Fire Sprinkler Design 1

[jon.torres89dtx]

Howdy to all,

I have been in fire protection for about 8 years.S Started out as a helper and moved my way up to becoming entry level designer. I just would like some explanation on SPRINKLER SYSTEM DEMAND for calculations.
I just would like an easy explanation, if there is one ,of what the flow required and pressure required (EX: 310.772gpm @ 32.924psi) is meaning, also what the flow of heads means when i see it in my calculations EX: Actual Flow and Required Flow, and what the safety factor means. I am good on layout rules, its just calc's are getting me a little stumped since i am going to start digging into them more to get me started. Like my past foreman taught me it meant something like you have to have so much water to flow so much but just needed some refreshing on it, thanks!

 

[TravisMack]

I would very much suggest a class on fire sprinkler system hydraulics.

If you are calculating based on density area method, then the required flow is the minimum required flow to meet the density for the sprinkler. Let's say you have a sprinkler covering 120 sq ft and a density of 0.2 gpm / sq ft. The req'd flow is 24 gpm. The actual flow may be something more than that due to friction losses of the system.

When you see flow/pressure required at the base of riser as 310 gpm @ 33 psi (there is no point in calculating out to that many significant figures as you note above), that is stating that in order to provide the minimum density over the required design area within the piping network that is indicated, you need 310 gpm @ 33 psi at that base of riser point.

Safety factor is the difference between minimum system demand and available water supply. For your example above, you need 33 psi at the base of riser. If your water supply can deliver 50 psi at 310 gpm, then you have a 17 psi margin between required pressure and available pressure. This is referred to as your "safety margin."

I can't stress enough how important it is for you to learn hand calcs. So many these days just put the information in the computer and just take what comes out. Having a better understanding of what the computer is doing with calculations will help you when it comes time to balance pressures in rack systems, foam systems, or even some attics. I was just helping a guy with an attic system that he could not get to calc. He is a NICET IV, but has no understanding of hand calcs or balancing. I showed him how reducing some pipe sizes actually helped cut down the system demand and allowed the system to work. He thought he needed a pump, when it was just a matter of knowing how to balance out pressure demands.

Memorize Q = k * sqrt(P) and how to manipulate that formula for any variable. Also, follow the calculation method in NFPA 13 for hazen williams calculation method. Then, take a tree system that you draw, calc it by hand, then calc it in the computer. You should come up very close. The difference should just be in rounding values. If you can't do this, ask some one to teach you or buy a textbook to learn it or do a seminar to cover it. But I promise you this will be very helpful in your career. Try calculating 49 foam grate nozzles that have a min pressure of 40 psi and a max pressure of 45 psi if you don't understand balancing. That is a real world example of a system I had to balance to an overhead sprinkler demand. You will be completely lost and spend days doing it.

Best of luck to you.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[SprinklerDesigner2]

I started 42 years ago before the age of computers when the great majority of our systems were pipe schedule.

But even in pipe schedule systems we still had calculations from the street to the base of the riser.

But in the early to mid 70's calculated systems were becoming more common so we all learned how to do hydraulic calculations by hand. It took a while to learn how to do it, and yet a while longer to actually understand what was happening, but once you did know it you would see computerized hydraulic calculations in a whole new light.

It also took a while to do some calculations. Trees were easy, loops were not that hard once you got the hang of it but grids could be a real time consumer. I always set my grids up to be ladder grids, no side feeds unless you had weeks (literally) to do the calculations which is why most of us avoided side feed grids.

But I feel I have an understanding of calculations that many who started on computers will never have. I can just look at a job before it is laid out and tell you where the trouble will lay. Another bonus is I don't need a program or computer to do hydraulic calcs. All I need is a pencil, a cheap $20 pocket calculator, paper and a couple hours and I can do most systems if I had to.

Travis mentioned "balancing" and that is something you learn to recognize real fast.

If you worked for me I'd make you do hand calcs until you understood it. You'd hate me for it but you would appreciate it farther down the road in your career.

 

[MatthewJWillis]

I look at calculations as the easiest hard thing we have to do with sprinkler systems. Maintaining obstruction rules to me is as challenging as calcs. I will agree 200% with the 2 post above.
I can now look at a water supply, and size my pipes during layout, with some minor tweaking at the back end.

If you would like a great starting point, NFPA 13D allows a simple one path calc. Worksheet is pretty straight forward. But to get really proficient, remember the old adage.., "How do you get to Carnegie Hall?

R/
Matt

 

[MatthewJWillis]

OH, and BTW,

You have just told everyone in this group IMO, that you will be a great designer!

R/
Matt