Industrial Engineering

The Requirements and Challenges of Residential Fire Sprinkler Systems and Water Meters

PME Magazine May 2020
By Dan Hubbard

Now that many jurisdictions across the country are mandating fire sprinkler systems in new-home construction, designers need to be aware of the requirements and challenges these systems can face in relation to water meters.

This article will reveal factors to consider when designing a fire sprinkler system for a home and how water meters or other flow-restricting devices can have an impact on system effectiveness.

Focusing on flow

Positive displacement (PD) meters are the most common residential water meters in new-home construction. (However, electromagnetic and ultrasonic meters are beginning to see more use due to their enhanced accuracy and durability).

Because PD meters are most accurate with smaller pipe sizes of 5/8 inches and 3/4 inches, water purveyors prefer smaller meter sizes to avoid lost revenue typically related to slow leaks or running toilets. Additionally, adding meter setters or meter yokes (that elevate the meter for easier reading and servicing) often incorporate smaller pipe along with fittings and valves that can restrict pressure and flow.

Why is this important? Because any devices that restrict flow can have a significant impact on how a fire sprinkler system is designed.

When looking at residential water meters, consider two important flow numbers: The safe maximum intermittent operating range and the recommended maximum rate for continuous flow.

Continuous flow relates to 24-hour constant flow, whereas maximum intermittent flow describes brief periods of flow. Because a typical flow time for a residential fire sprinkler ranges from 10 to 30 minutes, it falls within the “intermittent” category. Therefore, when evaluating water meters for a fire sprinkler system, it is important to look at the maximum intermittent flow rate, not the continuous flow rate.

Common maximum gallons per minute flow rates for residential water meters are:

  • 20 gpm for 5/8-inch or 5/8- x 3/4-inch meters;
  • 30 gpm for 3/4-inch meters; and
  • 50 gpm for 1-inch meters.

The National Fire Protection Association 13D Standard for the Installation of Sprinkler Systems in One- and Two-family Dwellings and Manufactured Homes requires system sizing to provide flow for up to the two most hydraulically demanding sprinklers located in the same compartment.

The most commonly used residential sprinklers are 4.9 k-factor pendant sprinklers. The minimum pressure and flow for these sprinklers is 7 psi at 13 gpm for spacing up to 16 feet by 16 feet. Hence, the most common demand for two sprinklers flowing is 26 gpm. Of course, this can vary depending on system requirements and the designer, and increasing sprinkler spacing or using sidewall sprinklers can increase the flow demand significantly beyond 26 gpm.

There are also low-flow sprinklers with smaller k-factors to protect 12-foot by 12-foot or 14-foot by 14-foot areas with flows as little as 8 gpm and 10 gpm, respectively. These can reduce necessary required flow to as little as 16 gpm, but can often require more sprinklers, adding to the cost of the system.

Finding the balance

To find the perfect balance of cost and effectiveness, let’s play out a scenario.

A typical fire sprinkler design requires 26 gpm, which is fine for 3/4-inch and 1-inch meters, but exceeds the maximum flow for 5/8-inch meters. One solution would be to use low-flow sprinkler heads to lower the system demand at or below 20 gpm. In many parts of the country, this is a great solution as long as the increase in the number of sprinklers is not excessive.

However, in some cases, such as two or more dwelling units sharing a supply line and a meter — or throughout the state of California — a problem arises. These situations require an additional 5 gpm applied to the underground supply. Shared supplies require the additional gpm at the point between the water source and where they split; the state of California requires the entire supply.

This additional 5 gpm puts the total system demand through the meter at a bare minimum of 21 gpm, exceeding the 5/8-inch meter’s maximum allowed rate. In this instance, designers would need to keep sprinkler spacing to a maximum of 12 feet by12 feet apart.

Also, using the standard system demand of 26 gpm could cause issues where the 5 gpm addition puts the total demand at 31 gpm, exceeding most 3/4-inch meter maximum flow rates.

The Fire Protection Research Foundation of the NFPA has researched meters’ capabilities of flowing well beyond their published maximum rate of flow. Its report titled, “Residential Fire Sprinklers — Water Usage and Water Meter Performance Study,” published in February 2011, concluded it is perfectly acceptable to exceed the maximum flow rate during the control of a fire event, as long as the appropriate amount of pressure loss is factored into the hydraulic calculations.

A real issue

Understanding how to talk to water purveyors about meters and other flow-restricting devices, such as meter setters and meter yokes, is critical to an accurate design. The purpose of meter setters or yokes in a meter assembly is to elevate the meter to a height that makes it more convenient for reading and servicing. Depending on the components used and the manufacturer, a 3/4-inch meter setter can lose as much as 25 psi of pressure at standard residential sprinkler system flow demands.

Companies that deal exclusively or conduct a majority of their work in dwelling and townhome installations understand the common components of these systems; water meters are part of every design. These design firms and contractors generally have a good grasp of requirements and potential additional components, such as setters and yokes, which can cripple a fire protection system.

Other companies that conduct a majority of their work in commercial spaces rarely see water meters as a component in their calculations. Typically, meters are a component in a detector check valve and are almost never a flow-through device in commercial applications.

When a commercial fire protection contractor designs a smaller-dwelling system, the calculations through a meter are often accidentally missed, or friction losses for meters are misapplied; i.e., the contractor will use “rules of thumb” instead of looking up meter-specific, friction-loss data.

It is not uncommon for reviewing authorities to miss mistakes like this, especially if they are more familiar with commercial-plan sets. If meter losses are not included, the result is a fire protection system that is undersized and doesn’t work. The only way these mistakes are uncovered is through a physical flow test of the system; or worse, a system failure in the event of a fire.

Liability concerns

In our society, contractors, designers and engineers all carry liability insurance to help protect their businesses. But when fire sprinklers do not perform as designed and lives are affected, who is liable?

Some argue that the Authority Having Jurisdiction that approved the plans showing the meter and the flow rates is responsible. But AHJs are rarely held responsible in these circumstances. For designers, engineers and installers, the question remains: Is the possibility of a system failure low enough to save money and go with a smaller meter?

Because water purveyors tend to charge large fees to upsize water meters from their standard installation sizes, projects typically go with smaller standard meters. And having standardized water tap sizes and meters helps keep costs down, meaning purveyors can minimize the amount of inventory on hand as well as the tools needed for installation. Plus, larger-sized meters start to lose efficiency, which can result in unaccounted-for water and lost fees, as stated earlier.

The good news

Here’s the good news: There are some viable water meter options to keep everyone satisfied and protected. And it is important to note that a 1-inch meter is not always necessary. Standard 3/4-inch meters are more than capable of handling the flow of a typical 13D sprinkler system. Moreover, when you use the manufacturer’s published pressure-loss charts, standard 3/4-inch meters do not even have a significantly higher pressure drop as compared with their 1 inch counterparts.

In fact, one meter manufacturer makes a 5/8-inch and a 5/8- x 3/4-inch meter rated at a maximum flow of 25 gpm, as well as a straight 3/4-inch meter rated at 35 gpm. Other manufactures are developing advancements in metering technologies that are also able to allow for higher flows than older-style meters.

As technologies continue to evolve and meter efficiencies increase, many of these problems will go away. Until then, it is important that water purveyors and AHJs have the correct information and are willing to expand their options to provide a reliable water source for these vital, life-safety fire sprinkler systems.

Reference: https://www.pmengineer.com/articles/94816-the-requirements-and-challenges-of-residential-fire-sprinkler-systems-and-water-meters