Issue



Designing a water system


10/01/2004







Quality and quantity both matter in the design of water systemsfor clean scientific laboratory facilities

BY DALE GORDON, MORGAN NORRIS AND GUY HART

Purified water is required for a wide range of applications within a facility, including cleanrooms. Water quality needs can range from laboratory grade to ultra-pure water, depending on the sensitivity of critical applications. The quantity of water needed from one point of use (POU) to another can vary from a few liters per day to several hundred liters per day. Understanding and then meeting these various requirements is a challenge but is critical in the design of a total water purification system for a cleanroom facility.

Design process stages

The design process can be broken down into four simple phases: investigation, approach selection, loop design and finalization.

Phase I: Investigation—A successful design outcome depends on the designer's ability to establish precisely the needs at each POU in the facility. Although investigating a facility's water requirements may appear straightforward, it is essential to have a well-defined plan.

Understanding a facility's water quality needs requires knowledge of various water quality standards and the organizations that publish them. Determining the correct standards to follow depends on the regulatory environment in which the facility operates and the specific applications at each POU.

Organizations such as the American Society for Testing and Materials (ASTM), College of American Pathologists (CAP), International Organization for Standardization (ISO), National Committee for Clinical Laboratory Standards (NCCLS), and United States Pharmacopeia (USP) all publish their own water quality standards. Typically, industries tend to follow the requirements of their affiliated organizations. For instance, in the medical diagnostics field, laboratories usually follow CAP or NCCLS standards. USP is written and followed by the pharmaceutical industry, and ISO standards are followed by quality organizations across multiple industries. In certain areas, such as the pharmaceutical industry and medical diagnostics, water quality standards are regulated and require compliance. To eliminate some confusion, we have defined water purification specifications that acknowledge regulatory agency standards (see Figure 1).

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After the water quality requirements have been established, the next step in the process is to determine the facility's water quantity requirements and how the water will be accessed. If the facility will have multiple POUs, then it is important to determine how much water will be used at each POU. In facilities with glassware washers, autoclaves or other equipment incorporated in the loop, the necessary flow rates and pressure requirements for this equipment must be considered when determining the total quantity of water required. Correctly identifying these needs and locations will make the difference in successfully designing a water purification system for a cleanroom environment.

Phase II: Approach selection—For many years, the conventional approach has been to plumb water throughout a building or facility in a single-loop configuration with a single make-up system. Often, the loops can run for thousands of feet. If the loop becomes contaminated or the make-up system fails, then the entire facility can be without water until the problem is resolved. Facilities can be shut down completely for several days due to water contamination issues.

Using multiple make-up systems (duplex design) to feed a loop lowers the risk of potential damages from a single make-up system failure. The duplex system approach adds redundancy to the configuration, but is similar to the conventional approach. If the loop becomes contaminated, then all POUs are out of service until the loop can be decontaminated completely.

Another approach is to use several smaller loops feeding individual floors or departments. The added redundancy reduces the risk of a total facility shut down. Having several smaller loops allows departments or individual floors to maintain and control their systems locally. Water is the most commonly used reagent in most facilities, leading end users to become more involved in the maintenance of the systems. The localized loop approach begins to put some control back in the end user's hands.

A hybrid approach uses a small central system with polishers at each point of use. The advantage is the ability to plumb a lesser grade of water in the loop and then raise it to the desired quality level at the POU. However, there is still an inherent risk of loop contamination and downtime, which also can shut down a facility completely.

When selecting an approach for getting water to multiple POUs with varying quantity and quality requirements, there are certain rules that should be followed. For example, it is important that the system continuously circulates water through a facility. The water must be kept moving continuously as stagnant water is a source of bacterial proliferation. Without a properly designed loop, the water cannot be circulated endlessly. Therefore, dead-legs (areas where water sits in the system) must be kept to an absolute minimum. Failure to do so is inviting bacterial contamination. Lastly, shorter loops typically result in less contamination. Once an approach is selected that incorporates one or more distribution loops, it is time to select the purification and monitoring equipment within the loop.

Phase III: Loop design—The appropriate materials of construction for the piping infrastructure and assembly are paramount in contributing to the preservation of the water quality within the loop. Identifying the most appropriate material and assembly method requires an analysis of the end user's application(s), water quality needs and desired cleaning and/or sanitization protocol.

Using the proper piping materials will minimize degradation of the purified water during distribution. There are a range of materials to choose from, including PVC, stainless steel, polypropylene and PVDF. Characteristics that differentiate the piping materials include material cost, installation cost, heat resistance and tendency to leach organic material into high-purity water.

For the majority of applications found in the cleanroom environment, polypropylene serves as an excellent choice for loop piping. It is a relatively inexpensive piping material with low installation costs and poses minimal risk of leachable contamination into the high-purity water. Polypropylene is not a heat-resistant material and these loops cannot be heat sanitized.

