Have hydraulic calculations ever kept you up at night?
I can usually tell when I’m carrying extra stress in my life because my nightmares looks suspiciously like a hydraulic calculation that, despite any refinement, just doesn’t calc’ out.
Ever had that?
I’ll refrain from sharing all my personal issues for now, but if you’re a sprinkler designer or FPE, you’ve had to experience a project where there’s seemingly no way to get the hydraulics in the black.
When I initially lay out a project I’ll have a rough idea of pipe sizing and layout type (tree, grid, loop) based on similar projects. I’ll lay out a remote area and ‘rough-in’ the rest of the design so that I can get to hydraulic calculations iterations as early as possible.
Iterations? Yes, iterations.
If you’re long versed in the sprinkler industry this needs no explaining. If not, the secret sauce of a high-value designer/engineer is all in the refinement and iterations.
If you’re a consulting engineer, perhaps you’re less interested in whether a system is efficient and more interested in whether a system ‘can work’.
If you’re on the install side of the trade, you can earn back good pay and more by calculating systems that are well optimized – that is – perform efficiently, use the right system type, sprinkler type, and allocate pipe sizing appropriately.
This week I’m running down a quick list on potential avenues to consider when you’re working through those calculations and need ideas on tweaks that could help.
A quick disclaimer – hydraulic calculations are an important part of ensuring that the systems we design will be effective in suppressing a fire. The categories below are important aspects to consider when conducting hydraulic calculations – not corners to cut – but rather ideas to get unstuck in optimizing a sprinkler system.
Probably most important consideration is the system type (grid, loop, or tree). Dry and pre-action systems have limitations (no grids allowed), but if a facility is big enough then moving to a loop or grid configuration may significantly help the system perform more efficiently (ie: smaller size pipe).
This perhaps could be the most often-overlooked impact on a sprinkler calculation. Are you using the right k-factor for the job?
If you’re always using K5.6 until you get into storage applications – there are better tools for the trade. Adjusting the k-factor based on the density and sprinkler spacing directly impacts the starting pressure within a hydraulic calculation.
If you haven’t tried it yet, use this tool that’s a part of the Toolkit to find the optimal k-factor for your job.
Perhaps the most obvious and classic go-to is adjusting the size of pipe diameters. The larger the pipe diameter, the easier (less friction loss) the water will experience when passing through the pipe.
This isn’t always negotiable – many specified projects will stipulate a pipe wall thickness – but be cognizant that the pipe schedule is (1) correct for the job, and (2) is considered in the hydraulic calculations.
I had long underestimated the impact that pipe schedule had on hydraulic calculations, but it’s major. Schedule 40 and Schedule 10 can have a major impact. Change from Schedule 10 to Schedule 40 and you’ll increase friction loss by 24%.
See the impact on friction loss with the Friction Loss Calculator as part of the Toolkit here.
Also not a negotiable part of a project – c-factor directly relates to the friction loss under the Hazen-Williams method of hydraulic calculations – its nonetheless important to get correct.
How can you improve the c-factor? The pipe type (plastic, copper, ductile iron or steel) and system type (wet, dry, pre-action, deluge) will impact the c-factor.
One project I worked on had major challenges, including a quadruple-slam of dry system, sloped roof, tall roof, and poor water supply. The only suggestion we had to avoid a fire pump was to insulate and heat the building such that a wet system could be used. That change had a number of impacts, but the c-factor change from 100 to 120 for the pipe had a major bearing on the new system working.
One major change that’s coming to the 2022 Edition of NFPA 13 is allowing a C-Factor of 120 to be used on new dry systems that are installed with nitrogen. I’ll be sure to explore this in more depth when the time comes.
Are your sprinklers maxed out at the greatest possible coverage area?
In some applications, this can hurt more than it helps. Yes, we might save on the cost of material and labor with a reduction in sprinklers, but reducing the area per sprinkler in a very tight calculation can have a positive impact with starting pressures and the densities achieved.
See the density vs. k-factor calculator as part of the Toolkit to see the impact with various sprinkler spacings.
Remote Area Size
There are some reductions in allowable remote area size in NFPA 13. A common one is the quick-response reduction, which allows a smaller remote area size for systems which have low(er) ceilings and quick-response sprinklers.
See the impact of remote area with the Remote Area Analyzer here.
Water Source Height
An often overlooked part of the calculation that has a major bearing on the result is the height of the water source. Many systems are designed based on a fire hydrant flow test. Is the elevation of that source accurate, relative to the jobsite?
I once worked on a job where a submittal showed the source (city water grid) to be at an elevation of 0'-0" relative to the first floor of the building. I charted google earth and paid close attention to the listed source from the flow test report. While a hydrant existed about at ground level elevation, the actual static/residual hydrant where the test results were gathered was at an elevation 27-feet below the project grade. It was a substantial hit to the hydraulic calculations (all calculations failed with the correct elevation shown).
Backflow Preventer Loss
On many systems the backflow preventer presents the worst pressure loss for any single piece of equipment on the system. It's easy as a designer to input a curve or a conservative static loss, run the calculation, and not return to the backflow preventer.
