A real-world, scientific solution for redundancy problems can be formulated without the need to add another pump or arbitrarily double the flow capacity.
by David P. Carrier
ASPE Plumbing Engineering Design Handbook, Volume 2: Plumbing Systems includes sizing procedures for domestic pressure booster systems in Chapter 5. When this information was initially included in 2014, it represented the first time that engineers were given a scientific procedure for calculating redundancy requirements for variable-speed boosters in an ASPE publication. This is something that I have been talking about for the past 18 years as I have traveled the country and listened to the challenges of plumbing engineers and designers. Some may ask why this is so important. The answer is simple: because until recently, no method was available!
What Is Redundancy?
Consultants understand that at some point in the history of a project, pump systems will require maintenance. Thus, there must also be a way to ride through these events, much like the way traffic is re-routed when a roadway is being maintained. The first thing to understand about maintenance is that it is temporary. The second element that must be considered is that this temporary modification of operation is brief. Unfortunately, the industry lacked a scientific method for engineers and manufacturers that addressed how to account for occasional service events, unless you believe that adding another pump or doubling the flow is “scientific.”
Designers must consider the worst-case condition during necessary maintenance, and although this can be a scary proposition of “what if’s?”, the answer can be found within the methods that have been used for more than 80 years. Although they were developed in the 1940s, Hunter’s curves continue to be the industry standard for booster sizing because they have been modified over the years for low-flow fixtures by organizations such as IAPMO, the International Code Council, and others. The essence of Dr. Roy B. Hunter’s work centered on the fact that plumbing demand is “never coordinated.” This means that all fixtures in a building will not be used at exactly the same time—the flow is random, and Dr. Hunter proved that these random events could be accurately accounted for and calculated as a result of the diversity of people’s interaction with the plumbing system. For example, when was the last time you took a hot shower while simultaneously washing your hands in the sink and going to the bathroom? Sounds ridiculous, doesn’t it? This is what Dr. Hunter surmised and then proved with sound mathematical models we now know as Hunter’s curves.
However, the Hunter’s curves that are used to select these systems include some redundancy, but they don’t consider the pump quantity available if a service event were to occur. This additional capacity (or redundancy) must be further determined by the designer.
Regardless of the building type, there is always diversity because there is never coordination. Even if you consider the type of structure that would have the least amount of diversity, it still exists. Consider the water usage in a stadium. We can all agree that a stadium is an extreme example of low diversity. How often have you waited in line during halftime or between innings at a sporting event? With all of the bathrooms completely full and a line to the water closet, you would assume that the delivery system was at 100 percent capacity, but according to Dr. Hunter, you would be wrong. The fact of the matter is that no booster is ever at 100 percent capacity. There is randomness in even the most seemingly non-diverse environment such as a stadium due to the fact that everyone is not flushing the toilet while simultaneously washing their hands at a lavatory. Furthermore, the super flush test that is performed on many of these stadium structures is, in essence, a completely illogical test since such an occurrence will never happen in the life of the building.
Application of Redundancy
How can we use the logic of human diversity to create better sizing methods? This brings us back to what happens when a piece is removed from the puzzle temporarily, such as when a pump is removed for maintenance. The first question to ask is: What are the odds that all of the pumps fail coincidentally at the same time? I have never calculated the odds, but they probably would be in the “once for every billion systems” range. In fact, much like the roadwork example above, when the capacity is re-routed, the system must ride through this event seamlessly without any indication to the user.
This brings us back to Dr. Hunter and his diversity values. If the system is sized per Hunter’s curves, we can surmise that the worst-case scenario would be that all fixtures would not be in use simultaneously. Based on Hunter’s diversity figures, the industry has accepted that boosters will never reach more than 70 percent of the total flow load. In this case, at 70 percent capacity a user would have no indication that a pump is removed when using a fixture. In other words, if we have a remaining load of 70 percent, the system will still temporarily serve 100 percent of Hunter’s real load with diversity until the pump can be re-installed.
Armed with this information, a real-world solution for redundancy problems can be formulated without the need to add another pump or arbitrarily double the flow capacity. In fact, adding another pump has no scientific basis, but it might help sell more, unnecessary pumps.
