Operational aspects

The following is a brief discussion of typical problems that can occur with storage bins, along with some common solutions.

2.10.1 No flow or erratic flow

Probably the most serious problem is one of no flow, i.e. the condition in which no material discharges when the feeder is started or the gate is opened. This is often due to arching or ratholing of the bulk solid within the vessel. If it is determined through testing that this problem is due to the cohesive strength of the bulk solid, changes should be considered to its moisture content, temperature, time of storage at rest and/or particle size distribution to make it less cohesive. Sometimes use of a chemical additive can improve the flow properties of a solid. If instead of cohesion the problem is one of particle interlocking, the solution is to decrease the particle size and/or alter the particle shape to make the particles more rounded.

If changing the properties of the bulk solid is not practical, consideration should be given to enlarging the outlet and feeder or using a mechanical flow aid such as an air cannon, external vibrator, vibrating discharger, internal agitator, flexible walls, etc.

Sometimes the problem of no flow can be caused by an outlet area that is not fully live.

This could be due to a partially opened slide gate, a feeder that withdraws over only a portion of the outlet area, or protrusions into the flow channel caused by mating flanges, access doors, poke holes, etc.

Another operational problem is erratic flow. This is a condition of alternating mass flow and funnel flow resulting in intermittent problems of arching and perhaps ratholing. The solution to this problem is to change the design of the vessel such that a mass flow pattern occurs.

2.10.2 Flow rate problems

If flooding, i.e. uncontrolled flow of fine powder through the outlet, occurs, the solution is usually to change the flow pattern from funnel flow to mass flow. If the flow pattern is already mass flow and flooding is still occurring, consideration should be given to decreasing the fill and/or discharge rates.

If, to the other extreme, the discharge rate is insufficient for the downstream process and changing the speed of the feeder does not correct the problem, the solution is to enlarge the outlet or add an air permeation system. If the mismatch between actual and required flow rates is so great that these alternatives are not capable of solving the problem, a fluidised or air-assisted discharge should be considered.

2.10.3 Particle segregation

If segregation causing downstream quality control problems can be isolated to a particular bin, the first thing to consider is the type of segregation that is occurring. If it is a side-to-side distribution of particles varying in size, chemical composition or some other attribute (generally the result of segregation due to the mechanisms of sifting, dusting, trajectory or dynamic effects), a mass flow pattern should be used. One should also consider using a distributor at the bin inlet to further minimise the segregation problem. If, on the other hand, the segregation profile is top-to-bottom (due, for example, to the fluidisation mechanism), it is necessary to first break this fill/segregation pattern. One way to do this is by using

tangential entry into the vessel. It will probably also be necessary to convert the vessel’s flow pattern to mass flow.

2.10.4 Excess stagnant material

Another operational problem is that of limited live capacity. This is usually one of the results of rathole formation. To correct this it is necessary to change the flow pattern from funnel flow to mass flow or at least expanded flow.

If the operational problem is one of degradation (caking, spoilage, oxidation), again the solution is to convert from funnel flow to mass flow or at least expanded flow.

2.10.5 Structural concerns

Sometimes there are problems with self-induced bin vibrations. If these vibrations are high in frequency but low in amplitude, an interesting phenomenon called silo music (humming) or silo honking may be experienced. This can be a nuisance to personnel or neighbours nearby but is usually not a structural concern. If, on the other hand, the bin vibrations are low in frequency but high in amplitude, the result is what are called silo quakes. These apply massive dynamic loads that most vessels are not designed to withstand. Structural failures have occurred due to this mechanism.

Structural problems are more common in bins and silos than in almost any other industrial or commercial structure (see Figure 2.29). There are three major causes of such problems:

(a)

Figure 2.29 Examples of structural failure.

(b)

(c)

Figure 2.29 (continued).

design errors, construction errors and operational issues (Carson 2000b). Within the category of design errors, some of the problems that often occur are:

r

Material properties and flow patterns not considered

r

Out-of-round bending of a circular cylinder

r

Large and/or non-symmetric loads caused by inserts

r

Lack of proper consideration of specific requirements for structure type, such as bolted metal or reinforced concrete

r

Temperature or moisture effects resulting in thermal ratcheting or particle swelling Construction errors include poor quality workmanship (incorrect bolts, incorrect placement of reinforcing steel, incorrect material type or thickness) and unauthorised design changes made during construction.

Operational practices that can cause structural problems include improper usage of the vessel (e.g. a change in the stored material that results in higher loads being applied to the vessel walls or adding additional outlets to the vessel wall, which changes the flow pattern).

Another operational issue is improper maintenance, such as allowing material to build up on surfaces or ignoring the effects of corrosion.

Whatever the structural problem, it is essential that, at the first sign of trouble, there be an appropriate response. Continuing to empty a vessel that is already starting to fail may well result in complete collapse of the structure. Engineers trained in structural issues as well as solids flow should be consulted to ensure that property damage is kept to a minimum and personnel are kept out of harm’s way.

