orifice flow calculations? - How to measure them
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by Dynodon » Thu Aug 26, 2004 11:06 pm
Hello to everyone, this is my first time at this forum. Just from what I've seen and read so far has answered alot of my questions. I've already built a bench from the PHR magazine article, but used a Dwyer incline manometer. But I didn't know that it would match the bench flow numbers because of the differences in the inh2o. But thanks to this sight I have seen the errors of my ways. My question is how to calculate the flow of an orifice!. I've read just about everything on this sight and am not sure what to think about the formulas. I've also tried the online calculator but am not sure that I've entered the right figures into the formula. Those values are for calculatinig liquids. And based on flow through a pipe. So what works? Thanks for all the info so far! Dynodon.
- Dynodon
- Posts: 2
- Joined: Wed Aug 25, 2004 6:06 pm
by 84-1074663779 » Fri Aug 27, 2004 9:13 pm
Welcome to the Forum, Dynodon.
That is the one big question that is a constant source of interest and discussion amongst flowbench people. The problem is that the flow conditions of the upstream air entering the orifice can have a pretty dramatic effect on the orifice flow coefficient.
There are really only two situations where a formula is going to come reasonably close to something you can duplicate on your own bench. First is where the orifice plate is mounted reasonably flush in a large flat surface, with a vast volume of completely undisturbed (room) air upstream. Provided the orifice diameter is much larger than its thickness, and the orifice is well made, flow and pressure drop will be fairly consistent.
Another way to do it is to mount a specified orifice in a long straight pipe of known stated diameter. This gets tricky. But it is useful for measuring flow in a real world engineering or process flow measurement situation.
I prefer the first method, but others here have quite different ideas. Provided there is minimum up stream turbulence, good results may be obtained if the pressure drop across the orifice is reasonably high. For instance, if your sloping manometer measures only a two inch orifice design pressure differential, the orifice will be far more sensitive to up stream turbulence than if it has a much higher design pressure differential. The problem of blower power comes into this. Generating high pressures at high flows takes a lot of blower horsepower and electrical power, the noise can be pretty extreme too.
For an orifice formula to work reasonably accurately you need to have well defined up stream flow conditions, and use as high an orifice measurement pressure as is practical.
Do not be surprised to find that say a one inch test orifice taped to the top of your bench, and a one inch measurement orifice in your turret measure quite different pressure drops. The first orifice sees undisturbed entering room air. The second orifice may flow more or less air. It might flow less because the entering air is highly turbulent, or it might flow more because a jet of high velocity air is blasting straight at it from the test orifice located immediately above.
If this is found to be the case, your measurement orifice is going to be severely effected by flow conditions created by whatever you mount on your bench immediately above it. Do the test and see. Until you can get identical pressure drops with identical sized test and measurement orifices, reliable bench calibration is not really possible.
In fact, the actual measured pressure difference between two identical orifices will give you a pretty good idea of the potential final measurement accuracy achievable of your bench once it is all set up. If the flow through the measurement orifice is turbulent and unstable, that needs to be fixed first before trying to find a final calibration figure for each measurement orifice in your turret. The largest one is always the most difficult to get right, so if that works well, all the smaller sizes will all work too.
That is the one big question that is a constant source of interest and discussion amongst flowbench people. The problem is that the flow conditions of the upstream air entering the orifice can have a pretty dramatic effect on the orifice flow coefficient.
There are really only two situations where a formula is going to come reasonably close to something you can duplicate on your own bench. First is where the orifice plate is mounted reasonably flush in a large flat surface, with a vast volume of completely undisturbed (room) air upstream. Provided the orifice diameter is much larger than its thickness, and the orifice is well made, flow and pressure drop will be fairly consistent.
Another way to do it is to mount a specified orifice in a long straight pipe of known stated diameter. This gets tricky. But it is useful for measuring flow in a real world engineering or process flow measurement situation.
