Watertightness - spray testing: The physics of leaks

[MintJulep]

A common method of demonstrating that a thing is "watertight" is to spray water on it.

Typically, there is a (seemingly arbitrary) set of requirements for water pressure, water flow rate, water velocity (usually not specified where the velocity is to be determined), and sometimes nozzle distance from the thing under test. For example, MIL-STD-810.

But what determines the necessary values for these parameters?

Or, the actual point of my question - if a certain test facility can't simultaniously meet all of the required parameters, what compensation might be made to allow for an "equivalent" test.

Is a 20 minute test with a nozzle velocity of 100 ft/min and a flow fate of 100 gpm "equivalent" to a 10 minute test at 200 ft/min and 100 gpm?

What is it that actually challenges a seal and causes a leak?

My thoughts are that you need to consider the kinetic energy impinging on the surface under test, but I'm not certain that's sufficient.

It's easy to imagine a pressure washer causing a leak that a garden hose wouldn't. But it's also easy to imagine something that would never leak when subjected to a garden hose - no matter how long you sprayed it.

Your thoughts on the subject would be welcomed, and actual references even more so.

 

[vc66]

Gotta love those "historical" test parameters.

V

 

[belowzero]

Jspisich -
You don't need a 'vent' to eliminate pressure/vacuum if you use a diaphragm, therby maintaining the 'atmosphere' within the unit. Obviouisly designed large enough to cope with temperature changes involved.
B

 

[DesignerGuy16]

I know, just saying it's another option using a membrane vent that won't allow moisture or vapor in.

James Spisich
Design Engineer, CSWP

 

[foglights]

I always thought that an equilavent was an eqvivalent was an equivalent but one should design a test most like actual conditions.

The box example second test failed because the Supports weren't strong enough, not the box (it survived the falling weight).

Anyhow, getting back to what I was saying. The test should test the seal effectiveness of the seal compound or material, as you have mentioned. So, in your case, a baseline test would have to be designed based on the sealing compound or sealing material's typical or actual use and presumed light, normal and heavy water conditions.

 

[IRstuff]

Anyway, for MIL-STD-810, the specific requirements are to envelope some rational construct of "worst case" conditions. For external military boxes, this boils down to two specific types of testing, already mentioned here; some sort of pressure spray environment, and a "dunk" test.

The pressure spray environment would occur as part of cleaning or decontamination, which may not be part of your specific UUT. The dunk test would apply to UUTs that might get submerged, like a military vehicle that must ford a river or shipboard equipment that might get overwashed with waves. These environments are all "soft" and uncontrolled, so many of the tests might well be "legacy", based on some historical or even mythical conditions.

Interestingly, the one test that isn't performed, is the "moisture getting sucked in by a pressure differential trick." We had a camera system that did that so well there was water halfway up the camera aperture.

TTFN

FAQ731-376

 

[MintJulep]

In fact, MIL-STD-810 recommends heating the water or precooling the TUT to avoid the cooling effect of the water spray from reducing the pressure inside and sucking water in.

I guess it didn't occur to the military that something that is actually hot inside when operating might get cooled when it starts raining.

At any rate, my test is not MIL-STD-810 per say, simply a similar generic spray test of unknown and likely arbitrary origin.

 

[foglights]

Good luck with your test examining the physics of leaks!! Its great fun making stuff up!

 

[foglights]

and so I have a question myself.
Will you be varying the volume and rate of the water water by the size opening of nozzle or at the pump producing the water's force?

 

[hansonbrest]

i am a little bit off topic here , but in case people want to look at leak testing for tanks , vessels , boxes etc

we often use compressed air on one side of the test object , and spray soapy water on the other side , it sometimes allows testing where hydrostatic or chemical penetrants like dyecheck cant be used.

either pressurise the vessel with 2 - 3 PSI , or run a blowgun along the internal seams while spraying soapy water outside.

hope its useful to someone

 

[griffengm]

Minty,
The test arrangements I encountered when working with automotive door seals made me believe that in order to assure leakproof joint one needed to have jets from multiple angles, volumes, and pressures. One of the more interesting stations resembled a car wash with the nozzles moving the streams in cone shaped paths to cover all angles from which water could hit the vehicle. Looked a lot like a carwash with the nozzles mounted on the top and each side of the carriage that usually carries the brushes. IIRC, the techs said that the velocities had to simulate 120 mph. That seemed unreasonably high until looked at in the perspective of 70 mph auto into a 50 mph wind.

Griffy