10.4 Type III Leak Testing
Key Takeaways
- Leak-test a low-pressure system with dry nitrogen at no more than ~10 psig; the rupture disc bursts near 15 psig
- Add a small trace of refrigerant to the nitrogen so an electronic leak detector can find the leak — never pressurize with refrigerant alone
- A standing-vacuum test pulls the machine to a deep vacuum, isolates it, and watches for pressure rise over time (often 24 hours)
- In a standing-vacuum test, a steady pressure rise that then levels off usually signals moisture boiling off; a continuous rise signals an air leak
- Tube and water-box leaks are critical: the vacuum pulls cooling water INTO the refrigerant, forming acid and copper plating
Leak Testing Without Blowing the Rupture Disc
Leak testing a low-pressure chiller is fundamentally a balancing act: you need enough pressure for a detector to find the leak, but a low-pressure machine cannot tolerate much pressure at all. The governing limit is the rupture disc, which relieves at about 15 psig. Therefore the maximum leak-test pressure is roughly 10 psig, giving a safety margin so the disc does not blow during the test.
The Correct Pressurization Method
- Use dry nitrogen as the pressurizing gas — it is inert, moisture-free, and cheap.
- Cap the pressure at ~10 psig. Going higher risks bursting the 15 psig rupture disc.
- Add a small trace of refrigerant to the nitrogen so an electronic leak detector (or halide-style detector) can sense escaping refrigerant at the leak site. Pure nitrogen leaks silently — the trace gives the detector something to find.
- Never pressurize with refrigerant alone. It is wasteful, costly, and venting it is a Section 608 violation; nitrogen does the pressurizing work and only a trace of refrigerant is needed.
This nitrogen-plus-trace approach is sometimes called the standing-pressure test, and it is the standard positive-pressure method for finding leaks on a low-pressure machine.
The Standing-Vacuum Test
Because a low-pressure system spends its life in a vacuum, the most telling integrity check is the standing-vacuum test (also called a vacuum decay or rate-of-rise test). It checks the very thing that matters — whether the machine holds a vacuum:
- Evacuate the system to a deep vacuum with a vacuum pump.
- Isolate the pump (valve it off) so nothing is actively pulling.
- Watch the micron gauge over time — often a 24-hour standing test.
- Interpret the rise, which is the diagnostic art:
| Observation | Likely cause |
|---|---|
| Pressure holds steady at deep vacuum | Tight system, no leak, dry |
| Pressure rises, then levels off and stops | Moisture boiling off inside — needs more evacuation, not a leak hunt |
| Pressure rises continuously without leveling | Air leak — atmosphere is bleeding in |
The key exam nuance: a rise that plateaus points to trapped water vaporizing (continue dehydration), while a rise that never stops points to an actual air leak (find and seal it). This lets a technician separate a moisture problem from a leak problem without ever pressurizing the machine.
Finding Leaks and the Water-Side Danger
| Leak location | Detection method | Why it matters |
|---|---|---|
| Shaft seals (open-drive) | Electronic detector, bubble solution | Classic infiltration point on older machines |
| Flanges / access plates / gaskets | Electronic detector, bubble solution | Large sealing surfaces on the shells |
| Tube sheets / water boxes | Pressurize refrigerant side, inspect water side for bubbles | Tube/water-box failure lets water into the refrigerant |
| Rupture-disc housing | Visual + electronic detector | A weeping disc admits air on the vacuum side |
| Purge exhaust trend | Monitor purge frequency | High purge rate = active air leak |
Why Tube Failures Are So Serious
Unlike a high-pressure system that leaks refrigerant out, a low-pressure system in a vacuum pulls cooling water IN through a failed tube or water-box gasket. That water reacts with refrigerant and heat to form acid, copper plating, and sludge, which can destroy compressor bearings. Detection tools include an acid test of the oil, a moisture test of the refrigerant, and eddy-current testing, in which an electromagnetic probe is run through each tube to find thinning or cracks so failing tubes can be plugged.
Example: A technician needs to pinpoint a slow leak on an R-123 chiller. They charge the machine with dry nitrogen to 9 psig, add a small trace of R-123, and sweep an electronic detector across the shaft seal and access flanges. The detector alarms at a flange gasket. Critically, the technician kept the gauge under 10 psig the whole time — had they pushed to 15 psig, the rupture disc would have blown, dumping the test gas and forcing a disc replacement.
For the Exam: Pressurize a low-pressure system with DRY NITROGEN to no more than ~10 psig (rupture disc bursts at ~15 psig), adding a trace of refrigerant for electronic detection. A standing-vacuum test that rises then levels off = moisture; a continuous rise = an air leak. Tube/water-box failures pull water INTO the refrigerant under vacuum, forming acid and copper plating.
What is the maximum pressure to which a low-pressure chiller should be pressurized with dry nitrogen for leak testing?
Why is a trace of refrigerant added to the dry nitrogen when pressure-testing a low-pressure system for leaks?
During a 24-hour standing-vacuum test, the pressure rises for a while and then levels off and stops. What does this most likely indicate?
Put the steps of a standing-vacuum (rate-of-rise) test in the correct order.
Arrange the items in the correct order