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Compressors - old or new ?

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threeReefs

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Compressors seem to be a topic that stimulates conversation on this forum, so here goes :

I'm looking to buy one and have the obvious choice of new or used. I would like an old one - partly because I like taking things apart when I buy them, and partly because older ones seem better made. So, something like this :
compressor-yes.png

(which is on eBay at the moment, I don't want a 100L one but just to illustrate the principle) rather than this :
compressor-no.png

(with apologies to readers who do have something sensible like that).

But I've heard unpleasant horror stories about elderly compressors that have been allowed to sit with water in the tank for long periods, and as a result have rusted with unpleasant explosive consequences. I'm very conscious that 50 or 100 litres of air compressed to 100psi represents a hell of a lot of stored energy.

So, how would I check an older compressor for safety ? doesn't look easy to inspect the inside of the tank. Tapping ? Pressure testing ? And while I'm at it, any good brands to look for ? Sealey ? Clarke ? Ingersoll Rand seems to get a lot of mentions although they seem quite high-end/large (This will be probably a 25- or 50-litre single-phase for simple paint spraying, amongst other things).

Thanks !
 

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Rorschach

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Unless you are prepared to pressure test it, buy new or at the very least buy a new tank and fit an old compressor pump.
 

Inspector

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You can pressure check the tank yourself but it naturally won't be certified if you need it for insurance coverage reasons.

Two methods.

First method I read about uses a pressure washer. You disconnect all the pressure switches drain lines etc., and add a pressure gauge with the range to read the pressure you need for the test. There should be a data plate with the working and proof pressure it was made to. You fill the tank to the brim with water and then hook up the pressure washer. Carefully pressurize the tank and stop when it gets to the test pressure, lock the pressure in with a ball valve. If there are no leaks and it holds pressure the tank is good. What I don't like about this method is depending on the pressure washer and tank you are doing it could be very easy to over pressurize the tank before you can shut it off.

Second method is much the same but you use a grease gun and some cheap grease to pressurize the water filled tank through a grease nipple. It won't take a lot of grease, half a tube if I remember the writeup correctly, and doesn't have the chance of over pressurizing the tank that the pressure washer does. After you are done you'll have to clean out the grease though.

In either case wear eye protection as a squirt in the eye might not be desired. :wink:

I haven't tried either method myself.

Pete
 

Trevanion

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I've always had my doubts whether a tank would actually spontaneously rupture like a bomb if it had rusted badly inside the tank. I would've thought you'd start with a pinhole leak which would gradually get worse and worse to the point it's emptying air quicker than it can fill it.
 

u38cg

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Explosions are possible, but I think you're right, in practice partial failure is the common mode. That said - my dad's compressor decided to autodismantle one morning - he walked in, turned it on, went for a coffee and just as the kettle boiled, bang. It had carefully rearranged itself as two endcaps and a flat, previously cylindrical, piece of steel. Never figured out why.
 

Rorschach

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Compressors are nowhere near as dangerous as a steam boiler for example but a spontaneous explosion is possible especially with a larger tank and not something I would want to risk with a second hand machine.
 

Duncan A

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The compressor is unlikely to go bang but the receiver most certainly can. The point of failure depends upon a number of factors but the area around the water drain is particularly vulnerable to corrosion (and erosion esp if blown down enthusiastically on many occasions).
The point of a water test as described by others is to minimise the stored energy. Water is incompressible therefore a pressure vessel failure, if filled with water, loses pressure almost instantly rather than continuing to blow out for a period of time during which a small hole may rapidly grow to a larger one - and possibly launch a fitting in your direction.
My advice would be to buy new or buy used with a known history.
Or inspect with an endoscope and/or ultrasonic thickness detector - but probably not easy to get hold of.
Duncan
 

Lazurus

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There are some very valid points made above, I will be looking for a larger belt driven compressor when I move - I have decided on the above it will be a new one, with certificate and warranty.
 

Rorschach

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I have a 250litre 4HP SIP compressor that I have been very happy with for the price I paid.
 

TFrench

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At work I've got a big hydrovane - its nice and quiet. I have since found out they like to be run constantly so I'm not sure how it will do with the sporadic amount of air we use. At home I've got a little sealey vertical. It's noisy but again, I don't use it constantly.
 

Rorschach

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A hydrovane would be lovely but sadly I couldn't afford one big enough to run my sandblaster.
 

threeReefs

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There - I did say it was a subject that provoked conversation.

Many thanks for all the input. I've got no problem spending some time and money sorting a used compressor out - I would be buying it as much to learn about the things as actually using it - pumps, joints, connections, etc. So I guess buying used, restoring/cleaning the pump and motor, and putting it on a new tank might be a good route, or as a minimum getting the existing tank tested. Sounds like - even though actually knowing the history of one might be hard - getting one that's been in regular use, rather than sitting idle for a year or three, would be a good plan.
 

mbartlett99

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I've never heard a hydrovane described as quiet, hate their noise. You can buy a timer kit which will run the compressor and get it nice and hot. Or you can just crack the drain cock like I do.
 

GrahamF

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threeReefs":3mmvk3xf said:
So, how would I check an older compressor for safety ? doesn't look easy to inspect the inside of the tank. Tapping ? Pressure testing ? And while I'm at it, any good brands to look for ? Sealey ? Clarke ? Ingersoll Rand seems to get a lot of mentions although they seem quite high-end/large (This will be probably a 25- or 50-litre single-phase for simple paint spraying, amongst other things).
Thanks !
For paint spraying, the bigger the better (25-50L rather small) and be careful when reading the specifications as they usually quote displaced air at atmospheric pressure, whereas spray guns quote required flow when compressed.

Having had large belt driven ones for spraying cars and small silent for pin nailers, I would say if you have the space go for large belt driven. If a second hand one has come from an industrial location, it should have been subject to annual tank inspection so ask about it. My last large one was a 14cfm Clarke with IIRC a 200L tank which was great for spraying.
 
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