Ball-valve leak identification by pressure decay method. 4

[Bharti__]

Hey there!

I am on project of designing of pneumatic-leak testing machine for checking of leak in 3-piece floating ball valves of size NPS 1/4" - 2".

While calculating pressure decay in system, for e.g. NPS 2" valve has volume of appx. 240CC so considering leak of bubble of dia. 0.5mm/ 1- sec i am getting 0.025mbar pressure decay in testing time of 15Sec. (as per my understanding calculations are correct)

So for that i need to monitor that pressure decay with time in testing valve, now problem is i need pressure transducer up to max. 50mbar sensing with allowable working pressure of 7bar, but seems quite awkward.

so i would like to have some suggestions from experts on that what can i do with that! or any alternatives to identify this leak? how to meassure this pressure decay? up to what pressure decay i can allow pressure transmitter to sense pressure decay.

Hope you will get my confusion! if any other info needed let me know.

Thank you in advance


Aiming towards Industry 4.0

 

[gerhardl]

A search in this forum by using the forums own 'sponsored search'- topp of page will already on its own give a lot of references til previous threads.

Another thing: you mention test parameters and suggest methods, but are the methods qualified for certifying valves after the relevant certified test criteria?

Leakage tests are most common visual for liquid. Tes for gas is also often made by liquids, or under liquids. The gas leakage is depending on gas type, air will leak less than hydrogen. It is also normal to test gas leakage by amount over a (prolonged) time required to be (far) less than required amount. New valves tend to be either almost tight or faulty (leaking fairly much).

I am at all not sure that your proposal for test would be one widely accepted. I might of course be wrong.

 

[Compositepro]

For your application looking for leaking bubbles under water would be the most effective leak test. Cheap and almost instant results. If you must quantify the result you can capture the bubbles with a upside down funnel and burette. Looking for a tiny pressure change in a high pressure, over time, is complicated and the instruments are relatively expensive. Temperature must be very stable, and this takes time.

 

[HitokiriAoshi]

pressure decay should always be used only as an assistance tool, this is because failure point is in electric, sensor, mechanical of machine, variation in material flexing, and temperature. volumetric is a much better way of detecting leakage or visual.

 

[Danlap]

Hello there,

Fully agree with above comments, however IMO indeed this might be also one of the way to test valves in the future. Some extra opinion:
-Due to pressure decay sensitivity:
As per Compositepro's comments, indeed it would take time to for the pressure to be stabilize. Valve consist of complex “pockets”
Before and after subject to be tested valve usually there is needle valves or regulator. This is also prone for small leakage, thus lead to hysteresis in reading.
Sensitivity towards gas used (normal air/Helium at certain percentage/Nitrogen of which all have different density at different temperature and pressure)
-Our standard is basically define leakage as either bubble, cc or noticeable pressure drop after 5-15 minutes (after pressure is stabilized). If pressure decay is the way forward meaning valve standard should be adjusted to with extra parameter for standardized gas used. I don’t think this would come soon.
This is like the transition from Barton chart to digital pressure record, which fully depending on inspector judgement. Finals assessment goes to third party / end user inspector which normatively refer to standard.
-Don’t forget that the idea of know leakage rate is for valve mechanic to have a sense whether for example only lapping required or major overhaul is needed. Is pressure decay capable of detecting leakage rate beyond Class B ISO 5208.
The idea of pressure testing is not just about to accept or reject valves.

I think bubble counter is still the best innovation so far that can be perform anywhere even with a straw, duct tape and bottle of water . And infrared bubble counter is one of the most helpful innovation afterwards.

Regards,
MR

https://nosuchvalve.com

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

http://www.eng-tips.com/faqs.cfm?&rat1=2&fid=376

 

[Strong]

Have you reviewed using ultrasound (passive detection at about 39-kHz) to detect bubbles from a valve leak with valve submerged in water tank or directly from escaping air/gas without tank? It can work quite well and with automation possibilities depending upon leak rate threshold requirements?.

Walt