Bulk solids applications provide many situations where modifications are needed to secure acceptable performance on newly installed or recently altered plant. The reason is almost invariably because the physical properties of the bulk material to be handled were not properly taken into account in the original design. The experienced bulk solids practitioner has many resources at his disposal to rectify the situation, but must also be aware of the immutable conditions that apply when undertaking a retrofit operation. Contract circumstances are totally different from original supply contracts, as are constraints and commercial factors. The search for solutions is also more demanding than an original design because the situation is compromised by the existing plant, pressure to achieve production and commercial and human factors. However, a combination of broad and specialised experience, state of art technology, innovative techniques and proprietary equipment, backed by sound engineering design, provides a comprehensive toolbox for the specialist to address this task.
Introduction
Industry handles and processes a massive variety of bulk materials in loose particulate form. The highly varied nature of these substances, and the wide range of operating conditions to which they are subjected, gives rise to many behavioral problems that detract from satisfactory process performance. It is therefore not uncommon for newly installed plant to need remedial action to secure acceptable production and/or a suitable quality of product. This situation has barely improved from a detailed study by the Rand organization in 1984 (1). The failure to achieve design objectives is inevitably the source of some disappointment and can lead to friction between the equipment supplier and the user. Time can also be a major factor driving the search for a solution when the anticipated output from a plant is not forthcoming to serve outstanding market demands. Relationships tends to suffer increasing strain if the position cannot be resolved quickly and economically by the parties concerned as the conflict of interest grows. An invitation to a third party to correct the situation is an admission that the task is neither obvious nor easy, but is ripe for dispute and inevitably places the task in a highly focused climate bounded by immutable rules. The management skills needed for troubleshooting are quite different from those needed to run normal projects. Issues involved with identifying and rectifying plant problems generally has to follow three concurrent tracks, the technical path, the commercial minefield and the psychological route.
The technical requirement is to define the objective, identify the root cause of the problem, secure and analyse the data required, formulate an acceptable solution, implement it and verify its validity. Commercial aspects are more sensitive than normal contracts, but are subject to conventional negotiation, whereas the psychological path involves multiple parties and follows the way of any other stressful situation. The sequence of emotions includes denial, anger, apathy and acceptance. Denial of responsibility is usually immediate, entrenched and can be virulent. A consequence is that the problem and its solution are considered to belong to others.
This will probably be true for some parties, but this attitude blocks the mind from seeking a meaningful solution, misdirects management effort and causes wasteful delays for those that require a solution. Denial of the difficulty of the problem often leads to the search for ‘quick fixes’.
A number of these may be attempted, and fail, before accepting that the situation needs a more fundamental approach. Emotions tend to be further disturbed by the lack of predictability, stress associated with dealing with the consequences and trying to solve the problem, potential loss of prestige, concerns for career prospects or financial cost to the company. Extended operation of poorly performing plant can reach the stage that it is accepted as the norm, in which case the ‘problem’ recedes, until refreshed by later circumstances. Attempting to fix a problem following earlier failures exposes personnel to career or credibility risks such that individuals may prefer to accept the situation than make further attempts. Pressure may be brought to seek an external party to absolve those involved from personal liability, but places the onus on the outsider to offer a solution that is technically, commercially and socially acceptable. Failure to meet all three requirements will not produce an acceptable solution.
Pre-existing conflicts, personal attitudes, and organisational culture may influence the position of a third party. Sources of inherent difficulties that he may face can arise from:
- Unrealistic performance expectations.
- Differences in procedure and communications style.
- Pre-existing stresses and positions taken by various parties.
- Psychological dissidence
- Personal discord and suspicion.
- Impossible objectives.
- Conflicting requirements.
These are discussed in detail elsewhere, (2), but a trouble-shooter must develop sufficient rapport and agreement of precise objectives with those involved to secure the framework for presenting a solution. An acceptable solution may be impossible, in which case the position must be identified as soon as possible and the ground rules changed or the action terminated.
