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Designing Storage & Conveying Systems For Problem-Free Bulk Solids Dosing

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Depending on the process requirements, a wide range of bulk solids batching, dosing and weighing systems are available in the market. It's a fact that, properly selected weighing systems will provide many years of a problem free service with minimum loss of raw materials due to improper measurement.  

By Süleyman Salihler, General Manager, Polimak Process Solutions Inc.

suleyman salihler

Süleyman Salihler, General Manager, Polimak Process Solutions Inc.

In general, end users are focused on the determination of proper configuration of dosing systems. Consequently storage and conveying systems that supply raw material to dosing systems are generally ignored or overlooked.

As it is essential to achieve the most appropriate configuration for minimum budget and shortest return of investment, these systems should be investigated in depth.

Raw Material Storage Systems

​The main points to decide initially are:

  • ​How much raw material is to be stored in the site?
  • ​How the raw materials are to be stored?
  • ​How the raw materials are to be conveyed from storage to dosing system?
  • ​How to protect the dosing system from disturbances of conveying systems?

​The amount of raw material to be stored in a site depends on the throughput of production line, ingredient delivery logistics, site location and operating budget. Dry bulk materials can be stored in silos, small temporary hoppers, octabins, sacks and FIBC / big bags.

Silos and big bags are generally preferred for major ingredients for ease of use.  If brought by a bulk tanker truck or inside a liner bag, raw materials are directly transferred to storage silos with minimum effort. If brought inside big bags, there should be a system for unloading big bags and filling the silos. 

There arise some questions about big bag handling:

- Are the big bags to be stored in a warehouse for some time?

- Are the silos to be filled as soon as possible?

- Is there enough space in the warehouse?

- How are the big bags to be handled? For example, by forklift or overhead crane?

- What about safety issues?

Plant managers should be very careful about answering these questions.

​The next question is determining the big bag discharging system. Big bag discharging systems are used for the safe unloading of big bags and filling the silos. The conveying capacity of big bag dischargers is a very important aspect here.

Equipment suppliers generally state the capacity on conveying systems between the big bag discharger and silo, however operator efficiency is much more important. Total time of handling a big bag includes big bag delivery from warehouse, lifting it, putting it on the big bag unloader, untying or cutting the discharge spout, waiting for the product to be discharged and removing the empty bag. Therefore, selection of a big bag unloading system should be made carefully. 

The same questions arise for ingredients brought in sacks or octabins. There are a wide range of sack and octabin emptying systems connected to pneumatic or mechanical conveyors in the market. Integration of these systems should also be studied in detail in order to have problem-free production. 

In addition to big sized storage silos, small temporary hoppers could also be used for storage. These hoppers are generally preferred if the total capacity of a plant is relatively low. Again, loading systems for these hoppers are to be designed accordingly. 

Conveying Systems

​All dosing and batching systems incorporate loss in weight and / or gain in weight units. These units generally have hoppers on the top to temporarily store the ingredient. Mechanical or pneumatic conveying systems are needed here to transfer ingredients from silos, sacks, big bags or octabins.

The capacity of conveying systems are to be determined carefully to supply needed material whenever required. Next, we go through the details of calculations to achieve a simple way of determining proper conveying capacity.

​Let's look at a simple dosing system that has both loss-in-weight and gain-in-weight units (Fig 1.) There are two major ingredients filled to the gain-in-weight hopper, which are then transferred to the mixer. And two minor ingredients directly fed into the mixer. Mixing starts after all ingredients are loaded into the mixer.

Figure 1. Layout of a Dosing System
Figure 2. Timing Diagram

 The system works as follows (Fig 2.):

> Mixer starts mixing of raw materials of previous batch.

> Major ingredient 1 is fed into the weighing hopper (gain-in-weight).

> Major ingredient 2 is fed into the weighing hopper (gain-in-weight).

> System waits for the mixer to finish mixing and then discharges the mixed product.

> Weighing hopper discharges ingredients 1 & 2 together with minor ingredient 1 & 2 at the same time to the mixer. Cycle finishes.

> New cycle starts, mixer starts mixing again.

 Therefore the total duration of one dosing and batching cycle can be written as follows:

(T = time consumed, Q = quantity)

Case 1:

If (Tmixing + Tmixerdischarge) < (Tmajor1 + Tmajor2) and Thopperdischarge < Tminor2 and Tminor1 < Tminor2

Tcycle = Tmajor1 + Tmajor2 + Tminor2

Case 2:

If (Tmixing + Tmixerdischarge) < (Tmajor1 + Tmajor2) and Tminor1 < Thopperdischarge and  Tminor2 < Thopperdischarge

Tcycle = Tmajor1 + Tmajor2 +  Thopperdischarge

Case 3:

If  (Tmajor1 + Tmajor2) < (Tmixing + Tmixerdischarge)  and Thopperdischarge < Tminor2 and Tminor1 < Tminor2

Tcycle = Tmixing + Tmixerdischarge +  Tminor2

Case 4:

If  (Tmajor1 + Tmajor2) < (Tmixing + Tmixerdischarge)  and Tminor1 < Thopperdischarge and  Tminor2 < Thopperdischarge

Tcycle = Tmixing + Tmixerdischarge  + Thopperdischarge

Tmajor1, Tmajor2, Tminor1, Tminor2 are the times needed to finish the dosing of each ingredient.

