Unheralded as it may be, specwriting is the foundation of a successful feeding system. This article provides a starting point for anyone charged with the responsibility of defining their feeding system requirements.
Specwriting is a mystery turned upside down - not even the author knows its solution till the last chapter. In a very real sense process system specifications are simply a carefully selected collection of clues. When a prospective customer requests a system proposal from potential sup¬pliers he is posing a problem for each of them to solve. His specifications form constraints to the problem’s solution, just as a mystery’s clues must fit the final explanation.
The specwriter’s intent is clearly to provide a basis for determining the best, most reliable and most cost effective system to meet his purposes. That single consideration demands the specwriter be especially diligent in fashioning his process requirements. And, unlike a mystery author, a specwriter must collaborate with his own people and com¬municate with his supplier audience, so his task depends as much on listening, interpreting, negotiating and responding as it does on simply conveying rigid facts.
Written with the assuredness that specwriting will never run short of its own innate twists and turns, this paper scrutinizes specwriting for process systems and, in doing so, sweeps away at least some of its mystery.
The Three Types of Specs
Generally speaking, specifications fall into three categories - each with its own purpose. Budgetary specifications are written mostly for systems rather than for individual pieces of equipment. Their function is simply to provide a ballpark cost for financial projections and an initial estimate of economic feasibility. Once a project is given the preliminary go-ahead, a detailed system specification is written encompassing all functional, per¬formance, testing, safety, delivery and other requirements relating to the system and the equipment comprising it. Usually written for the purchase of an individual piece of equipment, an equipment specification is similar to a system specification but on a smaller scale. An equipment spec lists the details of the application and the complete range of requirements to be met by the selected equip¬ment.
Specwriting for Budgetary Quotes
Because budgetary specs are written to gain a foothold in the uncertain worlds of financial projections and ‘out front’ process planning, specwriting specifics will be few and far between. The key to drawing up a reasonably plausible budgetary spec is to focus on con¬veying to potential suppliers the system’s goals in terms of its function and required performance. Also, ambient con¬ditions and any special relevant considerations (such as the need for explosion-proof motors in a munitions plant) should be communicated.
It is widely recognized that budgetary proposals are, by their nature, approximations. But a capable supplier has the advantage of his experience with other, comparable systems. Additionally, his experience attunes him to associated system costs the specwriter may be unaware of in outlining the basics of his project. A cooperative sup¬plier will not overlook these elusive cost elements (e.g. the anticipated requirement of optional equipment or special control devices). By focusing budgetary specwriting on the fundamentals of the proposed system, and taking advan¬tage of the experience of suppliers, a usable approxima¬tion of total system costs is possible.
Specwriting for System Quotes
Turnkey operations purchased from engineering con¬tractors relieve the customer of many specwriting chores, but more frequently in-house specwriters must obtain pro¬posals from at least a few individual system suppliers. In speci¬fying a feeding system the particulars of flow rates, materials, control, and a host of other factors must be con¬sidered. But from the point of view of structuring the set of specifications, nearly all process systems have three aspects in common: function, accuracy and conditions. By describing what a system must do (function), how well it must perform its function (performance), and under what cir¬cumstances it must operate (conditions), a system is ninety percent specified. The other ten percent specifies project administration, terms of sale, equipment test and inspec¬tion, delivery, and other contractual necessities.
Six factors describe a feeding system’s func¬tion, and each must be addressed to fully define the feeding task.
Basic Purpose - Although a feeding system is often one of the more critical systems in the process, it still can be looked upon as a black box with known inputs (control commands, material supply, etc.) and desired outputs (a blend, a product, a rate, etc.). What happens in that feeding system black box is the system’s basic purpose, whether it be the simple control of flow rate or the com¬plex combination of many materials. Describing a feeding system’s purpose in the most basic terms will begin the spec on the right foot.
Number of Materials - Leaving all other material specifics to a later section, the mere number of materials to be handled, processed or controlled is important in determining the system’s overall function. The number of materials in use during normal process operation may only partially describe the true situation, however. As is sometimes the case, a process line may have many more materials stored and ready for use as formulations change. Additionally, if circumstances permit, two materials may be fed on the same feeder, although rarely at the same time.
Rates - A feeder’s rate range, dictated by the demands of the process, should be specified not only in terms of its minimum and maximum rates, but also in terms of its nominal, or normal, operating rate. Often, however, the operating rate will be subject to wide and frequent variation, such as in many proportioning systems. In that case, a nominal rate loses much of its significance, and, if possible, a nominal operating range expressed by its upper and lower boundary values will assist the supplier in deter¬mining the appropriate feeder and sizing choice.
Proportioning Requirements - Most feeders can easily serve as either master or slave in a proportioning system, but when a feeder is a master (as opposed to some other piece of equipment serving as a master), the main ingredient feeder is usually assigned the leading role. A master feeder can provide the pace-setting control signal to the entire feeding system or just part of it, as required. Its influence can extend beyond the feeding system when it controls other flow-dependent process parameters such as the speed of a packaging line, for example.
