PTS Flowbench Forum

Hi, has someone tried to do an adjustable orifice? F.ex a sliding plate with an oval orifice that
slides over the orifice and changes the size. Then you have a calibrated scale of the overlapping
that represents what orifice size the positions represent.
/kimi

sounds complicated and i do not know with a tremendous amount of trial and error how you map the ever changing CD value as the orifice shape changes. Not that it could not or has not been done just not sure if there is any value add in this type design.

JMO

Rick

Hi rick,
If the slider is calibrated for five positions, then you know what reference orifice and cd it represents .
You then can easily change orifice size with quite simple mechanics and you can add more "orifices" later depending on your needs.
/kimi

Yes you can and you can also at each point adjust your calculations for the new orifice and CD
as I said above in my opinion I am not sure the complications are worth the net gain.

Rick

If you want to quickly select between various orifice diameters, best and simplest way to do that is to have several calibrated fixed orifice plates mounted in a revolving turret.
It does all become rather physically large and mechanically complex to do properly, but it is certainly one possibility if you are planning to build using water manometers.

These days, the Forum digital manometer works so well, it needa only one suitably selected fixed plate to cover the whole required range, and is definitely the simplest and by far best way to go.

The sf-1020 flowbench from superflow has an adjustable diamond shaped orifice. It can be done. It may be worth doing some searches for papers on square or diamond shaped orifices to see how they may compare on cd values.

stef-1 wrote:The sf-1020 flowbench from superflow has an adjustable diamond shaped orifice. It can be done. It may be worth doing some searches for papers on square or diamond shaped orifices to see how they may compare on cd values.

I have been following some threads on the sf-1020 and how their readings do not coincide with the earlier sf-600. Stef, you may have hit onto the issue. I did not know they had an adjustable odd shaped orifice. I have also read where the readings you get with the sf-1020 will not work with the formulas for finding other information from the airflow readings. I wish I could remember more detail about them.

Malvin and I have been talking about the cd of orifices and we both feel that the cd changes with test pressure and diameter of the orifice itself. in example a 1" orifice may have a cd of .587 and a 2" a cd of .597 and so on as you get larger. Malvin and I have both tried to photograph this phenomena but so far have not been successful. We are using a dye that shows up under UV or black light but so far have not had success. When one of us gets it to work we will post the results and videos.

John

jfholm wrote:I have been following some threads on the sf-1020 and how their readings do not coincide with the earlier sf-600. Stef, you may have hit onto the issue. I did not know they had an adjustable odd shaped orifice. I have also read where the readings you get with the sf-1020 will not work with the formulas for finding other information from the airflow readings. I wish I could remember more detail about them.

Malvin and I have been talking about the cd of orifices and we both feel that the cd changes with test pressure and diameter of the orifice itself. in example a 1" orifice may have a cd of .587 and a 2" a cd of .597 and so on as you get larger. Malvin and I have both tried to photograph this phenomena but so far have not been successful. We are using a dye that shows up under UV or black light but so far have not had success. When one of us gets it to work we will post the results and videos.

John

John, you are correct. The Cd value is made up of a number of factors; beta ratio, pressure ratio (Reynolds number), plate thickness, roughness of the bore, eccentricity of the plate to the wall, pressure tapping positions, etc.
Orifice standards set out to control plate construction, placement, pressure tap positions, etc. allowing these factors to be included in the Cd calculation so that only the beta ratio, the Reynolds number (pressure ratio) and perhaps an expansion factor are the variables.

If you build a flow bench using an orifice in a pipe you must use an iterative calculation method to determine the flow rate. You start with a "standard" Cd value, calculate the flow rate and then determine the Reynolds number through the orifice. This is used to then calculate a Cd value. The flow rate is calculated again and the value compared to the previous calculation. The calculations are repeated until the variation between flow rates is a minimum. It normally takes only 2-4 iterations to compute the "exact" Cd value and flow rate and can be easily accomplished on a spreadsheet.

The good thing about building a flow bench with large plenums is that when the beta ratio is very small, under 0.125, the Cd value vs Reynolds number is very stable over a large range. According to BS ISO TR 15377:1998, section 5.3, it means that you can use a constant Cd value of 0.596 for the flow rate calculation. The uncertainty value of this is 1% so Cd could vary between 0.590 - 0.600.

As the orifice size goes up and thus the beta ratio, the more likely the Cd value will change with the pressure ratio/Reynolds number.

I think its best to calc the biggest orifice size that you think you would need for testing and then determine the size of the plenums based on limiting beta ratio to less than 0.125.

I have just noticed that kimi had posted details similar to the above in another thread.
What is interesting, is that some years ago on the old forum, Tom V posted orifice sizes and Cd's for a Superflow 300 and 600 bench. These were:

Orifice sizes = 0.857", 1.227", 1.770", 2.507", 3.059" and 3.490"

Cd values = 0.593, 0.583, 0.593, 0.584, 0.592, 0.609

A little wider range than in my post above but overall in the ballpark.

Funny I posted that info a long time ago and have done a lot of testing since then and the numbers are still very close to being spot on.
The orifices for the SF benches are a known deal if you can measure them which I did and you have the actual superflow calibration data
which I also had for each range. And they match Roccos spread sheet calculations.

Tom V.