“Design Considerations for Potable Water”

Today’s engineers and designers must take much into account outside of the normal engineering concerns of load, flow rate, and pressure drop to design systems that protect public health.

by David Dexter, FNSPE, FASPE, CPD, CPI, LEED BD+C, PE

The primary purpose of the water distribution system is to provide adequate flow, pressure, and volume suitable for use at every device that requires water, even at the peak demand of the system. It is the designer’s responsibility to accomplish this in such a way as to ensure reliability, while being cost effective, code compliant, and safe for public use. Following are some thoughts on the design process and the competing and sometime conflicting constraints in the process of designing a potable water system.

In the past, codes were the basis for water system design as they were prescriptive in the descriptions and enforcement of the minimum requirements set therein. However, today’s water systems are designed on the basis of standards and engineering principles such as found in ASPE’s Plumbing Engineering Design Handbooks. While the plumbing code is a good starting point, many other conflicting and competing standards, regulations, and rules must be considered. Some of these other considerations depend on the facility type, but as engineers and designers, we must be aware and knowledgeable of all of them. One never stops learning to be successful and meet their obligation to protect the public’s health, safety, and welfare.

Water Source

Water distribution begins exterior of the building at some type of source, public or private. In the case of a pubic source, the local purveyor will have specific and detailed engineering standards to which your design shall conform to obtain water from that purveyor. If the source is private, such as a well or surface water, common sense must apply to both protect the source from possible contamination as well as ensure that the water is and remains potable to protect the public’s, users’, and client’s health and safety. As engineers and designers, we have the responsibility and obligation to protect the source from any possible contamination that our facility might create as well as to protect the facility and its occupants, visitors, and the public at large—hence, we utilize containment backflow protection (usually a reduced-pressure zone type) to separate the public or exterior service from the facility’s systems. Even if the local purveyor does not require such protection, our requirement to hold the public’s health, safety, and welfare above all other concerns mandates it.

Water Chemistry

As engineers and designers we must know and understand the water chemistry—pH, turbidity, harness, etc.—to appropriately select the materials for the system as well as to consider any necessary treatment. While the source water is most likely treated in accordance with U.S. EPA (Environmental Protection Agency) standards, we need to know about those treatment processes and the chemicals involved. We also need to realize that the current treatment can change at any time without notice. These changes may have adverse effects on the materials within our system and are beyond our control. However, one must be aware and proactive to best serve the public and our clients.

Conflicting Requirements

It used to be that as an engineer or designer, we had to consider things such as the fixture load that our system needed to serve, the availability of an adequate water supply with an acceptable pressure range and flow rate, and the material type that would best serve the client’s long-term needs while meeting the budget and protecting those who might come into contact with the delivered water. The local purveyor was contacted to coordinate the connection to the public service or, in some case, a private source. Our design would incorporate the purveyor’s rules and requirements, such as the proper material type for the service, meter location, and backflow (containment) protection, and the associated costs for tapping into the system. We would then go about designing our distribution system using the plumbing code, local ordinances, standards, accepted engineering practices, and experience to provide an optimal delivery system. This delivery system had to stay within an acceptable pressure drop across the system and within an appropriate velocity, so as not to damage the piping and fixture materials. Many engineering source on how to appropriately design and size a potable water distribution system are available. One of the better sources, in my judgment, is ASPE’s Plumbing Engineering Design Handbook, Volume 2: Plumbing Systems, along with other engineering textbooks.

Design has always been a balance between handling the peak load and maintaining reasonable pipe sizes that remain within budget. As we know, or should know, peak loading occurs only on rare occasions, less than 5 percent of the time, although no one wants to not meet the client’s and users’ expectation of having adequate flow and pressure at the point of delivery, every time. There is a balance between over-sizing to meet that desire and remaining in budget and providing a practical system.

