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Question for engineers on intake pressures?
On intake pressure how low do you pull it down when connected to a hydrant?
I am told to pull it down to 5 psi by our Assit Chief. All the training I have had for state certification said no less than 20 psi. We have poor water supply to our hydrants mostly 4 inch feeders on a loop. We have a 6 inch on the main supply with a static pressure of 65 psi on average (Town is on a hill), All the lines are from the early to mid 1950s. The most I have flowed is 350 gpm off a steamer to five inch LDH I could have pushed it a little more but not much. That was off of a dead end 4 inch.
Replies to This Discussion
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Permalink Reply by Ben Waller on
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Our department policy is that you MUST have permission from COMMAND to reduce the residual intake pressure below 20 psi. That policy is based on consultation with the engineers from the public service districts that operate our water system.
The primary scenario where we have this problems is where we have a forward lay of more than 800 feet. If that is the case, we normally have a second engine relay to the attack engine. If we're close to the hydrant, we supplement the 5-inch intake line with a 2-1/2, which also helps keep the residual intake pressure problems to a minimum.
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Permalink Reply by John on
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Thats exactly why we will not go below 20 psi is the fear of either collapsing our mains or the actual intake hose, From what I have heard here collapsing the inkake could possibly ,(if there is a weak spot on the LDH) suck some of the insides into the pump and damage it, as well as the cavitation problems. If our static is low we usually will have another engine boost the pressure to the attack engine, or like Ben said hook a 2 1/2 inch (ours would be a 3 inch) to help maximize our static pressures.
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Remember that what counts for the intake is VOLUME, not pressure - as long as you can maintain the minimum required residual pressure. If you have adequate volume, the pump will add the necessary pressure.
Sometimes that need gets confused. An old fire chief once told me "We extinguish fires with VOLUME, not with PRESSURE. If we extinguished fires with pressure, we'd just use compressed air."
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Permalink Reply by Chris Weber on
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We never go below 20 lbs. residual just for the safety factor.5 psi. seems like a mighty slim insurance policy.We do very little with hydrants but we do relay pumping.And yea,if the pump cavitates it can pull the line into the pump and really screw up the works.Best thing to do would be to experiment with your system and see how many lines you can flow without going under the 20 lb.residual pressure.Might have to change the way you do things a little bit.
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Permalink Reply by FETC on
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Well your assitant chief's thought is to rob Peter to pay Paul with the 5 psi theory. Your water supply system is basically lacking adequate flow for the fires you are sustaining today.
Funny you bring this thread up John, as I do alot of pump training around my way and have found very few engineers or (firefighter lever pullers) who understand how the new pumps work. I brought this up in another thread and had very few here say, "yes, I knew about that".
Old school 20 psi residual is the rule of thumb... "Old school" and the textbooks haven't caught up to the technology used today though.
If you are using a newer Class 1 Electronically Controlled Pump Throttle and Pressure Relief system, you need to use (40) psi rule of thumb for residual, if you are operating in the PRESSURE MODE. Now in this mode, it is the only time you are afforded a pressure relief safety feature. The computer will anticipate potential cavitation conditions and idle down the computer and pump even though you are not near actual cavitation.
If you are using the class 1 pump's computer in the RPM MODE, you are afforded no pressure relief protection BUT you can pull the residual pressure down to (0) actual cavitation. In your case, with the water supply issues you have, (average 65 static psi to start) you will have to use the newer pumps in the RPM mode when running off the hydrant system. Your pump operators should train on monitoring pre-cavitation conditions (difficult on top mount pumps) but with side mount, they should be leaning or checking on the stiffnes of the intake hose.
If you want to talk more about the subject feel free to check out my website and give me a call.
Bill
www.fetcservices.com
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Permalink Reply by CHRIS PUYLARA on
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We dont go below 20psi same deal....pullin the pipes outta the ground or suckin the truck into itself...lol
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The cause of broken water mains, commonly referred to as "collapsed mains" usually occur when we have excessive or violent water hammer operations.
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Permalink Reply by John Dobson on
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I never thought of a Hoseline collapse and ending up sucked in the pump! I have all ways worried about the towns a main collapse. It seems to me the age of the pipeing in the mains they may need pressure to hold there shape (rusted thin). We have always know at the start of a fire we don't have enough water so we call for, Tankers/Tenders whatever you wish to call them shuttle.
Are department has never had a formal engineer or even anyone who has had a state test until I took it. I would like to have more courses and teach it to the others. As far as I know I have had all that Indiana offers.
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Thinking about university of illinois fire school for there FAE class. http://www.fsi.illinois.edu/content/courses/programs/description.cf... and http://www.fsi.illinois.edu/content/courses/programs/description.cf... Any other ideas or classes I would be thankful for your input.
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Permalink Reply by Truckie Jay on
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Another reason not mentioned for not dropping the pressure below 20 is boil orders. If pressure in a water main drops below 16psi the city, water district whoever has to implement a boil order for 24 hours. This is required what is also required is they have to report the boil order AND reason for the boil order.
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