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Comment PDF Solids Handling

Selection of Silos for Bulk Storage

By Tom Nomady, Imperial Industries Inc. |

To determine what kind of storage silo is best for your site and your materials, begin by answering the following five questions

Selecting a silo for your bulk-solids storage needs isn’t like picking out a pair of socks. It’s a big decision. There are a lot of variables and plenty at stake, not the least of which is the safety of your colleagues and the success of your business. The high stakes can make the decision overwhelming. My experiences have taught me that there is a solution for every storage situation.

I have fielded every kind of question imaginable, from requests to simply increase storage capacity to those regarding the design of silos for storing unique materials — for instance, coal dust, plastic additives and others. Be confident that a solution exists. My recommendation is to begin the selection process by asking yourself the five questions discussed in this article before moving forward. All five represent critical variables that need to be addressed before a silo can be bid, designed, built or shipped. By first establishing the answers to these questions, you can turn what can be a confusing process into a rewarding one.

Figure 1.  Shown here are custom-made silos used to store materials in a high-technology plastics-manufacuturing facility

Figure 1. Shown here are custom-made silos used to store materials in a high-technology plastics-manufacuturing facility

 

What material will be stored?

Create a list of specific requirements for your storage application before you engage a silo vendor. Tell the vendor the characteristics of the material being stored, including density, abrasiveness and particle size (for example, is it a fine powder or granular material?). Consider how the material will be fed into and discharged from the silo. This will help determine the most cost-effective option. Additionally, determine whether the material’s composition is such that a silo will require flow-aid devices, such as bin activators, vibrators, aeration systems or mechanical agitation. Knowing the amount of the material needed to meet the supply-chain needs of your facility is also important.

No matter what chemicals or materials you are storing, most silos will likely require other components, such as dust collector flanges, manways, nozzles, flow control, stairways and peripheral conveyor supports. It is helpful to familiarize yourself with these terms in advance, if you are not already. For specific information on structural steel construction, reference books, such as the Structural Engineering Handbook and the Manual Of Steel Construction [ 1, 2], can be useful.

Understand that most silos are pneumatically fed, some are gravity fed, and others have mechanical feeding systems. The feed and discharge systems are separate entities from the silo weldment and vary from user to user, depending on what material they are storing, what equipment they are familiar with, and how they want (or need) to handle the product. The feed and discharge parts of the system are the responsibility of the company that supplies the material handling equipment.

What is the construction timeline?

The construction of the silo can and should be a straightforward process. The objective is to avoid last-minute changes prior to construction. Changes can happen for a number of reasons, and can be quite costly. Common changes include a sudden requirement to add capacity, which could involve a height change, diameter change or even additional silos. Also, an change of the handling system design can affect the silo design. This can include changes to silo connections, discharge clearances and overall dimensions. Changes in product bulk density can also have a significant impact. For example, if a silo designed for a product density of 40 pounds per cubic foot (PCF) changes to one holding a material at 45 PCF, that would add a dead load of 25,000 lb for a 5,000-ft 3 silo.

Give careful thought to your construction timeline, budget and expectations at the planning stage. Don’t rush anything. Build in plenty of time to adjust for conditions. Construction timelines will be influenced by the type of site you are building on: a greenfield site is new construction; a brownfield site is adding to an existing facility. When installing a silo next to existing equipment on a brownfield site, downtime can be very costly. Make sure you talk to your site contractor about all the variables, including the use of a crane, the need to set up scaffolding and more. Spending time for upfront planning can go a long way toward ensuring the construction runs smoothly and on time.

To provide a sense of the duration of project stages, it is typical to allow three or four weeks for an engineering timeline, during which you would consider and approve drawings, and eight to ten weeks for fabrication of the silo, following the final project approval from the end user. Of course, lead times increase and decrease according to the size and complexity of the project.

 

How will your silo look?