Retrofit Rules
The provision of retrofit modifications to overcome operating problems with new plant is therefore not a task to be lightly undertaken because of the manner in which the requirement has arisen. There is invariably some delicacy in dealing with a fundamental design deficiency, as determining who is responsible for the inadequate performance is a distinctly separate issue to affecting a cure, but is not without interest to those concerned. There is no shortage of opportunities for those prepared to indulge in this precarious activity. The science and technology of bulk material behavior, so painstakingly developed by the great pioneers of the past, is all too frequently ignored by those who believe that the main function of bulk solids storage and handling equipment design is a straightforward engineering task to provide a route for the material to pass through. The stubbornness with which recalcitrant particulate materials resist to traverse these paths often comes as a surprising, but common experience. The situation can quickly unfold of a theoretically completed project standing idle, with demented operators beating the living daylights out of some hopper or chute that refuses to disgorge its contents whilst frustrated managers run around in ever decreasing circles shouting at everyone.
A familiar pattern then develops and the first golden rule of retrofits becomes immediately apparent; Rule (1), ‘Whilst time is invariably of the essence and in short supply during the translation of a project from the conceptual stage to the final design specification there is all the time in the world for an inquest when things go wrong’. The main purpose of this exercise is to allocate responsibility for the failure and its rectification. This process usually boils down to belatedly examining the nature of the bulk material in minute detail, not to solve the problem, but to seek some specific feature that can be held different from some sample or description provided earlier in the project so that the equipment supplier can completely disown responsibility for the disaster. Contract conditions are scrutinized with infinite care and resources allocated with little restraint, to place the burden of rectification on the other party. By default, the second law or retrofits kicks in; Rule (2), ‘Far more time and money can be spent on assessing responsibility than on curing the problem’.
In many cases an answer of sorts is available, but is unacceptable in cost. Intensive efforts may be made to secure a more economical solution, but the passage of time generates escalating delay costs and increasing pressure for the situation to be resolved, bringing in the third retrofit rule; Rule (3), ‘The time spent in considering the problem before calling in someone new to correct it is inversely proportional to the time then available to produce a solution’. The search for a cure is generally hampered by impediments inherent in the layout of the equipment that then exists, leading to Rule (4), ‘Constraints of headroom and space, flexible at the initiation of a project, are cast in stone when retrofits are required’. By this time, initial budgets are not only overspent because of delay costs on the commissioning and subsequent lack of production, but additional funds are seen to be required to affect what should have been available within the original forecast. The commercial implications follow with Rule (5), ‘Money is far tighter for putting things right than for making them wrong’.
The project history dramatically focuses the buyer’s attention to plant performance issues when considering the authorization of a retrofit order. Apart from this incurring unbudgeted expenditure, which may itself raise embarrassing questions, there is an acute awareness of the consequences of an unsuccessful outcome should the debacle continue to be unresolved. Hence retrofit Rule 6, ‘There is invariably far more interest in performance guarantees for retrofits that there was for the original equipment’. Understandably, the buyers view is that the essence of the contract is to rectify the situation with the entire onus on the new supplier.
Miraculously, the situation transforms once a retrofit contract has been awarded. The difficulty of the task is then not relevant. What had been a meticulously examined document reflecting exceptional conditions and concentrating on tightly drawn, specific objectives, suddenly becomes a standard legal agreement, as indicated by Rule 7, ‘Retrofit contracts are very special entities during negotiations, but are considered to be simple, standard purchases once an order is placed’.
Assuming that all goes well and a suitable outcome is secured, it is noticeable that the status of the rectifier is relegated to that of a normal equipment supplier. Two features follow from a satisfactory outcome. Rule 8, ‘If successfully completed, little credit is given to the supplying company, as they have only supplied to order, and the uncomfortable circumstances of the original incident and potential on-going liabilities of the user are best and quickly forgotten’.
There is a natural reluctance to perpetuate memories of a distressing episode, but some praise may be allocated, acknowledged or claimed within the user organization, as Rule 9, ‘The credit for correcting the blunder normally goes to the person who called in those that affected the change’.
Overhanging the whole situation is the awful position of those who undertake a retrofit that fails to satisfy the full aspirations of the user, for what ever reason. The reason and extent of the original shortfall and the unsatisfactory position that became the starting point are totally discounted. With all its implications, Rule 10, applies, ‘If not totally successful, the retrofit supplier is hounded to secure the objectives that were not met when the equipment was first installed. His reputation is in tatters and all subsequent expenses are expected to be fall on his head to put matters right’.
