Does Max Pressure, Hazard Matter for Deluge?

7/24/2025

Does maximum pressure and allowed hazard classification for any sprinkler (such as light hazard, 175 psi) really matter in a deluge system?

I have a design situation where I need to use 8.0K extended coverage sidewalls listed for light hazard use, to achieve an FM Global Hazard Category 2 (HC-2), 0.60 gpm / 1,200 sqft, and I cannot accomplish that without exceeding 175 psi.

Since the heads are open, I would think that pressure and rating do not matter since the activation will be by electronic devices.

Does it matter?

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12 Comments

Pete H

7/24/2025 06:21:36 am

Did you use high pressure fittings? And valves listed for higher pressures as well?

Cause if say the threaded connections from the branch line to the open sprinkler head is only rated for 175 psi, and you have more than that, you will likely impact relevant system performance. The activation may be by electronic devices, but the delivery is still water pressure.

Brett

7/24/2025 06:52:43 am

Yes, they both matter and you'd be opening yourself up to liability if a component failed or the system didn't operate as intended. As far as the hazard classification goes you might need to work with an FM representative if you can't find a sidewall that's approved for HC-2 occupancies. If their datasheet specifically calls out an extended coverage sidewall for an HC-2 occupancy then the sprinkler must exist.

Glenn Berger

7/24/2025 08:21:35 am

Maximum pressure always matters. There are fittings, valves, and other components that still need to operate correctly.

Dan Wilder

7/24/2025 08:28:21 am

NFPA 13-19' 7.1.2 Rated Pressure - Components shall be rated for the maximum system working pressure...

3.3.216 System Working Pressure - The maximum anticipated static (nonflowing) or flowing pressure applied to sprinkler system components exclusive of surge pressures and exclusive of pressure from the fire department connection.

On the surface this would not be allowed.

A call to the MFG on the sprinkler capabilities could occur but I highly doubt anyone will sign off on an untested design pressure.

If the sprinkler was a density/area-based design method on the data sheet this "could" be acceptable assuming the sprinkler was rated for the higher pressure, but for an extended coverage sprinkler there is no data to support the density would be achieved by just using the SxLxDensity to get the needed minimum discharge.

Louie Lincoln

7/24/2025 08:30:29 am

It may matter even more on a deluge system as when it trips the surge is astounding.

Chris

7/24/2025 08:35:39 am

Not to de-rail the question, but code literally tells us to exceed the rated pressure of fittings for hydro-static tests. Most hydro's end up having a pressure of easily 220psi, and usually the field will pump it up a day before so that it is ready and settled by the time the actual inspection takes place. If this occurs on a Friday, I have heard that systems will go the weekend over 220psi with threaded fittings rated for 175psi and have no issues.

Code creates a single exception with hydro's where we exceed the 175psi maximum pressure, but it doesn't create any guidance on other exceptions that could take place. There are many times that when designing a fire pump system, the churn will bump a system pressure up above 175psi slightly, and I have had to debate the need of a Master PRV to keep the system below 175psi, even though it would only be >175psi for usually less than an hour.

The issue code seems to be trying to solve is that if a system stayed above 175psi for a year, it would likely create issues with threaded fittings. It would be very helpful if there was an exception for pressure exceeding 175psi for short duration's so that these exceptions can help save contractors from spending more unnecessarily.

If this was the case, this exception would allow the Deluge design listed above to be temporarily above 175psi, it would help lots of fire pump designs, and would make more logical sense as to why we hydro systems at 200psi for 2+ hours.

Anthony

7/24/2025 08:38:02 am

I'm not sure you should use sprinklers listed for light hazard in an HC-2 category design. FM might reject the design. FM has specific sprinklers/nozzles listings for exposure protection that might be worth a look.

Franck

7/24/2025 10:00:50 am

There is one big concern with high pressure: the water distribution pattern and the water droplet size.
Sprinklers best operate between 7 psi and 60 psi. Above 60 psi, the water droplet size tends to be non homogeneous and more fine droplets (not able to penetrate a severe fire heat plume) are created.
In addition, the distribution pattern tends to be more elliptical and all areas might not be fully covered.
This is some of the reasons why the spacing for EH commodities are smaller and why we need higher K-factor for storage occupancies when the density is above 0.2 gpm/sq ft.

On top opf that, sidewalls are normally not accepted for hazards that are more than the Light Hazard classification. This is also related to the efficiency of sidewall sprinklers that is not as good as standard spray sprinklers for high densities.

In your case, there are high chances that the AHJ will not accept the design, and the fact that it is a deluge system does not solve the problem.

David Kendrick

7/24/2025 10:18:23 am

Have you checked the possibility of using smaller orifice heads closer to the mains and riser?
Unless it’s restricted by the standard governing your project balancing a deluge system can be improved when smaller orifices are applied.

Jack G

7/24/2025 11:17:19 am

Yes it does matter.
First if mailable iron fittings they are permitted by 13 to be used with 200 psi and if new fittings they are probably mal. Iron rated for 250 psi. Grooved should be no problem.
Now hydraulics.
I ve found it’s best “ to allow the sizing and flow thru the pipes” to control the amount of water discharged.
This can be accomplished ( like the old days) for a center- center design——- sprinklers on lines, branches on mains. Center the supply. ( like as an exaggeration— if you had 5000 gpm going in, only the density times the area ( or as close as possible ) is discharged,controlled by the pipe sizes—- no safety factor— as that in itself causes overflow and pressure.
This should cut down on overflow and pressure drastically.
Now for my next “ tool” I perform a “ supply calc” - tweaking the pipe sizes as required to balance the flow/ pressure.
After getting it tweaked, I do a demand calc, and wha-la, the overflow is minimized.
So the design of the system could be causing it not to work.
( calc a normal system with 10 psi safety. Then add the 10 psi to the end head—- calcs overflow when excessive pressure is used)
And the system doesn’t hit the density or flow.

Kenneth Casey Milhorn

7/25/2025 09:28:51 am

Yes, as Franck said, it's not only about the activation element to "hold back pressure", but it's about the spray pattern. The spray patterns and droplet sizes can change dramatically at higher pressures. Also, many of these sprinklers are listed for 250 psi with the exact same "parts", manufacturing process, etc. it's just the fact that the manufacturers didn't spend the money to have them listed at the higher pressures because it wasn't a good ROI. Most likely, the rest of your components are listed to 250 psi or greater. Many times it does come down to the sprinkler head as the limiting factor for system pressure. Worth a call to the manufacturer to see if they will write a letter, especially if they have some documentation of testing they have performed on this head at higher pressures. You never know unless you ask.

Phil Watkins

7/25/2025 10:26:22 am

I'd be very surprised if you are going to see 175 psi residual pressure at any of your open sprinklers when the whole system is operating (although not impossible of course depending on total system flow rate and water supply performance).

If you enter your pump performance curve into the calculation you should see the actual pressure and flows.

Although its important not to deliver higher pressures than the nozzles are designed for (overpressure often means deterioration of the water droplet size etc) its often impossible especially where the system has multiple deluge systems of different sizes (water supplies designed for massive systems sometimes feed very small systems),

Quite honestly I've seen probably a hundred different deluge systems operating over the years and running pressure at remote nozzles are usually quite high but overall area water delivery when taken as a whole is more than satisfactory.

Have you considered using directional open nozzles (such as medium velocity) as they have alternative k-factors and spray profiles that might suit your application?

Does Max Pressure, Hazard Matter for Deluge?

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