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airsmith282
02-26-2009, 05:36 PM
can damage happen to an air compressor leaving the tank charged all the time with air in it

BillH
02-26-2009, 05:38 PM
Yes, air compressors are not supposed to have compressed air inside of them.

madman
02-26-2009, 06:15 PM
Ill let the air outa mine Immediately. I always wondered about that Thanx

dp
02-26-2009, 06:28 PM
This is one of those "there's no right answer so do it my way" situations. I have a shut-off valve at the tank that I keep turned off when I'm not using the air. That way if a hose breaks while I'm out hunting moose or running for VP it won't beat the snot out of everything in reach and burn out the pump motor if I've forgotten to turn the power off. If I'm not going to need it for some time I'll drain it of collected water then leave that valve open to let all the air out. They never explode when they're empty. Don't know though if the full stress range of zero pressure to operating pressure is worse for the tank than leaving it full of compressed air.

lane
02-26-2009, 07:30 PM
Mine just leaks down buy it`s self. Fill it up every time I go out to shop ,come back next day it`s empty. Must be the critters playing with the air hose every night.

hardtail
02-26-2009, 08:52 PM
Yes, air compressors are not supposed to have compressed air inside of them.

If you could please expand on this???????

Carld
02-26-2009, 09:06 PM
:eek: what? I have an industrial air compressor with an 80 gal tank. I never let the air out of it and have a ball valve to shut the air lines off. There is a check valve between the compressor and the tank but it still leaks down some.

Why would you want to let the air out and then later spend money to charge it up again? It takes about 15 or more minutes to air mine up from 0 pressure.

madman
02-26-2009, 09:28 PM
Mine Aleaks overnight also i thought it was some of those possums from Tunnel hill Georgia gnawin on the lines lots of small funny lookin holes. Dave Cofer said it wasnt his doing??

jmm360
02-26-2009, 09:32 PM
This is one of those "there's no right answer so do it my way" situations. I have a shut-off valve at the tank that I keep turned off when I'm not using the air. That way if a hose breaks while I'm out hunting moose or running for VP it won't beat the snot out of everything in reach and burn out the pump motor if I've forgotten to turn the power off. If I'm not going to need it for some time I'll drain it of collected water then leave that valve open to let all the air out. They never explode when they're empty. Don't know though if the full stress range of zero pressure to operating pressure is worse for the tank than leaving it full of compressed air.

Mmmmm - moose meat.

I do the same- ball valve at tank outlet closed every night, blow drain in the morning and during the day, I expect it to live a long time.

I suppose if you wanted it to last a couple hundred years you could pull as much of a vacuum as it could handle and heat it accordingly every night, but in the real world moisture is there and they'll still last many years with regular purging.

CCWKen
02-26-2009, 09:32 PM
It's not so much the air that hurts the tank, it's the moisture. If you bleed off the water or have a self purging valve there's no harm in leaving the tank full. As already stated, most compressors have a check valve so the reed valves are not loaded (held down). The check valve (actually a combination valve) bleeds the pressure off the output line so the motor doesn't have to start under a higher load.

If your compressor is not equipped with the combination valve at the pressure regulator then yes, drain the tank. Otherwise, it could shorten the life of your reed valves. You can tell if yours is equipped with the valve as it will release air after the tank is full and motor shuts off.

oldtiffie
02-26-2009, 09:34 PM
Where does it say that a compressor should be "blown down" to zero pressure every day?

Draining the condensate out - yes - but emptying the pressure vessel?

I buy my compressors new and replace them 10 years (maximum) after the test date as its time for a new one then.

My current compressor has a working pressure of 10 bar (= 10 atmospheres ~ 10 x 14.4 = 144psi) and the power to it off switched off, and all air-hose connections to it are disconnected until I need or am using it. I bought it not so much for the higher "switch off" pressure but for the higher "switch on" pressure (~7 bar x 14.5 = 101.5psi) as most switch on again at about 6 bar ~87psi which with line losses will be ~5.5 bar ~ 80ps to 5 bar ~ 70 psi at the "business/tool" end of the line which is a lot less than the 90psi (~6.2 bar) that many tools specify. A 105 psi compressor sure won't do for my plasma cutter which has a high air demand.

Most "domestic" and "light trade" compressors work at 7 bar ~ 7 x 15 = 105psi.

In OZ all compressors (and most pressure vessels) have to be tested before sale and at 10 year intervals - some may be more or less. The cost of the re-test just doesn't stand up against the cost of a new compressor. (Good excuse to get a new one too!!).

Also as required in OZ, my oxy-acet bottles are taken care of by my supplier (BOC) from whom I rent them. My car LPG tank was re-tested last year (10 years). All LPG bottles (small, 20#, 100# etc.) are to be re-tested every 10 years as well - as mine are.

CCWKen
02-26-2009, 10:31 PM
Most "domestic" and "light trade" compressors work at 7 bar ~ 7 x 15 = 105psi.

