Adding external air tank to existing air compressor to give equivalent bigger tank

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In psi.

Simply put, you'll get a longer run time and a longer tank-fill time. Your plan sounds fine as is.
HTH,
Twayne`
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Now that a new Harbor Freight has opened up about 9 miles, I went to the store and bought the tank for $37, an 8 ft 3/8" 200psi hose on sale for $4 and a couple of fittings for a few bucks. And I used a 20% coupon on the entire purchase...
So for less than $40 total, I have it all working beautifully.
The setup now looks like:
Portable tank-------> Tee ---> 3/8" hose --> Female quick connect | | Gauge Ball valve & pressure release (included)
(Note: I added the ball valve to allow for easy emptying rather than having to exert constant pressure on the built in release
Compressor ----> Drain ---> Ball valve ---> Male quick connect
When I want to fill the external tank, I quick connect from the tank to the drain, open the ball valve on the compressor drain and close the ball valve on the tank.
Then if I want to use the combined volume, I just attach my hoses and tools to the normal regulator side of my compressor and get a combined 19 gallon volume
If I want to use the portable tank remotely, I just disconnect the quick connect and then connect hoses and tools to the female quick connect on the portable tank (I may add an inline regulator later).
If I want to drain both tanks, then I disconnect the quick connect and open the ball valves on both the compressor tank and the portable tank.
Thanks for all the helpful comments that gave me the confidence to go ahead with my plan.
Now I just have to wait a few weeks until it's time to blow out my irrigation system.
Also, as a few others have mentioned, it will be good for high volume, relatively limited time operations like HVLP spraying.
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On 10/24/2011 6:40 PM, blueman wrote:

Be sure to report back and let us know how much difference it made.
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Thanks for the input. I am half considering buying the 6CFM HF $129 compressor so I can do it myself and don't have to hassle with the air compressor people. My configuration is different from yours. I have 9 sprinkler heads on a zone. The 3 heads at the end of the run are about 10 feet below the other 6 heads. The first two blasts of air will clear the higher up 6 heads. After that the blasts loose the pressure in the clear heads and I can't seem to build the pressure on the 3 low heads. I vacuumed the lower heads and got just over a cup of water. I am not sure that this would cause much damage. I am in my 70's so I am not sure how much longer I will be up to this.
Any further insight would be appreciated.

This year the price was up to $25. There are also a lot of people doing lawn aeration for $25. That seems like a lot more work than a blowout. Times are tough and a lot of people looking to make a few bucks.
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On 10/27/2011 7:42 PM, Ray wrote:

put a drain at the lowest end.

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OK - I used the above setup to blow out my sprinkler on Friday in advance of the upcoming October Noreaters snowstorm...
To maximize air delivery, I ended up using a 1/2 inch 25ft hose from my compressor to the water faucet with the hose pressurized to full tank pressure (115psi) and a regulator just before the faucet set at about 55psi (not my regular static water pressure is about 80-90 psi).
It all worked fine though surprisingly I didn't notice a major increase in the amount of time that the heads stayed open before falling back due to decreased pressure. In fact, as soon as I opened the valve the pressure almost immediately dropped off from the static psi of about 55 to approximately 20 psi. It then took about 30 seconds before the tank pressure dropped to ~80 psi and the compressor kicked in again. (note that the pressure lasted longer the first time through each zone when there was still water in the pipes).
So it seems like the 1/2inch air hose and 1/4 inch fittings (at both ends) were insufficient to supply enough pressure to maintain 55 psi through the irrigation system. I guess this makes sense since the irrigation system itself has 1" diameter pipes and the heads probably let through a lot more air than the 10 gallon/minute of water that it does when driven by water.
So, it still had no problem clearing all the zones (one-by-one) of water down to a slight mist. But I'm not sure the added tank made much of a difference. Still, for ~$40, it seems like a good addition to my stem that I can use for multiple uses in the future.
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I'm betting on the regulator being your bottleneck. If you're running 90psi normally, the 115 shouldn't be a problem, as it won't be that high for long.

It might have been more evident at the full rate, without the regulator. You could always fill and try it again to see.
-- Inside every older person is a younger person wondering WTF happened.
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It's a cheap regulator (Harbor freight). So if I understand you correctly, you are suggesting that the regulator limits the volume significantly and may be the problem here?
I was hesitant to run it higher than 60 since people always warn against using high air pressures in pvc pipe. Is high air pressure any more damaging (in the non pipe burst case) to irrigation control valves, irrigation heads, and backflow preventors then an equivalent water pressure? (I would think not but just checking)
But you are right that it won't be at that pressure long since as soon as the irrigation valve opens, the pressure will definitely drop even before the tank runs out just simply due to the resistance of the hose & fittings. Actually, the compressor while rated at 115 psi, the pressure switch shuts off closer to 100 psi.
Note: my water supply is at 90psi static and drops maybe to 80-85 with faucets open.

Still, I am a bit hesitant to put that pressure in the irrigation pvc pipe and fittings...
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I'm saying A) that you probably don't need the regulator at all and B) that they absolutely can have a secondary function as a flow reducer. First, run it all the way up to full pressure and check the run time. Then eliminate it and see if your flow changes. I'm betting that it will.

90% of it is underground and you already use 90psi water, so the difference is minimal. I'd wear eye protection in any case.

OK, then don't. <shrug> If you don't feel it's safe, don't do it. That's your option, and don't let anyone tell you differrent.
I wouldn't hesitate on my own system, but it's drip irrigation and I open the far end of the 1/2" poly anyway.
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Most of the commercial blow out people use 100 PSI. If you are concerned most air compressors have built in pressure regulators and you can dial it down. 2 regulators will reduce air flow.
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On 10/23/2011 5:48 PM, blueman wrote:

Absolutely reasonable thing to do. My son does exactly this when he blows out his irrigation system. A "T" and some extra couplers then 3/8" hose from the extra tank to the blow point.
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Well, I actually connected it to the tank directly (via the drain hole) so it goes before the regulator and insures both tanks are filled to full pressure...
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