Thread: Antique air compressor tank - inspect & test or replace?

As a P.E. too I must say that Joe has done a great job in advising all of you here regarding air receivers and the dangers they present.

From personal design experience in the air-conditioning industry, pressure components and piping etc used there have to have a test pressure of 5 times max working pressure.

The hydrostatic test is done with the test item and all hoses etc connected that are connected to it completely full of water - no air space anywhere as compressed air stores considerable energy and a sudden rupture releases all that energy in a very short time interval. The hydrostatic pressure testis safe only if it is truly hydrostatic. Our lab did these tests with the test item completely submerged in a vat or bucket of water to act as an energy absorber and provide an extra level of safety in case of failure of the test item. In one case, an inch diameter and 5 inch long component failed and the rupture resulted in the 2 gallon tank the item was submersed in being pushed down and completely through the laboratory bench top. Clearly the engineering test tech had not properly evacuated the test item prior to conducting the test. Luckily, he was standing not sitting at the test station. Needless to say, the test station was modified so no one could sit at it.

Point here is this - at 5 X maximum working pressure a two stage compressor operating at 175 psi requires a test pressure of 875 PSI and at that pressure the hydrostatic pressure test you would have to conduct to establish the safety of the tank has some inherent dangers too. Sloppy procedures can kill you.

The volume of the vessel is precisely determined prior to and after conducting the hydrostatic pressure test. If there is any measurable permanent change in the internal volume of the pressure test item, meaning a deformation of the test item has occurred, the test is failed. Likewise, if there is any leak it is failed.

From personal design experience in the air-conditioning industry, pressure components and piping etc used there have to have a test pressure of 5 times max working pressure.

Um. Not to dispute because your little corner of the technical world may differ - but 5X safety factor is common in many engineering productions where "disasterousness" of a failure drives it.

Such as steam boilers (ASME Section 1) and I expect air pressure tanks.

Safety factor in instances where failure is "detectable and addressable" before failure, such as most structural building elements, most automobiles, planes and trains, etc is commonly 3X.

But as Charlie Parker, one of my engineering mentors said - "You can have a platform hanging from a single support, and even have 5X safety factor, but if it doesn't 'look' safe to a casual user of the platform - then it isn't." Public perception of inherent safety is paramount - otherwise you're defeated at the start of your engineering - which is the creation of machines and structures of USE.

Now testing of boilers is mandated by the ASME Code to be 1.5x the normal maximum working pressure. Going above this was common, particularly in railroad and specialized applications - but usually didn't exceed 2x. In fact there is much discussion in Boiler Inspector circles about the inadvisability of going beyond the 1.5X operating pressure simply on the basis of possibly precipitating a failure. That rationale goes to the "let sleeping dogs lie" modus.

Opinion does vary, as it may in your particular industry.

SwageLock Co. as a general instrument fitting of common application advertises that they are TESTED to 10X the rating. But one assumes this is because the producer has little control of field conditions where their product is used, and this is a way of protecting themselves legally. But that is not usual. And it is advertising hyperbole which has no doubt established SwageLock in their superior market and performance position.

I once had the engineering design of a "nuclear reactor refueling seal ring hoist" thrust at me as a project while at Seabrook Station.

Nuclear reactors are normally run in a dry environment. The reactor vessel and cavity are in the air of the containment building. Air that is air conditioned actually to 76 degrees as a matter of normal cooling, extension of the life of plastic and rubber insulation, and those ever precious solenoids used as part of the control rod drive system. In a nuclear power plant, ironically the most comfortable place environmentally is in the reactor building DURING operation - providing you have personal protection against the neutron flux which otherwise might make your stay in containment - or your life - short.

But, the reactor is refueled wet - i.e. the reactor vessel is made watertight in it's cavity, the cavity is flooded to a depth of perhaps 25 feet to cover the fuel bundles as they are withdrawn from the reactor, and all refueling movement is done "underwater" as a matter of shielding, protection against airborne contaminants, and residual heat removal. Refueling activities tend to be warm & humid for those who do it.

To accomplish this a "reactor seal ring" is placed around the reactor head and bolted in place bridging the margin between the reactor vessel and the concrete containment cavity. The reactor refueling cavity is then flooded and the control rod drives and reactor vessel head are removed revealing the nuclear core. The seal ring to allow this is lifted into place using a hoisting rig and overhead polar crane. The seal ring has to pass over the reactor vessel and its control rod drives and allow the ring to be centered, lowered, and placed.

Passing as it does in its placement, the seal ring hazards the nuclear boundary of the reactor. A failure of the hoist or hoisting rig would be disastrous. As such the hoisting rig is over-designed to have a 10X safety factor by Federal Law. It also has to be simple in concept, easily and quickly assembled, clear the lift spaces and obstructions for its use, and fit into the available space. AND - lest the nuclear flux degrade it's properties and performance, it has to be removable from the containment building when not being used.

Few places in the world have the means for testing prepared cables of this particular size and strength. One of these is in Belchertown, MA where the rigging company has the equipment to element by element test the cables and connectors. For a price they will even "verify" a completed assembly - which is where we went with the Seabrook Reactor Seal Ring Lift Rig.

IIRC, while the design capability of the lift rig was 10X to failure, the actual number we settled on for testing was 3X. Even so, it was quite a test with weights hung at the three corners of the lift rig, each weight weighing as much as the seal ring by itself.

No verification is too expensive considering the disasterousness of a nuclear accident, of course.

But those cables, shackles, pins, clevises all had to be certified nuclear grade. The cables were like 1-1/4 inch - possibly the upper limit for manual moving into and out of the containment building - and all through a 7' diameter personnel access air-lock. A single shackle alone was 76lbs.

