How can I say this is the best size of pipe for sprinkler system 1

[yassinemzili]

I have I software (Elite Fire) that calculat the pressure and the flow needed for a automatic sprinkler systeme. for that, i have to choose the diameter of pipes, and I dont know how to choose the best size.

I know that changing the size will increase or decrease the pressure loss, but I don't know how to say "in this pipe i have i lot of pressur loss or not"

Please help, my boss told me to look for a standart pressure loss or velosity or pipe size that I don't have to exeed.

 

[b1ueshift]

The information about pressure drops should all be in the progam output.

Out of curiosity, which country are you in and are you working for a consultant or a contractor?

 

[yassinemzili]

I know that this information are in the program output
but when I read for example "in the pipes number 4 I have 2 psi pressur drop" how can I say if it's an acceptabale pressur drop or not.

I work in Morroco for I consultant(i think this mean a company that do study befor installing the sprinkler system)

 

[b1ueshift]

Ah Morroco. You have come to the right place to get information.

In my experience, American sprinkler designers are very good and you can learn a lot from them. They have the best training and certification programs and there are so many sprinklered buildings in the US. I'm from Australia myself and we are catching up. It's only recently that formal training in sprinkler design has become available. Most designers in my country seem to be self taught.

To answer your question, you just need experience to tell if a pressure drop is high or low.

Also why does your boss want you to calculate pressure drops? Did you know that NFPA 13 allows you to use pipe scheduling? Scheduling is the appropriate design method in some circumstances especially as your fee is likley to be the same whether you do a calculation or not.

 

[yassinemzili]

Yes your rigth about the American sprinkler desingers. that's why even if we are a French-speaking country most of company work with the american standard.

To answer at you idea about schedul methode, I prefer work with hydraulic calculation because il's widely used with no exeption we have a softwar that do all the calculations.
and for pressur drop, my boss wont to calculat the pressur drop to downsize the pipe at there maximum to reduce the price of the project( small diameter pipes are cheaper)

 

[sprinkler1000]

There are some things to take into account.
If you are doing hydraulic calculation one good thing to assume dimensions at first is the pipe schedule, then you check with the program.
Estimations as to what is good and bad comes with experience. You have limits. Many codes limit water velocity to 5 m/s.

Pressure losses should be checked. It is not good to loose 1kg/cm in a 5 m pipe for example.

 

[yassinemzili]

I agree with you but shedul method existe just for LH and OH, but we'll do if we have EH or Storage?

 

[b1ueshift]

I think most sprinkler designers work much the same way.

1. You design the system completely using a pipe scheduling system. This means that the pipe size depends only on how many sprinklers the pipe serves. You must be consistant such that if a 32mm pipe serves 2 sprinklers - this must be the same everywhere in your system. The pipe scheduling system does not come from NFPA 13 - you must decide what numbers to use based on your experience.

2. Select the hydraulically remote area.

3. Do a calculation

4. If the pressure drop is too high, then make some pipes bigger. If the pressure drop is excessively low, than you can make some pipes smaller - but still use a pipe scheduling system.

5. Repeat calculation untill it looks about right.

 

[DavidCR]

The velocity limit is not a requirement now, but can be a guide so for small pipes try to compare with the schedule method and velocities bellow 5m/s. You can look design examples to get a criteria based on velocities.

For big pipes (above 3") think of money and pressure losses reasonable; problem: only experience would tell.

Other criteria is based on imbalance of sprinkler flow i.e. the flow from different sprinklers should differ on a max percentage.

 

[TravisMack]

Pipe scheduling is not allowed for new buildings in excess of 5k sq ft.

If you are using a grid, there is no schedule to follow.

Basically, I try to find what gives me the smallest pipe sizes at the lowest costs. If I need to make a main 4" instead of 6", but increase a few branch line pieces to 2" instead of 1½", then it is usually best to go that route. I typically find that it is too much pressure loss to have 2 heads fed off of 1" pipe. I try to keep 1" pipe feeding one sprinkler, unless I have a lot of pressure to deal with (80+ psi).

Also, there are times when decreasing pipe sizing will be to your benefit as you will decrease the over-discharge in a system. It is a bunch of trial and error as well as experience that gives you the optimum pipe sizing.

Also, velocity is not mentioned in NFPA 13. As far as I can tell, there has never been a velocity limitation in NFPA 13. It was always an insurance requirement. Velocity is a self-limiting variable. As the velocity increases, the friction loss increases to a point where you need to upsize the pipe. This will then cause the velocity to drop.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[chillylulu]

1. Velocity used to be in NFPA 13. Originally, most of the data was imperically derived. I think the requirement was there because they really didn't have data to support high velocities, and they didn't know how it could affect the pressure loss in the pipe.

