Sistemas de control de lazo abierto y cerrado

High purity water and steam are the most widely used, and often the most expensive raw material or utility in a pharmaceutical facility. Improper sizing or selection of a steam or water system could limit or even shut down production if under sized; or compromise the reproducible quality and increase the capital cost if over-sized. However, system sizing is not the starting point in design. Proper definition of water quality require-ments and usage can save construction as well as operational costs.

Figure 3-2 shows a graphic representation of the system boundaries, limitations, and restrictions the de-signer faces when planning a pharmaceutical water system. Initial system planning reveals primary bound-aries that establish the cornerstone for design criteria. These primary system boundbound-aries are Water Quality, Use-Point Criteria, and System Criteria.

During initial planning, the limits of each boundary need to be established. The arrows encircling each bound-ary represent limitations that establish more specific operating strategies and ranges. When documenting these limitations, the designer should always indicate ranges of acceptability, rather than a specific value or position. This allows more flexibility in final planning and detailed design decisions.

The reality of certain restrictions will sometimes force a specific strategy. As long as the decision leads to an answer that is within the limits of the system boundaries, this is perfectly acceptable. An example is a facility where the use-point criteria require non-compendial water with microbial control. However, there happens to

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be an oversized WFI system in an adjacent area, so the designer decides to provide WFI to the use-point. In the example case, documentation defining water quality should identify the quality required for the product and why the WFI was used instead.

The primary emphasis of this section is to outline a systematic approach to planning a pharmaceutical water system. Figure 3-2 outlines a planning methodology that begins with the selection of water quality, given its own system constraints and limitations. Then the use point criteria are established, followed by an initial system planning exercise. Often, these sequential steps are repeated as information in the design process iterates, and further criteria about the overall system boundaries are identified.

Figure 3-2 Pharmaceutical Water System Planning

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3.3.1 Establish Water Quality

The first step in the evaluation of water systems is the selection of water quality required for the specific product and process operation. Selection is based primarily on the dosage and form, and the microbiological and chemical purity criteria set for the product for which the water is used. The selection must consider underlying factors that have impacts on quality control; installed and operating cost; maintenance and practi-cality.

See Section 3.2 in establishing possible water quality via development of the decision tree. Making notes as the water quality is designated for each use-point, indicating the basis for each decision. Simple annotations from the supporting documentation will be useful in later stages of the planning process. System design constraints may provide the motivation to challenge water quality or other criteria, particularly when it can be demonstrated that the change does not affect product quality or manufacturing controls.

3.3.2 Characterize Use Point

Once the initial selection of water quality has been established, the operational criteria should be character-ized for each use point. A matrix should be developed to outline the primary criteria required for system design.

Each use point should be annotated with the proper values for pressure, flow, and the temperature range of water entering unit operation, or process point from the water supply system. Establishing a range, rather than a fixed value, increases opportunities for system optimization by allowing a more flexible approach to final design.

This data can be organized in many ways, but a well-planned spreadsheet can simplify the planning process and provide clear decision pathways for future detailed design activities. Table 3-1 shows an example of a spreadsheet used to characterize use-point flow and system demand. Flowrate is primarily used to size lines, whereas Daily Use leads to storage and generation decisions. The Diversity Factor is one way to level-out anticipated usage, assuming that not all loads happen every day or at the same time. This table indicates a Clean in Place (CIP) system and stopper-washer that are both likely to be used on the same day, but never at the same time. Therefore, only the higher flowrate is relevant to loop sizing as shown in the Design Flowrate column. Demand flowrates are eventually used for branch line sizing.

FLOWRATE DAILY USE

Table 3-2 Use Point Criteria

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Once the location and qualities are finalized, the various properties can be charted on a requirements analy-sis histogram. This can be done with the aid of a computer and either simulation or spreadsheet software for larger systems, or done manually for small systems. At this point, basic Process Flow Diagrams also provide a good pictorial view of the water qualities, locations and the point-of-use properties.

Figure 3-3 Water Usage Chart

3.3.3 Establish System Criteria

Histogram analysis is beneficial for determining overall system peak demand(s), average demand, and the relationships between peak demand time periods and their flow rates. Figure 3-4 below shows a hypothetical storage tank profile using the 24-hour demand profile from Figure 3-3.

There is no “Rule-of-Thumb” for minimum water level, or the optimum water level to turn on a still. However, these charts provide the tools for creating various scenarios to simulate recovery times from a failure, future expansion or reduction capabilities and analyze other factors that allow design of a properly sized water generation, storage and distribution system.

System planning and analysis also reveals other restrictions that influence design, and often lead the de-signer to re-evaluate the primary boundaries as discussed earlier. These restrictions might include items such as:

• Must the system be available at all times?

• What are the constraints on a shutdown?

• Is the plant/personnel able to handle chemicals properly? Are permits in place?

• Is production batched or continuous?

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• Are the products dedicated or multiple product groups?

• How is campaigning between products handled?

• How much time is available for sanitization? Should redundancy be provided to allow adequate time for sterilization?

3.3.4 Revisit Water Quality

With all use points characterized for temperature range, and demand, the quality of water is revisited. A thorough review of use point criteria typically reveals a wide range of acceptable delivery conditions for the water. Since it is typically not practical to operate multiple water systems to provide the exact water conditions desired of the end product, compromises must be made. These compromises might include providing water of a higher quality than required to simplify the water treatment or delivery systems, or provisions for control-ling water consumption at a use point to limit peak demands. Whatever compromises are made, water must be delivered at conditions within the boundary limits.