Some observations may also be made on various reasons that some solids handling equipment fails to perform the task demanded because of problems related to flow. Contrary to the conclusions of a Rand Report, (3), I would suggest that this situation does not reflect a failure of R & D, but is due to a combination of more elementary factors.
- There is a wide lack of appreciation of bulk technology in industry and of the complex rheological nature of particulate materials. (4), (5). Project engineers tend to concentrate on the efficiency and reliability of individual process stages, viewing materials handling as independent, low-level, mature and proven technology, rather than being a sequential process involving differing states of the product.
- Standard items or standard designs offered by suppliers at relatively low cost, are often purchased on the naive assumption that they will handle virtually any bulk material.
- Purchasers that are reluctant to face up-front costs of pre-verifying equipment performance tend to be even more reluctant to accept responsibility for lack of it.
- Suppliers of standard equipment are often not equipped, willing or sometimes able, to undertake retrofits that involve non-standard designs.
- Deriving the physical properties of the bulk material for design purposes is often seen as the sole obligation of the equipment provider, and only then at a stage of the project when options are limited and there may be difficulties in establishing data that reflect the full range of material and operating conditions that are to be accommodated.
- Due technical diligence that includes an actuarial risk analysis of the potential liabilities that may be incurred by differing degrees of performance shortfall of the system as an integrated plant, is rarely conducted as part of a project evaluation.
- Items of low capital cost tend to receive little design attention, despite their critical function in a unique flow route. (By contrast, liquid plants frequently incorporate standby elements or emergency by-pass facilities).
A feature of retrofits is that a higher degree of technical and/or specialized competence and experience is generally required than that involved in the supply of original equipment.
Retrofit conditions
Although circumstances surrounding the need for retrofit of inadequately designed solids handling equipment are usually inhibited by many constrains of the site, installation, cost and time, there are features favourable to securing a solutions and a workshop of tools, resources at the disposal of a professional solids handling engineer that open the range of options for modifications to provide a working solution.
- The reason for performance failure due to errors in engineering design is usually clear-cut and the remedy and responsibility for rectification in such cases readily apparent.
- Operating difficulties due to the nature of the bulk material are more common, but the experience of a bulk solids specialist is the essential ingredient previous absent from the application. There is, or should be, recognition that the situation demands specialist attention and heed paid to expert opinion.
- Samples of the bulk material that cause problems, which may not have been available or appreciated as potentially troublesome at the design stage, are usually available in abundance for testing.
- The translation of initial information to the final design can be examined, an assessment can be made of the current design shortfall and attention directed to the shortcomings of the original design data.
- Commissioning problems and operating experience is usually available to highlight the specific nature of the problem.
- The original plant design was not based on the correct technology; otherwise it would have worked well, so scientific methods based on quantified parameters, offers a route to rectification.
Two factors that usually impinge on those undertaking retrofit work are time and cost. There is often a great urgency to secure a solution to problems when expectations of production have been rudely disturbed. Nevertheless, acceptance of the proper cost of rectification is often clouded by the allocation of responsibility, exhaustion of the budget allocation and overrun of commissioning costs prior to seeking external advice. Great pressure can be applied to secure an expedient, effective, low cost solution by compromising on some features of the work. This is a route to disaster, unless the risks are clear and fully accepted by all those concerned as merited to have a reasonable, but not guaranteed, prospect of success.
The first task of a retrofitter is to advise on the procedure to be adopted and forms of options that maybe considered and secure a clear basis on which to move forward. The second task is to collect initial data, undertake a preliminary assessment and formulate a time frame for a more detailed investigation and the submission of a report that will indicate the steps to be taken and specific recommendations.
Retrofit Tools
Methodology
The historical circumstances and pressure for rectification should not detract the engineer from adopting a rigorous process of investigation, separating ‘Cure’ from ‘blame’ and remaining totally impartial and non-committal. Unless specifically appointed to advise on aspects relating to the contractual responsibility, this aspect should not be allowed to cloud the investigation.
The first task is to gather all the relevant facts of the situation in a systematic manner:
- Establish the full circumstances of the problem, history of the contract, design process, events during commissioning and operation, actions taken and their results, current status.
- Examine the situation first hand, discuss with the various principals and operators.
