The issue of riveted tanks vs welded tanks is really not the cause for our concern. I routinely do engineering to evaluate steam locomotive boilers with riveted construction, determine fitness for service based on CURRENT inspections and ultrasonic thickness gauge readings.
Here is the low-down, not in any particular order:
1. Air receivers (tanks) are notorious for accumulating condensate (water + "tramp oil" from the compressor). This condensate lays in the bottom of the receiver and causes significant corrosion with resulting loss of material from the shell (or head if a vertical receiver).
2. Air is a compressible substance. When contained in an air receiver, it stores a tremendous amount of energy. If an air receiver has a rupture of its shell due to internal corrosion, the resulting escape of compressed air is usually explosive. The air expands back to its volume at atmospheric pressure, which is several hundred times the volume it occupied inside the receiver. In so doing, the common mode of failure is for the area which "blew through" to be the start of an "unzipping". Often, air receivers that fail in service will blast themselves into shrapnel packing considerable energy.
3. At the powerplant I retired from, we had some huge compressed air receivers (8000 gallons apiece x 12 of them, air at 140 psig). Each year, the insurance company would send out a team of engineers for "risk assessment". Being the senior mechanical engineer, I'd take these engineers around the plant and we'd go over any forced outages, equipment failures, improvements or modifications, and also go over the maintenance records. The insurance company engineers usually had a new horror story about some air receiver blasting itself to bits and taking out a good bit of the surrounding plant. These were WELDED construction air receivers, but had not had any inspection or testing done in ages. The failures were always due to internal corrosion from condensate.
4. In an ideal world, if a person were to evaluate an air receiver to determine its "fitness for service", here is how it would be done:
1. The receiver is inspected for any signs of damage, and for any alterations or repairs. Welded repairs "scabbed onto" old receivers, done by persons unknown,
are cause to cut that receiver up for scrap right then and there. Seeing pipes stabbed into an air receiver ringed with bird-shit welding and no compensation
(reinforcement) around the penetration thru the shell of the receiver is a big red flag.
2. Assuming the air receiver shows no signs of damage or modifications or sloppy repairs, the next step is to draw a grid on it with soapstone or similar, and lightly
grind off the paint in the center of each grid square. Once this is done, the thickness at each of these points is determined by ultrasonic thickness gauging.
3. If no working drawings exist for the receiver, a set of "as built" drawings has to be made based on measuring the reciever. Riveted seams present a little
more challenge as the diameter of the rivet shanks is not known. I take the dimensions/shape of the rivet heads and look up the shank diameters for those heads.
If several references show several shank diameters, I use the least diameter.
4. We use a Minimum Factor of Safety of 5.0 for calculations on pressure vessels. Without mill test reports on the steel the air receiver is made of, I would use
a minimum tensile strength of 50,000 psi, and with a FS = 5, this reduces the ALLOWABLE tensile stress to 10,000 psi.
5. I then run a full set of calculations for the receiver (or boiler). Riveted seams, as Joe in NH correctly states, have to have their efficiency calculated.
This efficiency along with the UT reading at the thinnest point will determine the maximum allowable working pressure (MAWP) the receiver can be operated at.
The efficiency of a riveted seam will result in the MAWP being a good deal lower than if the receiver were simply a cylindrical shell. I use an efficiency of
0.90 for welded seams (which is what the American Society of Mechanical Engineers calls for in Section I, the power boiler code). As I recall from calculations
I;ve done on numerous locomotive boilers, a longitudinal seam with "welt plates" inside and out, with three rows of riveting, will usually have an efficiency
around 84 %. A lapped seam of the sort on the OP's air receiver will have a much lower efficiency and corresponding reduction in MAWP.
6. In the 1920's, it was common for tank shops to build air receivers using riveted seams and then seal weld using oxyacetylene welding. I would not be put off by
welding along the seams rather than calking.
7. The internal condition of the receiver and the complete design of the longitudinal seam are unknowns. From the photo, the seam is a lapped seam, double row
riveted. In today's world we do have fiberoptic 'scopes and can get a look inside things like this air receiver. I would include an internal inspection as part of
evaluating this receiver's fitness for service. Ultrasonic thickness gauging is great, but it is not an absolute, and is only done at a series of points on the
receiver. If I get a low reading at any UT location, I move around that point and get more readings to either corroborate or determine if the low reading is an
anomaly. In old steel plate, "laminations" which are mill scale rolled into the plate at the mills, are common. These laminations do not hurt anything, but will
reflect the ultrasonic beam and give a lower thickness reading. Similarly, small pits will do the same thing. However, localized "wastage" or loss of metal
may be present, and this is why if I see a suspiciously low thickness reading, I "blanket that area" with UT readings.
8. Once I had determined the MAWP based on a minimum FS = 5 and using the minimum thickness readings, if there was enough left of the receiver to hold working air
pressures, I would give the receiver a hydrostatic pressure test (known as a "hydro"). This test is done using WATER. Water is incompressible, so if the tank
should rupture, the release of water will not produce a violent expansion and ripping apart of the tank.
9. The hydro test is done to 1.25 X MAWP, with MAWP determined by calculations base on as-found UT readings and physical inspection of the receiver.
5. The receiver, from the photos, has a "radiator drain petcock" on the bottom. This is the kind of petcock used to drain automotive radiators. Nothing wrong with it, but it is in an inconspicuous location, and not something that will pass a lot of water when opened. I'd bet that petcock was opened very infrequently. Each Sadie Hawkins Day would be a stretch, if that often. I am inclined to bet that there is significant loss of metal in the bottom of that air receiver (approximately 6:00 on the "barrel").
6. Blowing down air receivers to clear out condensate is something that has to be done daily. It also has to be done for a long enough period of time to get all the condensate out of the receiver. A restricted opening like that radiator petcock will spit and sputter and not really "blow clear" unless it is left open for a good few minutes. It is also a question of whether the receiver was set with a downhill pitch so that condensate ran toward that drain petcock.
What is a well settled point here is that old/unknown air receivers are potential bombs. Unless the OP is prepared to go the whole 9 yards as far as determining actual minimum thicknesses and running the numbers followed by a hydro test, my advice is to cut that air receiver lengthwise in two and use it for a planter or watering trough.
As Joe in NH notes, I also have seen old riveted tanks and boilers which held up remarkably well in service, and I've seen newer welded air receivers that went paper-thin due to internal corrosion.
As an illustration of what a "little homeowner grade" air receiver can do when it fails: a neighbor to my buddy's automotive shop bought a used air compressor at a yard sale. It was one of those homeowner specials, direct drive, noisy, and mounted on its own air receiver. The neighbor fired up this compressor outside his house, which is next door to my buddy's shop. My buddy said he heard a hell of a bang and then a thud. He went outside to see what had happened. The neighbor was standing in his backyard, looking up at the compressor which had landed on the roof of a one story portion of his house. The neighbor admitted that when he fired up the compressor, there was a "small leak" in their air receiver, but he figured if the compressor could keep up with it and maintain air pressure, he'd use it as it was. While he was thinking of this shrewd maneuver and seeing if the compressor could keep up with the leak, the receiver "unzipped", rupturing open. The resulting blast of expanding compressed air launched the compressor and what was left of the receiver up into the air, and it came down on the back roof of the house.
In short, if a person wants to use an old air receiver without properly determining its fitness for service, that is their choice. So long as they do it in a location where no one else gets taken along for the ride if the receiver blows apart, that is their own fault. I'd suggest the OP, if he is intent on using that old receiver without proper evaluation and testing, start reading his insurance policies to be sure injuries, death, or property damage caused by an exploding air receiver are covered.