Calculate (%) Safety Factor for Sprinkler Systems?

4/1/2019

For hydraulic safety factors (such as in fire sprinkler systems), determining whether a 5 psi safety factor is met in a design is easy. The Available Pressure must be greater than Required Pressure + 5 PSI.

However, for percentage safety factors such as 5% or 10%, is the percentage calculated based on the Required Pressure or Available Pressure?

As an example, if we have a system that requires 80 psi and the available city pressure of 88 psi, a 10% safety factor on the required pressure would be 80 psi x 10% = 8 psi safety required. In this case the minimum is met. However, a 10% safety factor on the available pressure is 88 psi x 10% = 8.8 psi safety required. In this case, the safety factor is not met.

This is an extreme example, but I'm curious what others would use when looking at determining safety factors.

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10 Comments

Wayne Ammons

4/1/2019 07:34:39 am

NFPA 13 does not mandate a safety margin for hydraulic calculations. However, it is a good idea to maintain some amount of safety margin to account for fluctuations in water supply.

Many companies and jurisdictions require a safety margin in psi and some require a safety margin percentage. In my experience working with jurisdictions all across the country, determining what this percentage is calculated from varies. Most often the percentage is of available city static pressure, sometimes it is of system demand pressure.

You will want to contact the company, AHJ, or municipality that is requiring the safety margin and confirm exactly how to calculate the safety margin.

Scott Walter McBride

4/1/2019 10:10:40 am

I would agree with Mr. Ammons. At the end of the day it is up to the AHJ of what the requirement is and how it is to be calculated.

Not to mention that there are safety factors that are built in to hydraulic calculations to begin with, but don't argue that with the AHJ. You will lose.

J Mcelvaney

4/1/2019 10:14:26 am

The City of Phoenix, AZ has the following design safety factor

507.4.2 Design safety factor.
The water flow test results shall be adjusted as noted in Sections 508.1.4.5.3 and 508.1.4.5.4 to account for seasonal and daily flow calculations in the water supply system.

507.4.3 Static pressure over 90 psi.
When the static pressure is over 90 pounds (621 kPa) per square inch (psi) the maximum design static pressure will be 80 psi (552 kPa) regardless of the actual test pressure. The slope of the original design curve shall be used even though the design pressure is reduced to 80 psi (552 kPa).

507.4.4 Static pressure less than or equal to 90 psi.
When the recorded static pressure is less than or equal to 90 psi (621 kPa), the system design shall include a minimum 10 psi (69 kPa) safety factor between the system flow and pressure demand and the available water supply.

Mike

4/1/2019 10:15:38 am

CBC requires 10% reduction of the available water supply flow test to be used. I use a minimum of 10% below that.

Most of the time it's just making, riser, riser nipples or starter pieces one diameter larger. This is usually very economical compared to the risk.

If you're doing a retrofit with many unknowns you can use a larger factor or larger starters for unanticipated offsets.

A really common oversight i have seen in when using flexible drops, the correct equivalent pipe length isn't used for the length of the drop and maximum number of turns.

I've also seen clowns put in a 4“ backflow in a 6“ supply to save a few $$$. This leaves no room for future changes.

Jake Zlomie link

4/1/2019 10:16:47 am

From my research, there is not a specific safety factor that is required for these systems, unless there is a specific ordinance adopted by a community or you are utilizing design criteria for a specialized industry. For example, the department of energy requires a 10% or 10 psi, whichever is greater based upon their DOE-1066 standard.

Some communities that also provide flow rates for your initial design, may reduce that by 10% to accommodate for a safety factor. When I researched this, many of those on the design end did not feel it was necessary based upon the safety factors already built into NFPA 13 design criteria as well as hydraulic calculations.

From an AHJ perspective, I think that the safety factor makes sense based upon when the flow was actually tested, fluctuations in the system, degradation and modification of the water supply, as well if the system was existing pipe schedule and then being hydraulically calculated.

Without an amendment or ordinance on the books however, the AHJ does not have a specific thread to pull in the event they find the calculations to be less than desirable. You could in theory submit calculations with a 0.3 safety factor otherwise.

4/1/2019 10:21:49 am

I have always used the following when asked for a percentage:

Safety factor psi divided by Available psi. For example:

Safety factor = 14.8 psi Available = 59.3 psi

14.8 ÷ 59.3% = 24.95

Dennis Clark

4/1/2019 11:16:39 am

The City of Sioux Falls requires the following:

The public water distribution system is subject to pressure fluctuation due to various seasonal and usage demands. Water pressure fluctuation is reported to be up to 10 psi annually as documented by the City of Sioux Falls Public Works Department.

A minimum pressure safety factor of 10 psi shall be added to all hydraulically calculated sprinkler pressure demands.

PETE

4/1/2019 11:22:18 am

It looks like I am late to the thread, but echoing some others above: a 10% safety factor would be on the demand pressure. That is, 110% of the calculated demand pressure would have to be less than or equal to the residual pressure in the water supply at the demand flow.

Alan A. Luce, NICET IV SET link

4/3/2019 05:06:33 am

10 psig equired in Georgia against the available water supply. However, remember to calculate available suction pressure on the inlet flange of a fire pump based upon 150% of the rated flow at 65% available boost per NFPA 20. This suctionnpressure must not be lower than 25 psig to protect the city water supply from potential backflow conditions.

Jessica Lutz

4/3/2019 10:04:54 am

In LA County and some other areas in SoCal, they require a 10% reduction of the pressure available at the supply.
In the Boston area, most specifications require a 10 psi safety between the demand and available pressures.

Calculate (%) Safety Factor for Sprinkler Systems?

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