Elevation calculations during sprinkler hydraulic calcs 3

[NewSprinklerGuy]

All,

This may be a really stoooopid question, but I'll ask anyway...

During hydraulic calculations for sprinklers, we have to account for water flowing up the pipe (such as risers and riser nipples) by multiplying the elevation difference by 0.433. Easy enough.

But what if we are "losing" elevation? For example, if I drop 1 ft from my branchline to my sprinkler head (arm over) do I ignore the 1 ft drop or do I now have "negative" pressure since the water is flowing down vs. up?

Up to this point I have been ignoring the "negative" pressure and just making sure I accounted for water rising up pipe.

How have you guys worked it? Am I doing it right or am I in trouble?

Thanks in advance.

Shawn Lee, CET
Fire Alarm Systems Level III
Sprinkler Newbie

 

[pipesnpumps]

The only important number is the elevation difference between the "source" (test hydrant or the test hydrants tap off the main) and the sprinkler head. So yes you can and should add back in the 2.31 ft/psi, 0.433 psi/ft for drops.

When doing it by hand some people keep track of the elevation change for each section of pipe, while others just apply the net difference once. I am not a NICET, but one of the gurus here can tell you how to keep track of it for whatever software you are using for your calcs.

You should look into getting the NFSA book on layout detail and calculation of fire sprinkler systems. $145 on NFSA.org (no affiliation yadda yadda)



Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.

 

[MGXFP]

You should account for the difference in elevation for each pipe. However in your example with a drop; if the drop is 12" or less then you can ignore the friction losses through the drop to the sprinkler. If the drop is over 12" then you will need to include the friction/elevation loss through the drop to the sprinkler. I believe the same is true for sprigs (I include the extra pressure required to overcome elevation in sprigs over 12"). Typically however I won't include the gain in pressure due to elevation in a drop because it is an extra margin of safety.

 

[b1ueshift]

As pipesnpumps says, you don't need to track the elevation change on every pipe and it will save you a lot of time and energy if you don't track the elevation change on every pipe.

In your first pipe, you can track the elevation change from your water source to the height of the sprinklers. In most cases all your sprinklers will be at the same level, if they are not at the same level, you may want to take the hight of the highest sprinkler to be conservative. In all subsequent pipes, you can ignore elevation changes. This works because every time you lose head going up hill, you gain the same head going down hill again.

There will be no loss of accuracy in your calculation by making this simplification and you will be less likely to make a mistake.

If you are using a computer program, your first node will be at ground level and all remaining nodes will be at the elevation of the sprinklers.

 

[SprinklerDesigner2]

I'll take the opposite view that elevations always matter between reference points. Just a little bit can make a huge difference.

Consider a single branch line having five sprinklers 12'-0" OC running up a 1 in 12 slope.

Spacing is 10'x12', we are using k=8.0 sprinklers with a density of .20 over 1,500.

The end sprinkler is 25' AFF and needs 9.0 psi to discharge 24.0 gpm.

12'-0" of 1 1/4" sch. 40 to the next sprinkler produces 0.75 psi friction loss and since we are going downhill you should add .43 psi so the pressure at the next head is 10.18 psi and not 9.75 not counting the elevation.

Discharge of head #2 would be 25.52 gpm when factoring the elevation loss but if you kept it flat, using 24' for all sprinklers to make it easy, you would only need 9.75 psi 24.97 or over half a gallon.

Doesn't sound like much but don't scoff to much before you continue running down that branch line. This can make a huge difference especially when dealing with marginal water supplies.

Yeah, if you have 110 static, 80 residual @ 1,300 is one thing but if you are trying to make it work with 45 static, 42 residual @ 890 that is a totally different.

To answer the original question it is yes. If we ran our branch line up instead of down the slope the second head would discharge 9.0+(0.75-.43)=9.32 psi for 24.42 gpm.

The difference between "going up" or "going down" becomes even more pronounced as you work your way down the branch line.

Get a large area, say a cotton warehouse with a design density of .25/3,000 with 30 heads can end up meaning a lot such as 50 or even 100 gpm difference. Add 100 gpm to your water supply plus the added friction loss of 100 gpm going through 300' of 4" pipe and it can easily add up to a 10 psi difference.

It is more work but I always have heads and reference points at the actual elevations.

 

[SprinklerDesigner2]

Concerning slopes please see the attachment showing calc results for for an identical system except on one the lines are flat, on another the lines are sloped down toward the main as is typically done but on the third set I put the main up in the peak of the building. Sloped the lines down towards a tie in drain, or perhaps a nipple and cap drain at the end of the branch line, and it makes a huge difference on to whether you have slope or not.