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.