2. I have been calculating fire sprinkler systems since 1983. I rarely start with a pipe schedule system, mainly because the pressures where I work vary substantially. I can have static pressures from 50 to 120 PSI, literally where one zone ends and the other begins across the street. Experience is the best way to determine starting pipe sizes.

That being said, start with something - pipe schedules can be great, but don't vary feed main or crossmain sizes. Use pipe schedules as a starting size for branchlines.

Look at the flow from each outlet - one of them will be driving the calculation. Try to keep the other outlets from over-flowing too much. Often, the answer is to decrease the pipe size to an over-flowing area - rather than to just start increasing pipe sizes. Sometimes your changes will move the remote area - so you may have to do more calcs.

Make sure you are starting with the right orifice size. Do not use a 5.6k head for high piled storage densities. Don't always used a 5.6k head for attics either. At 7 PSI, in an area that has a demand of .1 gpm/sqft, a 5.6k head will cover an area of 148 sqft. It is required to flow that much, even if the area you need to cover is less. IE; If you are protecting an obstructed combustible attic with a maximum coverage area per head of 130 SqFt - drop down a size and use a 4.2k head.

Look at the total flow through the system - is it within an acceptable range relative to the minimum required flow? For example, if you have a remote area of 1000 SqFt, and a required density of .1 gpm / SqFt - the minimum flow would be 100 gpm. I would be very concerned about a design that requires 175 gpm. This indicates an out of balance system. It also requires a larger supply main to provide 175% of the minimum required flow.

If a system looks unbalanced, check the outflow from each orifice first, then check the velocity through your piping system. Pipes with higher velocity will have a higher presure loss. Sometimes that is ok, for instance I would expect a small pipe right off the main, feeding a single hydraulically closer outlet to have a high velocity. In that case you want to reduce the pressure available to keep the outlet from overflowing. Balance the flow of each outlet to the minimum demand required.

Increase pipe size to large orifice sprinklers - including drop/sprig sizes. Don't normally try to force 40 gpm through 1" pipe, even for a couple of feet - it will kill your main sizes. There are always exceptions, so play with it a bit.

Another way to balance your system outflows is to have the most remote heads covering a smaller area. If you have five heads on a line, space them so that the head closer to the main has the highest flow requirement. NFPA 13 allows you to calculate the area of coverage for each head.

Remember that on hydraulically designed systems designed per NFPA 13, small rooms with one head such as bathrooms and closets can be ommitted from flowing in calculations. Turn them off. If you have 8 closets in a row, cover them with a single 1" branchline. Turn off double compliment sprinklers, and sprinklers under obstructions such as ducts. Don't flow more outlets than what are required. The trick is almost always in the exceptions.

Unless your system is laid out typically you should calculate the required minimum flow from each sprinkler. Use the SxL method unless light hazard and small room rules both apply. In that case take the room divided by the number of sprinklers to figure the area of coverage for each head. This is important - ideally, an 800 sqft room, meeting small room requirements at a density of .1 gpm / sqft, should require 80 gpm. If the room lay-out requires 5 heads, flow 16 gpm per head - not 22.5 gpm per head. Wall distance and distance to next head or line do not always matter in small rooms.

Remember that outlets with the same k-factor that are hydraulically closer to the supply will flow more than more hydraulically remote outlets. Modify your system lay-out, pipe sizes, head count, or anything to balance the required demand with the pressure available at an outlet.

The one thing you can't do is mix orifice sizes on closed head systems in the same area to balance calculations. You can and should vary orifices on deluge systems, but that's a whole other "Oprah."

Good Luck!

 

[chicopee]

Wow, I lived in Casablanca from the late 40's 'till middle 50's when the French were kicked out of Moroco. Yes, the US standards are one of the best in the world.

 

[SprinklerDesigner2]

Article concerning velocity in sprinkler pipe by Russel Fleming

http://www.sfpe.org/upload/080107_--_russ_fleming_picture.pdf

http://findarticles.com/p/articles/mi_qa3737/is_200005/ai_n8899216/

NFPA 13 doesn't contain velocity limits because the Committee on Automatic Sprinklers has long felt that piping velocities are self limiting. The pressure losses increase exponentially as velocities increase, so pipe sizes must be increased to make use of available water supply pressures. The committee also believes that hydraulic calculation inaccuracies using Hazen-Williams are minor compared to the inaccuracies associated with assuming a long-term pipe roughness factor. In one 1968 study of actual roughness of aged steel pipe, the measured C values ranged from 82 to 135.