IIRC the total bill on all of this was like half a million dollars.

So one should be sure one differentiates between "design requirement", "Safety Factor" and "test requirement" - lest their shackles become too heavy for even the stoutest laborer to pick up.

Joe in NH

Originally Posted by Just a Sparky

Well the whole issue with replacing the tank is that this is an antique. I'd like to preserve the cosmetics of it.

I suspected this.
Carefull application of a plasma cutter should "gut" that old tank.
install a new tank inside.
Might even hinge it half way up, or make the ends bolt on.

I'm looking at having a new tank fabbed up by a local tankmaker eventually. 16" OD seems to be a standard size so I imagine it should be a fairly run-of-the-mill job save the lack of any mounting provisions.

I'd like to keep this machine with me for a good long time. I'm wondering what sort of options I should go with when quoting a new tank. How long will a steel ASME tank last before it will need to be replaced again? Is it standard practice for modern tanks to be lined with some sort of corrosion inhibitor or is this an extra I would have to either pay for or handle myself?

Would it be practical/economical to have it done in stainless instead of mild steel?

Mill Valley splicing, Belchertown, MA Jim Byco, good guy!

Pressure vessel + standing H2O + 100 years old = Scrap.

Originally Posted by doug8cat

Pressure vessel + standing H2O + 100 years old = Scrap.

That's old news.

Originally Posted by Just a Sparky

I'm looking at having a new tank fabbed up by a local tankmaker [...] I'm wondering what sort of options I should go with when quoting a new tank.

Originally Posted by Just a Sparky

I'm looking at having a new tank fabbed up by a local tankmaker eventually. 16" OD seems to be a standard size so I imagine it should be a fairly run-of-the-mill job save the lack of any mounting provisions.

I'd like to keep this machine with me for a good long time. I'm wondering what sort of options I should go with when quoting a new tank. How long will a steel ASME tank last before it will need to be replaced again? Is it standard practice for modern tanks to be lined with some sort of corrosion inhibitor or is this an extra I would have to either pay for or handle myself?

Would it be practical/economical to have it done in stainless instead of mild steel?

Check out McMaster-Carr and other sources of "prepared" tanks. Unless you can convince this supplier to "donate" their time and tank.

Generally, ASME coded tanks are designed with the minimum of corrosion margin. Horizontal tanks typically 20 percent, but even that number might be too high given an "aftermarket" tank of probably offshore origin. (Harbor Freight provides ASME tanks with their Air Compressors, but doesn't specify actual corrosion allowance.)

Surprisingly, riveted historic boilers and tanks don't have an intended corrosion margin by design, but they do have "joint efficiency" - which in the case of a single row of rivets might be as much as 45 percent extra material. Possibly even more.

My O&S steam boiler was built "pre-code" - specifically 1907. About 20" in diameter the boiler is stamped with WP = 100psi, but the markings don't include the ASME cloverleaf, or a National Board number. Do the calculations based the shell thickness and a 55 percent lap seam joint efficiency and the usual working pressure for this boiler could be 210 psi. But even 1907, a "de-rating" of lap seam boilers to 100 psi was common - and this one apparently was.

As such, this boiler has about 80 percent "corrosion over design."

And this, or a similar rationale or design is why these frequently "ancient" boilers have lasted as long as they have. And probably are more "corrosion capable" than a modern welded boiler.

You may even have this sort of condition on your "obsolete" air tank. But only as you study it out, do some testing, calculate it out, and get this confirmed with an actual engineer. (My PE License was #8633 in NH, but I'm now "inactive" status.)

I will qualify this statement with "corrosion depends." Obviously if the heads of lap seam rivets are all rusted down to nubs, then this is a pressure vessel which shouldn't be in service.

In the case of a horizontal air tank with the riveted joint appearing in the upper quadrant, the corrosion USUALLY occurs in a line across the bottom, possibly a short distance up from a lower chord - but at a point where the waterline or moisture, when it exists, occurs. This would be a place to examine with an ultrasonic or magnetic thickness tester. If less than 60 percent of the riveted shell material remains, then the drum should likely be de-rated, or retired.

As to stainless, best advice I can give is - bring your wallet. Stainless is NOT the end-all-be-all in corrosion resistance since even stainless steels have their own unique modes of failure, chloride contamination being one of them this from my experience with Nuclear Service.

A commercially available carbon steel standard tank is VERY possible - done all the time in real life. Normally tanks of this ilk are not "internally protected" instead reliance upon the corrosion margin - and a pre-assembly coating of red-oxide paint. If you seek a "period" appearance, there are "applied" rivet heads (plastic) which give a very credible appearance when painted up.

Joe in NH

Just my two cents.
In short, dont trust the old tank unless you have the ability to ultrasonically test the thickness if the metal. A pressure test with water will only tell you that it is holding for now. You dont know how badly the tank might be corroded on the inside. In all things, better safe than sorry.

Sent from my SM-G930V using Tapatalk

Just A Sparky wrote:

"I've got other repulsion-start motors in my possession, but this Century takes the cake. It's enormous for it's power rating, makes a beautiful sound and emits a unique smell when starting up - almost like gunsmoke."

That smell is mostly ozone, created by the sparks at the brushes during the Repulsion-start sequence.

( It's also a "trolley car smell" - my electrician father, 1912-2009, and I were once at a working trolley car museum and he remarked that the ozone smell was a powerful memory trigger. )

This is a great thread with a lot of knowledge being conveyed by engineers with lots of practical experience.

John Ruth