croston_v1

Calculations for Silo Design and Hoppers

Mon, 03 May 2021 17:24:50 GMT

There’s more to silo operations than just putting material into a bin and removing it - calculations are necessary if you want a successful operation with minimal mistakes.

The discharge process is often overlooked, but this is one of the most detrimental parts of the whole operation. Things such as arching and ratholing can occur, so steps need to be taken in the design of the silo and feeders to prevent such events.

Keep reading to find out more about silos, and what needs to be considered when designing a silo.

What Is A Silo?

A silo is a large bin or tower that’s used to store bulk materials . Some materials commonly stored in silos include sand, cement, grains, and metals - but pretty much any bulk material can be stored in a silo.

Typically, silos are made using large sheets of steel for durability and strength. These sheets are often ribbed on the inside to ensure that material doesn’t stick on the silo wall, which reduces product loss.

Silos can vary in height to accommodate the amount of bulk material and the surrounding environment, but they tend to range from 10m to 90m tall, accommodating weights of up to 80 tons.

The removal of the material depends on your supplier and what the material actually is - however, silos usually have an access point on the roof to put the product in, and an exit point on the bottom for removal.

Some ways of removing the material include the power of gravity, an auger, or a pneumatic conveying system.

A typical silo features a cone-shaped design, which helps you gain control of the bulk material flow. If pneumatic conveyors are in place to remove the bulk materials, then a conveyor tube is attached to the bottom of the silo, to ensure no product is wasted and that the process is more streamlined.

In larger workplaces where a large number of materials are being stored and processed, multiple silos will be connected to the same conveying system to save time, manpower, and money.

Other silos are placed on a raised platform so a removal truck can park directly underneath and collect the load. This is a cost-effective method, but generally, you’ll find that multiple trips will need to be made to collect all of the bulk material from the silo.

Mass Flow

Flow issues can be prevented by making sure that mass flow occurs inside the silo or hopper (depending on the material). To achieve mass flow, you must make sure that the walls are steep enough, and the friction levels are low enough for the bulk materials to slide down.

To achieve this, you must make sure that the materials are tested and that the walls are measured - and then you must calculate the right hopper angle to allow mass flow to occur.

Mass flow bins are ideal if you want a reliable and steady flow out of the silo. However, mistakes can be made, and what you think is a mass flow bin can actually be a funnel flow bin, causing problems with the flow.

To overcome this, you will want to recalculate the angle of discharge and the minimum diameters - reassessing every calculation made to determine the silo or hopper measurements.

In a hopper or a silo, there are two main flow patterns that can occur.

Bulk Flow

In bulk flow, all of the solids are moving when the material is being emptied from the outlet, which stops material from getting stuck to the walls of the silo. This encourages the materials to flow in a ‘“first in first out” manner, providing a more structured exit, preventing ratholing and sifting segregation.

FIFO (first in first out) is more effective with bulk flow. Mass flow bins can be sensitive to arching, so this is something to watch out for during the process.

Funnel Flow

Bulk flow and funnel flow are very different flow methods. Funnel flow tends to be more chaotic and is more likely to worsen the segregation of materials. Funnel flow can also result in stagnant material being left inside (ratholes) the edges of the silo or hopper. Ratholing can lead to particle degradation, caking, and spoilage, which is not only a waste of product but a waste of time and space.

With a funnel flow, the product leaves the silo in the centre of the bin, creating a funnel shape. This can cause both arching and ratholing - we’ll go into more detail on arching and ratholing soon.

Calculation Methods

Powders can be difficult to control when stored in a silo or hopper and can slide and fall. When a large amount of powder is stored in a bin, it’s submitted to pressure due to the height of the powder and the weight of the powder on top pushing down on the powder below. This can have an impact on flowability.

Because of this pressure, solids tend to be cohesive. When this occurs, arching and ratholes can occur, which can be detrimental to the flow of the powder when discharging from the silo or hopper.

Arching and Ratholing

Arching and ratholing relate to the flowability of bulk solids and powders. Arching occurs when a powder is stored in a hopper that doesn’t have steep enough cone walls or a large enough outlet.

For improved flowability, powders need to be submitted to a constraint. The constraint depends on the outlet shape, and whether the powder can support it to consolidate itself and create arches. If the angle of the exit isn’t steep enough, then it’s likely that an arch will form.

Calculating the hopper wall angle is and the critical outlet diameter is critical in preventing arches from forming and if you want the powder to flow.

