REFINE THE HYDRAULIC CALC WITH THE IDEAL K-FACTOROne of the more fine-tooth aspects I look at is the k-factor used on the sprinklers. The k-factor for a fire sprinkler is the discharge coefficient, or in normal human terms just relates to the amount of water that is permitted through the sprinkler.The k-factor is dependent upon the orifice diameter of the sprinkler - a low k-factor (such as K2.8) restricts the flow of water, while a larger k-factor (such as K22.4, K25.2, or K28.0) permit much more water to flow through. K-factors were originally created to be multiples of the discharge of a K5.6 sprinkler. A K2.8 sprinkler, for example, is 50% discharge of a K5.6 sprinkler, while a K11.2 sprinkler is 200% of the discharge of a K5.6. NFPA 13-2016 Table 6.2.3.1 shows this well. USE IN DESIGNWe find K5.6 sprinklers in light hazard all the time. Residential sprinklers often have k-factors less than 5.6. ESFR and CMSA require minimum K11.2 (NFPA 13-2016 6.2.3.5). ESFR are tied directly to the hazard it protects. Back to refining the hydraulics in a system - increasing the k-factor of a sprinkler allows more water to flow through a sprinkler with less pressure loss. This becomes very important when trying to reduce pressure loss in a system. A LIGHT HAZARD EXAMPLEA light hazard system (0.10 gpm/sqft) with widely spaced sprinklers (at 225 sqft each) would require a minimum flow through each sprinkler of 22.5 gpm (0.10 gpm/sqft x 225 sqft = 22.5 gpm). In order to flow 22.5 gpm, a sprinkler with a k-factor of 5.6 now requires 16.1 psi to do so (Q=k √p, or rearranged, p=(Q/k)^2). This is 9.1 psi higher than 7 psi, or the minimum that NFPA 13 requires.In order to flow 22.5 gpm, a sprinkler with k-factor of 8.0 only requires 7.9 psi to do so, or less than 1 psi more than the minimum NFPA 13 requires. In this scenario, flowing the same amount of water (22.5 gpm) results in a 8.2 psi difference in the pressure required at the most remote sprinkler. Can 8.2 psi be important? Absolutely! OTHER SCENARIOSSimilarly, consider Ordinary Hazard Group 1 (0.15 gpm/sqft) and Ordinary Hazard Group 2 (0.20 gpm/sqft) systems. For Ordinary Hazard Group 1 and sprinklers spaced at 130 sqft, a K8.0 sprinkler requires 5.1 psi less than a K5.6 sprinkler (7.0 psi vs 12.1 psi). This same methodology applies to extended coverage sprinkler requirements, specific densities for traditional storage design, and more. A K-FACTOR SELECTORLast week we talked about the two drivers that set the minimum pressure. Does our k-factor and minimum pressure drive what our flow and pressure are at the sprinkler? Or, does the density and coverage area drive our flow and pressure at the sprinkler? We've created a tool that helps answer that in the Toolkit, which is the K-Factor selector. It's one of the more popular tools in the kit, and it quickly shows what the optimal K-Factor is to minimize flow, and what the optimal K-Factor is to minimize pressure: MINIMIZING FLOW OR MINIMIZING PRESSURE?Both flow and pressure can each be important in different applications. Most of the time, I want to minimize the starting pressure at the remote area. Reducing the starting pressure means I have more room to work with and can allow for more pressure loss in the pipe network. In essence, I can allow smaller pipe sizes and have a more efficient system if my starting pressures are as low as possible. However, I have had projects where flow was the primary concern. One was a wedding venue in a remote area without a water supply. We had a fire pump and a tank, and the owner's biggest concern was the tank size. There were limitations on how much space the tanks could take and the location in which they were located. Those tanks were our limiting constraint. Our fire pump in the building (a little downhill from the tank) could add just about as much pressure as we needed it to provide. However, the tank size couldn't. Our biggest concern in the hydraulic calculation was making sure we limited the flow overall to as low as possible. We dialed-in the k-factor to match the hazard and the density as best as possible to limit the overall flow, and make up for the pressure loss when we sized the fire pump. In the end, we were able to use the anticipated water storage tanks the owner provided only because we limited the amount of water we needed on the system. This was a rare case. Much of the time, minimizing flow and pressure at the remote area go hand-in-hand. If we reduce the starting pressure, then the next sprinkler down the line has a lower pressure and spits out less water than it otherwise would. That said - using a k-factor optimized for flow is often a different selection than a k-factor optimized for pressure. Close, but not exactly the same. You can see in the above image as the coverage area and density changes, so too does the optimal k-factor. The K-Factor can be an overlooked design decision for many buildings. When water supplies are difficult or situations are tight - understanding and honing in the best K-Factor can make a difference for a project. TL501 SERIES:- What is the Sprinkler Database?
- How to Use the Obstruction Calculator for Beams?
- How to Use the Obstruction Calculator for Soffits?
- How to Use the NFPA 13 Translator?
- What is Driving Design; the K-Factor or Density?
- How to Estimate Clean Agent Quantities?
- How to Calculate a Domestic Demand?
**How to Select an Optimal K-Factor?**- How to Quickly Calculate Friction Loss?
- How to Analyze Fire Pump and Water Supplies from a "Big Picture" perspective?
- How to Determine Fire Flow with the IFC Method?
- How to Quickly Estimate Hydraulics for a Sprinkler System?
- How to Estimate a Water Storage Tank Size?
- How to Calculate Hanger Spacing by Weight?
- How to Calculate the Size of a Trapeze Member?
- How to Summarize Notes for Fire Alarm and Suppression?
- How to Calculate Thrust Block Size?
- How to Quickly Classify Combustible & Flammable Liquids?
- Calculate the Volume & Air Compressor Size for a Dry System?
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