The presence of biofilm within a piping system is inevitable and exists to some degree in all water piping systems. A targeted flow velocity of 3-5 ft/s within the piping system is viewed as critical for minimizing the adverse effects of biofilm build-up and bacteria proliferation. This velocity effectively reduces biofilm because it creates a turbulent flow pattern within the piping. In turn, turbulent flow patterns make it difficult for bacteria to adhere to the piping walls. Flow rates well below this range create a laminar flow condition, where water flows with minimal turbulence through the pipe. This makes it easier for bacteria to adhere to piping walls and contributes to biofilm development.

In selecting the size and performance characteristics of the required distribution pump, one must balance the flow rate, pressure drop, pipe diameter of the distribution piping system, and targeted flow velocity of 3-5 ft/s. To do so requires the following actions:

1. Determine the expected distribution loop length (include fittings, valves, etc.) along with an accurate assessment of the number and flow rate requirements at each of the points of use.

2. Choose an initial pipe diameter (_", _", 1" 1 _", etc) reflecting the length and point-of-use profile of the distribution loop as determined above. For smaller loop configurations with minimal points-of-use and flow requirements, a smaller pipe diameter may be anticipated. For larger configurations, a larger pipe diameter might be expected. Keeping flow requirements and target velocity in balance by increasing or decreasing the pipe diameter is an optimal way for reducing pressure losses (see Figure 2).

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3. To calculate the required distribution pump pressure, begin with the minimum pressure needed at the most remote point of use. This can be achieved using a back pressure regulating valve at the end of the distribution piping. To this minimum pressure add the pressure loss through piping equipment and be sure to factor for piping elevation changes. The ideal total target loop pressure is 55-75 psi (90 psi maximum).

Loop monitoring components and devices should be incorporated within the loop design to verify and document performance. Conductivity or resistivity monitors should be used to measure ionic purity levels. Total Organic Carbon (TOC) monitors are used to measure the levels of organic carbon compounds present in the loop. The best designed make-up systems incorporate these monitors. Sanitary sampling ports or valves provide standard and clean protocols for collecting purified water samples to be tested and monitored.

The water produced by the make-up system is typically stored in a reservoir or distribution throughout the loop. Reservoirs are available in many sizes, configurations and materials of construction. Common practice is to use tanks constructed of molded polyethylene because it produces a very smooth finish inside of the reservoir and minimizes biofilm growth. Well-designed reservoirs also have a conical bottom that allows them to be completely drained and ensures that the water within the reservoir is completely turned over. Sanitary overflow devices, vent filters and CO2 traps may be incorporated to sustain the desired water quality levels within the reservoir by preventing the ingress of airborne contaminants. Properly sizing a reservoir requires selecting one large enough to meet peak demands during the day, but small enough that the water is completely turned over on a daily basis.

Once a loop design has been completed along with an assessment of high purity water needs, select a make-up system that meets the quantity and quality needs determined in the previous steps. Next, a water chemistry analysis should be performed on the incoming feed-water. The results of this analysis and the feed-water requirements of the make-up system will be used to determine the proper pretreatment needs.

Phase IV: Finalization—Further considerations when finalizing the design of the system include the physical facility, installation, validation, and/or ongoing maintenance. In evaluating the facility it is important to recognize the available space and access limitations. Commonly overlooked issues include access to the room where the make-up system will reside, especially stairs and doors that need to be navigated when entering the area. As the size of the water purification system increases, particularly the size of the reservoir, these issues become more important. The system designer also should check with the proper parties—either the facilities group or building architect—to ensure that the location has the required electrical, drain and feed-water requirements.

If the system requires validation, then the manufacturer should be consulted. Manufacturer validation packages tend to be more thorough and well designed because of their in-depth knowledge of the equipment. In most cases it is also a less expensive method because the manufacturer already has developed the necessary protocols.

Support services

Ongoing maintenance and required service will be needed for many of the components in the system. The manufacturer's service organization is the best option for this work because they know the system better than anyone else and have better access to spare parts and technical personnel that may be needed to maintain the system.

Investing in a total pure water solution is essential for any fully functioning cleanroom. In order to ensure success, we recommend a service plan that includes priority access to the service organization, routine validation and preventative maintenance visits. This guarantees that your system produces a high quality of water to keep the facility working productively.

Conclusion

The design of a pure water system should be an initial consideration when building a cleanroom facility. As essential as overall sterility, it can often pose contamination, productivity and financial risks if not managed properly. Deploying the four simple phases of investigation, approach selection, loop design and finalization establishes a smooth and reliable process.

Dale Gordon is regional business manager, Morgan Norris is product marketing manager and Guy Hart is regional custom systems specialist in the Lab Water Division, Millipore Corporation, 290 Concord Road, Billerica, MA 01821; for more information, call 781-533-6927 or e-mail dale_gordon@millipore.com.