Hazen Williams safety factor

[thomasfpe]

Hello,

I joined this board today after using it intermittently when I had a challenging project or problem. Now that I've been designing for a while, I have a question that relates to a 2011 post where I noticed that a member posted that the Hazen-Williams formula, if derived, has a 1.85:1 safety factor, or 85%.

From all the research I have done on this topic, I cannot find any substantiation of this claim. Further, it appears that, based on flow tests I have witnessed, that this is not the case. Instead, I've found that the exp(1.85) in the equation simply brings it up to reality for the calculation, and without it the equation doesn't hold. My understanding is that the equation is empirically derived, meaning that given a large set of flows, correlated with the proper C-factor, that the equation simply fits the flows, and that is why we have a 10%+ safety factor required by many AHJs.

Please forgive me if I am wrong, but I have a bet with a local AHJ on this and both of us have been through some extensive hydraulic calcs in the past... I have an MS in FPE, and about 15 years practical experience before that, and have never seen this safety factor in any literature on fire sprinkler flows.

Thank you

RT

 

[TravisMack]

There is no built in safety factor. It is empirically derived. I took a few of the MS FPE classes and we talked extensively about HW but it was never indicated it was anything other than empirically derived, if I recall correctly.

I don't believe the 10% is based on the HW calcs at all. 1 - How often do we get the seasonally adjusted lowest water flow test for a project? 2 - How often does a project go in EXACTLY as drawn without an extra elbow or anything? I think the 10% has just been a common factor to account for both 1 and 2 noted above.

Travis Mack
MFP Design, LLC

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[MGXFP]

Graduate Civil Engineer here. Hazen Williams is an empirical [based on laboratory results] equation. It is used to estimate friction loss only.

The truth is flows in pipes transition between laminar [smooth] and turbulent depending on where in the pipe the water flows and the relation to elbows, tees, etc. These transitions change the friction losses.

Hazen Williams simply assumes linear friction losses [psi/foot] which is wrong but in the safe direction i.e. long runs of trunk main pipe would have lower losses in reality than predicted by HW because turbulence is assumed everywhere [by HW].

I wouldn't call it a formal safety factor but HW is a conservative equation to use and an easy one given the large range of water temperatures it is valid for. Other wise we would need numerous calcs to figure the environmentally relevant range using Darcy Weisbach or other methods.

I'm not sure where the commonly used 10% safety factor came from. In addition, in post fire data I believe for most light/ordinary hazard occupancies 1 fire sprinkler deploys and in storage scenarios 4 sprinklers deploy on average. This also gives a safety factor of at least 4 on light/ordinary hazard [minimum 5 sprinklers in remote area] and 3 for storage [minimum 12 sprinklers on ESFR].

There are several ways of looking at safety factors in fire sprinkler design.

 

[thomasfpe]

Thank you Newton and TravisMack.

You both echoed what I was thinking in the OP, knowing that the equation is empirically derived, even if based in part on Bernoulli and a few other equations. I know that there are also certain parameters such as temperature and fluid speed that can really have some effects on the results. I would, however, stop short of calling it a conservative equation but only due to the limitations of the equation.

The 10% is more good practice, which in the fire sprinkler business helps to account for seasonal and other fluctuations in water supplies. From new construction to pipe repairs to pumps in the city supply, there are all sorts of things that can affect the daily residual and flow pressures.

Newton has a good point on the linear friction loss assumption, which I agree with. Find a section with a few zigs and zags and some seismic joints that require flex, and that could be problematic, but then is likely balanced by some long straight larger pipe main runs that can almost be considered laminar... Almost.

And thank you for understanding how many heads pop during a fire!!! One of those Cliff Claven (dating myself there) little known facts. Generally yes, 1-2 heads in light/ordinary 1 go off where 4-6 depending on the storage arrangement go off. Cooling of the ceiling jet and plume as well as wetting the surrounding area do help, where most people just think the sprinkler hits the fire and it goes out, which of course we aren't allowed (rightly so) to assume. I do quite a bit of smoke control too, and we just stop the fire growth at sprinkler activation but never assume it goes out or even gets knocked down, which is relatively conservative as well.

Great replies, I was thinking the same terms but saw a post with a VERY high assumption of conservatism and was glad to get this clarified.

 

[MGXFP]

Also, there is a diurnal usage effect on water supplies. If there is a homogeneous group of people where everyone uses the water supply at the same time i.e. showers, dinner, etc then the adverse effect on the water supply will be greater than if there is a mix of habits tapping the system.

This is something I look at when doing flow tests, to try and find a time when the water usage is at least at a local maximum. Some jurisdictions give me the model data for Max. Day from WaterCAD which we use instead of a flow test.

 

[thomasfpe]

WE get the same WaterCAD information in Northern California, mainly due to flow test restrictions in this extended drought. I find it odd that some jurisdictions still require full flow tests despite the water waste. It would seem to me that they would have some type of sensors out there to monitor flow and have a good idea about their supply capabilities. I have not asked if it was during max daily flow, but have to assume it is. I've seen pressures drop from 70-80 down to 50-60 due to a couple plants relocating in the area, so I understand not only the daily impact but what happens over the years with additional industry and other demands placed on the systems.

I'll be using that one soon, diurnal usage. Very good term to know.

 

[LCREP]

As an insurance carrier we always ask for 10psi or 10% which ever is higher, including hose demand. Always liked the cals with .05 psi safety with 5 year old water test 1/2 mile away from the job site

 

[MGXFP]

@thomasfpe

Max Day is a term commonly used in water resource engineering. This is the day of the year where demand is at its highest; usually in the summer but not always depending on where the site is.

Some might say this is a safety factor; using the Max Day numbers [it only happens once a year!] but I still apply the traditional 10% to this data just so no one can say it is wrong and I know the sprinklers will discharge at least their minimum densities. Typically it won't cause a change in pipe size anyways.