Water Demand - Pipe Schedule vs. Hydraulic Methods 1

[CanuckMiner]

I'm working my way through an automatic "wet" sprinkler design for an Ordinary Hazard Group 2 area. I'm having trouble definitively determining whether to use the pipe schedule method or the hydraulic method (7-2.2 & 7-2.3 in NFPA 13) to determine water demand.

It seems to me that sometime in the past I read that the hydraulic method was "preferred" by most modern designs, but can't recall the source of that comment. I've read and re-read NFPA 13 for clues as well as the Fire Protection Handbook from NFPA, but can't seem to find the reference.

For previous designs, I've used the Hydraulic Method and have spreadsheets already set up to assist me. But it seems that the pipe schedule design is a quicker, "cook book" design approach. One advantage of the Hydraulic method is that the water demand typically appears to be smaller (i.e. lower flow rate). This can be a bit of a concern in most of my designs, as the areas I deal with don't typically have access to "public water" and therefore the water must be supplied and "dedicated" to the fire suppression system by the owner of the property. This dedicated water supply costs money so the lower the flow rate, and the lower the total volume, the lower the costs.

I'm trying to develop a design guideline for myself (and my company) so that I don't have to wrestle through these comments every time I do a sprinkler design (every couple of years it seems).

Any comments?

Cheers,
CanuckMiner

 

[Slowzuki]

I believe if you read the latest editions of the Code, only existing Pipe Schedule Systems may be replaced. All new must be hydraulically designed/calc'd.

We always check over pipe schedule systems with a hydraulic calc to check. It all depends on the situation which requires more water.

Ken

 

[CanuckMiner]

Slowzuki,

Thank you for your response. I believe your statement is correct for the Extra Hazard Occupancy. Below is text from NFPA 13 - 1999 Edition. My interpretation of this is that the Pipe Schedule method can still be used for new systems of the Ordinary (and Light) Hazard Occupancies. I have based all of my designs on the hydraulic method, but was just wondering if I couldn't do it more quickly by using the pipe schedule method (if the available water supply was sufficient).

7-2.2 Water Demand Requirements — Pipe Schedule
Method.
7-2.2.1 Table 7-2.2.1 shall be used in determining the minimum
water supply requirements for light and ordinary hazard occupancies
protected by systems with pipe sized according to the
pipe schedules of Section 8-5. Pressure and flow requirements
for extra hazard occupancies shall be based on the hydraulic calculation
methods of 7-2.3. The pipe schedule method shall be
permitted only for new installations of 5000 ft2 (465 m2) or less
or for additions or modifications to existing pipe schedule systems
sized according to the pipe schedules of Section 8-5. Table
7-2.2.1 shall be used in determining the minimum water supply
requirements.
Exception No. 1: The pipe schedule method shall be permitted for use in systems
exceeding 5000 ft2 (465 m2) where the flows required in Table 7-2.2.1
are available at a minimum residual pressure of 50 psi (3.4 bar) at the
highest elevation of sprinkler.
Exception No. 2: The pipe schedule method shall be permitted for additions
or modifications to existing extra hazard pipe schedule systems.

 

[Slowzuki]

Just as a note the latest version of NFPA 13 is 2002 Edition.

I just read it over and recalled that the local AHJ is the reason we don't use pipe schedule. They do not permit it unless replacing existing and even then only if you can show a redesign to hydraulic would be very difficult due to pipe locations.

Quote from the NFPA 13 Handbook:

The use of pipe schedule design approaches is restricted to rather small systems, unless high residual pressures are available. While the pipe schedule approach had previously served as an acceptable design option for a wider range of buildings and spaces, it's misuse has resulted in its restricted application.

Ken

 

[LCREP]

NFPA 13, 2002, table 11.2.2.1 for a light hazard requires 15 psi at 500-750 gpm. A light hazard hydraulic system requires .10/1500 or about 150 gpm plus 100 gpm for FD hose for a total of 250 gpm about 50% less gpm then a pipe schedule system. Section 11.2.2.5 requires the residual pressure of 50 psi at the highest elevation if the system is larger then 5,000 sq. ft. I think the NFPA 13 committee is saying you can do it but why? 50 psi flowing 500 gpm on top of a 30 foot building!! and if the sprinkler system is a local alarm only, the flow is increased to 750 gpm as per section 11.2.2.7. I would think a hydraulic designed system would be much cheaper to install particularly if you have a marginal water supply.

 

[TravisMack]

Canuck,

With the advent of hydraulic calculation programs for fire sprinkler systems, pipe schedules are mostly a thing of the past. Addtionally, I have run into many pipe schedule systems that don't always calculate out.

If you are concerned about water supply requirements, you can calculate the system using quick response sprinklers (light and ordinary hazard wet pipe systems)and reduce the design area as long as the ceiling or roof deck (if exposed) is less than 20'. You can get between a 40 and 60% reduction in the design area.

I have not done a pipe schedule system in probably the last 10 years, and I do fire sprinkler systems every day.

I hope this is helpful. If you have any further questions, please feel free to email me.

T-Mack

 

[TSDavis]

Hydraulic calculations and systems designed with hydraulic calculations require an understanding of the effects the layout can have on the demand. A poorly designed system can have a higher demand than a pipe schedule system.

I've noticed with some of our newer designers that they don't necessarily understand the effect balance can have on their demand as the software is doing the work for them.

 

[TravisMack]

TS-

You are correct on that one. I remember being required to calculate tree systems by hand when I first started. We had the computer programs to do it, but the lead engineer wanted to make sure I understood what the computer was doing with the calculations. That was one of the best lessons I was given in the fire sprinkler industry.

 

[TSDavis]

Once you understand how it works you start to lay your systems out so that they balance. It's really fun with some of the programs available now. You can try different sprinklers, different pipe, different areas. Minutes to do what once took days. You can play with a pipe sizes, hit the button, and get instant feedback. Don't like the results just undo it. Technology, my favorite tool.

Terry S. Davis
TriStar Fire Protection
tdavis0114@comcast.net