Fire Protection Hydraulics of Grid Sprinkler Systems
Do you Peak?
Fire Sprinkler Grid Systems that is. How do you hydraulically peak your automatic sprinkler system? Do you depend on the program you use for fire protection hydraulic calculations and just trust it implicitly? I have learned not to!
The basic premise of hydraulically peaking a gridded fire sprinkler system is that the hydraulically most remote area of operation will be on the most remote branch lines between the primary and secondary cross mains. The branch lines furthest away from the point where the feed main connects to the primary cross main.
The programs I used did a great job of calculation as long as you understand the assumptions and limitations. Many hydraulic calculation programs assume that the system is a box with a common sprinkler head spacing and elevation. Then they just shift the remote area by adding the distance between heads to one side and subtracting it from the other. This is perfectly acceptable if the spacing is typical, but this is not always the case. So unless the system is spaced evenly, I peak manually. I just use the automatic peaking feature to give me a starting point.
Yes, I know what your saying. It’s Hard, and the Building Department, Fire Marshall or other Authority Having Jurisdiction are approving my calculations now. Well, That’s only because they assume several things. First, that since a computer was used it’s correct. Second, that you know what you are doing. But the day when the AHJ stops making these assumptions is coming. They have been learning!
Will the remote area be between the mains or will out board heads be involved. Many programs assume the remote area will always stay between the mains. But that depends on how many outboard heads the system has.
How many outboard heads can I have? The easy answer is one half the number along the line in the remote area. My experience is that as long as you follow this rule, even if the remote area should include outboard heads, the difference is marginal. However if design considerations mean that the secondary cross main has more heads outboard, then a calculation should be done using the outboard heads.
Will the remote area be on the end grid lines. Right now everyone assumes this is the case, but there are situations where it will not be. I have seen calculations where the end line was sized larger than the rest of the grid to make it work. But then the remote area actually will shift across the lines and not parallel to them. Most programs assume peaking is parallel to the lines. Also, if the grid is not typical, but has different spacing along the lines due to steel, or changes in ceiling grid, the remote area will not necessarily be on the end branch lines.
So be careful. Computer programs are great tools, but you are still the one in the hot seat.