Venturi Meter design guidance

[oxotx]

I've recently come across a Flow measurement application that seems suited to using a Venturi tube as the primary element (I need low DP and now sharp edges), I've used a lot of Square edged plates in the past but have never specified a venturi I have the proper software to do the calculations for a venturi, but is was hoping someone could give me some general guidance on the best practices for specifying a venturi. Such as what Dp/beta range you should target, How you design the pressure taps etc. Also if anyone has a comment on the best way to attach three transmitters to a single venturi I will need to do that in this application as well. And finally I would welcome any comments on who you typically by a venturi from.

 

[zdas04]

Venturi meters are the least forgiving devices I've ever encountered. You design one for X flow rate and somewhere around X+5% they go crackers (boundary layer separation causes discontinuities in the flow vs. dP graph). They are pretty good in Labs, but I would never put one in the field.

If I wanted low dP, low shear in a gas application I'd go with V-Cone. In liquid I'd look at MagFlow.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[ScotchWhisky]

I suggest ISO5167 parts 3 & 4 for guidance on venturi tubes

 

[hacksaw]

venturi's are fine, they are expensive and require engineering to insure proper installation, have put in them in 18" piping (steam)

what's your service and line size?

 

[oxotx]

The service is high pressure (850 psig) hot pure oxygen, we have various line sizes that we are considering using it it but the most immediate is a 4" line.

 

[hacksaw]

at that size, just buy a fabricated meter run in the proper materials for the service,
the throat/pipe taps are fabricated with the meter, all you need is the d/p installation

make certain you let recieving know a special order( with shipping date) is on the way, so that a designated person manages where it it stored and handled, it is not just a piece of pipe,

flow testing at the manufacturer is cheap, and you have the meter coefficient documented.

 

[Engdoitbetter]

Hi oxotx,

I'm into design of primary flow elements (my company has been working on them since 1970s), so I hope I can put you on the right track.

First of all, international standard for differential pressure flow elements is ISO 5167:2003. I would suggest you to take a look at part 1 (general principles) and part 4 (Venturi tubes).
You will notice that different limits for beta ratio and Reynolds number are stated according to the manufacturing process (casting, machining, welding). There are also some limitations on pipe inside diameter. One of the reasons for this is that a sufficient amount of calibration/flow data is needed in order to standardize geometry and flow coefficients.
Dimensions are also simply established starting from throat and pipe diameters, since inlet and outlet angles are fixed.
Flow calculations are even easier than orifice plates' since standard flow coefficient is constant (it doesn't depend on Reynolds/beta)

Pros and Cons of VT:
pros: low permanent pressure loss, shorter straight run requirements, no problems with two phase fluid (eg. suspended solids etc.)
cons: more expensive & larger than orifice plates and flow nozzles.

Outstanding references: Flow Measurement Engineering Handbook, R. W. Miller.

Regards,

Stefano

 

[oxotx]

I've been reviewing the ISO standard as recommended I noticed on the first page 5167-4, note 1 gives a caution against using Venturi's in high pressure gas service. Can anyone comment on their experience with this? Also I find no such note in 5167-3 for flow nozzles, I don't understand why they wouldn't have the same problem since they are so similar, would a flow nozzle fit the application and avoid the pitfall mentioned in the note?

 

[Engdoitbetter]

Dear oxotx,

I do not think we are dealing with a pitfall here. The note states only that, at the time the standard was being written, there were researchers investigating the use of Venturis with high pressure gases. As I pointed out, ISO 5167 is based on lots of calibration & flow data, but there are still cases (e.g. venturis with ID greater than 1200 mm) where those data are not sufficient for setting a standard. At the same time, the ISO standard does not put limits on upstream pressure, but only on Reynolds, ID, beta ratio; my guess is that the committee saw these devices (in this case machined Venturis) deviating from standard but didn't have enough evidence to support a different value of flow coefficient for high pressure gases. Hence they suggested to calibrate these devices individually, if small uncertainty is needed. To make a long story short, use the Venturi but with caution.

With regard to flow nozzles, first of all, they are not very similar to Venturi; I would say they are in the middle between orifice plates and Venturi, since they have features of both. So it is not surprising that they not suffer from the same limitations.

Hope it helps.

Regards,

Stefano