Sleepless night? Its your fault!!
Fordrules, I have been turning this over in my mind all night. There are a number of answers and questions that need to be considered. Since your firefighters are the most important parts of the system, let’s consider what they are encountering. An automatic nozzle delivering at 100 psi has a reaction force equal to half the gpm’s. The cross lays with fully opened bales at 200 gpm have a kickback of 100 lbs. The 1 ¾” hose with water weighs about 1 lb per foot plus a 5 lb nozzle, so we are now at 155 lbs of work. Add to this two SCBA’s and gear at 35 lbs each and the work load on the attack crew is 225 lbs. I try to keep the workload under 100 lbs per man, and 80 or less is much better. I know that you are instructing the nozzleman to open the bail just far enough to kill fire, so he is probably using about 150 gpm or 75 lbs of reaction. This gets the crew’s load back under 100 lbs per man. The question is now about the “Safety Factor” of 50 gpm or some other lesser value that you decide upon. Second question and answer concerns the effective reach of the line. Let’s assume the reach is 40 feet, so a 40 by 40 room 10 ft. high by the IOWA formula needs 160 gpm for 30 seconds. Almost an exact match for the 150 gpm delivery rate. If you and they are comfortable with an extra 30 gpm for safety, then 200 psi EP will be adequate for the crosslays.
Applying the same logic to the 2 ½” rear P.C.’s, a discharge pressure of 200 psi will be close to your specified 170 psi. This allows the pump operator to set a single pressure, and both interior and exterior lines the ability to adjust application rates to what they are comfortable handling. The nozzleman is supposed to be the one who controls application rate with an automatic nozzle. Let’s say the rear 2 ½” preconnect is being operated at 180 gpm (3rd bale click) and the P.O. gates the line to 175 psi. If the nozzleman needs more water and opens the bale fully, he will only get what can pass through the gated back valve. Thus a difference of opinion between the P.O. and the nozzleman.
Next thing that comes to mind is the engine RPM when delivering 240 psi. Since a single stage pump must turn at ever higher speed to achieve higher discharge pressure, just how close to the governed speed must the engine turn to achieve 240 psi.? A good rule of thumb is to keep it below 80% of the goverened “No Load” speed. A 2150 rpm Detroit should be kept below 1710 rpm if possible. 90% would be the absolute maximum at 1935 rpm.
In your last post, you mention working from a drop tank using front suction. Rarely, when teaching Rural Fireground Watermovement classes, have we been able to achieve more than 800 gpm. from a tanker shuttle on a continuous basis. The front suction should be capable of flows approaching 1200 gpm based upon your description, but you really should try supplying a master stream using the flow meter and increase the flow until you reach cavitation through the front suction. There are so many ways of plumbing that it is almost impossible to calculate the losses.
My personal opinion would be to operate at 180 to 200 psi engine pressure, leave the gates on both the cross lays and the 2 ½” lines nearly fully opened. Perhaps 10 psi drop, just to keep track of which lines are flowing water. Remember K.I.S.S. At 180 psi your pump is capable of about 1250 gpm and with all lines operating, including the bumperline, your delivery would be very close to that. The last piece of the puzzle is your ability to get 1250 gpm through the front suction. I don’t think you will get the 1100 gpm at 240 psi without pushing the engine pretty hard.
Kuh Shise
1 Attachment(s)
From Larry Davis' "Rural Fireground Ops #2
Tried reducing the file size, but I'm not sure it is small enough. Maybe too small to read at 87K
Yes! darn program just makes it unreadable...Guess I'll try Photobucket and go from there.
Try this location: http://s679.photobucket.com/albums/v...uctionhose.jpg
Kuh Shise
