By Larry Davis
Published Friday, February 20, 2009
| From the October 2006 Issue of FireRescue
One of the major advances in rural fire control in the last 30 years has been the wide acceptance of large-diameter hose (LDH)-mostly 4 and 5"-by rural deparments. However I continue to encounter folks all over North America who ask me how to convince naysayers of the benefits of LDH. This month's column is for readers faced with resistance from those unconvinced of the benefits of LDH.
The reasons most commonly cited for not using LDH are:
- Rural departments don't have enough water to fill up LDH;
- You can't use LDH unless you're pumping more than 500 gpm (a quote from a fire equipment salesperson);
- Anything larger than 3" hose is a waste of money;
- You can move the same gpm with dual 2 «" lines as you could with 5" lines and more than 4" because 2 «" + 2 «" = 5"; and
- LDH is just too expensive.
Demonstrate the Difference
I learned a long time ago that the best way to convince those who think LDH is impractical for use in rural operations is to conduct a demo that shows how the diameter of the discharge hose impacts the delivery capability of a pumper. The demo pits a small-capacity (400??-500-gpm) pumper discharging though LDH against a large-capacity (1 500-2 000-gpm) pumper discharging through 2 «" or 3" hose over a distance of about 1 000 feet.
The photos in this column show two such demos. The first was conducted during a water-on-wheels course for hazmat operations in Cleburne County Ala. in July 2006. The second was conducted during a rural water supply course in Miramichi New Brunswick Canada in September 2005.
The Cleburne County Demonstration
The Cleburne County demo used an Abernathy (Ala.) Volunteer Fire Department mini-pumper equipped with a 400-gpm at 150-psi pump and a 1 500-gpm pumper from the Carroll County (Ga.) Fire Department. The Southwire Corporation a major employer in Cleburne County provided use of its parking lot.
Each pumper was provided with a porta-tank from which to draft and a stand-by tanker to keep the porta-tank full. The 400-gpm pumper discharged through 900 feet of 5" hose to a portable monitor with a 1 «" smoothbore tip. The 1 500-gpm pumper discharged through 900 feet of 3" hose to an identical portable monitor positioned next to the one supplied by the mini pumper.
The engine speed (rpm) of the 400-gpm pumper was increased to its maximum. At this point the pump discharge pressure was 90 psi. The speed of the 1 500-gpm pumper was increased until the discharge pressure was 200 psi-the test pressure of the 3" hose being used.
The results of this demo were apparent at the portable monitors. The nozzle pressure on the 1 «" tip of the portable monitor supplied by the 400-gpm pumper was 46 psi for a flow of 452 gpm; the nozzle pressure on the 1 «" tip of the monitor supplied by the 1 500-gpm pumper was 36 psi for a flow of 400 gpm.
Because of the use of the 5" hose the 400-gpm pumper with a discharge pressure of 90 psi delivered 50 gpm more over the 900' distance than did the 1 500-gpm pumper with a discharge pressure of 200 psi. In other words a mini-pumper with 27 percent of the pumping capacity of the 1 500-gpm pumper delivered 13 percent more gpm than the 1 500-gpm pumper.
It's About Water Horsepower
Water horsepower (whp) is the measure of a pump's output which is calculated with the formula: whp = (gpm x psi) ö 1 714. To truly appreciate the Cleburne demonstration we must consider its pump output in terms of whp.
The 400-gpm at 150 psi pump is capable of delivering 35 whp [(400 gpm x 150 psi) ö 1 714 = 35 whp]. The 1 500-gpm 150 psi pump is capable of delivering 131 whp [(1 500 gpm x 150 psi) ö 1 714 = 131 whp].
In this demonstration however the pump output of the 1 500-gpm pumper was only 400 gpm at 200 psi (47 whp) and the pump output at the nozzle was 400 gpm at only 36 psi (8 whp). A full 39 whp (83 percent of the pump output) was expended forcing the 400 gpm through the 900 feet of 3" hose leaving the output at the nozzle only 17 percent (8 whp ö 47 whp) of that at the pump outlet.
The pump output of the 400-gpm pumper was 24 whp [(450 gpm x 90 psi) ö 1 714]. Because of the 5" hose only 12 whp (50 percent of pump output) was expended moving the 400 gpm through the 900 feet of 5" hose. The resulting output at the nozzle was 12 whp-50 percent (12 whp ö 24 whp) of that at the pump discharge.
As you can see pumps create whp and hose eats up whp. The smaller diameter of the 3" hose causes it to eat up much more whp than the 5" hose because of greater friction loss in the smaller tube.
The Miramichi Demonstration
In another demo I pitted a Miramichi Fire Department tanker equipped with a 500-gpm at 150-psi pump against a 1 500-gpm pumper/tanker from the Meguac Volunteer Fire Department.
Each pumper drafted from the Miramichi River. The unit with the 500-gpm pump discharged through 1 000 feet of 4" hose to a portable monitor with a 1 «" smoothbore tip. The 1 500-gpm pump discharged through 1 000 feet of 2 «" hose to an identical portable monitor. Both units were primed and run up to the maximum speed.
When we reviewed the results in terms of whp the 2 «" hose used up 91 percent of the 1 500-gpm pump's 43-whp discharge while the 4" hose used up only 46 percent of the 500-gpm pump's 63-whp discharge.
Demonstrations such as these can be set up anywhere to show the value of using 4" or 5" hose in transporting water in rural operations. If you must deal with those who suffer from HHS (hardened-head syndrome) when it comes to the value of LDH perhaps the information from these experiences can help.
For those who think that laying two 2 «" lines is the same as laying a single 5" line Figure 6 illustrates that when the diameter of a hose is doubled the area is increased four times. In other words laying a 5" line is the same as laying four 2 «" lines.
For those who say LDH is too expensive explain to them that when they buy fire hose they pay for the ability to transport water. Remind them that since 5" hose has four times the area of 2 «" hose that to move the same amount of water as 1 000 feet of 5" hose they must buy 4 000 feet of 2 «" hose. Lastly when you think about the time and effort required to pick up the 4 000 feet of 2 «" as opposed to that required for the 1 000 feet of 5" hose the 5" clearly is the better choice.
If you need any additional information or have any comments shoot me an e-mail.
'Til next time stay safe!
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