Ratholing can be avoided if you pay attention to calculations. The hopper must be designed so the outlet diameter is larger than the outlet rathole critical diameter. The rathole diameter is usually larger than the arching diameter.

This effect only tends to happen with powders, and the effect can be very different when liquids are stored.

The flow of powder in a silo/ hopper is determined by 3 things - the internal friction of the powder, how easily the powder can move on the surface of the silo walls, and how the powder compacts under the weight of the top-level powder.

How you measure these properties is important - if you want high flowability, you must follow the steps below.

Learn About The Powder

Knowing all about the powder and its properties is key if you want the powder to flow. You must learn the powder flow function, the static angle of internal friction, and the wall friction angle.

It’s important to know the characteristics of the product to discharge - and these can be determined by a shear tester. Once you learn these characteristics, you can calculate the critical discharge diameter. This is the diameter that the product will risk to arch - and different diameters can be calculated depending on the bin used.

Find Out The Angle For Mass Flow

Mass flow is imperative if you want a clean, rathole-free flow. The first step is deciding whether you need a conical hopper shape or a wedge hopped shape - but this depends on the environment and the material being stored.

In the food industry, conical hoppers are more desirable as they’re easier to clean - but in many other instances, wedge-shaped hoppers are used as they’re generally bigger. You’ll also have to determine the type of power feeder at the base and factor this into your operation. For example, star valves are far more difficult to fit in a wedge-shaped hopper, so this is something that should be considered.

Feeders

A feeder can help to control the flow of the materials exiting the hopper or silo. Feeders are also useful as they can dose the solid, and reach another unit while lowering the installation height.

The feeder should always be sized correctly - if the feeder is too small, the powder can flow poorly and chaotically and can result in a funnel flow bin instead of a mass flow bin.

A poorly fitted or constructed feeder can be the source of many other issues in the process - flow being the main one. You should be especially careful with hoppers that have slotted outlets, as the feeder needs to draw perfectly along the entire cross-section of the outlet. Although hoppers with slotted outlets carry more risk, hoppers with round outlets can still have uneven flow if the hopper interface isn’t designed and fitted with exact precision.

The design of the feeder is extremely important - it must be designed to bring the material out of the entire silo or hopper bin. A good feeder design will have large and steel pipe slopes to ensure the material flows quickly and efficiently, and will also be able to take the powder in the whole section of the outlet.

Three main feeders are used in most cases in the industry, and they are:

Butterfly Valves

Butterfly valves are the simplest types of valve positioned on the outlet of a silo. As opposed to other types of valves, butterfly valves are super hygienic - but they do have a disadvantage.

Once open, materials can be left in the passing area - and sometimes there can be enough material to form a new bridge. For very cohesive powders, you may need discharging aids and even vibrating valves to overcome this.

Screw Feeders

Although screw feeders are effective, it’s important to note that if the powder exits from this type of feeder unevenly, then structural damage may occur on large silos. If the screw feeder is placed below an elongated hopper, be sure to use a pitch to keep on taking powder in the flow direction. If you don’t the screw feeder will fill up pretty quickly and will only be taken out on one side.

Rotary Valves

Rotary valve feeders need to make sure that there’s a 2 diameter section of pipe between the hopper and the valve. This is to regulate and improve the flow and to hopefully result in a better mass flow.

This type of feeder is typically placed under hoppers that have round outlets. When you use a rotary valve, you must make sure that a short vertical spool is installed between the hopper outlet and the valve inlet to make sure that the flow pattern is correct.

Discharge Rate

Calculating the discharge rate is imperative if you want to improve your system. There are two widely used equations to determine this - the Johanson equation and the Berverloo equation.

Although these two equations can help you to estimate the flow, it’s important to note that they offer no certain accurate figure.

The Berverloo equation is direct, but for finer particles, you may get an overestimated discharge rate.

Although many people are skilled in liquids and gases, not many people know the ins and outs of bulk material handling. Knowing how a hopper or silo design can have an effect on a materials cohesive strength is imperative for a successful operation, and know knowing this can result in costly mistakes.

3 Different Silo Cleaning Methods

Thu, 29 Apr 2021 16:59:52 GMT

Silos can be as tall as 90ft, and store tonnes and tonnes of bulk material, so it’s never a straight-forward process getting them cleaned efficiently.

The methods of cleaning can change depending on what’s being stored inside it, as contamination, moisture, and clumping can all occur if the silo isn’t cleaned properly - which can lead to material loss.

If you haven’t cleaned your silo in a while, you may notice less material coming out than expected, or blockages may occur which can put a stop to your whole operation.

