Flow calculation acc. to ISO 5167-2 with too low Reynolds number 1

[finnp1234]

Hello

I measure flow with an orifice, and I am calculating the flow according to ISO 5167-2. However at some flows, the Reynolds number for the pipe is not large enough according to ISO 5167-2, but I would like to know if it is possible to calculate the flow despite of that, or perhaps calculate the increased uncertainty.

My system is:
Pipe ID=56,3 mm
Straight pipe length before orifice = 1200 mm
Straight pipe length after orifice = 600 mm
Orifice ID = 12,83 mm
Media = Air with a relative humidity of 40 %
Temperature = 20 °C
Pressure after orifice = 1013 mb
Orifice type = Corner tapping

I would like to measure at Dp 100, 300 and 600 Pa.

Thank you in advance.

 

[BigInch]

You can always calculate somthing, but flow must be within the prescribed range of validity of the formulas that you are using in order to get a valid answer. If flow through an orifice meter is too low, its accuracy will not be sufficient to use the resultant indications with any authority.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[hacksaw]

i have to agree with BigInch on this one, beyond that it is not really a measurement suited to orifice metering.

 

[btrueblood]

ISO 5167-2 will tell you if you can calculate the uncertainty or not using the standard. Generally, you need Re_D >= 5000. What Re are you calculating?

 

[hacksaw]

seems to be reynolds of 3000-4000 based on his max d/p

 

[finnp1234]

For Dp orifice 100 Pa Re(pipe diameter) = 1539
For Dp orifice 200 Pa Re(pipe diameter) = 2165
For Dp orifice 300 Pa Re(pipe diameter) = 2645

OK, it is very low(thats why I ask)

What other measurement methods can I then use, else than mass flow meter or volumetric gas meter. We are talking about air flow in the range of 4 to 10 Nm3/h which correspond to 100-1000 Pa Dp on the 12,83 mm orifice.
Decreasing pipe diameter is no solution, as it shall be minimum 50 mm acc. to ISO 5167-2.

 

[btrueblood]

Yeah, you are below the limits of applicability of the equation for flow coefficient in the standard.

That Re range is tricky, it's in that funky region where the Cd-vs-Re curve gets peaky. You can still make an orifice meter work there, but would need to calibrate it.

"Ok, fine. How do I calibrate it?"

Well, that's tricky too. I would do so by first purchasing a good quality mass flow meter....see the problem?

There are ways to make a first-principles flow calibrator/prover. Think of a large hydraulic cylinder, whose piston speed can be varied and controlled very accurately. You thus have a fixed volumetric flow source, or sink. But it gets kinda spendy to build those in large sizes.

Another method I've used quite successfully is to capture airflow into an inverted water-filled calibrated cylinder. Ancient chem lab technique, but useful for flows up to a few liters per minute. Gets tricky past that.

 

[finnp1234]

@btrueblood:
Yes, you are right. I prefer a first-principle method, to avoid expensive external calibration costs every year.
From 1 to 6 m3/h the cheapest (and only?)way is to buy a gas meter with calibration. It is same price as for a calibration of the old gas meter. But in the range 6-12 m3 there is no miracle cure.
Below 1 m3/h I use a soap bobble flow meter which is also a first-principle.
The water filled cylinder is interesting, but sound expensive.

I once calibrated a mass flow meter on Hydrogen(approx. 1 liter/minute as far as I remember), collected the Hydrogen in a Tedlar-bag and registrated the weight loss. It was a fine solution, but would require a pretty large bag for 6 m3/h, and more Hydrogen than I would like to be in the same room with.

 

[hacksaw]

honed meter tubes are available in smaller diameters, you can also use rotameter, mass flow (i,e, thermal type), and positive diaplacement meters.

 

[BigInch]

But do check their lower flow limitations as well.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[hacksaw]

most plant usage restricts normal pipe reynolds >20000 for reasonable measurements with square edged orifice plates even in non=metric pipe