Thopperdischarge is the time needed for discharging of the weighing hopper.

Tmixing is the time needed for mixing all the raw materials.

Tmixerdischarge is the time needed for discharging the mixer.

In a wide range of applications, dosing can be done during the mixing process. Some materials can be mixed first and some additives can be added whilst the mixer is working. Time is also required for stabilising the weighing hoppers after filling or discharging. Calculations are to be done accordingly. 

Now we can calculate the time available to fill each ingredient hopper. For example the time available to fill a hopper with one ingredient is: 

Tfilling1 = Tcycle - Tmajor1

Capacity of a conveying system can be calculated as:

​Capacity = Qingredient1 / Tfilling1

​Where Qingredient1 is the amount of material to be used in one batch.

​As an example; if 50kg of calcium carbonate (CaCO3) and 100kg of PVC is to be used in a mixing process;

TfillingCaCo3 is 5 minutes and TfilingPVC is 8 minutes; Tcycle is 12 minutes

Capacity of a CaCo3 conveying system = 50kg / 5min = 600 kg/hr

Capacity of a PVC conveying system = 100kg / 8min = 750 kg/hr

Number of cycles = 60min / 12min = 5 cycles / hour

= 120 cycles / day

If the plant runs 24 hours in a day:

Daily consumption of CaCo3 = 120 x 50 = 6.000 kg

Daily consumption of PVC = 120 x 100 = 12.000 kg

Assuming that CaCo3 is delivered by 25-tonne bulk trucks, one truck of CaCo3 is enough for 4.2 days of production. So there is a need for a bulk truck delivery every 4 days. A 25-tonne capacity silo is required for this operation.

There should be some amount of reserve in the silo to prevent any production loss in case of any delay in logistics operation. Therefore 30 tonnes of capacity will give approximately one day of extra time. Depending on site conditions, plant managers could decide on purchasing higher capacity silos to decrease the truck traffic.

​If PVC is brought in 500 kg big bags, there will be a need for handling 24 big bags per day. At this point, there are two options for PVC handling: First, big bag discharge stations could be directly connected to a dosing system so that the PVC can be transferred directly from the big bags to the hoppers (Fig 3.).

An operator should put a full big bag on the discharger unit and remove empty big bags every hour. The capacity of the conveyor installed between the big bag discharger and the dosing system would be 750kg per hour as stated above. 

The other option is connecting a big bag discharge station to a storage silo and using higher capacity conveyors, e.g. 6 tonnes per hour. (Fig. 4.) 12 big bags are to be discharged per hour and two hours of loading would be enough for daily production. The importance of determining the capacity of the PVC silo is the same as calcium carbonate.

Figure 3. Big Bag Discharger Connected to Dosing System
Figure 4. Big Bag Discharger Connected to Silo

​Small sized sacks or bags are widely used for minor ingredients. These bags are generally kept close to weighing systems so that operators can fill the hoppers manually whenever needed. A good automation system should warn the operator before the hopper is empty. Therefore level sensors or similar equipment are needed to monitor the level of product inside the hopper.

Preventing Errors 

To have problem-free weighing, all upstream and downstream equipment should be isolated from the weighing system.

​The following points are to be taken into consideration:

  • ​​Mechanical vibrations coming from conveying systems are to be isolated. Flexible couplings, vibration dampers might be used, the production line is to be designed accordingly.
  • ​Air pressure effect is another issue. Powdered materials can be loaded by pressure or vacuum conveying systems. Any pressure difference between the weighing system and connected equipment might cause measurement errors. Venting the air and using a flexible connection between chambers would decrease the negative effects of air pressure.
  • ​Inlet port of weigh bin should not absorb any of the load the scale is trying to weigh.  It is important to check how the material is loaded onto the scale. Although a good weighing scale compensates errors caused by off-centre loading, it is good practice to distribute the load evenly.
  • ​Storage and conveying systems are to be cleaned easily in case of recipe changes. Mechanical design of these systems should be done accordingly.
  • ​There should be minimum tension on connected electrical cables, compressed air hoses and similar utilities.
  • ​Being the most important component of weighing systems, load cells are designed to compensate temperature changes, but there are minimum and maximum temperature limits for load cells. Maintaining relatively constant air temperatures would prevent reading errors.

Achieving optimal dosing 

Determining the configuration and system selection are typically budget-driven and the return on investment provides the justification for both supplier and equipment selection.

A wide range of engineering solutions are available in the market and plant managers have to choose the most appropriate ones in terms of expandability and flexibility. 

Solving a technical problem at hand is not an option for this type of investment. A good and detailed study at the beginning allows for future growth and many years of problem free service.

Process Industry Informer

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