When a single pump (because they don’t fail at the same time) is removed for service in a multi-pump booster system, the remaining pump or pumps should still be capable of 70 percent of the total gallons per minute (gpm). If the initial intent is to size a duplex pump system with one pump removed for service, the remaining single pump must be able to maintain 70 percent of the load. If the Hunter’s curve gpm is 100 gallons, if one pump fails in a two-pump system, only one pump remains to carry the load. In a duplex arrangement, the redundancy gpm would need to be increased to offset the lack of redundancy in the quantity of pumps available as follows:
100-gpm demand = 70 gpm × 2 pumps = 140-gpm total redundant load
Each pump must be sized for the full 70 percent redundancy since if one pump fails or is removed for service, the other pump must still have a 70-gpm capacity (70 percent of 100 gpm).
Consider what happens with a triplex (three-pump) system instead of a duplex system. If one of the three pumps fails, there is actually greater redundancy since two pumps are still fully operational to serve the 70 percent load capacity. By starting with three pumps instead of two, you have a greater redundancy than the duplex system. Thus:
100-gpm demand = 35 gpm × 3 pumps = 105-gpm total redundant load
The total load is reduced due to the additional redundancy of three pumps vs. two pumps. If one 35-gpm pump in the triplex example were to be removed for service, there are still two remaining pumps at 35 gpm. These two pumps combined yield the same 70 percent redundancy but are calculated based on the remaining pump or pumps. This is the “magic” of the 70 percent method: it sizes for real-world conditions, not for an occasion that will simply never happen.
The Science of Redundancy
Having a methodology to satisfy ride-through demands, we can now scientifically determine the number of pumps required to better match the load of the system at any time. In 1994, a colleague and I coined the “80/20 Rule” for booster pumps.* Simply put, 80 percent of the time, a typical plumbing booster is at 20 percent or less. After all, what is a booster other than a glorified jockey pump? Once the piping system is filled with water, the primary job of the booster is to maintain pressure in the piping riser and branches.
In fact, consider that most of the water available for fixture demand is already stored in the piping distribution system. All the booster has to do is make up any water that leaves the piping system as a toilet is flushed or a lavatory is opened (remember Hunter’s curves?). Armed with this data, rather than choosing pump systems based on personal preference in pump quantity, designers can load-match the demand to the selection. Consider the example of a duplex system vs. a triplex system. Assuming a duplex system is selected, it requires two 10-horsepower (hp) pumps. Using the triplex system, since the pump quantity is higher, additional redundant flow is not required, as shown below:
Duplex selection: 2 pumps × 10 hp/pump = 20 total combined hp
Triplex selection: 3 pumps × 5 hp/pump = 15 total combined hp
Clearly, from the example above the lesser hp selection would be better since the combined load is less, but that is just the tip of the iceberg. Since 80 percent of the time only one of the pumps will be operating, the duplex system must run a 10-hp pump whereas the triplex unit will only run a 5-hp pump. Therefore, if the triplex system is chosen over the duplex, half the load is required 80 percent of the time! The only item that changed in the formula is the quantity of pumps. This example is the scientific proof that the method adapts based on the design conditions used.
This is why the 70 percent method can be proven mathematically, which is the hallmark of any scientific method. Other methods, such as adding another pump or arbitrarily adding flow, cannot be proven scientifically—thus the 70 percent method becomes logically obvious.
I have used this sizing method for the past 18 years and have never undersized a booster pump. In fact, my company guarantees the sizing when International Plumbing Code (IPC) water fixture units (WFUs) are used along with the 70 percent method. We can do this since it is scientific. The past 18 years along with 80 years of Hunter’s curve history are the industry’s proof of concept.
About the Author
For more than 30 years, David Carrier has worked in packaged pumping and controls. Starting as an independent representative in 1984, David called on engineers, distributors, and contractors. He is the author of several publications regarding packaged pump systems and specializes in the application and promotion of variable-speed packaged system design. A CEU Provider and ASPE instructor, David is passionate about the market and continuing education for engineers and designers. David is the “hands-on” director for all sales and marketing for QuantumFlo, in addition to being its Chief Executive Officer.
The opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.
*See the 1994 Armstrong Pressure Booster Systems Handbook, 2009 QuantumFlo Engineer Handbook, or 2018/2019 ASPE Plumbing Engineering Design Handbook for more information.