Gee, warm water expands. If you are doing a dynamic test then one is using pressure release valves set for the bleed off pressure, bleeds off as you load it. If a static leak down test, if the pressure release valves are set for the max for the test, then thermal expansion is not important as it will be let off anyway. For a leak down test all you have to do is chart the time and pressure while looking for the obvious water trail.
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As CCDW stated ductile brittle failure is a real subject. In the sophomore engineering class Material Selection, you quickly learn that no material is truly a perfect fit it. To get what is hoped is the best result both engineering and other factors must be considered. It is important to understand the anticipated service conditions today and in the future.
A history of the Charpy V Notch test is here https://www.labtesting.com/resources/te ... y-process/
The behavior of ductile brittle transition can be found with liberty ship accidents such as the SS Schenectady in Portland, Oregon, on January 16 1943, or in the failure of the fleet oiler USS Ponaganset. For a highway bridge failure in cold weather see the failure of the Silver Bridge collapse in December 15, 1967. For a brief description of Ductile Brittle Transition Temperature (DBTT) see https://www.jeremyjordan.me/ductile-to- ... materials/
Many of the elements that effect welding such as phosphorus and sulfur also effect the DBTT. Take this material behavior and then add corrosion and what you built deteriorates to a level that the structure can suddenly fail in ways that are quite dramatic. Unfortunately, the following is only true in cartoons https://www.youtube.com/watch?v=vVZ73o3O7Jo . What we do not know can still kill us.

Robby Peartree

Not necessarily....
Textbook NDT failure....

"Pressure Vessel Failure during Hydro Test
Nanjing Yuchuang Co, China

28th November 2007

Please find attached a couple of photos of a pressure vessel that recently failed whilst under hydrotest during post fabrication testing. This vessel was manufactured by Nanjing Yuchuang Co. in China and the plate was of Chinese mill origin. Unfortunately this is another example of serious equipment/material failures with equipment being sourced out of the rapidly developing economies such as China, Eastern Bloc and others. These examples are becoming almost a weekly occurrence now and are exhibiting failure modes not seen in the mature manufacturing economies since the 1930's. Again we need to ensure vigilance in the acceptance of manufacturers and once more I stress the need to know where the base materials are sourced from.

Apparently this pressure vessel had reached fifty percent of the required test pressure when the shell ruptured. A metallurgical failure report is not available however from the photographs a number of observations could be made regarding the quality of the material and the welding. If you check out the welding (undercut/poor visual/arc strikes) it is unlikely that this vessel would have be conforming to any Pressure Vessel code, let alone be ready for a hydro.

As others have said in relation to this particular failure example I bet there would be more than the one contributing factor ("The Material") associated with this failure."

I have seen Chinese-made pipe fittings, such as schedule 80 pipe plugs, fail at low pressures. In some, the castings were so porous, they would barely hold at atmospheric pressure.

There is no evidence in the photos that you posted that the system being hydro-tested "exploded". A rupture or weld failure during a hydro test is just a rupture. IF it had been a failure under high pressure steam, then that could have resulted in what many would call an "explosion".

What Matt reported might, in fact, have qualified under the sense in which I used the word "explosion", a dangerous rapid structural energy release as a consequence of testing. Moreover, I easily see propagation as occurring at, perhaps faster than, the speed of sound in steel, which might make this tantamount to detonation kinetics...

When boiler explosions proper were less well understood, and boiler materials, pressures, and fabrication were different, it was noted that there were two broad results of explosive 'steam explosions' -- those in which rocket effect and expansion of supercritical water produced the damaging effect, and those in which the boiler material was observed to shatter, often without respect for plate boundaries or connection, precisely as if shockwaves were directed into a brittle material. This represents a potential third class related to the second: an "explosion" (or more appropriately perhaps a 'rapid unanticipated deconstruction', due to unrelieved strain in the metallurgical structure, or developed during the stress of the testing, that abruptly and probably very loudly progresses to much of the structure beyond an initial hydraulically-induced point of initial failure.

My initial brief understanding of the failure in Mr. Austin's example was that high stress abruptly relieved in the initial 'hydro-induced' failure created stress raisers in the surrounding metal that then propagated prompt cracking through much more of the material, perhaps at high speed, perhaps like cracks in tempered glass or similar material. Considerable noise and energy could be expected to accompany this, although not likely even a spray of high-pressure water through much of the evolving crack, and not of course the cumulative damage from expansion of a large mass of supercritical content.

I had not considered the possibility of this kind of event in assessing hydro testing 'risks' and I'm grateful to Mr. Austin for bringing it up with a valuable example.

It meets the National Board/engineering definition of an explosion.

"AN EXPLOSION has sometimes been referred to as a sudden random disassembly of an object. There is no question as to the suddenness of the occurrence - it takes place with frightening speed. The object affected, separates and is destroyed in a random pattern. The fact that the object itself, the building, equipment and people that were in the area, may also be disassembled by an explosion is well documented."
per
Richard E. Jagger, Assistant Director of Inspections,
The National Board of Boiler and Pressure Vessel Inspectors,
at the National Board's 53rd General Meeting,
Charlotte, North Carolina, on April 24, 1984.

MECHANICS OF AN EXPLOSION

I certainly don't want to appear xenophobic, but I will echo Q.C. problems that I have observed in pipe fittings marked with CHINA as country of origin, such as thread tolerance issues so awful that either the fitting is wrench-tight with only one thread of engagement or it is hand-loose until the threads are almost completely engaged, non-standard geometric relations between centerlines in tees and elbows, lousy castings that look like the work of an apprentice foundryman's first attempt.

Some of this may be due to rounding errors in converting dimensions that are sourced in Imperial Units to the metric equipment that is being used in manufacturing, but mostly it is just a lack of care about inspection and quality control processes on the factory floor.

I think this is being a bit pedantic. By that definition, a bridge collapse is an explosion too. I think most people understand the word "explosion" to mean an event accompanied by a violent explusion of projectiles. And getting back to the original point, the reason why hydostatic tests are done in the first place, with hydro, as opposed to pressurizing with air or steam, is that liquids are essentially incompressible. There is much less potential energy stored in a tank full of 200psig water than air or steam.

We have a locomotive in our collection, SBK #4. Its main reservoir was an old riveted tank I'd say 18x60, mounted in a vertical position. On its last day in service on the SBK, the compressor governor failed. Or, depending on the account you listen to, it was sabotaged. The safety valve was either tampered with or was just "frozen" with rust and hadn't been tested in forever. The air compressor was much stronger than the reservoir. The latter exploded when the rivet circle connecting the end bell (at the top) to the side sheet failed. This not only destroyed the tank, it left a giant hole in the roof of the car, which is made of steel plate of, I would estimate #6 ga. Now, that is an explosion!

So would a plane crash or perhaps an overpressure test on a refueling tanker. IF you want to get lost in semantics....

To be further lost in semantics, many people consider a loud BANG! to be the hallmark of an 'explosion', whether or not being struck by projectiles instead of just shockwaves from whatever produced the excitement.

I don't know if anyone has pictures of a failed high-pressure hydraulic cylinder, but I assure you that when those let go there can be ample shrapnel, if that is to be a criterion...

We might also note that 'adequate' energy to cause potential human harm can be orders of magnitude less than the catastrophe that is a prompt release of supercritical-water latent heat. The rubble doesn't have to bounce for the strike to be effective...

Here's an explosion, or at least that is what Trains Magazine is calling it, in Utah yesterday (01/09/2021), North Country Fire Department (UT) photo: https://trn.trains.com/news/news-wire/2 ... locomotive Not sure what would do that back there, but I believe the compressor is located in that area.