Hydraulic Calc - In-Rack Sprinklers 1

[Haldorson]

Hello,

When performing a hydraulic calculation that captures both ceiling and in-rack sprinklers, do you calculate them at the same time?

Or do you calculate the in-rack sprinklers on their own to figure out their psi and flow demand, then calculation the ceiling sprinklers with the in-rack demand added at the connection point?

I appreciate any help.

Thanks.

 

[pipesnpumps]

All at the same time. NFPA 13 tells you how many inracks have to be considered flowing.
They've done a good job writing the storage sections. Spend some time reading the section for your storage type and chosen protection type and it will become clear.

If you want a specific answer like how many inracks should be flowing for your particular job you'd have to list the complete design parameters. If you already have the complete design parameters then determining what the inrack reqts are should be trivial by comparison..

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.

 

[skdesigner]

NFPA 13 2010 22.8.2 Water demand of sprinkler installed in racks shall be added to ceiling sprinkler water demand over the same protected area at the point of connection.

22.8.3 The demand shall be balanced to the higher pressure.

Either method you've stated will work. Any calc program I've used can do the first option with ease (and even balance to within .5 for you).

If you were using a simpler program or doing it (gasp) by hand, your second method would be the way to go. Assign a node to the point your in rack line connects to the overhead system, calculate your in rack demand to that point, and then use 22.4.2.4.3 to express it as an equivalent K factor. Calculate your overhead system, showing the connection point as a flowing node, with the K factor you've calculated for it. Balance to the higher (ceiling) pressure.

 

[SprinklerDesigner2]

What is interesting about rack sprinklers is the sprinklers in the are usually calculated assuming the farthest heads away from the riser are the most remote but the reality is oftentimes they are not.

Imagine a system with an overhead grid that requires 70 psi at the point of connection with the rack sprinklers. If the rack sprinklers psi is identical to the overhead, at the point of connection, you can add the 500 gpm plus 200 gpm for 700 gpm or whatever.

But what if the racks require 40 psi @ 250 gpm and overhead of 70 psi at the point of connection? It's going to require a lot more than 250 gpm.

Sometimes you need to "march" the rack most remote up and down the line to determine with certainty what constitutes the hydraulically most demanding area.... it could be the farthest from riser, closest to the riser or somewhere mid point along the rack.

 

[SprinklerDesigner2]

If I am wrong somewhere feel free to correct me.

Consider the following rack sprinkler

http://img219.imageshack.us/img219/5673/racksprinkler.jpg

Not that it really matters but we'll go Class III nonencapsulated commodity. Six rack sprinklers required to be calculated but what six heads? Answer is it depends.

Calculation for the six sprinklers farthest away from the point of connection

http://img28.imageshack.us/img28/3182/rack1t.jpg

Calculations for the six sprinklers closest to the point of connection

http://img8.imageshack.us/img8/9476/rack2a.jpg

Of course never say never but nearly always you are going to want your rack sprinkler demand pressure less than your overhead at the point of connection or RP17 as shown on my sketch.

Calculating rack sprinklers #1 through #6

http://img28.imageshack.us/img28/3182/rack1t.jpg

we find we need 134.4 gpm @ 51.2 psi at the point of connection to the overhead.

k=134.4/51.2^.5
k=18.78

Calculating rack sprinklers #10 through #15

http://img8.imageshack.us/img8/9476/rack2a.jpg

we find we still need 134.4 gpm but at a pressure of 33.7 psi.

k=134.4/33.7^.5
k=23.15

Assume the overhead requires 585 gpm @ 72.8 psi at RP17.

@ 72.8 psi rack sprinklers 1 through 6 will discharge 160.2 gpm (q=18.78*72.8^.5) but rack sprinklers 10 through 16 will discharge 197.5 gpm (q=23.15*72.8^.5) showing the hydraulically most demanding rack sprinklers are nearest the riser.

It's obvious if your overhead requires less pressure than the rack sprinklers @ point of connection then the reverse will be true; your most demanding rack sprinklers will most likely be farthest from the riser.

In the real world it's only 37.3 gpm and isn't going to make a hill of beans anyway and to avoid argument you may as well give the AHJ what they are used to seeing at anyway. Remote, far away.

 

[skdesigner]

SD2, thanks for pointing that out. My response was overly simplified to answer his immediate question.

The formula Qadj=Qlow(Ph^.5/Pl^.5)

where:

Qadj= Adjusted flow to the lower pressure line
Qlow= required flow in the lower pressure line
Ph= higher pressure
Pl= lower pressure

illustrates your point nicely. The greater the imbalance between the high and low pressures, the greater the flow into the lower pressure system.

As you've shown, the lower the pressure required from the in-racks, the greater the imbalance between the in-racks and the overhead, resulting in a higher adjusted flow into the in-rack line.

It's nice to pull out all these formulae once in a while rather than letting the computer take over!

 

[SprinklerDesigner2]

Postskdesigner,

For the first ten years of my career all my calculations were done by hand. We didn't have personal computers until 1985.

Trees and loops were easy enough but grids not so easy. I always tried to get an end feed to make it simple and after a week I would get to where I needed to be.

I feel fortunate, all that gave me an understanding that those that used the computer from day one will never have.

 

[TravisMack]

SD2:

I fully agree about the need to understand what is happening when you calculate by hand. I started in 89 and we had a Wang machine for calculations. However, my boss req'd me to calculate every tree / loop by hand for the first 6 months. I was able to use the Wang terminal for grid calcs because of the amount of time it would take to get those calc'd.

When I worked in a contractor's office, I used to require all of our new guys to calculate their first few jobs by hand. They all hated it, but now realize the benefit of doing so.

As far as balancing the rack / overhead, I try to get the pressures to be as near to each other as possible at the point of connection in order to reduce the over discharge. If I have a 50 psi demand for in-racks and a 70 psi demand for the overhead, I will reduce the bulk main size for the in-racks to get the demand as close to 70 psi as possible. Doing those calcs by hand also allowed me to see that sometimes, reducing pipe sizing is going to be better for the system than always increasing pipe sizing. Reducing over-discharge can be more effective at making a system work than increasing the pipe sizing.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com