- Determine constraints, objectives, timing, commercial position, priorities and who is the decision maker.
- Secure appropriate samples of the full range of material to be handled, identify ‘worst’ condition and obtain measured values of the relevant physical properties.
The second stage is to examine and interpret the findings
- Analyse data and establish conclusions as to reasons for, and extent of, the operating deficiency and determine the essential requirements.
- Review the options for expediency, emergency, temporary or low-cost action and what is needed for a permanent, guaranteed performance.
- Prepare report and recommendations.
Depending on the urgency and complexity of the situation, this process may be completed in a short time or may take some time and effort to collate the information. With full knowledge of the criteria and clear objectives, a constructive proposal may be considered for authorization. This usually takes the form of a report on which a decision may be taken. It is often a mistake to dispense with the further services of the specialist at this stage as the translation of his recommendations to implementation can often loose sight of the crucial details that are inherent in a sound design.
The third stage should be to oversee the implementation of the design changes, ensuring that key features are not degraded in the detailing, fabrication, alterations or final assembly.
Finally, the modified plant has to be operated, to verify that the performance complies with expectations.
a) Technology: Quantified Design
Where operating problems are concerned with the flow of material in hoppers, chutes and transfer points, designs based on measured values of the relevant physical properties of the bulk solid concerned are well proven. Knowledge of these measurements allows the engineer to determine whether modifications are practical that utilize classic design principles within the constraints of the installation. This does not necessarily mean that this approach is the most economical option, but it provides a fall back position for a guaranteed solution for solids flow problems. Jenike’s work allows comparison of the respective benefits of mass flow over non-mass flow and of plane flow over radial flow regimes, so that these conversions may be considered in total, or as part of the solution, if circumstances and viability permit.
b) Brute force Techniques
Not recommended, but often the first recourse taken by operators to solids flow problems is to apply Brute Force to the equipment. Persuasion is rarely gentle and is applied to any accessible metal surfaces that may be linked, however remotely, to anywhere a blockage may be thought liable to occur. This often becomes a routine duty for an operator. Unfortunately, repeated unsophisticated attacks on the equipment by frustrated operators tends to cause further deterioration in the geometry of the flow channel, leading to a vicious circle of escalating violence. The resulting ‘Hammer Rash’, so commonly seen in solids handling plants, bears mute testimony to the acceptance of this technique as a permanent feature of much equipment operation.
An equally un-enlightened approach with more gentle use of force, at least to the equipment, is to apply vibration or substantial quantities of air to stimulate flow without a thorough analysis of the problem. Vibrators are usually positioned where the wall of the container is able to deflect easily. Cones are stiffest at their smallest diameter, and usually reinforced by an outlet flange, so vibrators tend to be placed some way up the cone. Arches, of course, are strongest at the smallest spans in the flow channel, so the effect of vibration above an arch location can do more harm than good by consolidating static material. Cones also vibrate like a bell, with stationary nodes at 90 degrees to the point of maximum flexing so, if a second vibrator is to be fitted, it should be located at 45 degrees to the first in the plan view and on the same level. Operation of the two units should then alternate for a short period each to create a fully ‘live’ wall action, but only when a flow stoppage is detected.
Impact vibrators are useful for stimulating flow by sending a shock wave through the bulk, but tend to be noisy. Rotary electric vibrators are mechanically efficient and varying their speed can tune the alternating force to a natural frequency of the container system. However, they operate at a relatively high frequency and do not stop quickly when the power is turned off, so they are often left running all the time that flow is required rather than applied only when necessary.
b) Sophisticated Force.
A more effective use of a vibrator is to bolt an inclined, resonant ‘reed’ against the inside of the hopper wall where the vibrator is fitted. This should face down to the region where an arch is likely to form, to transmit the energy of vibration into the sensitive region of the bulk material via the cantilevered end. The technique is even more effective if designed to modify the flow regime in a conical hopper by shielding the outlet and forming a type of flow annulus that includes a shielded region under the reed in the form of a ‘Bates’ Insert. Vibration of the reed end against the hopper wall de-stabilises any potential arch at the end of the reed. Preferential extraction from the sheltered peripheral area under the reed generates a weak sector in the circumference that undermines the stability of the hoop stress normally opposing reduction of the cross section in radial flow. The flow regime above the insert is thereby changed from radial to a transversely relaxed, plane flow form
The combination of applied vibration in the critical arching location, substituting a pseudo-plane flow channel with circumferential relief for the original radial flow regime and also reducing wall friction by vibration of the contact surface, can radically transform the flow reliability of a troublesome cone outlet. The externally mounted vibrator should be of the rotating, out-of-balance quadrant type, mounted with the axis parallel angle to the internal reed. The top end of the reed should be clamped with a tapered pad between the reed and the hopper wall to form a robust mounting for the reed inclined across the axis of the cone.
