6.1 Strength & Leak Pressure Testing using OFN

Oxygen-Free Nitrogen (OFN) is the industry standard and legally mandated gas for pressure testing refrigeration and air conditioning systems. Unlike compressed air, which contains moisture and oxygen, OFN is completely dry and chemically inert. If compressed air were used, the oxygen could form an explosive mixture with compressor oil, and the moisture would contaminate the system, leading to acid formation, copper plating, and eventual compressor failure. OFN prevents all these issues. Because it is stored in cylinders at extremely high pressures (often up to 200 bar or 230 bar), safety is paramount when handling and regulating OFN.

Strength Testing vs. Leak Testing

There are two distinct types of pressure tests performed on a refrigeration circuit, usually conducted in sequence: the strength test and the leak test.

Strength Test The strength test is designed to prove the structural integrity of the system and its components. It ensures that the pipework, joints, and vessels can withstand the maximum pressures they might experience during abnormal operating conditions. This test is typically performed at a pressure higher than the system's normal operating pressure. According to standard BS EN 378, the strength test pressure is calculated based on the Maximum Allowable Pressure (PS) of the system. The strength test must be performed first to ensure the system doesn't rupture.

Leak Test Once the strength test is successfully completed, the pressure is reduced to the leak test pressure. The leak test is intended to prove that the system is entirely gas-tight and will not leak refrigerant into the atmosphere. This test is held for a longer duration, and the engineer meticulously checks every joint, flare, and braze for any signs of pressure drop or physical leaks using a leak detection spray.

Equipment and Safety Devices

When testing with OFN, the extreme pressures involved mean that specialized, well-maintained equipment is legally required.

Never use an oxygen regulator for nitrogen, and never connect an OFN cylinder directly to a system without a regulator and a pressure relief valve. Doing so could cause the system to over-pressurize and explode, resulting in severe injury or death.

Test Pressure Limits and Procedures

The Maximum Allowable Pressure (PS) is a critical value determined by the manufacturer, typically found on the system's data plate. It represents the maximum pressure for which the equipment is designed. Systems often have different PS values for the high-pressure side and the low-pressure side.

1. Preparation: Ensure the system is empty of refrigerant. Connect the OFN cylinder to the system via the regulator, PRV, and manifold gauges.
2. Calculate Pressures: Determine the strength test pressure and leak test pressure based on the PS.
3. Step-by-Step Pressurization: Do not open the regulator fully all at once. Increase the pressure gradually in stages (e.g., 3 bar, then 15 bar, then final pressure). This allows you to check for major leaks at low pressures, saving nitrogen and reducing risks.
4. Strength Test: Bring the system up to the strength test pressure. Hold for a short duration (e.g., 15 minutes) to ensure structural integrity. Stay clear of the system during this phase.
5. Leak Test: Reduce the pressure to the leak test pressure. Apply a proprietary leak detection spray (bubble spray) to all joints, connections, and suspected areas. Leave the system on test for a specified period (often 24 hours for large systems) to monitor for pressure drops.

Temperature-Pressure Corrections

A crucial aspect of a prolonged leak test is understanding the relationship between temperature and pressure, as dictated by Gay-Lussac's Law. If you leave a system on a pressure test overnight, the ambient temperature will likely change. If the temperature drops, the pressure of the nitrogen inside the system will also drop, even if there is no leak. Conversely, if the temperature rises, the pressure will rise.

To accurately determine if a pressure drop is due to a leak or simply a temperature decrease, you must correct the final pressure reading. The formula relies on absolute pressure (gauge pressure + 1 bar) and absolute temperature (Kelvin, which is °C + 273).

For example, if you pressurize a system to 10 bar (gauge) at 20°C, and return the next morning to find the gauge reads 9.6 bar at 10°C, you must calculate the expected pressure. If the expected pressure based on the temperature drop matches the actual gauge reading, the system is leak-tight. If the gauge reading is significantly lower than the expected pressure, there is a leak that must be found and repaired.