Water Flow Switches in Calculations

[bdk]

I am currently reviewing some shop drawings from a local contractor. In there calculations they have not included the fixed pressure loss for water flow switches. The project is being designed to NFPA 13, 1999 edition. I believe that a flow switch is considered a “device that affects sprinkler discharge”. After reading the Memo from Potter dated 11/24/03, that explains the friction loss for a flow switch is 3 psig for sizes 4 in. or less and 1 psig for sizes exceeding 4 in.

Under the '99 code, section 8-4.4.5 Pipe friction loss shall be calculated in accordance with the Hazen-Williams formula with C values from Table 8-4.4.5.

(a) Include pipe, fittings, and devices such as valves, meters, and strainers, and calculate elevation changes that affect the sprinkler discharge.

In the ’02 code, section 14.4.4.5, the above was changed to “(1) Include pipe, fittings, and devices such as valves, meters, flow switches in pipe 2 in. or less in size, and strainers, and calculate elevation changes that affect the sprinkler discharge.”

I am wondering what the over all consciences is of requiring them in calculations?

 

[stookeyfpe]

So deduct the pressure loss from the calculations. If the hydraulic demand is satisfactory and can be satisifed at the design pressure, you are done.

If the system is designed with a 3 PSI or 1 PSI safety factor you got more problems than the friction loss of the water flow switches.

 

[SprinklerDesigner2]

What stookey said.

Georgia requires all sprinkler systems to be designed with a 10 psi safety factor where there is a backflow preventor installed and a 15 psi safety factor where there isn't.

Not a bad deal at all, let's me sleep better at night, and as long as everyone is made to do it (it appears to be this way) it's the way to go.

The 15 psi safety factor is required with the thought the local water perveyor may someday require a backflow preventor.

I've been in the desogm end of this business since the early 70's and in my younger days I used to strive to get it 'as close to the line" as I could. Looking back most of the systems I "super tweaked" might have save a couple hundred bucks at most and what is a couple hundred bucks on a job selling for $30 or $40K? Kind of silly if you ask me.

Did a storage building a while back; a 40,000 sq. ft. building with a 6" riser, 4" mains and 2" branch lines on a grid. I toyed with the idea of running the lines 1 1/2", if would have worked, but in working up the savings the difference between 1 1/2" and 2" didn't amount to $700 and this was a $48K job. By running 2" I had a 25 psi safety factor. No problems on that job for me.