c) Pneumatic Force
Short-term, massive air injection, as by air cannons, will cause a major local disturbance that can break down the side of a rathole or dislodge a bed of residue sticking in a corner. This is essentially an intermittent operation and produces a significant change in the condition of the material displaced. The energy release is substantial and an essential requirement is that the stored product can expand from the injection point to a local unconfined surface, preferentially adjacent to the outlet.
This approach does not address the fundamental cause of a flow stoppage, therefore is only considered to be appropriate for occasional use, such as clearing residue in bins that will not completely self-clear, and not to be used as a regular operation to overcome arching problems. Drawbacks of this method, apart from shock loads, vibration and noise, are that their need is only apparent when flow ceases and that the radical change made in the state of the bulk material is far from conducive to securing consistent discharge conditions or improving flow.
The general sales philosophy regarding air cannons is that if one does not produce the required result, then increasing their numbers will eventually reach a point that flow must take place. This policy sometimes leads to hoppers resembling porcupines in their proliferation. Success of sorts may be achieved, but this route serves to perpetuate inadequate design and poor solutions rather than addressing the underlying fundamentals to secure improved designs.
A common alternative use of air is to employ fluidising pads, ‘magic mushrooms’, ‘air lances’ or other local means of air injection. The pressure to overcome initial penetration resistance may be high in a settled, fine particulate material but significantly reduces once a pathway for the air has been developed.
Air supplied at this initial penetration pressure will tend to find a weak channel within the bulk and, if the supply volume in not controlled at the original pressure, can fluidise material in the developed gas route whilst others regions remain cohesive. The provision of an uncontrolled, continuous supply is almost invariably far in excess of the amount needed to maintain a stable, ‘flowable’ state of the bulk material, generally being more dependent on the size of connection supplied and the pressure setting of a regulator.
High pressure air jet may be used to mechanically displace static material in locations of high strategic importance in the required flow channel, but with the penalty of causing excess local dilatation and uneven density condition, apart from the venting demand and possible dust.
d) Sophisticated Pneumatic Force
Recognition should be given that the void space in a particulate solid does not have a fixed value and will naturally change to suit settled bulk conditions or the dynamic circumstances of flow. Bulk flow requires a degree of re-ordering of the packing structure compared with the close-packed particle to particle contact stresses that support the overpressure of a superimposed mass in a settle bed. The void demand of expansion of the bulk to change from a settled to a flow condition creates a reduced internal pressure that is not relieved until air permeates the bulk from ambient. The pressure differential inhibits expansion and is a prime cause of hoppers storing fine powders being slow to start discharging and having low discharge rates. By contrast, a bulk material that is maintained in a slightly dilate condition by a positive pressure in the voids is able to expand with the benefit of a positive pressure differential and with reduced opposition of the particle-to-particle attractive forces that develop with high contact pressure in a closely packed bulk material. Sustained air injection is generally effective only in applications of fine powder storage, because the rate of percolation through a bed of coarse particles tends to nullify significantly effects.
A problem is that pre-handling, especially by pneumatic conveyor supply, tends to deliver the material into the bin in a highly dilate condition with a propensity to ‘flush’ or be difficult to control on discharge. This is hardly the state that one would normally consider adding extra air. If the material does not settle to a stable condition sufficiently quickly to avoid ‘flushing’ problems, then means should be adopted to accelerate the de-aeration of the bulk, (4).