In OZ all compressors (and most pressure vessels) have to be tested before sale and at 10 year intervals - some may be more or less. The cost of the re-test just doesn't stand up against the cost of a new compressor. (Good excuse to get a new one too!!).

No such requirement for compressor tanks here. (US) The tank is certified when built but it's longevity is left to the owner.

105psi ?? My shop and paint compressor is 175psi but I have the cut-off pressure set back to 150. Cut-on pressure is about 120. The "BIG" compressor is only 125psi but it will fill an 80 gallon tank in about four seconds.

BillH
02-26-2009, 11:12 PM
Geez, you guys don't recognize sarcasm when you see it.

radkins
02-26-2009, 11:27 PM
can damage happen to an air compressor leaving the tank charged all the time with air in it


You will shorten the service life of your tank if you allow it to drain completely on a regular basis, leave it pressurized! Certainly allowing all the pressure to drain off occasionally is not going to hurt anything and of course this is going to happen. When a tank is pressurized from empty it will expand, not a lot but it does expand, and each time it de-pressurizes it will go through this expansion/contraction cycle which over time will cause cracking (usually at a weld around the motor/pump mounting flange or the tank mounting feet) due to metal fatigue. A tank will expand/contract a small amount during normal run/recharge cycles but nowhere near as much as if it is completely discharged. In addition to the possible tank damage think of all the wasted run time it will take to recharge an empty tank if this is done every day!

oldtiffie
02-26-2009, 11:37 PM
Geez, you guys don't recognize sarcasm when you see it.



Yes, air compressors are not supposed to have compressed air inside of them.

Touché Bill.

Thanks for the kick in the *rse for not reading your first post properly - the more so as I use a lot of sarcasm as well (p'raps you noticed).

Ian B
02-26-2009, 11:41 PM
We have a utility air and an instrument air system on the platform out here. The tanks have been pressured up almost continuously for the past 25 years, no problems so far.

The main thing is to blow any condensed water out via the bottom drain.

Ian

oldtiffie
02-27-2009, 12:04 AM
A couple of points.



CCWKen It's not so much the air that hurts the tank, it's the moisture. If you bleed off the water or have a self purging valve there's no harm in leaving the tank full. As already stated, most compressors have a check valve so the reed valves are not loaded (held down). The check valve (actually a combination valve) bleeds the pressure off the output line so the motor doesn't have to start under a higher load.

If your compressor is not equipped with the combination valve at the pressure regulator then yes, drain the tank. Otherwise, it could shorten the life of your reed valves. You can tell if yours is equipped with the valve as it will release air after the tank is full and motor shuts off.

The valve CCWK refers to is an "unloading" valve as it unloads/vents the receiver-to-regulator line so that the motor starts under no/minimum load. I can tell that its needed as if I don't follow the correct start-up and shut-down sequence the line is not vented and the motor is trying to start under potentially full load - and it doesn't like it one bit!!

One thing I forgot to mention in my previous post was that I unload the regulator (reduce outlet/line) pressure to zero each time I shut the compressor down so as to minimise leakage and to ease the tension on the regulator outlet pressure control spring.

I have to have the compressor switched off at the regulator before I make the power switch as the outlet to ensure that the unloader is not by-passed. I then make the power switch at the regulator to start the motor and adjust the outlet/line pressure as required. I plug in the air-line/hose before start-up.

Shut-down is the reverse - except that I often drain the condensate just before I shut down. I try to remember to leave the compressor idle for 30 minutes or more to give the air in the receiver time to cool down to ambient shop temperature so that I get the best result from the lower "dew" point and so precipitate the maximum amount of condensate.

Another point too as regards compressor capacity (pressure as well as volume/minute - cubic feet or litre per minute at a rated pressure) is the power supply limits. I have my compressor and plasma cutters on separate circuits as both draw a lot of electricity. The cutter relies on the continuity of the air supply not just for cutting but for cooling the torch as well. A limited air supply can really reduce the cutter current and duty cycle and a loss of cooling air is a risk I don't take. It really doesn't matter on my other machines as the air supply is only for cleaning etc. and so its duty cycle is low and I can get by with 70psi but air drills and impact wrenches as well as some air grinder require a good 90psi supply - at the tool - so "line-losses" are important.

John Stevenson
02-27-2009, 04:20 AM
Yes, air compressors are not supposed to have compressed air inside of them.

Mine has air inside it and outside it, in fact every morning when I go in the shop, the shop is full of air :)

An aside note, I never switch mine off, stays on all the while. I have two compressors up in the hay loft 45 gallon capacity mounted side by side and with linked tanks.

The original one is a direct drive and is unbelievable noisy, that noisy you don't want to be in the same post code when it running. It was OK initially as I only ran up once in a morning for blowing off.

