12-04-2008, 07:46 AM
I have had a few FF's ask me what the advantages are in running LDH instead of 2 1/2" supply. I thought it had somthing to do with volume of water being moved can be accomplished with lower pressures as the 2 1/2 would need higher pressures to move the same amount of water. Anyway could sombody clearify this for me? Thanks
12-04-2008, 07:57 AM
In basic terms think of 2 1/2 as 250gpm supply line and LDH 4 as 1000GPM supply line, LDH 5 as a 1250 GPM supply line.
Your hydrant lay should match the capacity of your pump.
On the other hand 4 inch is great for as you mentioned lower friction loss etc, but you need a waer source that will provide that much water.
12-04-2008, 10:46 AM
Large-diameter hose (LDH) has a remarkable ability to move large volumes of water at a fraction of the friction loss encountered in conventional, smaller-diameter hose. LDH can be thought of as an aboveground water main laid into an apparatus pumping at a fire or simply as a very long soft-suction hose.
The advantages of LDH are very convincing when compared side by side with smaller-diameter hose. For example, five-inch hose can flow the equivalent of three three-inch hoselines at about the same friction loss.
Five-inch LDH can deliver 1,000 gpm at about six psi per 100 feet or 1,500 gpm at 15 psi (friction loss). With figures as impressive as these, someone is bound to ask, If five-inch LDH is so good, why doesn't every fire department use it? or Even though four-inch LDH is also available, why would any department purchase it when five-inch hose flows 70 percent more than the four-inch hose?
LDH virtually eliminated the need for engine companies to reverse lay hoselines from a fire and pressurize them directly from a pumper connected to a hydrant. As a result, all of the water flowed at many fires ended up being supplied by just one line of five-inch hose.
Forward (hydrant to fire) hoselays permitted our engines to operate at the fire scene instead of pumping at a hydrant down the street. This allowed us to use more extensively our preconnected attack lines and apparatus-mounted heavy stream devices. Also, all of the tools, appliances, and ladders on the apparatus are close at hand.
Complicated relay operations involving more than two engines pumping "in line" could almost become a thing of the past. Consider that an engine pumping 180 psi at a source of water can deliver more than 1,000 gpm to an engine pumping on the fireground through almost 3,000 feet of five-inch hose or 500 gpm through 9,000 feet of five-inch hose. This practically eliminates the practice of spacing engines at 1,000- to 1,200-foot intervals to overcome the friction losses commonly found in 21/2- and three-inch hoselines.Hard Lessons
...it took some hard lessons to learn the limitations of LDH and make the changes in our procedures, apparatus, and training necessary to use LDH safely and most effectively. We learned the following while ascending the LDH learning curve:
LDH can give you a false sense of security. Sure, a single five-inch supply line may deliver all of the water used at most fires, but isn't that putting all your eggs in one basket? We now know that was a mistake. The failure of that single hoseline can cut off the entire water supply to a fire-and it can happen easier than you think.
Of course, a single LD supply line connected directly to a hydrant cannot possibly meet the water flow requirements of every fire. We learned this lesson at large fires when we used long hoselays or any time hydrant pressure alone was insufficient to push the necessary gpms through the supply hose. When the demand for water exceeds the available supply, fire control becomes doubtful, and cavitation and pump damage become real possibilities. Strong hydrants can also fail to supply sufficient water flow: Although five-inch hose can flow an impressive amount of water, one five-inch supply line may not tap the potential gpm available from a hydrant.
12-05-2008, 06:21 PM
Thank you for the information, dont think anyone could have made it clearer for me. Thanks once again.
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