Flow meter selection help 1

[Prototyp]

A flow meter should be placed in an grinding oil supply pipe.

System data
Fluid: grinding oil
Supply pipe: ND150 (6 inch)
Flow rate: 150 to 2300 l/min
Flow pressure: 3.5 to 8 bar

In this case I can only chosse between paddle wheel and vortex type, so other flow meter types don't need to be considered due to contractual resasons.

In this context 2 questions:
1. What is the better option regarding the given sytem data: paddle wheel or vortex type?
2. Which nominal diameter should the flow meter be. Dose it have to be ND150 because the pipe is ND150 or smaller size also possible respectively recommendable?

 

[danw2]

Both paddle wheels and turbine meters have vanes that rotate, but paddle wheels are cheapies and turbine flow meters are industrial grade. Pay your money, take your choice.

Both turbine and vortex are velocity meters whose measurement capability and accuracy is great at high velocity but performance and accuracy drops off at the low end of the velocity scale. In fact, the vortices that a vortex meter measures drop off so much that vortex meters have rule-of-thumb low flow cutout at 1/10th of max flow rate, where the reported flow rate is zero for flow rates 10% of max or lower. Now the actual low flow cut-off point is application-specific, 10% is rule-of-thumb. But beware because the your low range of 150 l/min is at the point that I'd be concerned whether a flow rate in that low area would be reported as zero or 150.

Any legitimate flow meter vendor will 'size' the meter for your conditions. It is common with velocity meters to size the meter lower than pipeline size to achieve a temporary higher velocity for a better measurement. That comes out in the sizing process. Of course, the question then is, how much pressure drop can you tolerate?

A typical vortex accuracy chart looks like this:

Carefully check the lower left corner. That is not cartesian coordinates (0,0), it is (15.2,1.6), the minimum flow rate. Any lower flow rate is reported as zero. For this particular meter, 75 was max, 15.2 was minimum so this meters low flow cut-out 1:5, not even the rule-of-thumb 1:10.

Turbine meters like clean liquid. Dirty liquid will kill the exposed bearings and degrade accuracy before causing outright failure. I don't know if grinding oil is clean or dirty.

 

[LittleInch]

I agree with all that.

A turndown of 15:1 is not easy and you will need some sort of PD type meter to do this to any level of accuracy.

Do you mean a paddle wheel like this?

https://www.blue-white.com/paddlewheel-flowmeters-and-why-they-are-popular/

They have flow velocity limits a bit like the vortex meter so both might just not pick up your low flows. I can't be bothered to work out your velocities, but see whether it matches the requirements

Also be aware for vortex meters you need to be well above the bubble point of your liquid or they don't work either (been there)

Most meters are smaller because they cost less, the pressure drop isn't that big and the higher the velocity, the better most of them work.

why are you limited to these two? if all you have is an insertion point there are other types available, but just because someone wants a meter to do 15:1 at some unknown level of uncertainty, doesn't mean it can be done if you limit the type of meter.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

You may have to select a 3inch vortex meter (VM vendor to confirm) subject to pressure drop considerations, if you wish to enable getting a better turndown, but even in the best case, agree it would be difficult to get down to 230litres/min. Vaguely recall you can ask for the VM not to auto cut out at 10% of calibrated range in this application, but continue down to say 200litre /min, and accept a reduced accuracy below 10% of calibrated range.

 

[Prototyp]

Thanks for advice.

Littleinch: yes, something like that

Is there some rule of thumb how far the flow meter diameter can be reduced compeared to the pipe diameter?
What is the limiting criteria, is it head loss?
For example is reduction from ND150 to ND100 ok and how much head loss would result from this reduction?

 

[danw2]

There's probably some velocity limitation inherent in bluff body/vortice formation physics, but I don't know what that is.

Vortex is tough for straight run compliance to meet an accuracy spec because Vortex is very sensitive to flow profile perturbations. Vendors advise minimum 20 diameters upstream of the meter for a pipe reducer.

Go ask your flow meter vendor to size a meter (he'll want to know fluid viscosity and density at flow conditions) and he'll tell print you out chart of pressure drop over the flow range.