Estimating Sprinkler Pressure Loss from Friction?

11/13/2025

I have a quick question about estimating the pressure requirements for an automatic wet sprinkler system during the initial design phase.

Qm = d X (Area per Sprinkler)
= (0.15) X (130sqft)
= 19.5 gpm

Estimated Total Flow (ETF) = Qm X N X F plus hose demand
= (19.5 gpm) X (12 sprinklers) X (1.30 for spray pattern overage) + 250 gpm (Outside Hose Stream)
= 554.2 gpm

Minimum pressure needed for the system.
= (19.5gpm/5.6K)
= 12.13 psi for the most demanding sprinkler + another 20-35 psi for potential friction loss throughout the reference.

The total estimated pressure needed would be 47.13 psi, not factoring in elevation change. The 20-35 psi for potential friction loss is an engineering judgement based on experience.

What is the recommended potential friction loss for an initial design phase, and where would it be referenced at?

I need to reference the source of recommended potential friction loss for the project. SFPE, NFPA or etc?

Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe

5 Comments

Anthony

11/13/2025 08:37:23 am

Generally I use remote area size * demand +20% + HA to estimate. (sqft x gpm/sqft *1.2+250) Ex: 1500sqft x 0.15 gpm/sqft * 1.2 = 270 + 250 = 520 gpm. Looking at induvial sprinklers at the estimating phase can lead to trouble later and takes too long.

I use the following items as my major pressure loss factors:

Backflow Preventer
Elevation
Sprinkler Min Operation+ 20%
Friction Loss

I have the above in an excel table and for fiction loss I have an excel calculator that runs Hazen-Williams. So everything plugs and adds up automatically. friction loss is usually 10psi + my anticipated or known UG and main sizes. That 10 PSI might fall off and I might get a bit more granular adding feet and pipe diameters together. Its super easy to set up the HW calculator (I think Joe sells one too as part of the tool kit?)

So I'll take those items and get a "good enough" calculation. Throw that on an N^1.85 chart and you can get a decent first pass feasibility report. (I believe Joe also has an excel chart in his tool kit too)

But if I'm feeling lazy I add 15 PSI for valves and pipe. That's usually good enough for anything OH-2 or lower. Pipe can always get big enough to have negligible friction losses That ends up being a value engineered item not a will the system work item.

Jack G

11/13/2025 08:52:51 am

They may be referring to SFPE5th edition chapter 41. " Hydraulics ".
L1 = equivalent length of pipe fittings plus valves from the remote sprinklers back to the city supply .
P1 is the average end head pressures at end of remote area.. head pressures ( d x a ) then q =(k) (p) *.5.
Then simply take the friction loss available ( minus safety factor ,minus elevation liss ). and divide it by the total equivalent length of the path .
In SFPE they estimate that to ( .15 psi per foot )
Then I look at the flow, starting at the end head ) with a flow/ psi chart for pipe sizes and pick pipe sizes that don't exceed the psi per foot that I calculated, SFPE indicates .15.
In the old grinnel black book there is a page that walks you thru this ( I have my Father's from the early 50 s)..
It's a pretty good way to get pipe sizes.
Now a days the cad programs have tree estimators and grid and loop estimators that do the same thing basically by just punching in the design data and pipe lengths averages.
So as a reference I would use the sp]e text or cad estimator hydraulic program.

Wes

11/13/2025 09:22:32 am

I would express some caution to using a "rule of thumb" for system friction loss as a blanket value. Some systems could be very long (long pipe runs), have unique pipe types (CPVC vs steel vs copper), vary in system type (dry, wet, preaction), and vary in arrangement (tree, loop, grid).

It's too easy today to mockup a hydraulic calculation, or even do a full hydraulic calculation, to validate a preliminary design.

If you're unsure, or need to validate your numbers - just do a calc. A program-based preliminary approach like HASS or HydraCALC can be put together in about 20 minutes. You could build an excel spreadsheet to calc it (just like the NFPA 13 forms). The MF Toolkit has an Estimator for trees that takes seconds. Or, you could do a CAD/Revit layout and run it through a full-fledged program.

We've gotten into a couple very hairy project situations where a consultant did 'rough numbers' but no actual calculation. In one case, they were suggesting that it was 'possible' to achieve a 3rd floor, 0.10 and 0.20 over 2,500 sqft, with a borderline water supply if we just increased the main size - largely predicated on this type of 'rough loss' analysis. When we performed a full hydraulic calculation, it still couldn't work with 8-inch dual-feed mains and 3" gridded branchlines (extremely unrealistic for a dorm anyways).

I would just suggest that if you have a calculation that is clearly easy to achieve, or clearly hard to achieve, these preliminary 'rule of thumb' estimates are fine. But if you have a borderline situation, dig a little deeper and either run a calc through an estimation program, or just do a calculation. It's a healthy and enlightening process that'll give you the validation and the confidence in your result that you're looking for here. Just my take.

Jesse

11/13/2025 02:47:37 pm

Gotta garee with Wes here. I have this conversation with architects a lot when they ask us to estimate FL. The consultants "rough numbers' are no match for a calculation.

11/13/2025 02:57:57 pm

The “ rules of thumb “ is used to calculate the pressure left after all the critical losses and divide that by the footage of pipe to the remote area from the supply .
It gives you psi/ ft loss.
When you pick a size for your max pipe flows per that friction loss factor you get an estimation that you put into your hydraulic program and rarely have to go back and change sizes.
This method is outdated( goes back to the late 50 s) that fire protection estimators used to estimate the system.
There are too many easier methods now a days. ( I remember using it in 59, from my grinell black book)

Estimating Sprinkler Pressure Loss from Friction?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES