Low-Side Float Level Alarm After Load Change
You operate a single-stage ammonia plant with a flooded evaporator fed by a low-side float valve. At 06:00 the plant runs at 40% load with 4 of 10 evaporator fans running and the suction pressure steady at 2.1 barg. At 06:30 all 10 fans start and three new blast-freeze rooms come online, raising the evaporator load sharply. Within 20 minutes the suction pressure falls to 1.4 barg, the low-side float alarm chatters, the accumulator level drops below the low-level sight glass, and the compressor discharge temperature rises from 95 C to 130 C. The liquid pump is running and its discharge pressure gauge reads 3.0 barg. No alarms on the high-side receiver level. The plant supervisor wants to open the liquid make-up valve to the accumulator by hand and leave it open.
1. Diagnose the most likely cause of the falling accumulator level and rising discharge temperature, and state whether the supervisor's manual make-up plan is safe.
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The rapid load increase pulled the suction pressure down from 2.1 to 1.4 barg faster than the float valve could replenish the accumulator, so the evaporator is being starved and the compressor is drawing superheated vapour. Rising discharge temperature (95 to 130 C) supports a starved, high-superheat condition rather than a flooded one.
Holding the make-up valve open by hand is not safe as a standing practice. A float valve regulated by level is the only thing preventing liquid carryover to the compressor; a hand-held open valve can overfill the accumulator once the load drops again, and the operator may not be watching. The correct response is to reduce load or bring fans on in stages, confirm the float valve is actually passing liquid (strainer, ice, or debris on the seat), and only then make up liquid under controlled observation with high-level protection verified.
2. The pump discharge reads 3.0 barg while suction is 1.4 barg. What does this 1.6 bar differential tell you, and what should you check before blaming the float valve?
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A pump differential of 1.6 bar is plausible for a running liquid pump but is on the low side if the pump is meant to overcome the evaporator circuit and riser. A low differential with a running pump suggests cavitation, vapour in the pump, a partly closed valve, a fouled strainer, or a worn impeller.
Before adjusting the float valve, verify pump suction is solid liquid (accumulator level above the pump inlet, no vortex), check the pump strainer differential, confirm the pump discharge valve is fully open, and listen or look for cavitation. If the pump is cavitating, the evaporator is being underfed for a pump reason, not a float reason, and opening the make-up valve will just push liquid through to the compressor. Fix the pump feed first.