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Thread: Pneumatic Conveying of Bulk Materials

  1. #1
    snorcros Guest

    Pneumatic conveying of bulk materials

    I am working on a project to increase the rate of off-loading of finished product (dry, granulated) from a bulk road tanker (~25 Te loads). Currently discharge is carried out at 8-12 psig and can takes between 1.5-2 hours, depending on the reception silo design and location relative to the off-loading point.

    Discharge is currently carried out through a 4 inch diameter pipe. Increasing the pressure (to say 28 psig) will increase the rate of off-loading, but does anyone have any practical advice as to the effect on the silo with regards to over-pressurisation? Does higher pressure discharge require increased dust filter area in order to prevent over-pressurisation? Are there any dense-phase systems in use for off-loading road tankers?

    I would be most grateful for any suggestions or comments.

    Many thanks,
    Steve.

  2. If you have a granular material it is unlikely that you will be able to achieve dense phase conveying to off-load your tankers. Dilute phase conveying is probably your only option and for this you will probably need a conveying line inlet air velocity of about 15 m/s as a minimum, regardless of the conveying line inlet air pressure. So if you use air at 12 psig you will need about 450 cfm of 'free air'. If you want to use air at 28 psig you must have an air supply of about 725 cfm in order achieve the same pick-up velocity.

    Filters are rated on the volumetric flow rate of air expected and the pressure drop across the filter for that flow rate will be in inches of water and so over pressurisation in this situation will not be a problem if the filter is maintained. You should not overload the filter, however.

    The material flow rate that you will get through your pipeline will certainly increase with increase in air supply pressure but only if you maintain a similar pick-up velocity. The flow rate achieved will then depend upon conveying distance, the number of bends and vertical lift. With 12 psig I would expect about 10 tonne/h through a 300 ft long pipeline with 8 bends, increasing to 24 tonne/h with 28 psig. If your pipeline is shorter than this you should improve on these discharge rates. This, of course, is little more than a first approximation, for the flow rate depends very much upon the properties of the material being conveyed.
    David Mills

  3. #3
    Cement is routinely offloaded from road tankers in dense phase with a pressure of about 2 bar at coveying rates of between 70 to 100 TPH.

    Please send further details of your product so we can see what can be done. Petrochemical pellets have been conveyed in dense phase albeit with a relatively low material to air ratio ( about 20)

    Please visit our website www.scorpioengg.com and send us an e mail at scorpioengg@vsnl.com

  4. #4

    Truck Unloading

    The rate you are reporting is not out of line with the line size and pressures. Not knowing other details such as material characteristics, airflow, length of line, connection type to the railcar etc. it is difficult to give a detailed analysis. Adding higher pressure conveying air is not always a solution and can cause problems such as leakage at the pick-up, excessive wear in the convey line etc. At the receiving tank, you should be at atmosphereic pressure, so the silo should not be in danger unless the filter is undersized, which is a common mistake in unloading systems. Most engineers don't properly account for the conveying air and don't recognize the problems at the end of the convey cycle.

    In most cases the unloading rate can be increased by a couple of different methods. If you want to contact me at rwalsh@deltaducon.com with additional information, we can discuss the options.

    Visit our website at www.deltaducon.com

  5. #5
    Smoot_West Guest

    Pneumatic Conveying

    Increasing the pressure in the tanker would allow you to increase your truck unloading rate, assuming all of your hardware would tolerate this adjustment. Most PD trucks in the U.S. (and some internationally) are rated only at 15 psi, if this is the case with your trucks, other solutions would be necessary to increase your unloading rate. Most PD trucks have a pressure drop through they truck of about 2-3 psi, which if you have a 15 psi truck and air supply (pd blower?) you have about 8-10 psi of pressure to actually break the product loose and begin steady state conveying. As has been mentioned by some of the others who posted responses to your question, one of the factors used when calculating line lossed is the material friction factor. This is a factor that can change dramatically the level of pressure necessary to convey XX lbs / min through XX feet and XX elbows through a X in. dia. pipe with a material pickup velocity high enough to convey your product. The factors that you control are distance, elbows, and pipe diameter. (Example) If you are getting only 12-15 Te / hour unloading rate through a 180 ft. line w/ 5 elbows @ 12 psi with an average product, you may be able to achieve the same rate through the same distance at about 2 psi less pressure if you pipe run could eliminate 2 ebows, the other side of this is that you could maintain the pressure and likely increase your convey rate by about 27%. There are other methods that may yield a real increase in unloading rate, feel free to give me a call.

  6. #6
    Xavier Meyrigne Guest

    Truck unloading

    I can help you for your problem of unloading, I am specialised for pneumatic transfert and mainly for pneumatic unloading of wagons and trucks. I have done special applications in dense phasing for trucks unloanding, long distance unloading of trucks (about 500 meters with less than 1 bar), automatic unloadings stations for trucks an wagons, unloading of dangerous marterial...

  7. #7
    Xavier Meyrigne Guest

    Truck unloading

    I can help you for your problem of unloading, I am specialised for pneumatic transfert and mainly for pneumatic unloading of wagons and trucks. I have done special applications in dense phasing for trucks unloanding, long distance unloading of trucks (about 500 meters with less than 1 bar), automatic unloadings stations for trucks an wagons, unloading of dangerous marterial...

  8. #8
    Xavier Meyrigne Guest

    Truck unloading

    So, it seemed difficult to finish these explanations, To help you, it will be necessary to know the material to unload, the geometrie of the circuit, your actual surface of filtation and techic of cleaning for filters..., you can contact me at xavier.meyrigne@wanadoo.fr or fax 33 (1) 42 38 00 76 or phone 33 (1) 42 38 00 25 to give you an help adapted to your exact problem.

    Best regards Xavier Meyrigne

  9. #9
    Author Guest

    Pneumatic Conveying

    With regards to over pressurization of the silo I suggest the following:
    1) Calculations must be made to see if the silo can handle the pressure.

    2) Increase in dust collector capacity is very critical. Many pneumatic conveying supplier to do not take into factor the expansion of the air as it leaves the collector through the venturi's and bags.
    For example, truck with a blower for cement - normal operation 100-200 SCFM @ 8PSI. When the "blows off" (empty and clean the truck) it can go to ~ 500 CFM @ 15 PSI. Normally we use a 4 to 5:1 air-to-cloth ratio for cement. This ratio is very acceptable to the cement industry in the US. 500 CFM divide by 5 = 100 S.F. of material. However when the air from pressurization expands into the clean air plenum of the collector it will expand. This corrected figure is ~ 1,500 CFM @ 1-2" w.g. Now we call for a collector that requires 250 S.F. of area. Bin Vents or collectors for this service have to be a least 225 S.F. by law of South Coast Air Quality Management District for years now. This is the governing agency in So. Calif.
    So for 28 PSI take the SCFM of the system and multiply by 4 times (aprox.)

    3) To the best of my knowledge these are companies that can do truck unload with dense phase.
    A) Smoot Co.
    B) Cyclonaire
    C) US Systems
    D) Macawber.
    E) Dynamic Air?

    Good luck. Write to me if you need more help.

    Ross Jamison


  10. #10
    A cost effective method to increase truck or railcar unload rate and decrease unload time is to increase convey line size. Along with increased line size a automated air bypass system is required around the truck to keep air velocity within limits on the 4" truck portion of the unload line. A different blower would be required for the increased air flow. Convey pressure would remain in the 8 - 10 PSIG range. Silo filters would need to be addressed for proper sizing.

    If you would care to discuss further please e-mail me.

    sporsd@macequipment.com

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