Cleaning may cause temporary downtime, but it’s a lot better than your whole operation suddenly failing.

Keep reading to find out more about why you need to get your silo regularly cleaned, as well as 3 different silo cleaning methods.

Why Do I Need To Clean My Silo?

Silos need regular cleaning not only for hygiene purposes but for safety and to protect the bulk materials stored.

When bulk materials are stored inside a silo for long periods of time, they can begin to clump and gather around the sides of the silo. Some silos have preventative measures against this such as ribbed interiors, but many don’t.

The materials built up around the side of the silo will need to be knocked off and cleaned off the interior walls - as this won’t be coming out during the emptying process and results in material loss.

If you notice that less material is coming out than expected, then you know it’s time to get your silo cleaned.

The built-up materials inside the silo can also change the pressure, which causes off-centre loading. This can damage your equipment, so it’s best to get it checked on a regular basis.

It’s also a legal requirement to keep your silos maintained to an acceptable standard according to the Confined Space Regulations Act 1997, which also means you should find a qualified and component contractor to ensure this for you.

Silos should be cleaned at least once a year, but more frequently for larger operations. It may be time-saving to get your silo cleaned along with maintenance and servicing, as this will lower any down-time saving you money.

How Do I Clean My Silo?

Flour Silo

Flour silos and other silos containing powders can be difficult to clean, as the materials tend to clump together - especially when wet. When the material is clumped together, it’s a lot more difficult to control and can develop a paste-like consistency.

Rope access tends to be the norm for cleaning flour silos, but this depends on the design. Some have an opening on the roof, or near the roof, as well as the silo floor, which makes for easier access for cleaning and servicing.

It’s highly recommended that the silo cleaning team be trained for cleaning in dangerous conditions and confined spaces, as it can be a dangerous task if not completed correctly.

The cleaning process begins with a dry scrape down of the interior walls and surfaces to remove any clumped flour or materials stuck on the inside. Then, hot water or steam will be blasted at the walls with pressure to ensure any residue is removed.

Once the hot water pressure wash has been completed, the silo walls should then be treated with a food-safe sanitiser.

You should always be careful what products you use on the inside of a silo that stores consumable products, as many products can be dangerous when ingested.

Grain Silo

Silos should always be emptied as much as possible before the cleaning process begins, and a brief inspection should be done to gather knowledge of entry points, safety issues, and the overall structure.

It’s necessary to get silos containing consumable goods cleaned regularly as any residue can attract insects, bugs, and mould, which can contaminate your grains. Insects and bugs can also consume the grain, which will both contaminate and destroy your product.

A common method of cleaning silos for grain is by vacuuming the interior to remove any residual dust.

Once the silo has been vacuumed, you need to check over the floors, flanges and seams have no dirt or dust. If you do notice any dirty areas, use a clean water wipe and rub them down.

It’s important to thoroughly dry the area after cleaning, as damp can lead to mould which will contaminate your grains.

Cement Silo

It can be a lot more difficult to clean a cement silo than a silo for powders and grains as cement can harden around the silo interior. Evaporation and moisture can cause clumping in the silo, which can lead to clogging issues.

Before you start on the inside of the silo, the silo exterior should be rinsed with water to remove dust and dirt. If rinsing doesn’t remove all the dirt, then try cleaning tools and agents for a harsher clean.

Safety should always be considered, so when possible, use equipment that negates the need for human-entry. CO2 blasting is a great way of removing the clumps of cement on the silo interior and exterior and minimises the loss of material.

This method can also help to remove blockages in pipes and storage vessels and reach places that a cleaning team won’t be able to by hand.

Once the silo has been cleaned, it must be dried efficiently. Any damp areas may lead to rusting, which in time can lead to structural failure.

Conclusion

Cleaning your silo is imperative and can prevent blockages and complete system failure. Whether you get your silo cleaned when you get it serviced, or you just do it as and when needed, it’s important for safety and hygiene reasons.

Thanks for reading our post today. Do you have any tips for silo cleaning? Let us know in the comments section!

What Is A Bulk Material Handling System

Thu, 29 Apr 2021 16:45:25 GMT

Handling tonnes of bulk materials can be a difficult task, and lots of planning must go into it to make the process streamlined and efficient. An effective bulk handling system will speed up the transport of your bulk materials, saving you time and money.

At Croston Engineering, we’ve got decades of experience handling bulk materials and can help you out every step of the way.

Keep reading to find out what a bulk material handling system is, the various steps of the process, and what equipment is typically used.

