Tricks of the Trade for Hydraulic Calculation

by Mike Lehner, P.E.

This article is part of Wood Harbinger’s newsletter series.

NFPA 13, the standard for sprinkler system installations, requires that the sprinkler system designer perform a hydraulic calculation to verify that the system is sized correctly for its specific application. This involves determining the total friction losses in the system and ensuring that these are not higher than what is available at the water main. NFPA-13 uses the Hazen-Williams formula to calculate the friction losses in a section of piping.

So…what is a hydraulic calculation and why is it important? Let’s take a look.

First Things First

Before we can perform the hydraulic calculation, we have to have a system to test. A sprinkler system designer begins by laying out the sprinkler heads and piping to create a sprinkler system for a specific building or structure. Once the system is laid out, the piping can be sized. Pipe sizing starts with the facility’s design criteria (requirements) and the available water pressure in the city mains.

Starting the Hydraulic Calculation

The sprinkler code is fairly prescriptive in defining how many sprinkler heads must flow in a specified area (the remote area) and at what density (water flow per square foot) as well. In addition, the sprinkler designer consults the fire department or water department to obtain fire flow test information, which includes the available water pressure/flow on site. Armed with this information, the sprinkler designer can run the calculation and adjust pipe sizes to get the system to work.

Typically, the available water pressure from a city main ranges from 50-pounds per square inch gauge (psig) to 80-psig. Sometimes they go as high as 150-psig or more. These pressures usually enable a sprinkler designer to create a simple sprinkler system in a one or two-story building that meets the NFPA-13 standard. In many cases, they may even work for taller buildings as well.

However, in some cases the water pressure may be too low to meet the standard. For example, if the building is on a hill, or is near the water tank, the available water pressure will be affected. For certain facilities, the design criteria may require more water pressure, more flow, or both. This could include high-bay factory spaces, high-piled storage applications, and other occupancies that require a higher sprinkler density. What does a person do if there is not enough water pressure to operate the sprinkler heads?

What Now?

The first thing to do is recheck the calculation to ensure all the information is correct, including the water supply, the remote area, the sprinkler density, pipe sizes, and the system layout. Even the choice of piping can impact a calculation.

If all is correct but there is still not enough water pressure, there are a few things that can be done. The obvious answer is to provide a fire pump, which adds pressure to the system at the required flows. Fire pumps can be diesel engine driven or electrically driven. However, both of these options are very costly and require maintenance over the lifetime of the fire suppression system. Before you go out and buy a fire pump, we fire protection engineers have a variety of tricks that may help.

Look for the Pipe Squeeze

The challenge with fire sprinkler systems is getting the water through the supply mains and out to the furthest sprinkler heads (the most remote area) while maintaining the required pressure. Here, we take a closer look at the calculation to see if there are “squeeze points,” in the piping system where pressure losses are very high. In some cases, simply upsizing the sprinkler main can alleviate pressure problems.

If that doesn’t work, take a look at the branch lines and cross-mains as well. Sometimes upsizing the branch lines can help. Flowing water for three sprinkler heads through a one-inch pipe can eat a lot of pressure; a larger branch line to the second head may fix the pressure issue.

Another option to mitigate squeeze points in a wet-pipe sprinkler system is to “grid” the system. This means that there is a grid of branch lines connected on both ends with a cross-main to allow multiple pathways for the water to reach the remote sprinkler heads. This strategy is often used to accommodate high sprinkler density requirements and/or if the shape of the sprinkler system allows for it. Gridding can be a simple way to reduce pipe sizes and still provide adequate water pressure and flow to the sprinkler heads.

Save Water

With sprinkler systems, water savings can result in lower water flows through the piping, which reduces friction losses. It is important to balance the sprinkler system so a similar amount of water is discharging from each sprinkler head. For example, you have a half-inch sprinkler head, flowing 15-gallons per minute (gpm), at a minimum pressure of 7-psig, with three of these heads flowing on a branch line. If the piping is small, then there are larger pressure drops between the sprinkler heads. This would force the two sprinkler heads closer to the cross-main to flow more water at a higher pressure to ensure that the furthest head is still flowing at 7-psig. As such, the sprinkler head at the end of the line would “drive” the system. With the branch line sized to minimize pressure losses, the difference in flows is smaller and the efficiency of the system is increased, thereby reducing water waste to a minimum.

Make it Smaller for Each Sprinkler Head

Standard spray sprinkler heads can cover areas from 100-square feet (sf) to as much as 225-sf for light-hazard occupancies. However, these are the maximum allowable coverage areas per NFPA for standard sprinkler heads. Each sprinkler head has a “K factor,” which is a characteristic for each sprinkler and is used to calculate the required pressure to flow enough water for the area the sprinkler head covers.

If you reduce the area each sprinkler head covers, you can reduce the required pressure (and flow) through that sprinkler head, down to the minimum start pressure of 7-psig. For example, you have a requirement to flow a sprinkler density of 0.15 gpm/sf. A half-inch sprinkler head would need to flow about 19.5-gpm at about 12-psig to cover a 130-sf area. If you reduce the area that the head covers, say to 100-sf, then the half-inch sprinkler head would need to flow about 15-gpm at about 7-psig. This can result in a 5-psig drop in the system pressure requirements. The downside is that there are more sprinkler heads, but that may be an acceptable alternative.

Conclusion

When calculating a sprinkler system, it’s important to check the input to ensure the data is accurate. If your system still needs more pressure, then explore the options to reduce friction losses in sections of the piping, improve water efficiency (reducing water flows), and reduce sprinkler head operating pressures by reducing sprinkler coverage area. By adjusting the sprinkler layout, pressure savings can be obtained, and project costs can be kept down while meeting NFPA-13 requirements.

 

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