The external appearance of the silo needs to be acceptable for both aesthetic and pragmatic reasons. Paint systems for silos can range from no coating at all to sophisticated multi-coat paint systems. The paint coatings not only determine the appearance of the silo to the public, but also serve as protection against corrosion and other damage. In most cases, coatings are essential for storage performance that meets the requirements of the application, such as corrosion and abrasion resistance. Higher-quality coatings mean more protection during the life of the silo.

The most common silo coatings tend to be epoxy primer with polyurethane topcoats. Custom coatings, which are usually user-specified, can range from primer only to high-end zinc prime/epoxy mid-coat/polyurethane topcoat. Custom color match is usually available and coating life usually can extend up to 20 years with proper maintenance.

It is important to take into account the impact of weather on the silo coating. Rain can cause inadequate coatings to delaminate from the surface of the structure. Heat and cold cycles can cause the silo’s metal to expand and contract, which could cause the coatings to separate. The sun’s ultraviolet rays can accelerate coating deterioration and chalking. If all of these factors are considered, the right coating can make the difference between a high-performaning silo and one that will fail prematurely.

Some coatings require inspection in order to satisfy federal, state and city regulations. With such a wide variety of paint options, your vendor needs to be a specialist in the field who can guide you through the decision-making. The good news is a quality vendor will be experienced in planning for all these scenarios.

 

Where will the silo be located?

Consider the size of the area where the silo will be located. This will help determine the silo type and construction method. Once the location is determined, consider how the silo will be maneuvered into place. Make sure the silo vendor is capable of transporting and installing the silo in the given area. It is essential to ask detailed questions about the logistics of the transport, including those about specialized trailers, which allow for maneuverability and can be lowered or raised in the air depending on site circumstances. Silos generally can be manufactured up to 16 ft in diameter and shipped in one piece up to 90 ft long with a 90,000-lb weight. Construction, size and weight are defined by the structural requirements more so than the size requirements.

In geographic terms, the location of your silo can be affected by local building codes and other safety regulations that apply to the project. This affects the design, construction and installation of your silo and needs to be considered upfront. Some locations may require a permit, too. Vendors follow the current International Building Code standard building codes for the specific job site location in question. There are no specific design codes for dry bulk silos, so a combination of American Petroleum Institute (API), American Society of Mechanical Engineers (ASME) and American Water Works Association (AWWA) standards are used at certain times to calculate allowable stresses and other design factors.

 

How will the silo be maintained?

The objective here is to consider what will be needed to maintain high performance at minimal cost throughout the life of the silo. For example, make sure your silo has easy access to internal components for routine maintenance. Periodic inspection by plant maintenance personnel is required to keep up with paint touch-up and other minor issues. As your silo ages, look to your manufacturer to provide inspection services to ensure its safe and lasting operation. Eventually, many years down the road, these inspections can help you assess whether to repair or replace your silo.

There are more factors that go into the silo selection, such as the user’s budget for storage and expectations about having the new silo in place before needing additional storage or material handling equipment. Also, it may help to include future needs, but if you address the questions mentioned here, you will position your company for selecting a storage silo that is quality-built and pleasing to the eye, and that will serve the company effectively for many years. n

Edited by Scott Jenkins

 

References

1. Gaylord, E.H., Gaylord, C.H., and Stallmeyer, J.E., “Structural Engineering Handbook,” 4th ed., McGraw-Hill Education, New York, 1996.

2. American Institute of Steel Construction (AISC), “Steel Construction Manual,” 11th ed., American Institute of Physics / Springer, College Park, Md., 2017.

Author

IMG_0434Tom Nomady is general sales manager at Imperial Industries Inc. (550 West Industrial Park Ave, Rothschild, WI 54474; Phone: 1-715-359-0200; Email: salesinfo@imperialind.com). Nomady has a bachelor’s degree in industrial engineering and product development, and additional training in welding and painting. Before joining Imperial, he worked for a global food company, specializing in custom machine design and facilities planning, and a large sanitary stainless company as a sales engineer for custom-designed and fabricated food-processing equipment. He has been with Imperial Industries since 1993.

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