High Liquid Level Cutout Testing

In industrial ammonia refrigeration, high liquid level protection is one of those safety functions that cannot be treated as a simple checkbox item. If a vessel reaches a high enough liquid level to allow liquid refrigerant carryover to compressors, the associated compressors must shut down or be prevented from operating. That safety function is critical for protecting equipment and reducing the risk of serious mechanical damage.

IIAR 6-2025 Section 6.1.2 makes that expectation clear: The High Liquid Level cutout devices and shutdown circuits shall be tested such that if a high liquid level in a vessel could result in the carryover of liquid refrigerant to compressors, those associated compressors will shut down or be prevented from operating.

The Goal of Testing

The purpose of the test is not just to prove that a sensing device can move, click, or change state. The goal is to verify that the actual high liquid level shutdown function works as intended. In other words, when the device is activated, the connected shutdown circuit must cause the associated compressors to stop running or prevent them from starting. That is what makes it a functional test. Many vessels incorporate mechanical float switches for high liquid level protection, but mechanical float switches are not the only option. For example, level probes or level sensors are also acceptable methods for providing a control that shuts down equipment to prevent damage from liquid carryover.

Acceptable Testing Methods

IIAR 6-2025 Section *6.1.2.1 states: High Liquid Level cutout devices and shutdown circuits shall be functionally tested to activate the high liquid level shutdown while taking care to prevent vessel overfilling or causing carryover of liquid refrigerant to associated compressors. So far this doesn't provide any specifics on how to perform these functional tests and since *6.1.2.1 has an asterisk next to it, this means that more details are found in the appendix of IIAR 6. Keep in mind that the appendix section only provides recommendation information.

IIAR 6-2025 *6.1.2.1 section gives examples of methods that may be used to safely functionally test these devices. These include:

1. Raising the liquid level above the cutout point

One method is to raise the liquid level in the column above the high liquid level cutout device by metering in liquid refrigerant of similar temperature and pressure to the liquid in the level column.

This approach can simulate an actual high-level condition, but it must be done carefully. The facility should evaluate the system design, operating conditions, and the potential consequences before using this method.

2. Activating the float switch electrically while still connected

Another acceptable method is to use a magnet or metal rod to activate the float switch electrical circuit while the tested device’s wiring remains connected to the compressor shutdown circuit.

This is an important detail. The switch must remain tied into the actual shutdown logic, so the test confirms that the entire chain works as intended, not just the sensing element by itself.

What Does Not Count as a Valid Functional Test

A major takeaway from the appendix language is that removing the float switch or single-point level sensor from service and bench testing it by itself does not constitute a valid functional test if its wiring is disconnected from the compressor control circuit.

A valid functional test must demonstrate more than the device alone. It should also confirm that the related wiring, relays, fuses, control logic, and shutdown circuit remain intact and will actually shut down the compressors when needed. This distinction is especially important in real-world facilities. It is possible for a sensing device to appear to operate normally during a disconnected bench test while an issue still exists somewhere else in the shutdown chain. A failed relay, broken wire, blown fuse, or logic issue could prevent the compressors from shutting down even though the float itself appears fine. That is why a true functional test must verify the full protective path.

Safety Considerations During Testing

The appendix also recognizes that raising liquid level in a vessel or level column to test the integrity of a mechanical float may introduce additional hazards depending on system and equipment design. Because of that, each facility should evaluate its own risks and determine the safest method for conducting the test. The key is to minimize the risk of introducing liquid refrigerant to a compressor while still verifying the shutdown function.

Final Takeaway

A proper functional test must confirm the shutdown action while also being performed in a safe manner that does not create the very hazard the device is intended to prevent. Nothing in IIAR requires that the liquid level be raised in the vessel or float column for this test. Can you do this to ensure your float functions properly? Yes, that is perfectly acceptable. However, when you close the bottom inlet valve to the vessel level column for this test, you have effectively removed the very safety function you are trying to verify. There are real-world examples of personnel closing the liquid inlet to a float column and then forgetting to reopen the valve, resulting in liquid carryover into the compressor suction after the test.

So, regardless of how you choose to perform the functional portion of this test, always keep potential hazards in mind and ensure you have controls in place to manage them.

Stay tuned to the MySafety blogs. Reach out to us if you have additional questions or concerns. Our team will be pleased to assist you.

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High Liquid Level Cutout Testing