Hydraulic Calculation for Fire Protection 2

[waien]

Hi,

I am new to hydraulic Calculation and need some one to post step by step procedures to carry out the calc manually.I have studied the NFPA-13 procedures as well as several materiel available on the net but i haven't been able to crack the Hazen - williams eqn for FP system.

My main area of concern in the procedure in NFPA -13 is that do we have to assume the pipe dia and calculate the friction through it. or we have to assume a certain friction rate and then calculate the pipe dia.

A hand worked and each assumption explained in the equation would defiantly be helpful and if any materiel is sent it would be appreciated.

 

[cdafd]

Why do you want to do hand calcs

 

[cdafd]

You might call and see if they have a book on hand calcs

https://nfsa.site-ym.com/store/default.asp

 

[waien]

The reason for the Hand calc is that it gives a person thorough understanding of the calcs as such his confidence,ability and precision is enhanced.

I prefer to always have a good grip on manual calcs before utilising softwares to do the same. So again the request if you do have something let me have it.

 

[cdafd]

Let me check

 

[TravisMack]

I did it just by following the formulae in NFPA 13. Use the report sheets in NFPA 13 and it is pretty straight forward. You have to make assumptions on the pipe size, and then figure the friction loss based on your assumptions.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[Sdpaddler50]

"I prefer to always have a good grip on manual calcs before utilising softwares to do the same"

Wise words. Check out FM Data Sheet 3-0. You can register and get access to these if you dont already have it. It will give you some examples. As you probably know, figuring out manual hand calcs takes a lot of time. When i first started with FM 25 years ago, they had us doing these old school calcs for literally hundreds of hours to master it. I no longer work for them, and use a computer these days for the rare occassion when i run calcs, but taking the time to learn at the least the basics aspects of manual calcs is a good thing. Good luck.

 

[SprinklerDesigner2]

The reason for the Hand calc is that it gives a person thorough understanding of the calcs as such his confidence,ability and precision is enhanced.

Kudos to you!

Over the years I've trained 2 other layout technicians from scratch and before moving on to HASS I've always insisted they first be able to hand calculate trees, loops and very simple girds because in my view that is the only way a trainee can actually see and feel what is happening.

I've got plenty of N^1.85 graph paper and no end of the old work sheets given out by Viking we learned how to use by attending one of Jack Woods three day class in Hastings. I did my first calculated system in 1976 and it wasn't until 10 years later computers that could do the work were made available to the general public.

And I got really good at it too where simple grids weren't a problem except they did take some time.

For example we're doing this one job now that I've taken a lot of grief on because nobody has seen done what I did because I learned it doing hand calcs a long time ago.

I have a building measuring 120' by 100' across having 14' eaves and a 3 in 12 pitch placing the peak @ 26'-6".

The water supply is horrible. 45 psi static, 27 psi residual @ 628 gpm. I used a 400 gpm @ 55 psi pump because in Georgia you better not run the suction, which is measured at the pump suction flange, below 20 psi with the pump discharging at 150% or you fail the test. Yeah, right... how do you fix that "mistake"?

Because of the roof pitch we were designing to a density of .20 over 1,950 sq. ft plus 250 gpm for hose. The trick here is to keep the sprinkler water requirement as low as possible so that sprinkler plus hose does not draw the suction below 20 psi remember I have a backflow assembly that is going to cost me 3 psi.

With a system using 430 gpm with 250 hose I have 24.1 psi left at the street. Obviously any water I use above 430 is going to really cost in terms of being doable so I resorted to a little trick I learned doing hand calculations that may not be apparent to those who never did hand calcs.

To make it work everything I did is backwards. The main is at the peak of the building and instead of using one size sch. 10 welded pipe I opted for threaded sch. 40 because I wanted to "balance" my flows.

I think I did pretty good given the peak... the system requires 415 gpm for a density of .20 over 1,950 sq. ft.. I didn't care about pipe size, the size I selected was to achieve as close to a friction loss of 1.3 psi between heads to offset the elevation pressure gain and we all know what that does. The worst thing to do would be to try the traditional way with mains at the eave and lines running up the slope. Whatever you did, whatever size pipe you ran even if it was 3" lines, you were going to pick up 1.3 psi between every sprinkler and your water requirement is certainly going to bust the 680 with hose you got to work with.

Sure, it's rear end backwards but it works, people at work are looking at me like I came from mars, but it works. Not something I do often but sometimes it is the only way or the way that will give you the most "safety factor" which I want.

Anyone else ever have a job where you did it this way? Run the main in the peak because the water was so tight?

Sorry to get on my soapbox but I am the #1 proponent of all technicians learning first to do it by hand.

 

[TravisMack]

SD2:

I recently had a hangar project. It was a foam job. Per NFPA 16 (I think that is the foam standard), you can only discharge between 100% and 120% of required flow for the sprinkler. Now, this hangar had a pitch to it, so we had to do some similar things to what you describe. It actually had to work out to a crazy looped system with "unique" sizing to get it all to flow at the required densities and such. The elevation was killing me in trying to keep that 100-120% discharge criteria. I could have made it work all day long with "standard piping," but I would have heads discharge 16gpm - 24gpm and that was not going to fly. If 16gpm was my minimum, 19.2gpm was the maximum I was allowed to discharge. So, I feel your pain of having to do a unique piping situation to adhere to maximum water discharges. I believe that my understanding of hand calcs is what allowed me to understand how to get a piping configuration that could be installed and worked. It was not just a matter of increasing pipe sizes to get under the curve. I had to get creative. Some pipe sizes increased, some had to decrease. It was not much fun, but after a day or 2, I finally got it all to balance!!

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[DavidCR]

Most of the jobs I´ve done deal with foam, deluge and hose systems, so checking uniform discharge is a must. For this I love the "THE" software, because it is like doing hand calcs.

My coleagues used to critize me using such an "old" tool , but when I teach them to use it, they start to like it. It has limitations for sprinklers sytems compared to other programs. We also use Autosprink but I lke using THE because it gives you a better grab of the flows in the pipes with just a glance to the result tables.

 

[Sdpaddler50]

I was searching through 13 for something else, and found some good examples of manual hand calcs in Supplment 2 in the NFPA 13 handbook - all access, on-line version.

 

[bmlxd40]

I think its awesome that you are taking the initiative to learn hand calcs. I consider myself very blessed to have had the opportunity to learn hand calcs from someone gracious enough to teach me. I would be hard pressed to think of anything else that has helped me more in my career, and I'm not even a designer. As a project manager, I have been able to throw out some ideas that have helped us, or see the potential for a problem, and without a knowledge of hand calcs, that wouldn't have happened.