Manual fire hydrant calculation

[Civilchica]

I have a project with an existing 12" water main out front, and an existing fire hydrant coming off a 150-foot long 8" dead end main onsite. We are expanding the existing building and the plan checker has asked for calculations proving we have residual pressure of 20psi for a flow of 1500gpm at the existing hydrant. We just did fire flow testing using two hydrants on the 12" main out front. Results are 70psi static, 40psi residual, 32psi pitot, 950gpm flow. I want to just do a quick hand calculation using the energy equation, but I am unsure about one thing. For my velocity at the fire hydrant, do I use the velocity based on the 2.5" hydrant orifice, or based on the 8" pipe. In the past I've used WaterCAD or the water district ran the calculations, and I don't think they factor in the fire hydrant orifice size. But I believe the fire hydrant testing does, so I don't know which to use and it makes a huge difference. Any feedback would be appreciated.

 

[CarlB]

The basic equations for just the fire flow test can be found at

http://www.firehydrant.org/info/ftest4.html

Using 2.5" orifice, 32 pitot, C=0.9, the calculated flow is 949 gpm, so agreeing with your number.
Then calculating flow at 20 psi residual yields 1250 gpm. So it appears that 1250 gpm is the maximum fire flow at the tested hydrant, and will be less at the on-site hydrant.

Regarding the energy equation, you will be accounting for the additional head loss in the 150' 8-inch leg, so you would use that length & diameter.

 

[Civilchica]

Yes, I agree. I ran all the numbers that the fire flow test provided and they all work out nicely. What I am struggling with is when we have the water district run the calculations and they provide their letter to the fire department, or when we use EPANET, I don't believe that this accounts for the 2.5" hydrant orifice. Using the velocity from this discharge point is what increases my losses so substantially. Does anyone know if EPANET or WaterCAD accounts for these losses?

 

[CPENG78]

Civilchica,
I have used WaterCAD in the past and where I currently work, we use WaterNetworks by Boss International. When running a fire flow model, available pressure and flow is measured at fire hydrants and other points of connection of interest (ie connection to building sprinkler system). In my models the hydrant will be represented by a point that is serviced by say a 6 inch service lateral in the case of a fire hydrant. Then I place a demand and check residual pressures throughout the system, making sure I meet the 20 psi requirement. In other words, I have not used the size of the hydrant's orifice within the model; only the service lateral. I have however, checked the maximum flow available for the type of hydrants been specified for the project by either using CarlB's calculation or obtaining the data from the hydrant manufacturer.

I don't think the programs outer are not that detailed and I don't recall WaterCAD been any different. Unless I am completely off, programs used by water agencies would also not be any different. Any others outer, know differetenly; I am curious to know. Thanks.

 

[Civilchica]

CPENG78,
That is what I suspected. The confusing thing is that in some cases the water agencies provide letters or certifications stating that 20 psi residual pressure is available with a specified fire flow. Unless they are specifically calculating it using the higher losses at the hydrants, I don't understand how they can do this. It seems like there is a disconnect between the water system model from a water agency and the fire hydrant calculation required by the fire department.

I'm going to try to get some feedback from the local water department on this in the next week or so as well.

 

[CPENG78]

Civilchica,
Your post and thread bring up interesting points in modeling and practical issues at the same time. I suspect that such letters from water agencies provide pressure and flow requirements with residual pressures high enough that both the 20 psi requirement is met and where a more detailed calculation may not be needed. That has beeen the case in the letters I have received in the past. I imagine such calculations would be more applicable when the existing system is already in trouble (pressure and flow wise). I come across such letters from agencies such as EBMUD here in the San Francisco Bay Area. Where are you located? What agencies do you deal with?

I will follow up with some research on this on my end by asking the same question to our senior engineers here in the office.

 

[fel3]

I have done several projects for a fire hydrant manufacturer that included hydraulic testing to verify that a particular hydrant met some agency's minimum criteria for approval.

We measured the pressure drop between the inlet to the bury elbow and the 4-1/2" outlet, then corrected it for velocity head, elevation difference, temperature, etc. On one project, a 1500-gpm flow rate resulted in a corrected pressure loss of about 6.5 psi (the criteria was 8.0 psi loss). On another project with a slightly different hydrant assembly, a 1000-gpm flow rate resulted in a corrected pressure loss of 2.25 psi against a criteria of 2.5 psi.

My caution is that a result of 20 psi at the main/lateral connection, or even at the base of hydrant, isn't 20 psi at the nozzle. On the other hand, the 20 psi requirement includes a safety factor for water quality protection and for the fire department. You just don't want to eat into that safety factor because it beongs to someone else.

 

[CPENG78]

Civilchica,
So it took a few days but I wanted to confirm my research with our senior engineer who has been out of the office.

In reading one of the last letters I received from EBMUD (one of the water agencies here in the area), one of the paragraphs after reporting their test results reads as follows:

"The flow and pressure (static and residual) is available at the street main connection. It is recommended that the fire sprinkler designer incorporate allowance for pressure losses through the fire service meter, backflow prevention, and the lateral from the street main to the fire service meter"

EBMUD does not carry out a physical fire flow test, rather they run their computer model and report their results. Like I mentioned before, I have not ran into projects where the available pressure was so low that the pressure loss had to be calculated that close. However, I have run the hand calculations to verify those losses just to be on the safe side.

What I did fail to mention last time was that within WaterNetworks by Boss International, the losses in a stretch of pipe can be entered when defining that pipe. That is losses due to bends, contractions, expansions, entrance and exits, etc can be incorporated into the model by adjusting the loss coefficient of that pipe run. This would be the place within the program where you could account for those orifice losses. However for the size of projects that I have been involved with, the hand calculation has sufficed. I would incorporate those hand calculations into the program if I was modeling a much larger project.

Everyone,
The attachment includes an excerpt from the 2006 IFC which describes the reasoning behind the 20 psi requirement. I have encountered fire department officials that will allow dipping into the 20 psi requirement but like FED3 mentions, that is their safety factor and therefore they have the final say. Now typically this will be a 2 o 3 psi drop below in the 20 psi requirement. In the last project I was involved, the pressure dropped to 11 psi when applying the required demand. This loss was also affected by the topographical features of the site and therefore a pump will be installed to mitigate this problem.


I hope this helps.

 

[psnyder]

I am located in NYS, and the fire flow for projects here must meet the design standards in the Recommended Standards for Water Works (also known to us as the 10 States Standards). Section 8.2.1 (see below) states that the residual pressure must meet 20 psi at ground level at all points in the distribution system. I take that as meaning the residual pressure must meet 20 psi at the hydrant nozzle (ground level). For the entire version of 10 States Standards go to this website:

http://www.10statesstandards.com/

8.2 SYSTEM DESIGN

8.2.1 Pressure

All water mains, including those not designed to provide fire protection, shall be sized after a hydraulic analysis based on flow demands and pressure requirements. The system shall be designed to maintain a minimum pressure of 20 psi (140 kPa) at ground level at all points in the distribution system under all conditions of flow. The normal working pressure in the distribution system should be approximately 60 to 80 psi (410 - 550 kPa) and not less than 35 psi (240 kPa).





PAUL S SNYDER, P.E.