However, if you're seeing high pressure losses, shop around. There are a variety of backflow preventers on the market, and using backflows that are straight (horizontal/vertical, not N- or Z-type) and use OS&Y valves instead of butterfly valves can offer some hydraulic savings.
There's a whole database we've created on available backflow preventers as part of the Toolkit here: BACKFLOW DATABASE*
Just like backflow preventers, valves introduce pressure loss into the system. An OS&Y valve will remove the water-blocking paddle from the water stream, allowing water to pass through mostly unimpeded. Other valves like butterfly valves leave the paddle in place, causing some pressure loss.
Sprig, Drop, On-Pipe, Flexible Drop & Return-Bend Configurations
Ever looked at the difference between using sprigs and not using sprigs (on-pipe fittings or outlets) has on a calculation? If you're at a higher density, it can be significant.
Hydraulic calculations are usually not a driver in whether sprigs are used, or whether return-bends vs. side outlets vs. bottom drops are incorporated - however - these have an impact on the calculations and can introduce pressure loss in between the sprinkler and the branch pipe.
One notoriously high friction loss arena is use of flexible drops, which can add pressure loss with equivalent 1-inch pipe lengths of 20-70 feet of pipe. These friction losses can vary significantly among manufacturers and models.
Riser Nipple & Sprig Diameters
Have a storage calculation with high densities and a high sprinkler k-factor? It may be worth adjusting the sprig diameter to see the impact of the 1-inch diameter pipe.
Similarly, riser nipples in-between a main and branch line bear the full flow from the branch line to the main. These pieces, while many times shorter than the spacing between sprinklers, can still introduce a pressure loss to the system that a stepped-up diameter can help.
Special Application Sprinklers
Lastly - is there a sprinkler specifically designed for the application you have?
Many manufacturers over many decades have dialed-in and created sprinklers that are built for specific purposes (special application sprinklers). These product listings can allow different starting pressures and design criteria, which, as a whole, can help reduce the water burden on a system.
Two that I often use that come to mind are residential and attic special application sprinklers. In both cases, use of those sprinkler types within their respective hazards dramatically reduce the water required at the remote area, thereby allowing smaller mains and equipment back at the riser.
See a list of all available sprinklers on the market (filter & search) on the Sprinkler Database in the Toolkit.
In the next post I'll look to put together something that's a little more handy as a checklist for entry and intermediate designers.
In the meantime - what am I missing here? What aspect of hydraulic calculations do you think are often overlooked yet carry a big impact?
2/3/2021 10:25:22 am
Are you psychic or something? I literally had to review a calc from a contractor that worked, but not very well. And I looked at the incoming service; 4" for a Extra Hazard Group 2 scenario. I looked at the pressure loss in the 4" pipe, and the pressure loss in 6" pipe. With the flow from the hydrant flow test, I put back 25 psi into the system. My other favorite is in Light Hazard, if all the sprinklers are in 2 x 2 center of tile, then the area is 196 instead of 225. Did I miss the reduction in area of the calc if the ceiling height is less than 10 feet? That's been a savior for me sometimes. Thanks for all the great input you provide! Love this site and this forum!
Brian Gerdwagen FPE
2/3/2021 11:42:26 am
One big tool that was taught to me early in my career was to take the computer calculation and chart the water flow from the sprinkler to the source and account for all the pressure losses. Start pressure, elevation, backflow, and each pipe type as a separate line item. It helped with finding errors, but helped with efficiency.
2/3/2021 12:16:10 pm
In addition to the bullet points above,
2/3/2021 12:21:04 pm
Great topic, and great checklist - I'd be hard-pressed to come up with any more ideas... but I'm going to give I some thought. I was once taught that if you are scraping for every crumb, since you are accounting for the equivalent feet loss in the fittings, don't add the length of the fitting again. Avoid reducing Vics if you have to (IF you even remembered to add the RV loss). I'm finding too many designers take for granted what the computer-generated hydraulic calc or seismic brace calc tells them. Like Brian said above, examine the output data, look for potential improvements, where you are burning-up pressure; where the velocity is hurting you (e.g., second-to-the-last sprinkler on the branch line). Consider looping or bird-caging. Look where you can afford the pressure loss and massage the pipe diameters. Nothing beats the experience of doing hand-calcs, and especially with some design programs doing the calcs, I feel it is becoming a lost art. If your calcs do work, you shouldn't stop and call it good there. You need keep watching for your company's bottom line by considering most efficient in terms of material, fabrication, joining methods, and installation. What makes your company competitive. That isn't always what calcs best. That computer program doesn't know fitter rates, what length you can stuff through the trusses, and it tends to like grooved tees.
2/3/2021 12:21:59 pm
Get a new hydrant test.
2/5/2021 05:07:20 pm
Hydrant flow tests can be tricky. Flow tests may vary with the time of day or time of year depending on the municipality. In my jurisdiction, we have one area that fluctuates from 90 to 60 psi static pressure depending if the water utility is filling an elevated storage tank. Designers should do their best to ensure they are using the most conservative hydrant flow test so that the calcs always work. No matter how robust or awesome the municipal water utility is, there will be fluctuations.
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Joe Meyer, PE, is a Fire Protection Engineer out of St. Louis, Missouri who writes & develops resources for Fire Protection Professionals. See bio here: About