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.
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.
No man can be both ignorant and free.
Thomas Jefferson
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.
No man can be both ignorant and free.
Thomas Jefferson
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.
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.
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.
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.
We are here on Earth to fart around ......Kurt Vonnegut
You can be anything you want......As long as you don't suck at it.
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
UA Local 602
A man who says that he can't learn anything in a class is exactly right.
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.
"The only true wisdom is in knowing you know nothing" Socrates
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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Originally Posted by VTP99