When I got the CNC which often has to run late and it's on air clutches and tool change I bought a Hydrovane compressor, very quiet and doesn't affect anyone. Last week I bought another Hydrovane to act as a backup instead of the direct drive unit and this will be plumbed into the same two tanks.

I do have 1/4 turn ball valves on the CNC and each of the 3 shops. The CNC is always off unless working as it does leak a bit from the clutches and I throw the main ball valve overnight.

It doesn't leak tank side so it never strikes up, Weekends it does get shut off but on a Monday it's only dropped to 80 pound or so.

I have reduced max pressure from 120 to 100 as I don't use impact wrenches etc and the power required to go from 100 to 120 is as much as going from zero to 100.

I may leave the original Hydrovane at 100 and have the new standby still set to 120 and a change over switch so if I do need the extra pressure I can get it easily instead of going in the loft and giving the unloader switch a couple of turns.

.

A.K. Boomer
02-27-2009, 06:57 AM
You will shorten the service life of your tank if you allow it to drain completely on a regular basis, leave it pressurized! Certainly allowing all the pressure to drain off occasionally is not going to hurt anything and of course this is going to happen. When a tank is pressurized from empty it will expand, not a lot but it does expand, and each time it de-pressurizes it will go through this expansion/contraction cycle which over time will cause cracking (usually at a weld around the motor/pump mounting flange or the tank mounting feet) due to metal fatigue. A tank will expand/contract a small amount during normal run/recharge cycles but nowhere near as much as if it is completely discharged. In addition to the possible tank damage think of all the wasted run time it will take to recharge an empty tank if this is done every day!



Ditto on this, the worst thing for a tank (besides never draining it) is cycling it to extremes -- leave it sit full or leave it sit empty.
Also what you dont want to do is leave it on when your gone is case of an air line rupture -- its not the big blow out leaks that are the worry, their the easy'st on the comp. and motor as they will run forever without hurting themselves due to all the resistance being removed -- its the leak that keeps them right on the edge of max shut off pressure that can kill them.

gnm109
02-27-2009, 08:24 AM
:eek: what? I have an industrial air compressor with an 80 gal tank. I never let the air out of it and have a ball valve to shut the air lines off. There is a check valve between the compressor and the tank but it still leaks down some.

Why would you want to let the air out and then later spend money to charge it up again? It takes about 15 or more minutes to air mine up from 0 pressure.


Yeah, Gosh. I've had my 80 gallon compressor pumped up to 135 psi for the last 25 years except for when I let the water out of it. Who knew that could damage it? Gee whiz!

I learn things like this here all of the time.

:confused:

Carld
02-27-2009, 10:14 AM
Yes, mine is a 1956 Kellogg American and as far as I know it has been in service all it's life under pressure. It is a compound compressor for high pressure.

Jeffw5555
02-27-2009, 11:38 PM
Ditto here. I have an early 70's Kellogg American two stage 5HP on an 80 gallon tank. I bought it for $150 as it had a bearing knock & was 3 phase. It had been run almost continuously for almost 30 years. I rebuilt the pump and put a new motor (single phase) on it, repainted. I expect it to outlive me in my hobby shop. BTW, I always keep it full, with a ball valve on the outlet. Pump kicks on at 135 psi, off at 175. Yes; 175 psi puts more stress on everything, but it still will outlive me I'm sure.

Plus, higher pressure is better for water removal. (all air leaves the compressor at 100% RH, as it cools water condenses out, when pressure drops, the air reduces in RH....)

wierdscience
02-27-2009, 11:53 PM
I look at it this way,I paid good money for electricy,valves and piston rings to compress that air,so I am not going to waste it.

There is no good reason to blow down a tank unless maintinace is being done,long as the moisture is bled off everything should be fine.

radkins
02-28-2009, 12:09 AM
There is no good reason to blow down a tank unless maintinace is being done,long as the moisture is bled off everything should be fine.


But, there are several good reasons NOT to bleed it down unless necessary!

dp
02-28-2009, 12:15 AM
So as I was saying, it's a "do it my way" kind of issue.

I have an older compressor - a 1915 Curtis, in fact, and the tank is pretty badly battered after all the time since. It's been welded, brazed, patched and worse. A new tank and it's all good as new.

http://thevirtualbarandgrill.com/machinery/uglypump/

Today I went to Harbor's Frights and bought one of their $10.00 auto drain kits. Nicely done, more less, but the connection to the power-on pressure drain pipe is a no-go. They use plastic pipe between the bits but the as-installed is 1/4" or so aluminum tube. No place for the plastic nibs to attach, so off the the big orange box tomorrow to buy a new tee and proper plumbing for it. The plastic looks like trouble waiting to happen. I need a 90 degree elbow for ground clearance, too.

Here's a how-to I found: http://www.paragoncode.com/shop/compressor/

And here's the valve kit:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=46960

Probably not made in Pennsylvania.