Silos and bins should be inspected on a routine basis to anticipate potential flow and structural problems before these become major.

2.10.6 Process problems

If the operational problem is one of cross-contamination, this is most likely due to a funnel flow pattern. If this is the case, changing to mass flow should correct it. It should be recognised that even with mass flow there will be a velocity distribution across the vessel, so achieving perfect first-in-first-out is seldom achievable.

If there are problems with inventory control, this is usually a signal that the flow pattern needs to be changed from funnel flow to mass flow (Pittenger et al. 1999). Sometimes a weighing system is needed in order to provide proper inventory information.

If the bin is being used as a processing vessel such as for drying, purging or conditioning of the bulk solid, non-uniform processing may occur. This is usually an indication that a funnel flow pattern needs to be changed to mass flow. Sometimes an insert is used in such applications, but this must be done with caution (Carson et al. 1995).

2.10.7 Abrasive wear and attrition

If the bin wall is wearing abnormally, one should first determine where the wear area is occurring. If it is in an area where impact is occurring, it is important to minimise the opportunity for the bulk solid to impact the vessel walls, such as by maintaining a minimum level in the vessel as much of the time as possible. Other options include reducing the fall height of material as it enters the vessel or redirecting the incoming stream to a wear plate.

If the wear problem is due to abrasion, the first step should be to test the abrasive wear characteristics of alternate liners (Johanson & Royal 1982) and then consider changing the wall material or liner.

Another operational problem is unacceptable attrition. If this is occurring, consideration should be given to using a letdown chute, reducing the size of the bin or changing the type of feeder to one that provides fewer pinch points.

2.10.8 Feeder problems

Finally, if the operational problem involves the feeder such that it cannot be started, requires excessive power to operate or experiences excessive wear, these all could be indications that the flow pattern is incorrect for the material. The importance of the feeder interface cannot be over-stressed, particularly for elongated outlets.

References

ASTM D6940-03 (2003a) Standard practice for measuring sifting segregation tendencies of bulk solids. In:

Annual Book of ASTM Standards, Vol. 04.08. American Society for Testing and Materials International, West Conshohocken, PA.

ASTM D6941-03 (2003b) Standard practice for measuring fluidization segregation tendencies of powders. In:

Annual Book of ASTM Standards, Vol. 4.08. American Society for Testing and Materials International, West Conshohocken, PA.

Bates, L. (1997) User Guide to Segregation. British Materials Handling Board, Elsinore House, Marlow, England.

Beverloo, W.A., Leniger, H.A. & Van de Velde, J. (1961) The flow of granular solids through orifices. Chem.

Eng. Sci., 15, 260–269.

Carson, J.W. (2000a) Step-by-step process in selecting a feeder. Chem. Process., Powder Solids Annu., 38–41.

Carson, J.W. (2000b) Silo failures: case histories and lessons learned. Proceedings of the 3rd Israeli Conference for Conveying and Handling of Particulate Solids, Vol. 1, pp. 4.1–4.11, Dead Sea, Israel.

Carson, J.W., Purutyan, H. & Rotter, J.M. (2003) The dangers of relying on wall friction values in codes. 4th International Conference for Conveying and Handling of Particle Solids, Budapest, Hungary.

Carson, J.W., Royal, T.A. & Goodwill, D.J. (1986) Understanding and eliminating particle segregation problems.

Bulk Solids Handling, 6, 139–144.

Carson, J.W., Royal, T.A. & Pittenger, B.H. (1995) Mass flow purge and conditioning vessels. Chem. Process., 58(8), 77–80.

Jenike, A.W. (1964) Storage and Flow of Solids. University of Utah Engineering Experiment Station, Bulletin No. 123.

Johanson, J.R. & Royal, T.A. (1982) Measuring and use of wear properties for predicting life of bulk materials handling equipment. Bulk Solids Handling, 2, 517–523.

Pittenger, B.H., Carson, J.W., Prescott, J.K. & Purutyan, H. (1999) Uniform purging of resins in contact bed purge vessels. Polym. Eng. Sci., 39, 1802–1811.

Rotter, M.J. (2005) Silo design loads from stored bulk solids: the provisions of the new Eurocode EN 1991-4 (2005). Proceedings of the IMechE Seminar on Specifying and Designing Safe Storage Silos, IMechE.

Royal, T.A. & Carson, J.W. (1991) Fine powder flow phenomena in bins, hoppers, and processing vessels.

Presented at Bulk 2000, London.

Troxel, T.G., Carson, J.W. & Bengston, K.E. (2005) Proven techniques for air-assisted handling of powders in bins and hoppers. 7th World Congress of Chemical Engineering (C13-008).

In document bulk solids handling (Page 106-112)