My Bench..... taking ages!!!

[jsmith]

[98-1074649673]

Well in the Spirit of R&D why not build your bench the way you feel it is going to be most accurate for your application and give feedback on whatca find? Once you have something built you can always work on your design and refine it. Just make sure you make the bench so you can modify it to test different design ideas. I think thats what most everyone here who built their own bench have done. I don't think anyone has "got it right" the first time out.

I know personally it took me a couple of years till I got to the point I am now with the accuracy I am comfortable with. But, I'll probably still be making changes as I build my new bench.

A divergent cone entrance will have a big effect on your flow through the orifice plate! Thats the whole idea behind velocity stacks. You want to stay away from this in your design you do not have enough length to get the flow back to usable velocities before entering your orifice plate. This is the whole basis on my testing on venturi and velocity stacks on my carb R&D work. Yes u can make more flow go through a specific size hole then the math says you can.

[Terry_Zakis]

Hello Fellow Flow Bench Enthusiasts,

I'm new here, but new to flow, engineering, and motor work.

I think that jsmith is really onto something here with the concept of the flow inside of a pipe, as compared to plenum flow, especially when using an orifice plate. The same would apply to all other bernoulli devices (venturi, pitot tube, anubar, etc).

The previous unregistered guest was also exactly correct, in stating that the Superflow style (and MSD type) benches will show differences in flow, with changes in head orientation on the bench. This is due to the proximity of the orifice to the flow discharge, and the fact that the flow is not stabilized before entering the orifice plate in relation to the plane of the pressure taps.

I've been a practicing Engineer for 15 years (power plants, steam turbines, heavy-duty gas turbines, and more recently aero-derivative gas turbines of Pratt & Whitney), and from all of my research, there are only two industry recognized codes for measuring flow with orifice plates in closed pipe. There is a separate code for plenum flow that the vacuum motor people use, but that's a separate case, and I don't think it can provide the level of accuracy of a properly set up orifice system.

The most recognized code is the ASME MFC-3M, which deals with measuring flow inside of closed pipes, with orifice plates. Another piping designer which I read comments from (in another post here) was also onto this with estimations of straight lenghts of pipe before orifice plates.

Estimating, compressible gas flow accurately is the most difficult thing to do in process measurement, as there are numberous chances to introduce uncertainty into the measurement.

The beauty of the MFC-3M code, despite it's complexity, is that measurement of flow within closed pipes, that follow the code requirements, can determine flow within +/- .5 to .8%, depending on the Beta Ratio (Orifice ID / Pipe ID), and most importantly, do that without a separate calibration.

Frankly, and with all due respect to those running benches, I don't believe all the hype circulated about being able to measure overall flow to within 1/2%, as so many claim. The +/- .5% to +/- .8% that you can acheive from the MFC-3M code is also based on an U95 Uncertainty. Meaning that there is a 95% probability that the "true" flow value is within a band of +/- .5% to +/- .8%, but this is also dependant on the beta ratio, the uncertainty of the differential pressure transmitters, and gas temperature measurement (i.e. Superflow type benches that heat the air stream).

The need to be so close on determination of flow is typically called "custody transfer" and is the highest level of flow measurement accuracy available, and is used for flow measurement of fuel flows (for power plant and gas turbine acceptance testing). So if specially designed flow sections that determine pass/fail criteria on multi-million dollar contracts can only measure within 1/2% at best, we shouldn't go on fooling ourselves that we can do better.

On a practical side, the key of course is repeatability. I think we can all live with a bench under or over reporting flow by 1%, as long as the data is repeatable. That way you can determine changes to port flow on an accurate basis.

As we comtemplate these flow determinations, keep in mind that flow straighteners are a big help. Watch for recommendations that call out to have 10 upstream lengths and 5 downstream lengths, without disturbance, from your orifice plate. Most deisgners are aware of this, but the fine print states that these distances are required when suitable flow straighteners are in alredy in place, and when all all bends are in the same plane.

There are published data, ASME Fluid Meters, Part-I, which you can't even purchase anymore, that show flow can be upset for as much as 80 diameters, after out-of-plane turns, without flow straighteners.

For these reasons, I think use of some type of plenum is beneficial, so that your flow measurement is not affected by the discharge from the port. Transitioning back into a pipe smoothly is another factor, and then you still need to get your straight lengths in.

Sorry about the long post. Wish it were more simple. Best case is to develop a flow bench, and use a separate flow "prover" system to tell you what the bench is doing. This is how I'll be setting up my flow bench/lab. I'll have one wall of my shop (26 ft.) which is have ASME MFC-3M, code compliant flow sections, with four pipe diameters, four different orifice sizes to optimze Beta ratio. Air flow will be routed in series, from through the provers, and then through the flow bench. This way I'll be able to try different designs and see which more closely match the flow measured by the provers.

My flow bench? Not there yet. Almost. I've spent over $7,000 in the last 1.5 years acquiring all of my components and doing research. The only thing left for me now is to decide on the geometry of my design. I will be using orifice plates.

Well back to work!

Best Regards,

Terry_Zakis

:O

[84-1074663779]

All of what you say is valid, but we amateur home flowbench constructors are working under a few practical difficulties that the professional engineer may not have.

Probably the biggest difficulty is gaining some sort of verifiable result without access to an accredited gas flow laboratory, or expensive calibrated flow reference standards.

Probably the easiest check is a home made very thin sharp edged orifice plate that has either been punched or turned on a lathe to a precise well finished round size. This can then be used as a test piece flat on top of the bench with still ambient air upstream of the orifice plate. This will come the closest to ideal measurement conditions that each of us can duplicate and be reasonably sure that we are all going to see a very similar result with the identical test setup.

The real problem comes with the measurement orifice or pitot tube within the bench. My own philosophy is to try to duplicate the same conditions for the measurement orifice as above, that is to have a very large settling plenum before the measurement orifice. If you do it right, it is possible to get almost identical pressure drops across both orifices, and if you get that far, the flow of different sized orifices will be predictable from diameter alone. For instance one orifice 1.4142 times the other will have exactly half the pressure drop, and this can easily be tested and confirmed. Also, the orifices can be swapped over and the figures again confirmed.

This sort of thing is something that only requires patience and a bit of time, and each of us can check his bench over a wide range of orifice sizes. If practical measurement agrees with the results predicted from theory, it is a good way to gain confidence in your bench.

If you place the measurement orifice in a pipe there are all sorts of correction factors that need to be applied, and my thinking is that the more the numbers need to be fiddled, the greater the danger of significant error. As you mentioned, the Beta factor of pipe diameter over orifice diameter is an important factor as well as upstream flow conditions in the pipe. There are also the physical problems of building ten or more straight pipe diameters without using acute elbow bends.

In a commercial process monitoring situation where fluids are racing down pipe that may be hundreds of feet long, flow conditions may be a lot more stable than in a table top sized flow bench with several acute pipe bends.

Any measurement should be as simple direct as possible. For instance, water manometers are ideal. Water density and gravity do not change much so the results are going to be very stable and repeatable with very basic equipment. Some sort of fancy electronic instrument that reads to five digit resolution might be telling you lies, you have to just assume it is correct.

The same with some flow formula that requires you to plug in six different measurements and than multiply by some "coefficient" fiddle factor to get a final flow number. Maybe you can trust the results, and maybe not. But how do you know ?

I believe the KISS principle applies here, "Keep It Simple Stupid", and the more simple the design the more you can trust the results.

[Terry_Zakis]

Tony,

Thank you for your feedback. I admire your posts here, and a lot can be said of keeping the approach more simplified.

As you can gather by now, struggling with where to make comprimises has been one of my greatest challenges in moving to construction on this project. I'll be keeping your advice in mind.

Thanks Again,

Terry

[bruce]

Terry welcome to the forum! We have such a varied experience level here and value everyone's input.

The posts that are showing up as Guest unregistered are mine somehow my name got deleted from the members and I had to re-register. When I did that it did not go back and change the guest back to Bruce . . . UGH

[84-1074663779]

Yes welcome Terry, your professional experience is extremely valuable to us here. I think most of us are learning as we go along, and feel a bit intimidated by the maths and the thermodynamics, I know I certainly am.

On one hand following what other people have already done is probably not a bad way to go about it, I think there is also a lot to be gained by being a bit innovative.

Two areas that I think can be improved upon are blower design and basic bench layout.

Everyone seems to use multiple vacuum cleaner motors, I am probably the only person here running a single large centrifugal forge blower at high RPM. While something like a Vortech centrifugal supercharger would be ideal, it is just too expensive for most of us. But a large diameter junkyard forge blower might be found very cheaply.

The other thing is that nearly every bench design seems to use some complex method to reverse the flow through the whole bench. My bench flows in one direction only but has two test holes with one sucking and one blowing.

I am sure there are many other ways to improve things or do it slightly differently. This Forum is a great place to swap and discuss ideas, thanks to Bruce's hard work.

[Terry_Zakis]

Thank You Bruce and Thank You Tony.

####, alot of it's intimidating to me. I worked professionally as a Plumber for 5-years before I went for my Engineering Degree! So not much of this come easily, which is why I've been looking into this in my spare time for over a year, and don't have the bench built yet!!

I always like to have a feel for what I'm doing, which is why I respect your straightforward approach Tony. Your experience in being on your third build is quite valuable to the group.

Tony, I like your idea of the single blower. I looked for one for a while and didn't find anything suitable that wasn't prohibitively expensive. I also like the idea of flowing in only one direction, but my thoughts of using the flow provers in series with my bench (for research) keeps me away from one-direction only. At least at present. Once I convince myself on a few things I may go to a single direction.

On the matter of vacuum motors, I've done a lot of work in this area and have 2 Excel files that I'll post here, and also in the Motor Help section. Let me know if there is another place they should be posted or archived.

Heck. Didn't work. How do you post Excel files?

Best Regards,

Terry_Zakis