Dear Faisul.

This information can be asked from the bulk truck manufacturers or from the company that is going to serve you with the cement shipments or the customer, you work for.

Engineering is also gathering data from the right source, if it was only to be sure that your design matches the plans of the user and that afterwards no discussions can arise about misunderstanding.

Your existing bag filter (pulse air cleaned) of 160m3/min should have a filter area of approx. 80 m2.

A de-dusting filter load for cement is approx. 0.5 to 0.75 m3/min, resulting in a total convey air volume of 0.5 * 80 = 40 m3/min to 0.75 * 8 0 = 60 m3/min.

A modern standard cement bulk truck has an onboard compressor of approx 15 to 18 m3/min.

The number of trucks that can load the silo simultaneously is therefore 40/18=2 to 60/18=3.

This compressor volume is sufficient for a 6” pipeline.

Do not forget the silo pressure safety valve.

Success

Teus ■

Beware Faisal,

The truck driver will open his compressed air valve fully when the product has been delivered, if he is in a hurry, to empty the residual air in his tank. The silo filter needs to be oversized, maybe 2 or 3 times, to accommodate this flow.

Michael Reid. ■

Faisul,

The tanker driver should be instructed to relase the residual compressed air in the tanker gradually to avoid damaging the filter, but that is an unrealistic expectation.

The sizing of the filter depends on the volume of the tanker, the pressure and the rate at which the tanker is emptied (after product delivery).

A rule of thumb is to size the filter to handle 4 times blower flow or calculate the flow based on 125 m/s in the delivery pipe.

Good luck,

Michael Reid. ■

Dear Faisul,

I assume from your reply, that the (self cleaning) filter assembly is without an extraction fan.

That means that there will be an overpressure in the silo when bulk trucks are unloading.

This over pressure (safe guarded by the safety vent) has to be kept within safe limits.

In pneumatically loaded cement silos, the rule of thumb is a filter area of 2 times the airflow with an extraction fan of 2 times the filter area.

That means for your case:

Area = Number of trucks * 2 * 18 = 36 * Number of trucks = 160

Number of trucks = 160/36 = 4.4 cement trucks.

However, the extraction fan (not existing in your installation) should reduce the over pressure in the silo and for reducing this over pressure by a factor 2, the incoming airflow is to be reduced to approx 70% (0.7 *0.7 = 0.5) or 3 bulk trucks simultaneously.

The effect of the tanker, emptying the residual pressure, which, as Michael stated already, can be significant, depends on the free volume in the silo, the volume of the tanker and the residual pressure in the tanker after the cement is completely unloaded and the pipe size and length.

Normally, the residual pressure has dropped already because of the reduced cement flow towards the outlet and the discharge pipe act as a flow resistance, ramping the pressure gradient.

Therefore, the safety vent is crucial in such an installation.

In practice, these rules are not always obeyed and I have seen small silos with a 25 m2 filter area, without a fan, operating with standard bulk trucks that worked fine. (for most of the time).

Take care

Teus ■

Hi

There are a few rules of thimb going around

De dust volume of 50m3/hr per tph of conveying . so 50tph unload rate > at A/C 1.5 >> 27m2 of filter area

2 x FAD of the blower (min) plus calculae at the line temperature and not ambient

20m2 min or preferably 30m2 filter area per tanker unload line

You could look at the peak flow at tanker end of unload - lower line resistance > large tanker volume.

So..I think that 30m2 seems to be OK per unload line.

Tues > do you have experience with fan less silo vent filters....I am looking at a grain silo conversion to fly ash and inter venting ducts are going to be $$, so thinking of bin vent filter on each silo and no fans (save 8 fans/motors/drives)...any wise words.

Cheers

James ■

James,

The relation between the de dust volume and the tph is the Solid Loading Ratio.

And the SLR is lower for longer pipelines.

Therefore, this rule of thumb is not reliable.

That is the rule of thumb that I used also, although with the use of a fan.

Applicable for the average bulk truck unloading station and therefore not safe for other than average installations.

Almost all cement storage silos at concrete mix plants are with fan less silo vent filters.

The silo pressure, stabilizing at a filter airflow equal to the conveying air flow, is used to force the airflow through the filter.

A dirtier filter requires more silo pressure.

The over pressure in the silo requires a non leaking roof and roof connection, to prevent dust escape from the silo

A fan should keep the silo pressure negative to prevent dust emissions through possible gaps and at the same time create a buffer for transient air surges, generated by a bulk truck releasing 45 m3 air at 1.5 bar pressure in a few seconds and thereby keeping the silo pressure within the structural limits.

The filter pressure drop is delivered by the fan and divided in f.i 250 mmWC vacuum in the silo and 250 mmWC over the filter.

If the maximum silo pressure is +250 mmWC, then the fan increases in airflow until the new airflow creates a higher filter pressure drop and the in and outgoing airflow equalizes again.

In the absence of a fan, the filter pressure drop should be chosen approx the pressure drop with a fan, to cope with the air surges.

Pressure drop is proportional to the square of the air flow, resulting in SQRT(0.5) = 0.7.

Thus, for a fanless filter the rule of thumb becomes 2/0.7 * FAD = 2.8 * FAD

Balancing investment $$ against equipment and possible consequences (dust clouds) is a matter of evaluation.

Installing filter fans is the safe way to go and avoid risk and arguments later on.

Success

Teus ■

Dear Faisul,

Already replied:

Your filter area = 160 m2

Your filter fan = 330 m3/min

Resulting in a filter load of 330/160 = approx. 2, which is OK

The number of cement tanker trucks that can unload simultaneously is:

Area = Number of trucks * 2 * 18 = 36 m2 * Number of trucks = 160

Number of trucks = 160/36 = 4.4 cement trucks.

Reducing the rule of thumb factor (2) a bit results in maximum 5 bulk trucks simultaneously.

Then you need 5 pipelines, one for each truck and do not forget the silo pressure safety valve.

Take care

Teus ■

Dear faisul,

This rule of thumb works well for fly ash.

Though, fly ash has a smaller particle size and therefore the filter fabric or filter material is usually chosen denser for fly ash than for cement.

Your calculations are correct.

The filter size is OK

The fan volume is 3.3 times the convey air flow, which under normal unloading conditions should also be OK. (A rule of thumb is not absolute)

However, the silo being relatively small, the pressure fluctuation in the silo due to the air surges from the tankers at the end of their unloading (especially when 2 tankers are reaching that stage at the same time) could be more. Even so when you increase the fan capacity.

High and high/high level testers can keep the minimum volume above the silo content to a required value.

The air surge at the end of a pneumatic bulk truck unloading has to be considered the worst situation in the unloading process.

Depending on the silo size and the minimum void above the full silo content determines whether you can run 2 or 3 unloadings at the time.

If there are 2 unloading bays, then you can unload 4 bulk trucks/hr, where you need 3 bulk trucks/hr.

Have a nice day

Teus ■