Top Ten Reasons to Use 100 mm Diameter Pipes in North America ^†

Abstract

North American fire insurance and the American Water Works Association (AWWA) recommend that no pipes smaller than 150 mm in diameter are used in water distribution networks, mainly for reasons of fire protection. However, the 150 mm requirement reaches back more than one hundred years, cited by the AWWA at least as far back as 1916. In general, North American fire flow requirements are shown to be conservative compared to other places in the world. We show how pipe cost, water loss, water age, disinfection byproduct formation, contamination risks, and transients can all be improved with 100 mm diameter pipes. Perhaps most compelling is the fact that 100 mm pipes are used widely elsewhere. Perhaps it is time to consider permitting 100 mm pipes as the minimum recommended size for use in North America, especially in modern residential service.

1. Introduction

The American Water Works Association Manual M31 states that the “minimum size of water mains for providing fire protection and serving fire hydrants is 6 in. (150 mm) in diameter.” [1]. Fire insurance rating also plays a large role in water supply. In conversation with an insurance representative, the authors were told that networks using 100 mm pipes in our region “would not be considered”.

This work provides ten reasons why 100 mm diameter pipes should be explored as a design alternative, especially in new, residential service. At the very least, it would be beneficial for other researchers to explore what would happen if 100 mm pipes were permitted in network design in North America, in optimization or system-improvement studies.

2. Ten Reasons

2.1. The 150 mm Requirement Is from a Different Era

The 150 mm (6 inch) requirement may have its origins in the aftermath of the Chicago Fire of 1871, which destroyed 17,500 buildings, resulted in 300 deaths, and caused USD 5.4 billion (in 2022 dollars) of property damage [2]. As early as 1916, the Journal of the American Water Works Association (AWWA) describes the practice of removing all existing 100 mm (4 inch) pipes and replacing them with 150 mm (6 inch) pipes for fire protection reasons [3].

Fires cause loss of life and property, and over the last 100 years, there has been a great incentive to develop new ways to reduce this cost. Improved building codes are likely to have been a key factor (e.g., most Chicago houses were closely spaced and constructed of unfinished wood), but there are also smoke alarms, sprinklers, and highly trained firefighters that can arrive in minutes. Despite 100 years of progress, the infrastructure to supply water for firefighting has not evolved in North America.

2.2. Fire Flow Is Likely Sufficient

For family homes, the USA-based AWWA and Insurance Services Office (ISO) recommend between 32 and 95 L/s (500 to 1500 gpm) depending on separation distance. However, homes in the UK require just 8 L/s (127 gpm), which is 75% less than the lowest US requirement. The French requirement for single family homes is approximately half the US requirement [4]. The fact that many millions of Britons and French function perfectly well in their homes with much smaller fire flows may suggest that there are generous safety factors, perhaps created in bygone eras, built into the US fire flow requirements.

2.3. Lower Cost

Many cities are strapped for cash. Smaller diameter pipes should cost less to purchase and install. The change in installed pipe cost has been reported to change with diameter raised to the power of 0.87 [5]. According to this, 100 mm pipe should cost approximately 30% less than the cost of 150 mm pipe.

2.4. Reduced Water Age

A fresh donut is worth a lot more to a consumer than a day-old one, which often ends up in the garbage. Similarly, water age can have a big impact on water quality. The time that water spends travelling from the water treatment plant to the consumer is, on average, the length of the pipe divided by its flow velocity, neglecting storage tanks. Since velocity is the flow divided by the cross-sectional area (πD²/4), transit time decreases according to the square of the pipe diameter at any given demand. Switching from 150 mm pipe to 100 mm would reduce the associated transit time by a factor of (100/150)², or 44%.

