At a 130t/h CFB cogeneration plant, the operators proposed adding UPS power for motor-operated valves and actuators, arguing that after a total plant blackout these devices cannot be electrically operated. Should the request be granted? Forty replies on the engineers' forum settled the matter — in almost all cases, no. The correct defense is dual power supplies plus fail-safe design plus a manual-operation plan.
Why Not (the Mainstream View)
- Cost and upkeep are prohibitive: one engineer's numbers — a small UPS in the water-treatment area carrying less than 1kW lasts only 20 minutes, costs tens of thousands, and needs battery replacement every half year; another plant budgeted two 80kVA UPS units just for actuators and management killed it
- After a plant blackout, operating MOVs barely matters: with station service lost the unit is already tripping down; "the plant is dead — what are you still driving valves for?" is the bluntest and truest reply. What must be protected are the DCS, the emergency sections and key DC systems — not ordinary MOVs
- Absurd at scale: if MOVs get UPS, why not motor protection devices too? Where does it end; one reply quips it is "fitting ABS to a tractor"
- Manual is the answer: MOVs and actuators all have handwheels; after a blackout you crank them by hand
Smarter Alternatives
- Dual supplies from different station-service sections — the baseline design, far more effective than UPS
- Emergency/secure-section supply: important MOVs belong on the secure bus; solve UPS centrally upstream, not per-actuator
- DC or inverter supplies for key circuits (standard practice on the DCS side); diesel generators are the standard answer for blackouts
- Actuator self-capability: intelligent actuators with absolute encoders keep valve position without power (battery serves only the display) — that solves "position memory", not "powered operation after blackout"
- Case-by-case for the few valves that genuinely need post-blackout operation
Buying UPS units for every MOV is expensive medicine for the wrong problem. The layered defense: dual supplies → secure bus → position-preserving/three-fail protections (pneumatic) → manual handwheel. See also our article on where valves settle after loss of power or air.