Thus, one key to determining which piece of equipment to specify as the master involves deciding which controllable process parameter best dictates all of the remaining controllable parameters. Flow rate is often the best choice, but it is not too unusual to find other process parameters in the driver’s seat: temperature, extruder speed or even moisture content. It all depends on what’s critical and controllable in the process.
There is another key to specifying proportioning systems, and, since it has to do with system dynamics, it sometimes goes unnoticed. Because each element in a master/slave system has its own dynamic response characteristics, a change in the master’s control signal must not occur faster than the slowest slave’s capability to keep pace with it. One solution strategy is to confer master status on the slowest responding piece of equip¬ment. That may not always be feasible or desirable, however. A second approach involves choosing an ac¬curate and highly controllable piece of equipment as the master, and then taking appropriate measures to guarantee that any change in its control signal occurs no faster than the slave’s ability to keep up. This can be ac¬complished by time-ramping, either manually or automatically.
Control/Data Requirements - These requirements are usually straightforward, but, like an iceberg, ignoring the part that lies beneath the surface can be fatal. Under the surface of control requirements lies a family of con¬siderations which relate to controlling the system when it is not in a normal operating mode. Some of these considera¬tions include control at start-up and shut-down (both nor¬mal and emergency shut-down). Questions of inhibiting or enabling equipment operation under specified conditions also emerge, as do questions of coordinating time delays possibly required to prevent off-spec production during rate changes.
Determining data requirements hinges heavily on how and to what extent the performance of the process is to be monitored and managed. Generally, data is required for two purposes: control and record keeping. By assessing data requirements from both perspectives, most data needs will quickly become clear. To assist in that deter¬mination process, review the literature of equipment sup¬pliers to see what data output provisions are offered (both as standard and optional). Those provisions will often be more than sufficient.
System Interface - As mentioned earlier, a feeding system can be looked upon as a black box. That perspec¬tive underscores the importance of a system’s interfaces ¬both mechanical and electrical. Since the specwriter must orchestrate many separately specified systems into a unified whole, it is critical he take special pains to ensure that the various pieces to the puzzle fit.
Up to this point all of the factors presented center on purpose, not performance. The requirement to perform to a certain prescribed standard has a great ef¬fect on the choice of equipment a supplier will propose. As a result, it is of the utmost importance that the spec¬writer be clear and precise in stating performance stan¬dards.
Usually the specwriter will be given a set of product specifications from R&D or the Quality Control department. Just as often these specifications will have to be translated into a form that is compatible with the statement of per¬formance standards relevant to each system in the pro¬cess.
To say that the accuracy of an ingredient’s flow should be to within 1/2 % of the desired value is simple and understandable, but is the leeway expressed in weight or volume? And at what statistical significance is the specification drawn? A complete feeder accuracy spec must precisely address both repeatability and linearity per¬formance. For example, a well written weigh-belt feeder spec might read like this: Repeatability: ±0.5% of sample average weight at 2 Sigma based on 30 consecutive samples, each of which is taken over a specific time interval (typically one minute); Linearity: ±0.5% of set rate based on 10 consecutive weighed samples taken over a specific time interval (typically one minute) and throughout a range of 20:1 from full scale.
As mentioned above, stating performance standards affects the type of equipment a supplier will propose. Again, feeding systems are a good example. Since there are basically two categories of feeders - gravimetric and volumetric - a supplier may propose a volumetric feeding system if the specwriter’s required accuracy is not too stringent, but may propose gravimetric feeders if the ac¬curacy spec is tightened.
System dynamics also affect accuracy specifications. Here’s an example: Two feeder-controlled flows are to pass through a mixer. The effective residence time in the mixer is, say, one minute. Question: Over what time period should the feeders be expected to perform to the desired accuracy standard so the blend itself is on spec? Certainly less than a minute, but how much less? Will the feeders need to perform to spec on a second-to-second basis if their flows will be averaged by the blending action of the mixer? If not, how would you express the accuracy standard? Achieving high accuracy over very short time periods is more difficult and costly than achieving the same high ac¬curacy over longer periods. Specwriters need to consider that fact carefully when devising performance standards.
Anticipating all the conditions to be faced by process hardware is difficult at best. Operating conditions do, however, conveniently categorize them¬selves into three pigeonholes: ambient, material and input/output conditions.
Ambient Conditions - Most often ambient temperature presents few problems to a feeding system unless temperature is extreme or varies widely. Suppliers simply need to know the ambient temperature and its range to forestall any problems. Vicious extremes should be avoid¬ed, though, if at all possible. Also, outdoor installations should be identified as such so the weather-related con¬siderations of wind, rain, icing and so on may be taken into account.
Vibration is a common condition in many processes, and it can present difficulties for some weigh-feeding systems. Since it is often impossible to precisely quantify the frequency and amplitude of the vibration, it is best to be conservative when dealing with the condition. Gravimetric feeders should always be mounted on rigid supports (usually supplied by the customer) to prevent the transmission of ambient vibration to the weighing ap¬paratus. Provisions such as shock mounts and flexible con¬nections should also be specified to further isolate the feeding equipment, even if severe vibration is not ex-pected. Consider it insurance.