2.5. Lower Risks Associated with Network Contamination

Low pressures can result in intrusion of contaminants into the network. Also, intentional acts of terrorism, spills, or other events can introduce undesirable contaminants into the network. The longer these contaminants remain in the network, the higher the risk that they will be consumed. Smaller diameter pipes mean higher velocity and reduced transit time at any given demand. As a first approximation, the time these contaminants are in a 100 mm pipe waiting to be consumed is reduced by a factor of 44% compared to a 150 mm pipe.

2.6. Cleaner Pipes and Less Sediment

Switching from 150 mm to 100 mm pipe increases the average velocity by a factor of (150/100)², or 225%. Though this may raise concerns about head losses, other research has shown that the flow velocities of standard 150 mm pipes in regular residential service are very low (i.e., <0.05 m/s) and the head losses remain inconsequential at 100 mm [6]. Increasing pipe flow velocity to improve water quality will also reduce the probability of red water events and more effectively remove sediment from the pipes [7].

2.7. Lower Disinfectant Byproduct Formation

The formation of disinfection byproducts and the loss of disinfectant residuals can often be modelled as a first-order process where C(t) = Co exp(±kt) [8], in which C(t) is the concentration at time t; Co is the initial concentration; and k is a rate constant. We have already argued that to switching to 100 mm pipes can reduce water age seen by the consumer by a factor of approximately 44%. Substituting this new water age into the first-order equation, we see that switching to 100 mm pipe can be expected to reduce concentration of disinfection byproducts that reach the consumer by a factor of 57%.

2.8. Less Water Loss after a Burst

Conservative fire flow requirements in the North American network of gridiron layout large-diameter pipes mean that a huge amount of flow is always almost instantly available. As a result, when there is a pipe burst, vast quantities of water are wasted until the pipe is isolated. In addition to the loss of product, this can have number of other expensive consequences including local flooding, traffic stoppages, and interruption of other services like gas and electricity. According to the Hazen–Williams equation, the amount of water lost from a pipe at any given head is proportional to the diameter to the power of 2.63. Switching a supply system from an all-150 mm pipe system to one of 100 mm could reduce water loss after a burst by a factor of approximately (100/150)^2.63, or 34%.

2.9. Less Severe Transients during Pipe Bursts

The magnitude of transient pressures can be estimated using the Joukowski equation where the change in pressure (ΔP) is equal to the product of the fluid density (ρ), the wave speed (a), and the change in velocity (ΔV). As described above, when a pipe bursts, the flow is reduced by a factor of 34% for 100 mm pipes. This reduction in flow effectively decreases the change in velocity (ΔV) experienced in 100 mm pipes after a burst, making the transient pressure oscillations less severe.

2.10. It Is a Low-Risk Change

It can be risky to try something new. In this case, much of the world is already using 100 mm pipes for water supply. From an informal survey of our colleagues in the industry, 100 mm pipes have been used for many years in England, France, Australia, New Zealand, and the Netherlands. Many other countries not on this list are likely to be using them as well. The UK alone has a population of nearly 70 million. If there were serious shortcomings associated with 100 mm pipes, especially related to fire protection, one would think it would be well known by now.

3. Summary and Conclusions

This work summarizes what we think is a strong case for permitting a minimum of 100 mm diameter pipes in modern North American residential service. There is evidence from other parts of the world that these smaller pipes will have little effect on fire protection. The potential benefits may include lower cost, reduced water age, lower contamination risk, cleaner pipes, fewer disinfection byproducts, and lower water loss and less severe transient pressure variation during pipe bursts. Overall, it seems to be a low-risk and high-benefit proposition.

As for next steps, we hope other researchers will explore this idea when seeking to improve, make better trade-offs, or optimize new networks. Automatically applying the constraint that all pipes must be 150 mm in diameter or greater may be hiding a range of undiscovered possibilities and performance improvements.

To implement this in the real world is more complex and challenging. Water utilities, insurance companies, researchers, regulators, contractors, and firefighters all have a stake in this issue. It will take open-mindedness, cooperation, and collaboration from all these groups to realize, or at least to test, some of the potential benefits reported here.