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Safety Factor

Safety Factor

(OP)
All, can anyone give input on how they personally design the safety factor percentage when required? I can't seem to find consensus on what is used.

Ex. We have a system demand of 65 psi. Available water is 100psi. We have a safety factor of 35psi.
This is the easy part.
The tricky part (at least for me) is....what is the percentage a factor of?

If we say "how much "extra" water is there over the demand, I'd divide 35/65 and have a safety factor of 53.8%
If we say "How much of the "extra water" is in relation to the supply, I'd divide 35/100 and ahve a safety factor of 35%.

Just curious, as we have some AHJ's that mandate (10%) but don't stipulate what that is off of. It's cut and dry when your required to decrease the supply by 10%, or an AHJ states 10psi, but when a generic 10% is the altered code, we have a little debate in the office right now.

RE: Safety Factor

The 10% is generally accepted as the margin in the water supply. When doing this, you need to have parallel supply curves. A 10% margin requires you to reduce your static and residual pressure by 10% of the static while the flow is the same. This creates parallel lines when graphed on n1.85 paper.

In the example above, I would say you have a 35% margin.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC
www.mfpdesign.com

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