
    Pretty close to heatingmans original numbers. I'm not sure why they're not exactly the same as heating mans numbers but I assume it is my choice of 62.4 lb/ft3 for water and 0.07 lb/ft3 fir air.

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  20. #17
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    Interesting problem. Seems the measurement would depend on the density for the fluid and the pressure. As y'all know, air measurement is affected by altitude (pressure) so I would expect other fluids pressure would affect the readings by affecting density.

    I can see why different pipes would need different sensors. The smaller a pipe the harder it is to get accurate. Like traversing a 4" round. I believe the pipe roughness would also play. I know it does when sizing pipe for some applications.
    The probe also would make an affect on partial blocking of the pipe.
    I would guess this stuff and the math would come with the probe.
    Doesn't someone make a device that uses a Hall Effect sensor.
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  21. #18
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    Gas Flow Measurement

    Hi Heatingman,
    I know that I am late to this party, but I have a question.
    Why don't you clock the gas meter for the building? The gas meter has a test hand, either two cubic foot or five cubic foot. You would time the test hand to determine how much time it takes to consume the measured (2 cubic foot or 5 cubic foot) amount of fuel. All other gas consuming appliances must be turned off. This is the most accurate way to determine the amount of gas flow to the power burner. The gas meter is accurate because it is what the gas company uses to bill the customer.
    I have used this method many times to test a gas furnace in a commercial rooftop unit.
    The thing to remember about measuring gas flow using the gas meter is the gas volume is in cubic feet per hour. You need to know what the gas pressure is at the gas meter, low pressure gas which is up to 14 inches of water column or high pressure gas 2 psi or higher.

    Good luck,
    Allen
    Last edited by Allen; 02-13-2022 at 08:22 PM. Reason: Left out my name
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  22. #19
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    PV=nRT
    Almost all gases behave the same at the same pressure, temperature and volume.

    So, if you have 2 cylinders of gas (one air and one natural gas at at the same P, V, T), then the amount of moles (n) or molecules present would be the same. I believe the reason for changing the density correction is because you’re changing pressure, volume or temperature.

    The mass of the molecules would be different, but the same amount of molecules would be present.
    Mass of methane ~16 g/mole. Just add up the atomic numbers on the periodic table.
    Mass of air ~29 g/mole.

    So, if you’re just measuring volumetric flow, you shouldn’t have to correct if using standard T,P, aka STP.
    Measuring mass flow would be different.

    If there are any physics professors out there, please correct me if I’m wrong. It certainly wouldn’t be the first time.
    Last edited by mgenius33; 02-15-2022 at 02:19 PM.
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  23. #20
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    Quote Originally Posted by Allen View Post
    Hi Heatingman,
    I know that I am late to this party, but I have a question.
    Why don't you clock the gas meter for the building? The gas meter has a test hand, either two cubic foot or five cubic foot. You would time the test hand to determine how much time it takes to consume the measured (2 cubic foot or 5 cubic foot) amount of fuel. All other gas consuming appliances must be turned off. This is the most accurate way to determine the amount of gas flow to the power burner. The gas meter is accurate because it is what the gas company uses to bill the customer.
    I have used this method many times to test a gas furnace in a commercial rooftop unit.
    The thing to remember about measuring gas flow using the gas meter is the gas volume is in cubic feet per hour. You need to know what the gas pressure is at the gas meter, low pressure gas which is up to 14 inches of water column or high pressure gas 2 psi or higher.

    Good luck,
    Allen
    Im familiar with that procedure, however:

    Most buildings I deal with dont have dedicated meters to the boiler, so no real way to know whats using the gas and not practical to isolate everything. That and most commercial building meters have the relevant dials blocked off, and the meters are significant distances from what Im wanting to clock. Like several hundred feet away, or down at the street level when Im 20 stories up.

    In an ideal world, a dedicated meter just for monitoring flow would be installed for each boiler, or burner. In industrial and process they have started doing just that, but I suspect mainly for EPA permit purposes.

    And Im talking about large equipment mainly. Like for schools, hospitals, large buildings, etc..

    Traditionally I just go by stack temperature to find high and low fire. And its good enough.

    But Im always wanting better.

    Ive found some options, but most are either too expensive, or far less practical then id like.


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