What Is a Bulk Material Handling System?

You need a planned system in place in order to transport bulk materials on a quick and constant basis. The materials are received from either ships, trains, or trucks, and then unloaded to a storage facility or a site, most commonly using conveyor systems.

The bulk materials will be deposited into a stockpile, and can then be retrieved by the stockpile using equipment such as automatic reclaimers or reclaim tunnel hoppers and feeders. Once reclaimed from the stockpile, the materials will then be transported for processing at a facility.

It’s important that your bulk material handling system is tailored to your environment and your requirements, which is where we come in. At Croston Engineering, we have plenty of experience in this area and can offer our services from start to finish.

Belt Conveyors

Belt conveyors are one of the most used pieces of equipment in bulk material handling.

Different conveyors such as screw or chain conveyors can be used, but belt conveyors tend to be the most effective as they’re designed for the continuous transport of bulk materials, which reduces manpower, time, and money.

Belt conveyors consist of a system of belts and pulleys that work together to get the bulk materials moving. The belt wraps around a pulley located either end of the conveyor.

A pulley is connected to a drive unit which gets the materials moving. There can be more than one pulley depending on your requirements. The length of a conveyor also depends on your requirements - it can vary from a few meters to tens of miles.

Belt Conveyor Maintenance

The belt is one of the key components of a belt conveyor, and one of the most expensive. Unfortunately, belts tend to be exposed to impact, corrosion, and elongation, so it’s important to work with a belt that can withstand damage.

The belt contains the materials being conveyed and transmits the force to move the load.

Regular maintenance is necessary to keep the belt conveyor functioning quickly and efficiently. If poorly designed, the system can easily become frail and break under harsh conditions.

Some components of a conveyor belt have a short life even well maintained properly - for example, rolling element breakings can only function well for a few months, so it’s important to get your system serviced on a regular basis so any flawed components can be fixed or replaced.

The belt should also be cleaned using the appropriate equipment to increase the lifetime of the system as well as for safety reasons. The feed and discharge points should be a cleaning priority to increase the longevity of other components such as rollers and idlers.

Belt Conveyor Design

There’s a lot to consider when designing a belt conveyor. Not only does the system need to withstand the amount of weight you expect to be moving, but other factors such as dust presence and corrosion need to be considered.

Superior seals should always be used for bearings and other components otherwise damage is likely to occur. All components should be correctly designed to fit your needs, and if designed correctly, your bulk materials will be transported with no problems for years.

With a standard conveyor design, a direct-coupled motor gearbox or a parallel-shaft drive supplies the power which operates the drum. Some designs use a motorized drum, where an electric motor, bearings, and a gear unit form a complete drive unit inside the pulley drum shell.

This powers the belt directly and keeps the drum protected from external forces. Motorized drums can have power of up to 150kw, but this varies depending on your expected load weight.

The components used in a belt conveyor should be lightweight and compact, but this can make servicing the components more difficult.

Reclaimer Machines

Once the bulk material has been transported from the ship or truck and transported to the site (usually using a belt conveyor system), the bulk is usually placed in stockpiles and then reclaimed when needed.

Stacker and reclaimer machines are used to speed up the stacking and reclaiming process, which reduces costs as well as manpower.

A boom conveyor is used to reclaim the bulk materials from stockpiles and feed the materials to silos, bags, and other storage apparatus.

A common method of reclaiming is bucket wheels or rail tracks. Bucket-wheel mechanisms are usually powered by a motor of variable frequencies that can be adjusted by modifying the rotational speed of the bucket wheel.

Over-force protection units are used to prevent any overpowering accidents that can halt the entire process. The bucket mouth is designed using wear-resistance materials as it can experience severe damage over time.

Tungsten carbide tends to be used, as it’s a structurally rigid material that withstands wear and tear. The bucket teeth also experience wear, so rigid and strong ferromagnetic materials are used for this too.

The speed of reclaimer and stacker machines vary depending on the design and your requirements. The speeds for standard operations can vary from 5 to 12 metres per minute, but this can increase to 30 metres per minute for larger machines.

The faster the machine, the more unstable the process, so a low speed is encouraged unless higher power is needed. At Croston Engineering, we can help you determine which design and speeds will work best for your process.

Important Reasons For Regular Silo Servicing

Thu, 29 Apr 2021 16:23:52 GMT

Silos can store pretty much any bulk materials, from consumable goods like salt, flour, or grain, to oil, coal, and dust. They can accommodate up to 80 tonnes of bulk material, and it can be very costly if any of the material is wasted or damaged.