I prefer the first method, but others here have quite different ideas. Provided there is minimum up stream turbulence, good results may be obtained if the pressure drop across the orifice is reasonably high. For instance, if your sloping manometer measures only a two inch orifice design pressure differential, the orifice will be far more sensitive to up stream turbulence than if it has a much higher design pressure differential. The problem of blower power comes into this. Generating high pressures at high flows takes a lot of blower horsepower and electrical power, the noise can be pretty extreme too.
For an orifice formula to work reasonably accurately you need to have well defined up stream flow conditions, and use as high an orifice measurement pressure as is practical.
Do not be surprised to find that say a one inch test orifice taped to the top of your bench, and a one inch measurement orifice in your turret measure quite different pressure drops. The first orifice sees undisturbed entering room air. The second orifice may flow more or less air. It might flow less because the entering air is highly turbulent, or it might flow more because a jet of high velocity air is blasting straight at it from the test orifice located immediately above.
If this is found to be the case, your measurement orifice is going to be severely effected by flow conditions created by whatever you mount on your bench immediately above it. Do the test and see. Until you can get identical pressure drops with identical sized test and measurement orifices, reliable bench calibration is not really possible.
In fact, the actual measured pressure difference between two identical orifices will give you a pretty good idea of the potential final measurement accuracy achievable of your bench once it is all set up. If the flow through the measurement orifice is turbulent and unstable, that needs to be fixed first before trying to find a final calibration figure for each measurement orifice in your turret. The largest one is always the most difficult to get right, so if that works well, all the smaller sizes will all work too.
- 84-1074663779
by Dynodon » Fri Aug 27, 2004 10:41 pm
Thanks for the reply Tony. Sounds like you know abit about these things. My incline is a 6 InH20 Dwyer with .826 specific gravity oil, but I will be changing it to the more expensive 1.91 oil @$30 per 3/4 ounce bottles! So that will give me a 13.9 InH20 test pressure differential. I'm also looking to build a different bench to use the pipe approach method. I've called and talked to some orifice plate manufactures about their plates and they said that you need to have 10 tmes the diameter of the pipe in length going to the orifice and 5 times the length after the orifice plate. ie: 4 in dia.*10=40 in. long and 4 in dia.*5=20 in. long . Then the pressure taps are to be 1 inch from the face of the plate on each side. This is to give you that smooth flow that you say is needed. Their only consern is the use of vaccum instead of pressure like in a gas line pipe.
My idea to check the calibration is to use a ford mass airflow meter to flow and test an orifice and read the output of the maf sensor and look at the transfer funtion for said meter, which I have for several different meters. This will be done with a 4 inch pipe set up like stated above, and tested with 13.9 dp manometer. The orifices will be the type you can buy from the manufactures I mentioned. I've found them for as little as $17 ea. Once I test this out, I'll let you all know how it works.
My idea to check the calibration is to use a ford mass airflow meter to flow and test an orifice and read the output of the maf sensor and look at the transfer funtion for said meter, which I have for several different meters. This will be done with a 4 inch pipe set up like stated above, and tested with 13.9 dp manometer. The orifices will be the type you can buy from the manufactures I mentioned. I've found them for as little as $17 ea. Once I test this out, I'll let you all know how it works.
- Dynodon
- Posts: 2
- Joined: Wed Aug 25, 2004 6:06 pm
by 84-1074663779 » Sat Aug 28, 2004 5:58 pm
Testing airflow meters is an interesting exercise. Once you have hooked up a few and compared the results it will shatter your belief that these things are either repeatable or accurate. The most telling test is to string several supposedly identical airflow meters in series and directly compare the output voltages.
They are all non linear by design, the output being some sort of curve with much greater sensitivity at low airflows than at maximum flow. For this reason alone they are pretty useless to use for anything other than the intended application on an engine.
They are all non linear by design, the output being some sort of curve with much greater sensitivity at low airflows than at maximum flow. For this reason alone they are pretty useless to use for anything other than the intended application on an engine.
- 84-1074663779
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