CFM requirements to push a pull string

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joe_sts

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Say I wanted to push a string through as opposed to pulling it with a shop vac. For 1/2 or 3/4 150 to 200 foot run would a 6 cfm compressor work? I know that is on the low side but I was curious if I could pull this off with a 12 volt compressor such as the ARB dual tank model. I'm often doint this at sites where power is unavailable or inconvenient.
 
Probably not, as the rat gets further down the pipe, that much more space has to be pressurized, and the rat would stop until it built up enough pressure again. Probably slow going on that long of a run. Yesterday I tried my Milwaukee 18 volt vac on a 230' run of pvc, just didn't have enough umph to do it. Unfortunately the store had just lost utility power, and I didn't have my inverter on the truck to run a shop vac. Utility came back on about thirty minutes later, and we were able to use the shop vac.
 
I've always used a vac myself. Greenlee actually uses a vac in reverse with their blower systems and those are around 100 cfm. Thing is that a compressor is designed for high pressure, low volume whereas a vac is low pressure high volume. You could try and see what happens but use a foam piston and lightweight fishing line.

I have seen instances where large compressors (such as what is used for jack hammers) were used and those can deliver a high cfm. Overkill, but it was on site.

-Hal
 
I had a thomas jet line which is a bottle of compressed gas for blowing lines.
The greenlee line vac was overall the best, but of course needs power, it was much better than a shop vac, it could blow or vacuum.
A generator and vac would work.
 
joe_sts said:
Say I wanted to push a string through as opposed to pulling it with a shop vac. For 1/2 or 3/4 150 to 200 foot run would a 6 cfm compressor work? I know that is on the low side but I was curious if I could pull this off with a 12 volt compressor such as the ARB dual tank model. I'm often doint this at sites where power is unavailable or inconvenient.
Click to expand...
The difference in atmospheric pressure that the shop vac creates on one end pushes the mouse and string into the pipe at the other end so if your compressor needs to create pressure greater than that to work. A canister of compressed nitrogen works great for blowing a string into a pipe.
 
ActionDave said:
The difference in atmospheric pressure that the shop vac creates on one end pushes the mouse and string into the pipe at the other end so if your compressor needs to create pressure greater than that to work. A canister of compressed nitrogen works great for blowing a string into a pipe.
Click to expand...

Thanks for the tip, I did not consider other pressurized gases. Why nitrogen? After a quick Google search I examples of using C02 but not nitrogen...I'm wondering what the difference is.
 
Seems like I need to get a power fishing system and run of the genny, I was looking at greenlees offerings
I like the size of the porta blower but I cant find a CFM rating and Im guessing its the lowest of the options given its size. Since my requirements are on the smaller end of the rated conduit size (1/2 and 3/4) Im guessing any of these would be feasible? Also not a requirement but it would be nice to have something that can clear out a little bit of dirt or a couple small rocks.
 
joe_sts said:
Thanks for the tip, I did not consider other pressurized gases. Why nitrogen? After a quick Google search I examples of using C02 but not nitrogen...I'm wondering what the difference is.
Click to expand...
Nitrogen has to be regulated when the pressure is released or you could over-pressurize the line locally and bust it, especially if PVC. The problem is that most regulators you can find for bottled N2 won't get you the volume you need. CO2 is not going to be over 700-800 psi unless you put a heat source on the bottle.
 
joe_sts said:
Thanks for the tip, I did not consider other pressurized gases. Why nitrogen? After a quick Google search I examples of using C02 but not nitrogen...I'm wondering what the difference is.
Click to expand...
I did some work at a gas plant and that's what they had around for purging vessels. Based on what gadfly says CO2 would be better.
 
joe_sts said:
Thanks for the tip, I did not consider other pressurized gases. Why nitrogen? After a quick Google search I examples of using C02 but not nitrogen...I'm wondering what the difference is.
Click to expand...

For this application, Nitrogen, CO2 are essentially equivalent as inert gases, and so the difference will be 'what equipment do you have'?

As gadfly56 mentions, Nitrogen in its tank is at higher pressure than CO2, but for this application both need to be regulated and your flow rate will depend on the regulator you have.

CO2 liquefies under pressure, so you can get more mass of gas into a given size tank.

Other gases would be suitable but vastly more expensive (Argon, Helium, etc.)

The big thing is that the mouse doesn't create a tight seal and doesn't need much pressure, so you need high volume low pressure. If you a using bottled gas, some sort of air amplifier would be great, using the high pressure gas from the bottle to entrain air and get a high volume low pressure flow.

-Jon
 
Several years ago I decided to try blowing a foam mouse and kite string into a conduit by reversing the hoses on our vacuum. I started the process and after watching the mouse and string go in the pipe for a minute or two, I stopped and radio'd my help at the other end. Nothing. It should of been there by now. I decided to pull back on the string. Stuck. "@#$%". Tried to snag it with a fishtape from the other end. Didn't work. "@#%#"

We ended up digging down to the conduit. About 120' into a 150' run, there was our mouse stuck in the conduit with a 6"long tight plug of string right behind it all wadded up tight. I bet we had 20'+ of string wadded up in that 6" space next to the mouse.

That was the last time I ever tried blowing a string through.

Now we use a Rigid shop vac, or our personal favorite, the Greenlee vacuum that sits on a 5 gallon bucket. We have two of them. They pull a higher vacuum than a shop vac.
 
Years ago we had a Gardner Bender blower that had three switches for three levels of blow.
It had a right angle fitting to connect the hose to the conduit that had a hole for the string to feed into.
I was the go to instead of the vacuum for a while.

looks like they still make it.

Jetline Super Blower Fishing System

The B60 Super Blower was designed for use in long underground single bore conduit from 2-1/2" through 6". The B3000 Power Unit must remain above ground for safety reasons, and the 2-1/2" diameter x 25' long hose and seal-off are used in the manhole. The portable triple motor power unit provides...
www.gardnerbender.com www.gardnerbender.com
 
tkb said:
Years ago we had a Gardner Bender blower that had three switches for three levels of blow.
It had a right angle fitting to connect the hose to the conduit that had a hole for the string to feed into.
I was the go to instead of the vacuum for a while.

looks like they still make it.

Jetline Super Blower Fishing System

The B60 Super Blower was designed for use in long underground single bore conduit from 2-1/2" through 6". The B3000 Power Unit must remain above ground for safety reasons, and the 2-1/2" diameter x 25' long hose and seal-off are used in the manhole. The portable triple motor power unit provides...
www.gardnerbender.com www.gardnerbender.com
Click to expand...

Ya I saw that one but I don't think it has the right adapter for diameters under 2.5"
 
The main reason the Greenlee or other blowers designed for this purpose work better than average shop vac is their blowers have more "stages" on the blower wheel and therefore can build more pressure and deliver more volume of air flow.

I have also found a leaf blower works great most the time , especially for 2 inch or larger lines and of course gas powered for those sites that don't have power yet.

I have also found the "mouse" or "conduit pistons" work best with a high volume blower but the lightweight plastic shopping bags work pretty well with lower volume blowers/vacuums like a typical shop vac.

Also found one should install the pull line ASAP after installing underground raceways even if not intending to pull conductors right away. Condensation or any other reason to have water in the line makes blowing a line in more difficult if you wait too long and this water has accumulated.

If you don't have a high volume blower and water has gotten into raceway, the shop vac is probably easiest way to eliminate the water, even if it won't pull all the water out leaving it sit a little while with vacuum on it does dry it out as water will evaporate more rapidly under the negative pressure.
 
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