It’s imperative to get your silo serviced at least once a year - but why? Keep reading to find out why it’s important to regularly service your silo and for information on how silos are cleaned and maintained.

Why Do Silos Need Servicing?

As well as regular maintenance, your silo should be designed to withstand the elements and to increase longevity. Preventative maintenance is just as important as scheduled servicing - maybe more so.

Many things can lead to silo damage, including improper filling and unloading of the silo, and weak foundations. These issues can go unnoticed if your silo isn’t regularly serviced, which is why you should get your silo serviced at least once a year.

Proper care and inspection of your silo will increase the life of the silo as well and reduce the need for repairs.

To Protect Your Materials

Some silos are made using large steel sheets that have ribbing on the inside, which helps to reduce the build-up of product on the sides of the silo. However, this doesn’t mean that there won’t be any build-up.

After some time bulk materials will build up and become compacted on the sides of the silo, which will need to be knocked off during servicing. This clumping can cause blockages and means less material will in the emptying process, so you’ll be losing material.

It also lowers the predictability of your offloading process and causes off-centre loading, which can change the amount of pressure on the walls and damage your equipment.

This can all be avoided by getting your silo regularly serviced and keeping a close eye on each step of the loading and offloading process.

To Identify Issues

Regular maintenance will pick up on issues such as wear, blockages, and structural failures. It’s a way to pick up on minor issues before they develop into more costly or dangerous problems.

Even if durable materials are used in the silo design, weather and time can still cause wear on the silo body. Degradation can show differently depending on the silo’s construction and the bulk materials stored in it.

Concrete silos may exhibit cracks and crumble, whereas steel silo issues can include the build-up of material and corrosion.

Once inspections have been completed, you’ll receive results and maintenance will be performed - which is a lot better than waiting for something to break and having to break the bank fixing a whole system collapse.

Thankfully, most silo failures don’t occur suddenly - your silo will exhibit signs of distress and you’ll pick up on these problems and get them serviced. It’s important to catch potential failures early so they can be quickly addressed with minimal damage.

For Safety Reasons

Silos can be as tall as 90m and as wide as 90ft and can store tonnes of bulk material, so if there are any structural failures, lives could be at risk.

Regular maintenance can pick up on any potential structural failures, as well as corrosion and accidental damage. Every silo, whether it be made of steel, fibreglass, or concrete, has a support structure that will need to be regularly inspected as a safety precaution.

Although there have been no reported fatalities in the UK from a silo failure, that doesn’t mean that it can't happen. In Europe, there have been cases of loss of life from silo failure, which could have been prevented with regularly scheduled silo maintenance.

What Needs To Be Serviced?

Your silo should be serviced once a year at least, but professional inspections can be as little as every 5 years - although it’s not recommended.

Some things that need to be inspected closely include:

Roofs

Roofs are one of the main parts of the silo and work with the silo body to keep the bulk materials dry and protected. If there are any problems with the roof, leaking can occur which may lead to contamination, lowering the quality of your product.

Any roof leaks can lead to corrosion on the inside of the silo, as well as the roof beams - which can cause structural damage.

Flaking on the roof beam pockets can expose the beams and cause the roof to collapse, which is something that can be avoided by checking this regularly.

Silo Exterior

During in-house or professional inspections, exterior surfaces are always something you should inspect closely. Look for any cracks on the walls, roofs, and foundations as this is an indicator of stress.

It’s important to catch these things early before they develop into holes or total wall failure and collapse - which would be a lot more expensive and damaging than a regular checkup.

You should also look out for any signs of corrosion and rust, as this can lead to silo failure if left untreated.

Silo Cone

The cone construction and design can hugely affect the performance, and if best practices and safety precautions are not adhered to, silo cone failure can become a costly problem.

The silo cone inspections should be left to professionals as safety practices need to be followed.

Silo cone failure can be easily prevented by routine silo servicing. However, silo cone inspections should be left to professionals as safety practices need to be followed.

The inspector will look for ways that the silo can be improved, as well as the materials used and the thickness of the silo wall.

Suspended cones are no longer best practice, as this type of silo, unfortunately, has a high failure rate, and will need more repairs than the new standard. The inspector will recommend that you update this, and it should be addressed during servicing.

Bulk Material Buildup

The buildup of material is likely to occur if the silo hasn’t been cleaned in a while, or if the interior doesn’t feature ribbing.

The buildup can lead to clumping, which can cause a skewed flow and unbalanced pressure, potentially damaging any equipment and losing material.