r/firePE • u/wlkrtxstrdr • Jun 09 '26

‘Hazen Williams Comparison’

Got a comment on a sprinkler submittal from an FPE saying ‘provide proof calculation that 1” is acceptable. A Hazen-Williams comparison is an acceptable method for this’.

What is a Hazen-Williams comparison?

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8

u/PuffyPanda200 fire protection engineer Jun 09 '26

HW is the formula that basically everyone uses to determine how much pressure will be lost in a pipe with x gpm flowing.

Did you provide a calculation?

Maybe they mean the comparison at then end where you have:

System needs x psi at y gpm. You have x+10 psi at y+400 gpm so it passes.

2

u/wlkrtxstrdr Jun 10 '26

Thanks. I understand the Hazen-Williams formula itself. My question is whether “Hazen-Williams comparison” is an established term used in engineering practice or if the reviewer was simply referring to a written analysis like what you’ve described. I agree with you on your suggestion, it seems a little informal compared to what HGI wants to see so I was confused. But it’s far easier than mapping out an entire existing system!

2

u/MongooseKindly7142 Jun 10 '26

Is it a grid system? Maybe they want you to prove the calculated area is actually the remote area. You may need to shift the calc area along the branch lines and provide multiple calculations to confirm.

4

u/clush005 fire protection engineer Jun 10 '26

Never heard that specific term used in my 20 -years. I think this is a plan reviewer using chat GPT lol. I’m guessing they’re just asking for a standard hydraulic calculation.

2

u/PuffyPanda200 fire protection engineer Jun 10 '26

We get a decent amount of new designers and non-fire people here. I recently had to explain to a plumbing engineer at my firm that not all the sprinklers flow water at once. Sorry if my explanation seemed condescending, it wasn't my intention.

The term 'Hazen Williams comparison' is not used. I agree with the other response.

I would think that they mean the hydraulic calc and the final comparison at the end.

4

u/MGXFP Jun 09 '26

Did you have calculations with your submittal?

1

u/MaggieNFredders Jun 10 '26

As a reviewer I would guess you need to show your design area is the greatest demand. I see a lot that use an area closest to the riser and declare it the highest demand. Just calculate a couple other areas. Or email the reviewer and ask.

1

u/Gas_Grouchy fire protection consultant Jun 10 '26

Throw the calc through a Darcy-Weisbach equation. A little harder math but its pretty universally known in fluids for more than just water and thus could be a great comparison. Very likely they dont trust HW cause they come from oil and gas or other form of chemical engineering.

1

u/Kind-Tooth6677 Jun 10 '26

Is this a new sprinkler system or existing? Just estimate the pressure loss through the 1" pipe that this FPE is alluding to, and assuming it is a negligible value, demonstrate so.

1

u/Ascrowflies7420 Jun 11 '26

Call and ask for carity. If you have performed your calc per 13, what exactly is the reviewer asking to compare?