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Thread: Measuring Conveying Velocity in a Dilute Phase System

  1. Measuring Conveying Velocity in a Dilute Phase System

    Hi:

    We have a number of conveying lines for plastic pellets, fluff, or chips. Occasionally, line clogs occur which are directly attributable to either the performance of the receiving bin vent, or the condition of the air intake filter.

    I would like to add an early warning system, which monitors the velocity in the conveying line, and tells the technicians when the feed conditions have become marginal.

    Solids in the airflow are tough on pitot tubes, so I am looking for an alternate method. I’ve thought about measuring the pressure differential across the air mover, but I’m not sure if this would give me what I need.

    Any recommendations?

  2. #2
    You will need to know the volume of air coming in from the blower, the volume of air that would leak from the rotary valve, and the internal cross-sectional area of the conveying line. With this information you can write an equation that would calculate the conveying velocity and display it on your control panel. In this calculation you would have to consider compressible gas flow.

    I can help you in this calculation if you would like.

    Regards,

    Amrit Agarwal (Tim)
    Pneumatic Conveying Consultants
    polypcc@aol.com

  3. Tim:

    There are ways to do this, which are more accurate, and more difficult. We already have Dwyer magnehelics on the pressure side of the air movers.

    I therefore already know that, in general:

    Less than 10 psi = OK
    10-20 PSI = something is amiss.
    Over 20 PSI = line clog imminent!

    By installing transmitters & tying into our DCS, I might have what I need.

    I was just hoping that by converting to velocity, the effects of pipe resistance, material feed rate, and other line unique variables could be negated. One line may be OK at 5 PSI, and another at 15 PSI.

    Would air flow meters give me a more accurate number?

    Thanks again.

  4. #4
    Dear Bob,

    If you have a positive displacement blower, its incoming volumetric flow rate is fixed. Its discharge flow rate will depend upon the pressure at the blower outlet. You already have on-line pressure measurement. You can use this pressure measurement to convert the intake flow to discharge flow. This could be done by a simple algorithm stored in the DCS.

    Using a flow meter will do the same thing if that would cost less.

    One thing to remember is that solids velocity in a conveying line is always less than the gas velocity. By the flow meter or through the DCS calculation you can get the gas velocity but not the solids velocity.

    Solids velocity in a conveying line depends on a number of variables such as particle size, shape, density, pipe line bends, etc. Let me know if you have any questions.

    Regards,

    Amrit Agarwal (Tim)
    Pneumatic Conveying Consultants

  5. #5
    Dr M Bradley Guest
    I agree that use of the air mover characteristics together with rotary valve air leakage characteristics and measurement of pressure is simplest.

    I don't agree with the statement above that the volume of air taken in by the air movers is constant, though.

    Get a copy of the blower characteristics from the manufacturers, and take a look at it - you'll see that volume drawn in reduces as pressure goes up. However, you can write an equation for that characteristic (based on a straight line equation against pressure) then subtract the rotary valve air leakage (again dependent on pressure - easily calculable from my equation in Proc IMechE part E Vol 214 pp 185-196). That gives you the free air volume going into the pipeline. Divide by the absolute pressure in bar (gauge pressure in bar plus 1) and divide by the pipe cross sectional area, and bingo - you have the line inlet velocity. Programme the calculations into a calibratable display connected to a pressure transducer and you get a continuous display. Easy peasy. Let me know if you need more help with it!

    Mike.

  6. #6
    For lobe type positive displacement blowers the incoming gas flow depends upon the slip or "leakage" between the rotor tips and the casing. This slip increases as the discharge pressure increases resulting in reduced incoming gas flow. Blower curves given in the vendor catalogs are based on this slip flow. Most vendors can also give equations for calculation of the slip flow based on the standard clearances.

    The discharge flow from the blower depends upon the discharge pressure and temperature.

    Regards,

    Amrit Agarwal (Tim)
    Pneumatic Conveying Consultants

  7. I would suggest to install particle moving detectors in your line.
    This detectors are available in different sizes and types.
    The simplest are adjusted by your Pls and indicate if the particle flow out of the desired range. This sensors are available for about 700 $ or less.

    If you want to determined the velocity exactly you have to install a more comfortable system (the sensors are the same).

    Please visit the following websites for more informations:

    http://www.dynatechnik.com

    or

    http://www.swr-engineering.com

    Best regards
    Klaus Schneider (schneider@enviro-engineering.com)

  8. #8
    Its not possible to measure the solids velocity in pneumatic conveying systems. We can measure the gas velocity but not the solids velocity.

    I think that what you have is for measuring emission of solid dust particles from dust collectors, etc. to comply with emission regulations.

    Regards,

    Amrit Agarwal lTim)

    Pneumatic Conveying Consultants

  9. Dear Mr. Agarwal,

    It seems that you have not visited the websites I recommend in my last post. Please have a look for example to the Dynatechnik-website. They sell equipment for measuring particel velocities since several years.
    The technique is not new and well known since about 20 years.



    Best regards
    Klaus Schneider

  10. #10
    Dear Mr. Schneider,

    I am fairly conversant with devices such as made by your company. I have not seen applications of these devices in pneumatic conveying lines. It may be a good idea if you give some examples of commercial installations where these devices are being used for ACCURATE measurement of solids conveying velocity in pneumatic conveying.

    Best regards,

    Amrit Agarwal (Tim)
    Pneumatic Conveying Consultants

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