Fire Sprinkler Head - Minimum Pressure Requirement 1

JBDuffy · Jan 22, 2009

I am reviewing Hydraulic Calculations for a wet type fire sprinkler system to be installed in an existing building. The new sprinkler system will connect to an existing fire pump fed by a 25,000 gallon underground water storage tank (the tank is dedicated to fire protection).

From the hydraulic calculations, it appears that the designer is using a minimum pressure of 14 psig at the sprinkler heads located in the Hydraulic remote zone.

I though that NFPA required a minimum fire sprinkler pressure of 7 psig at the sprinkler head. I do not own a copy of NFPA.

Our agencies fire suppression specifications require a 10% safety margin for pressure. The hydraulic calculations are based on connecting to the existing fire pump. Using the performance characteristics of the existing fire pump and a minimum sprinkler head pressure of 14 psig, a safety margin for pressure of only 5% is achieved (per the submitted calculations). The safety margin is calculated back at the pump discharge point. In other words, if the pressure at the sprinkler head is set at a minimum 14 psig (lets call it the required residual pressure), what pressure must be available at the pump discharge point (+10%) to achieve this, taking into consideration friction losses between the two points.

The calculations show that a new pump or new larger diameter piping from the pump to the new sprinkler system would have to be installed (or both). This is cost prohibitive, considering built out nature of the surrounding building.

Is 7 psig at the sprinkler head the NFPA minimum pressure?

Sorry for the long winded question. This is my first post and I often see responses that start with "you need to provide more information"!

Thanks.

RemoteControlFF · Jan 22, 2009

Check the listing of the individual head (I assume they sent you a material submittal along with the calcs) to see what the operating pressure of that specific head is.

More important than that, however, is checking how much water they are flowing out of the most demanding head. This is determined by hazard classification of the area the head is protecting and how many square feet the head is covering.

Once you have the hazard level you get the minimum GPM/ft^2 from NFPA 13 FIGURE 11.2.3.1.5 and multiply that by the square footage being protected and you have the required flow. Then you use Q=K*[√]p where:
Q = your flow
K = the K factor of the head (should be on the material submittal, the plans and the calc sheets)
p = your pressure

and you can determine the minimum psi.

Don't take this the wrong way, but how are you reviewing sprinkler plans if you don't have a copy of NFPA 13? What are you checking for, just that the math is correct or are you looking at spacing, obstructions, etc.?

stookeyfpe · Jan 22, 2009

Without the sprinkler data sheet and the hazard/occupancy classification of the area, it is almost impossible to verify compliance with NFPA 13. If you don't have a copy of NFPA 13, seek professional assistance.

I wonder if the sprinkler designer may be using some CMSA sprinkler, which means the calculations should be based on "x" sprinklers in the most demanding area, rather than a design area.

PEDARRIN2 · Jan 22, 2009

When reviewing sprinkler layouts and hydraulic calcs, I always require the cut sheets for the sprinklers being installed. That way I can verify the flows, pressures, K factors, temperature ratings, etc.

We typically use 5% safety factor on the pressure.

Request this information and then determine if it is acceptable.

TravisMack · Jan 22, 2009

7 PSI is the bare minimum for standard spray sprinklers. If you are in OH2 and covering 12'x10', the minimum flow is 24 gpm. With a K5.6 sprinkler, you will be looking at 18.4 PSI as a minimum EHP just to get the req'd density.

Without knowing more, you may be looking at some type of ex cov sprinkler with a specific pressure req'd for the spacing.

Also, without knowing the piping sizes / routings / elevations / etc, one can not determine what the pump discharge pressure would need to be.

Since you don't seem to have access to NFPA 13 and NFPA 20, I would suggest you get help from some one more "fluent" in fire sprinkler system design / review.

Good luck with the project.

Travis Mack
MFP Design, LLC

fireguy519345 · Jan 22, 2009

nfpa.org will be pleased to sell you copies of NFPA 13 and NFPA 20. You will need to know what year is enforced in your jurisdiction. Even though my company does only service and repair work, I would be lost with out my NFPA Standards, and constantly refer to them.

RemoteControlFF · Jan 23, 2009

fireguy, don't say "only does service and repair work"... I'm pretty sure that is the only place anyone in the industry is making money at this point, with the way people are starting to bid work.

JBDuffy · Jan 23, 2009

Thanks to all for the excellent responses. I liked the part about "fluent"! I fully agree that each technical discipline on a project should be left to the experts.

I am just trying to do a sanity check (I should not have said "reviewing", that was done by our fire engineer) on the calculations before we use them as the basis for making an extensive modification to the construction contract (Design/Build) and the building itself. I am a civil engineer who has moved into construction management over the years. I have a decent understanding of hydraulics, but have to admit that the particulars of Fire Suppression engineering and NFPA code are outside of my scope. It seems to me that the calculations are generally very conservative given that many of the actual pipe lengths (not the Hazen-Williams “Equivalent Lengths”) used in the calculations for the piping from the pump to the sprinkler system are higher than the field measurement.

The k-factor for the sprinkler heads that are being installed is 8.1 (English units). There are 8 sprinkler heads in the hydraulic remote zone. Of the 8 sprinkler heads in this zone, the one with the highest flow rate requirement requires a flow rate of 27.38 GPM. Using the formula Q=k*(Square Root P), this results in a minimum pressure requirement of 11.43 PSI for proper sprinkler head function.

I guess I'd better stop avoiding the inevitable and start drafting the modification.

Thanks again for the knowledgeable insight. I definitely learned something new and valuable (the modified orifice formula used by fire sprinkler manufacturers was one thing - I mistakenly thought this was a minor K loss for the sprinkler head). Look forward to reading more postings.

cdafd · Jan 23, 2009

JBDuffy

as stated above the manufacture sheet should give you what pressure is required if non standard head.

Can you post the brand and model name/ number of the head to be used and the sq ft it is intended to cover????

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Fire Sprinkler Head - Minimum Pressure Requirement 1

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