18.3 Level Glasses, Float Switches, and High/Low Cutouts

Every liquid-containing vessel that can send liquid to a compressor or starve a pump needs level you can trust. On an industrial ammonia recirculator that usually means two independent methods: something you can see (reflex or transparent glass, armored column, bullseyes at known elevations) and something electrical (float switch, displacer, differential-pressure transmitter, or guided-wave device). IIAR 9’s existing-system evaluation language and IIAR 2 design practice both treat visual indicators as protected devices with internal check-type shutoffs on linear glasses so a broken tube does not become a hose stream of ammonia. Bullseye glasses are a listed exception to some of the linear-glass rules, but they still must be ammonia-rated and guarded from forklifts and ladders.

CIRO does not ask you to recite a catalog. It asks whether you know which trip does what, and whether you can smell a false level before you jumper a float.

Dual independent: glass plus float or transmitter

Operating level control keeps the recirculator in a working band so evaporators have liquid and compressors have vapor space. That may be a modulating high-side feed, a solenoid and hand expansion, or a high-side float dumping into the vessel. Safety level is not the same loop.

Independence means a single failed equalizing valve, a single frozen column, or a single transmitter zero-shift cannot both hide a high level and keep the feed valve happy. Good packages use:

• A visual column with frost shield or heated section so a human can confirm level on a round.
• An analog transmitter (or multiple probes) for the controller and trending on the plant screen.
• Dedicated high-level and low-level switches, often floats in a stilling well or a separate cage, wired as cutouts in the compressor and pump starters—not merely as HMI alarms.

If the only “high level” is a software setpoint on the same 4–20 mA that the makeup valve uses, a failed transmitter can overfill the drum while the screen still says 50 percent. That is not dual independent. The second method must fail differently: a float that opens a hardwired contact, plus a glass that does not need power.

High cutout: stop the compressor, alarm the room

High liquid level in a recirculator, accumulator, intercooler, or suction knockout means the vapor space is gone. The next slug is liquid carryover into wet suction and into the compressor. Screws can ingest some liquid better than recips, but neither is a pump. High-level cutout is compressor protection:

• Stop compressors that take suction from that vessel (or from a common suction that the vessel protects).
• Typically stop or inhibit liquid pumps as well so you are not still shoving liquid into a packed vessel through returning overfeed.
• Alarm to a monitored location. A silent trip that someone finds on day shift is not an alarm.

High level is also a process clue. Causes include a makeup valve failed open, a high-side float stuck open, defrost liquid returning faster than compressors can pull, a sudden load collapse, or oil occupying volume so the ammonia surface rides high. After a high-level trip, do not restart compressors on a “maybe it’s a bad float” theory until the glass, independent switch, and vessel pressure/temperature all agree there is vapor space.

High-level control (throttling feed) should sit below high-level cutout. The cutout is the last fence. If operators run with the high float jumpered because it “nuisance trips on defrost,” the plant has chosen compressor destruction as the backup plan.

Low cutout: stop the pump

Low liquid level destroys pumps first. As section 18.1 showed, NPSHa on a saturated drum is the liquid height. Low cutout:

• Stops recirculation and transfer pumps taking suction from that vessel.
• Should be above the elevation where NPSHa < NPSHr, not at “the pump suction flange is dry.”
• Alarms so makeup failure, a leak, or a rectifier stuck open is found before the entire charge is on the floor or in the still.

Low cutout does not usually stop compressors by itself. A low recirculator with vapor still returning can keep running in a starved-coil mode—bad for product, but the compressor is not the first casualty. Some plants interlock compressors at low-low after pumps are already down; know your ladder. CIRO cares that you do not reverse the trips: high is wet compression, low is dry pumping.

A low-level trip with a full-looking glass is a gift. It usually means the electrical device is telling the truth about ammonia inventory and the glass is lying because of oil, isolation, or frost. The wrong response is to jumper the float so the pump can “catch up.”

Glasses that lie: frost, oil, and isolation

Cold ammonia columns frost. Frost hides the meniscus. A completely white glass can be half full or liquid-full. Practices that keep glasses honest:

• Frost shields, heated glasses, or wiping a small window on the round (without damaging guards).
• Bullseyes at high, normal, and low elevations as a backup to a long column.
• Never using “the frost line on the vessel shell” as the only level method on a vessel that also holds oil—the shell frost line is crude and lags.

Oil in the glass or column is the other liar. Oil is darker, may not frost, and sits on the bottom of the column. You can read a “healthy” ammonia meniscus sitting on an oil pad and believe the drum is 40 percent full when the pumpable ammonia is already at the low cutout. Oil can also stick a float high or low. After oil draining, expect level devices to need time and sometimes a controlled blow-down of the column (procedure, PPE, check valves on the glass) to get ammonia back in the indicating legs.

Isolation valves on the glass or float cage that were closed for service and never reopened produce a dead indication. Internal check-type shutoffs on linear glasses are there so a break closes; they can also stick. A glass that never moves while the transmitter hunts is a closed valve until proven otherwise.

IIAR 9 checklist language is worth the operator’s time: linear indicators get physical protection along the whole tube; they are not to be installed where hydraulic shock is expected; armored flat glass is preferred over tubular glass. A sight glass on a hot-gas inlet or a defrost liquid hammer location is a missile, not an instrument.

Redundant instrumentation and what “independent” costs

Redundancy is not two numbers on one HMI fed from one DP transmitter. It is:

• Separate process connections where practical, so one freeze or one oil-clogged tap does not blind every device.
• Cutout switches that open to trip (fail safe on broken wire), with compressor and pump circuits that need the contact closed to run.
• Test procedures under IIAR 6 mechanical integrity: trip high and low on a schedule, prove the starter actually drops, restore valves, sign the log.
• After a float replacement, a PSSR mindset: equalizing valves open, wiring landed on the correct high versus low switch, and the glass in service before the jumper comes off.

If the recirculator is the compressor’s only knockout, high-level redundancy is as important as oil-pressure cutout on a recip. CIRO ladder items often combine oil-pressure, high level, and pump low level on the same elementary. Read which contact is in the compressor coil versus the pump coil. Mixing them up on the exam is the same mistake as mixing them up at 2 a.m.

When glass and transmitter disagree, stop adding liquid and stop assuming the pump is fine. Pull the safer trip (treat as high if either says high and compressors are at risk; treat as low if either says low and pumps are at risk). Then prove the column: valves, frost, oil, and a second visual. That habit is the difference between a level chapter you memorized and a plant you can still walk into.