Water flow to an aerial....

Need some help understanding why you would feed identical aerials, with 2 different supply lines.

I was out of state visiting a friend that is also a fire mechanic, when he got called out for some problems with a fire truck. He asked if I wanted to go, and I said yes. When we got on scene, I noticed that one engine was feeding one aerial with a 5" LDH, and yet saw another engine feeding its twin through a wye with 2 2-1/2 lines.

The only difference between the two engines, is the one feeding with the 5" LDH was also shooting water through its deck gun. Both engines had the same number and sized handlines out flowing water.

Wouldn't there be a difference in flow rates through the monitors on the tip of the ladders with the different lines hooked up, or does it matter??? If it matters, both monitors were the same on the ladder.

FM1

There is a good discussion about this somewhere in here. Since most places use supply grade LDH, rated only for 200 PSI (185 max working), it might be more advantagrous to use twin 2 1/2 or 3" lines to overcome the pressure loss in the aerial piping, since this hose is rated for anywhere from 300 to 400 PSI (285-385 working pressure). Some aerials have as much as 150 PSI of pressure loss or more in their piping.

The possible reasons are endless. It could be a matter of what hose and what adapters were handy. Other reasons could range from mere convenience to some real operational issues.

All of this assumes that the ladders do not have pumps and the water was being fed by engines directly to the ladders.

If the water flows aren't great, two 2-1/2" lines may suffice and be easier to handle. For example, let's just say 600 gpm was being flowed. Head math friction loss would be roughly 18 psi/100' (300 gpm x 2). OK if you only have a couple of hundred feet out, but take it out much farther and you can get into some serious flow capability problems in a rush. Add elevation loss (head math 1/2 psi/ft., actual, 0.434), waterway loss, etc. Do the math.

The same flow in 5" would have pretty minimal friction loss. I use a number of 7 psi/100' at 1000 gpm. That's actually a high number. Some older woven hose had that much. Newer woven hose has less, and extruded hose is usually considerably less.

Now flip this whole thing over and look at it from another perspective. All of the same losses are there. Most 5" hose (certainly not all, and ours is one of the exceptions) has a maximum operating pressure of 185 psi. Just say they're using a fog nozzle that calls for 100 psi at the tip, and they're 80' vertical from the ground. There's a loss, something on the order of 20 psi (wildly variable) at the swivel joint where the inlet meets the ladder's waterway. Then there's the waterway loss itself. Every one of them is different, so giving a pat amount of loss isn't happening today. Start plugging in the numbers and you can see that you can easily be on the edge of what a 5" line should be doing.

You sure that wasn't a couple of 3" lines, rather than 2-1/2?

Quote:

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Originally Posted by chiefengineer11

The possible reasons are endless. It could be a matter of what hose and what adapters were handy. Other reasons could range from mere convenience to some real operational issues.

All of this assumes that the ladders do not have pumps and the water was being fed by engines directly to the ladders.

If the water flows aren't great, two 2-1/2" lines may suffice and be easier to handle. For example, let's just say 600 gpm was being flowed. Head math friction loss would be roughly 18 psi/100' (300 gpm x 2). OK if you only have a couple of hundred feet out, but take it out much farther and you can get into some serious flow capability problems in a rush. Add elevation loss (head math 1/2 psi/ft., actual, 0.434), waterway loss, etc. Do the math.

The same flow in 5" would have pretty minimal friction loss. I use a number of 7 psi/100' at 1000 gpm. That's actually a high number. Some older woven hose had that much. Newer woven hose has less, and extruded hose is usually considerably less.

Now flip this whole thing over and look at it from another perspective. All of the same losses are there. Most 5" hose (certainly not all, and ours is one of the exceptions) has a maximum operating pressure of 185 psi. Just say they're using a fog nozzle that calls for 100 psi at the tip, and they're 80' vertical from the ground. There's a loss, something on the order of 20 psi (wildly variable) at the swivel joint where the inlet meets the ladder's waterway. Then there's the waterway loss itself. Every one of them is different, so giving a pat amount of loss isn't happening today. Start plugging in the numbers and you can see that you can easily be on the edge of what a 5" line should be doing.

You sure that wasn't a couple of 3" lines, rather than 2-1/2?

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Pretty sure it was 2-1/2 inch lines, and in 100' length. The 5" LDH was green and 100', so I am assuming that it is the high pressure stuff 400psi (??? numbers escape me).

I've been to several of our own multiple alarm fires and noticed that all of the ladders are fed 5" LDH from the engines. So I was curious about why the use of 2-1/2 inch lines to feed this one.

I've only been doing this for 4 years exactly next month with the FD. So I am trying to get a grasp of some of the ins/outs/whys of fire operations. As for friction loss, I'm working on getting and understanding it.

Thanks guys.

FM1