Space is an ambient condition frequently overlooked. Considering only how a piece of equipment will physically fit into the process ignores an important point: elbowroom. Maintenance people need access, and the equipment may need free space if it is to be disassembled for clean¬ing or repair.
And finally, fumes, vapors, dust and the like are present in many processes. A complete statement of operating conditions requires that corrosive or explosive atmospheres be described so appropriate measures may be included in the supplier’s proposal.
Material - Because of the vast diversity of materials and the broad range of physical conditions in which nearly any material can exist, all materials to be used in the pro¬cess must be specified as completely as possible. If feasi¬ble, samples of the materials should accompany the specification when it is sent to candidate suppliers. Sup¬pliers must know: the name of the material, its bulk density (packed and loose) or specific gravity if a liquid, its form (powder, granule, paste, slurry, fiber, etc.), its particle size, its flowability (floodable, hard-to-flow, dusty, cohesive, etc.), and its safety-related characteristics (toxic, explosive, corrosive, hygroscopic, etc.). All these questions must be answered if suppliers are to respond with a thorough proposal for a workable system.
Input/Output - Each system in a process and each piece of equipment in a system faces input and output conditions, all of which have to be specified in advance. For feeding systems in particular, these conditions include pressure, material supply, and the characteristics of neighboring equipment. Some applications require the feeding system to be slightly pressurized to prevent the in¬trusion of the outside atmosphere. Some others involve the possibility of blowback of discharged material into the feeding zone, and still others require the feeding system to operate smoothly even when faced with an intermittent supply of material.
Specwriting for Equipment Quotes
Specifying on the equipment level is the last and most detailed phase. Equipment specifications are often written as an integral part of a system quotation, but just as fre¬quently they are created to replace equipment in an ex¬isting line. Understandably, many of the points that com¬prise a system specification also comprise an equipment specification, and thus do not require repetition here.
A complete feeding equipment spec not only helps determine what feeder a supplier will propose, but also what configuration it will take, what accessories will be re¬quired, and even what materials will be used in the con¬struction of the unit. If the specwriter has carefully reviewed his equipment needs from the perspectives of function, accuracy and conditions, his list of requirements will cer¬tainly be long, but it might not be complete. Unless the specwriter is familiar with suppliers’ offerings and capabilities, he may miss some requirements he doesn’t intend to ignore.
From supplier to supplier, differences in feeder design and construction result in different levels of performance for otherwise identical applications. Being aware of dif¬ferences in operating principle, control design, material handling capabilities, weighing system, and overall con¬struction can greatly assist the equipment buyer in defining his needs.
The thorough specwriter will also investigate suppliers’ optional equipment to clue him in on needs he may be missing. For example, the inclusion of a viewing port to conveniently inspect the feeding zone may be a desirable requirement - especially if the system is under pressure or the material is at all toxic. If the material is dusty, a collec¬tion system may be required to keep the feeder free of build-up. Or if the feeder is to undergo frequent cleaning, the specwriter will benefit by knowing that some suppliers offer feeders with sanitary duty or quick clean design features. Similarly, if the application involves feeding cor¬rosive materials, stainless steel may be specified as the material of construction. Also, feeders may be painted to specification and have special motors fitted to guard against explosion in combustible atmospheres. These are just some of the accessory items available from suppliers. There are more, and it will pay the specwriter to be familiar with them when he sits down with pen in hand.
Finally, reliability and maintainability are always impor¬tant considerations and therefore deserve a place in any complete equipment specification As he familiarizes himself with the ins and outs of feeders, the sharp spec¬writer will identify the equipment features and design characteristics he thinks might increase reliability or ease maintenance.
If specifications have been written reflecting both the needs of the application and a familiarity with existing or procurable supplier technology and hardware, the customer will most likely be in the enviable position of hav¬ing to select from among several feasible proposals. Cer¬tainly, the major question is whether and to what degree the technical specs have been met. (It is the exception rather than the rule when a proposed system unequivocal¬ly meets all the specifications.) After a process of clarifica¬tion, accommodation and negotiation, most technical wrinkles can be ironed out, and, in this scenario, several dif¬ferent but attractive proposals may still remain.
Peripheral (but not unimportant) factors then come in¬to play. They include the reputation of the supplier (his reliability, experience, references, etc.), the adequacy, responsiveness and accessibility of his service force, his ability to train operating personnel, the clarity and thoroughness of his manuals and other documentation, as well as delivery and price.
While the technical aspects of a pro¬posed system must be given the greatest weight in evaluation, strong consideration should also be given to these related factors because, as good as a system may look on paper, its continuing operational success will de¬pend as much on the support of the people who made it as it does on the integrity of the equipment itself.
As one of the foundation documents for the creation of a process, a specification inevitably requires the invest¬ment of a great deal of thought, time and effort. Making sure that investment pays off demands a balanced perspective of the ‘big picture,’ a familiarity with suppliers and their equipment, and a diligence in clearly, thoroughly and precisely communicating your needs.