The limiter operates in two situations, under 95% of maximum speed where the compressor
can run more efficiently in the unloaded plug valve mode and at 100% of maximum speed to
protect the compressor. Speed between 95% and 100% is the speed hysteresis.
If speed < 95%:
If Vi < 0.80 * fixed Vi ratio: unload plug valve
If Vi > 0.85 * fixed Vi ratio: allow loading plug valve
This part of the Vi limiter can be enabled or disabled in the menu Compressor block.
See subsection 6.4.3. Compressor block 5: VI comp. plug valve.
If speed = 100%:
If Vi < 0.65 * fixed Vi ratio: unload plug valve
If Vi > 0.70 * fixed Vi ratio: allow loading plug valve
When the plug valve is unloaded a Low Vi alarm is given. This part of the Vi limiter cannot be
disabled.
Compressor control and surveillance
Engineering manual - Unisab III 1.10
001930 en 2014.09
93/319
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