This alone, however, is likely to exacerbate the ultimate flow condition by developing a strong, settled condition. To avoid this, a small, controlled volume should be continuous added at the same time that the material is being de-aerated. This injection will not significantly extend the de-aeration period, as the volume added is trivial in relation to the initial exodus of excess air. Controlled volume injection will prevent the settlement of the mass to a poor flow condition by replenishing the air loss at a stage that the material has partially settled and maintain a positive pressure in the voids that offers partial support from storage overpressures, (5).
The added air can diffuse through the expanded interstices prior to the gaps closing as the material settles, sufficient air being introduced to replace that leaving at the stage that the material is in a non-fluid state but in a condition at which flow can easily occur.
The air supplement needed can be identified by simple settling tests that indicate the rate of volume change at the required flow condition categorized by its density. The key lies in volume control of the addition at sufficient pressure to overcome any resistance.
Typical decanting curves showing natural void pressure decay, accelerated de-aeration and a combination of controlled injection with accelerated de-aeration. The location of limited-volume air injection for ‘powder state control’ should be at least two orifice diameters about the outlet to avoid short-circuiting of escaping air. The sustained void pressure will maintain the bulk in a ‘flowable’ condition and provide a differential force above ambient to encourage the material to discharge when the outlet is opened.
Exploitation of Fundamental principles in Powder Technology
The flow benefits of Mass Flow and plane flow are well established, as are the frictional values for wall slip and self-clearing. Wall friction testing is an essential tool for quantified design. Measurement of this value on relevant surfaces will indicate whether a different contact surface will make a positive change. The crucial outlet region of a cone or pyramid shaped hopper that is not steep enough to generate wall slip can be transformed by creating an ‘Expanded Flow’ type of flow regime. based on a plane flow region that extend from the outlet to a span greater than the ‘critical rathole dimension’. Allowing a small degree of transverse expansion in the flow channel permits slip to occur at reduced wall inclination and flow to take place through smaller openings than in a conventional Vee slot. As will be seen on a yield loci diagram, reducing the minor principle stress, σ2, results in lowering the major principle stress, σ1, by a similar amount plus double the radial difference of the respective Mohr’s circle.
‘Cone-in-Cone’ inserts generate mass flow in hoppers that have shallower walls than that required for classical designs. Vee-in-Vee inserts allow similar conversions to be made in plane flow, provided extraction takes place across the total outlet area and the minimum slot width is greater than the critical arching dimension. Interfacing such construction with screw or belt feeder places higher design demands on progressive extraction. This burden can be eased with a form of ‘expanded flow’ shape, where an insert system is followed by a short, classical mass flow design.
Proprietary Equipment
A wide range of discharge aids are available, from agitating devices such as bin activators to controlled volumetric discharges such as screw feeders. Mechanical discharge aids are mainly used where the size of the final discharge opening is inadequate to provide reliable flow or the required discharge rate. To some extent, the proliferation of the flow aids industry is mute confirmation of the demand for overcoming flow problems. This are no substitute for a scientific design but the facilities provide additional tools for dealing with obdurate flow problems.
A key criterion affecting the design requirements is whether mass flow is needed to avoid stagnant pockets of storage or is employed to secure flow through a small outlet. Broad experience is required to secure and optimum solution that may involve a combination of state of art technology with flow aids incorporating air injection, vibration and/or proprietary mechanical equipment.
Summary
The undertaking of retrofits can be a challenging activity, placing demands on commercial, psychological and technical experience. As appreciation of the implications of these three fields, with a sound background in the technology and best design practice in the solids handling industry, will equip the professional practitioner with the tools to engage in a fulfilling and rewarding activity.
References
‘Linking R & D to Problems experienced in Solids Processing’. Rand Report. Murrows. E. 1984.
‘Troubleshooting and Human Factors’ Sloley A.W. A.I.ChE Spring National Meeting. 3 – 13 Mar. 1997.
‘A quantitative assessment of R & D Requirements for Solids Processing technology’.
Rand Report. Merrows. E. 1986
‘The need for Industrial Education in Bulk Technology’ Bates. L, Bulk Europe. 2006
‘A multi-attribute characterisation diagram for bulk solids’. From E. McGee Thesis. Caledonian University. 2006
‘The Use of Flow Inserts in Bins’, Bates. L.
‘Dealing with aerated bulk solids’, Ajax Equipment publication.
‘The use of air to stabilise flow condition’. Miles, et al. Warren Spring publication. 1968.
