10.4 Type III Leak Testing

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, rated at the chiller's maximum working pressure (Trane, for example, rates the disc on its R-123 CenTraVac chillers at 15 psig ±2 psig). Stay within the manufacturer's leak-test limit, below that rating, so the disc does not blow during the test.

The Correct Pressurization Method

• First choice: controlled hot water or a built-in heating/pressurization device (such as Prevac), which raises the refrigerant's own pressure — this is EPA's order of preference.
• Second choice: dry nitrogen through a regulator with a relief valve — inert and moisture-free. Never use air or oxygen.
• Stay below the rupture-disc rating and within the manufacturer's limit.
• If nitrogen is used, 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.

A nitrogen-plus-trace test is a positive-pressure method; on a low-pressure machine EPA ranks it after the hot-water or built-in heating methods.

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:

1. Evacuate the system to a deep vacuum with a vacuum pump.
2. Isolate the pump (valve it off) so nothing is actively pulling.
3. Watch the micron gauge over time — often a 24-hour standing test.
4. Interpret the rise, which is the diagnostic art:

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

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 first raise the machine's pressure with its built-in heating/pressurization device, then sweep an electronic detector across the shaft seal and access flanges. The detector alarms at a flange gasket. Throughout, the technician keeps the pressure within the manufacturer's limit, well below the rupture-disc rating — pushing past it would blow the disc and vent the charge.

For the Exam: Raise a low-pressure system's pressure for a leak test with CONTROLLED HOT WATER or a BUILT-IN HEATING/PRESSURIZATION DEVICE first, then DRY NITROGEN (with a trace of refrigerant for electronic detection), always below the manufacturer's rupture-disc rating. 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.