But, as we should all know, design and our understanding of the components involved in that process are always changing as our knowledge and understanding grow. Today we have a better understanding of such things as water chemistry, the growth and development of biofilm within our piping distribution systems, and the effects of various chemicals on the water within the system and their potential impact on the human body. We also have many competing interests that must be considered during the design of a distribution system: the U.S. EPA, CWA (Clean Water Act), energy conservation involved with the movement of water, LEED (Leadership in Energy and Environmental Design), as well as the requirements associated with specific building types. As an example, healthcare facilities require approval and certifications from the Joint Commission on Accreditation of Healthcare Organizations (JCAHO), Centers for Medicare & Medicaid Services (CMS), Veterans Administration (VA), Occupational Health and Safety Administration (OSHA), as well as others that are required to review and approve the design and installed systems. A facility may also be subject to the owner’s insurance carrier’s requirements. So, as an engineer or designer, we must research and ask many more questions to ensure that our design will be accepted and approved. The requirements of some of these organizations may conflict with another or even the code in effect. It is the responsibility of the engineer/designer to balance all of these requirements and design a system that meets the intent of all of the various organizations, while meeting the owner’s/client’s expectations, remaining in budget, and protecting the public’s health, safety, and welfare. Life is never simple, is it?

Competing Thoughts

Today there also are many competing thoughts on the best approach to designing a system, some with merit and some not so much. It is the responsibility of the engineer/designer to review and evaluate them, resolve conflicts, and arrive at an acceptable design that takes into consideration energy conservation, maintenance of the sanitary condition of the water and piping systems, allowance for future changes and expansion of the system, etc., all while protecting the public good and the client’s interests.

The typical plumbing pipe size begins at ½ inch nominal, meaning the internal bore is one-half inch, but we know that the typical supply line is a 3/8-inch flexible tube or hose in today’s installations so some have pushed for using a 3/8-inch pipe size as the minimum size. Their argument is that there would be less surface area for biofilm to develop on and higher velocities to scour it away. While these may be viable reasons, this argument only looks at a small piece of the distribution system. It also virtually eliminates the concept of expansion or revisions of the line.

Balancing All of the Components

Given today’s concerns for biofilm and the potential for pathogen development within an enclosed water source, as engineers and designers we must take a hard look at pipe surfaces exposed to the water within the system, the elimination of any dead ends that can allow water to stagnate, the temperature of the water that might encourage pathogenic growth, the water velocities needed to scour the piping walls, and the water chemistry that can maintain a sanitized condition of the contained water. As you can see, much has to be taken into consideration outside of the normal engineering concerns of load, flow rate, and pressure drop.

After all, having responsibility for the design as well as protecting the public’s health, safety, and welfare can be challenging process. We must attack it bit by bit while maintaining our focus on the public’s health, safety, and welfare above all other interests. It is a challenging endeavor, but as professional engineers we are unequally up to the task. We just need to remember to think outside of the box, balance the competing constraints and holding the safety of users above any other outcomes. The engineering and design remain the same, but other constraints must be considered as we balance all of the competing interests: current needs, future revisions or expansions, budget, client expectations, and the public good.

While this may seem to be a demanding process, break it down into its smaller bites and proceed to advance the design. Try to consider all of the options and the constraints that might occur. We have been advancing the design process for years and will continue to do so—it is engineering in a living and ever-changing world as we learn more. There is no such things as a poor design, just a learning experience. Use your knowledge to advance the profession and protect the public good.

David Dexter, FNSPE, FASPE, CPD, CPI, LEED BD+C, PE, is a Registered Engineer, Certified Plumbing Inspector, and Certified Plans Examiner with more than 40 years of experience in the installation and design of plumbing systems. He specializes in plumbing, fire protection, and HVAC design as well as forensics related to mechanical system failures. Dave serves as Chair of ASPE’s Main Design Standards Committee, Chair of the Bylaws Committee, and Co-Chair of the Professional Engineer Working Group. He also was the 2010–2011 President of the Ohio Society of Professional Engineers.

The opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.

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