Pneumatic conveying of bulk materials

(not verified)
Posted in: , on 9. Jan. 2002 - 19:44

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,


Re: Pneumatic Conveying Of Bulk Materials

Posted on 10. Jan. 2002 - 12:30

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

Re: Pneumatic Conveying Of Bulk Materials

Posted on 12. Feb. 2002 - 04:15

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 and send us an e mail at

Truck Unloading

Posted on 15. Feb. 2002 - 02:34

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 with additional information, we can discuss the options.

Visit our website at

Bob Walsh
(not verified)

Pneumatic Conveying

Posted on 15. Feb. 2002 - 02:59

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.

Xavier Meyrigne
(not verified)

Truck Unloading

Posted on 15. Feb. 2002 - 03:12

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...

Xavier Meyrigne
(not verified)

Truck Unloading

Posted on 15. Feb. 2002 - 03:12

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...

Xavier Meyrigne
(not verified)

Truck Unloading

Posted on 15. Feb. 2002 - 03:19

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 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

(not verified)

Pneumatic Conveying

Posted on 15. Feb. 2002 - 04:11

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>

Re: Pneumatic Conveying Of Bulk Materials

Posted on 15. Feb. 2002 - 04:31

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.

Re: Pneumatic Conveying Of Bulk Materials

Posted on 18. Feb. 2002 - 01:33

You will need to make sure that dense-phase is the better option and whether indeed the material can be transported in dense-phase (and type of dense-phase). Also, to increase capacity, you need to make sure that the tanker and its discharge arrangement will not limit feeding rates and also that the method of aeration/air injection is suitable for the application (and avoids over-filling the line). Are there any particle attrition issues?? As for the silo/filter, a higher tanker unloading pressure will result in a higher purge air flow at end of cycle (requiring a larger capacity filter). Clean blow cycle also needs to be checked. By knowing/estimating the prime mover specs (eg steady-state air flow), tanker pressure and pressure decay rates during unloading, tanker volume, product properties, suitable filtration areas can be determined. Each case cannot be generalised and needs to be worked out properly (for the material(s) in question and the various pipeline configurations). I would look at what may be possible with the existing 4" line (before exploring larger line sizes - which would increase air flows/filter sizes dramatically). Note we have designed tanker unloading systems up to 80 t/h for granular materials (and these systems are BIG) - some tanker designs had to be uprated for this purpose. You will need to find out where the bottlenecks are. I am hoping to publish a paper later this year on pneumatic road tankers - should be useful for you.


(not verified)

Pneumatic Conveying

Posted on 19. Feb. 2002 - 08:42

Good Morning Steve.

I am not very familiar with the dense phase system. I am sure they can give

you much more accurate info.

However, let me add 2 cents worth.

Increasing the pressure to 28 psi through a 4 inch line would require more

air, or more product being forced into the fill pipe. You need a special

blower if you want to go to 28 psi.

The only time you will need to increase the filtration area that allows the

air to escape from the silo, and keeps it from exploding, is if you add too

much more cfm to the system. A filtration system does not care about the

amount of pressure you use to get the product up the pipe. It only cares

about the amount of air you are trying to put through it.

If your filtration unit is set up to handle 1000 cfm of air and you

increase your blower size to 2000 cfm, there will be a problem.

If you have a 800 cfm blower and put enough product in the line to generate

a 10 psi load on the blower, or if you have the same blower and put enough

product in the line the generate 20 psi, the filtration system is not


The dense phase system, I believe, uses less air at higher pressures.


Ken Swaving

Walinga Inc.

(not verified)

Pneumatic Conveying

Posted on 21. Feb. 2002 - 06:00

Dear Mr. Steve,

We have supplied similar dense phase systems for off-loading road tankers.

You need to have at least 50% more filter area for the bag filter. Also I

presume that the silo is having breather valve.


R.H. Kulkarni

Re: Pneumatic Conveying Of Bulk Materials

Posted on 15. May. 2002 - 03:47

Let me try to answer your specific questions:

Effect of higher pressure on the Silo:

Pressure in the silo will depend upon its back pressure. If the silo is vented to the atmosphere, its pressure will be unaffected by the pressure in the tanker. Pressure drop across the dust collector (bin vent filter) would increase due to the higher air flow. You should check the air to cloth ratio and keep it under 10.

Dense Phase Systems for Off-Loading:

Yes, there are dense phase systems for off-loading provided your material is not so cohesive that it will pack up in the conveying line and form a solid impervious plug. You have not given the name of your material or its physical properties, so it is not possible to give you a final answer.

Other options to Increase Rates:

You may look into the present air flow rates. If the air flow rate is too high, you can reduce it provided your conveying velocity is above saltation throughout the conveying line. Reduced air flow will reduce the pressure drop and allow you to increase the conveying rates.


Amrit Agarwal (Tim)

Pneumatic Conveying Consultants

(Visit our Web Page)

(not verified)

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Posted on 13. Oct. 2003 - 11:55

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