Pumping Question

[vfdguy]

I've got a quick question for you guys. I'm on a smaller volunteer dept, I'm one of the younger guys, and though I'm off my probation, I'm still the rookie of course. Anyway, on to the question.

At a drill the other night we were doing the NFPA engine evolution 1 - forward lay from hydrant, pull two attack lines and flow water, pretty simple stuff. Due to space constrains we only had 50ft of 3" supply line from the hydrant to the engine. I was on the pump, and I'm certainly no pump expert or anything, but we make sure everybody can at least get water flowing if need be. Anyway, I'm pumping along, when one of our guys decides he would like to pull another line and asked me if we could. I saw I was running 50psi residual pressure so I said go ahead. However, one of our older guys standing nearby told me to be very careful not to let the residual pressure drop too much or you would collapse the water main. I said you didn't have to worry about collapsing the main because first you would simply collapse the supply line. His reply was "with the old mains in this part of town you have to be real careful". He also said you never want to pull a vacuum on a hydrant - I though you could only pull vacuum on a hydrant if you were using hard suction. I let the issue drop since he was my senior, but I've been wondering about this since.

I always try to stay over 20psi residual when I'm on the pump, but I'd always thought that as you start getting below that you would collapse your hose and didn't have to worry about the mains. So the question is - is it possible to collapse mains in a scenerio like this?

[Bones42]

Theorectically, yes. It's possible to not be pulling a vaccum but have the pressure in the main low enough that ground pressure could collapse a really old delapidated main.

Again, possible...yes. Chances of happening....very very very very slim.

[vfdguy]

It's possible to not be pulling a vaccum but have the pressure in the main low enough that ground pressure could collapse a really old delapidated main.

However, wouldn't that be the same as when the water department shuts off mains for repairs, hookup, or whatever? Our water dept, shuts off different sections of mains for all sorts of things all the time and never has any trouble.

[Bones42]

Theoretically, if the water is still in the main, maybe not.

Do note, if your water mains are in that bad of shape that collapsing them is a worry...I would not count on them as your water supply for a fire.

[stretch13]

We keep pumping lines as long as our residual is above 10.

[vfdguy]

As you said, we don't rely on hydrants in a fire. The first in engine does forward lay into the residence, however, a first alarm on a structure brings at least three tenders, two of which carry 1750gal, and one carries 2500gal, so we should be able to handle your average residential fire with the water we carry to the scene if the hydrant should crap out. On anything where we need a lot of flow we don't even mess with a hydrant we go straight to drop tanks and a tender shuttle.

Only a very small portion of our coverage area is hydranted so we primarily rely on hauled water anyway, I was just interested in the hydraulics of this scenerio.

[rfdlou]

contamination of the public water supply

We keep pumping lines as long as our residual is above 10.

Keeping the residual is above 10 PSI also prevents contamination of the public water supply. If there are any underground leaks in the main you want the clean water to leak out not the ground water sucked in. This is also the reason plumbing codes require backflow preventers on all garden hose connections and boilers

[Dickey]

It is always good to keep 10psi of residual pressure.

However if you use hard suction hose on the hydrant (I have seen it done) then yes, you can collapse the main like a plastic straw.

If you are using LDH or 3" cotton hose, you will collapse the hose before you hurt the main. Yes, it's possible, but so is winning the lottery.

[don120]

[QUOTE=vfdguy].

His reply was "with the old mains in this part of town you have to be real careful".


He is absolutely right. With older mains they can break with simply a sudden
pressure drop...I've seen this happen. We connected to a hydrant with one
50' 2.5 inch hose. opened the hydrant without any flow out of the pump and
boom the main broke.In that area the mains were about 35-40 years old.
What happened was a "water hammer ".

Don

[THEFIRENUT]

He is absolutely right. With older mains they can break with simply a sudden
pressure drop...I've seen this happen. We connected to a hydrant with one
50' 2.5 inch hose. opened the hydrant without any flow out of the pump and
boom the main broke.In that area the mains were about 35-40 years old.
What happened was a "water hammer ".

Don

Just for clarification:

Water hammer is a force created by the rapid deceleration of water. It generally happens when someone closes a nozzle or valve too quickly.

[ntmd8r38]

As many said, collapsing a main is possble but not probable, I'm the operator for the water dept, in my town, we're more worried about the water hammer you could cause by shutting down to quickly that could rupture a main. I'd rather see you keep at least 20 lbs, of pressure on the line too, in MO anytime pressure drops below 20 lbs. state law requires a boil order to be issued and bacterialogical sampling to be done.

I'd suggest you dept. contact the operator of your water system and find out what the conditions of the system are. In my town as the operator I own the hydrants and do all the maintenance ans flow testing which I provide to the FD, which I am also a member of. I won't allow a pumper to be hooked directly to a hydrant, with only 4" mains the system won't flow enough to keep pressures up. I have a 6" hydrant and main near the water plant that we use for filling tankers about 5 minutes to fill a 2000 gal tank.

[Kevin65]

I saw I was running 50psi residual pressure so I said go ahead.

A lot of very good points are being brought up on this thread, but remember hard suction is designed to support a vacuum not pressure. Also remember that your 50 psi residual pressure in only relative to your starting static pressure.

[BirkenVogt]

The old, thick, black hard suction (non convoluted) could do vacuum or pressure and it was used to make large diameter connections to hydrants before the advent of soft sleeve LDH. They did it in those days not to pull vacuum on the hydrant but simply because that was the only large hose they had for that initial connection from hydrant to engine.

As far as pulling the pressure too low, consider if your district has elevation differences. If you pull the pressure down to 10 psi in the bottom of a valley and the water is coming to you from up on a 50' hill, then you could have as low as 11.7 psi (24 in. Hg.) vacuum on the main up there. This actually happens in certain parts of my district.

Birken

[vfdguy]

If you are using LDH or 3" cotton hose, you will collapse the hose before you hurt the main. Yes, it's possible, but so is winning the lottery.

Like I said, we were using 3" cotton hose, so we should have been okay there. I guess we were both right I said you would collapse the hose, he said you could possibly collapse the main, so we both had valid points.

I really appreciate all your guys' input.

[chadf652]

I dont mean to take this thread but im taking a firefighter class and i forgot the question how long does the air pack last??? i think its 30mins or less depends and how much you suck in the air am i correct????