Valve Sequencing & Overlap
Turning Load Reference Into Multiple Valve Positions
Modules 3.1 and 3.2 covered how the control system calculates an overall target and moves an actuator to hit it. On a nozzle-governed turbine with multiple governor valves (Module 1.1), that overall target has to be translated into individual position commands for each separate valve — and the order and pattern in which those valves open is exactly what this module covers.
The Sequential Opening Pattern
A sequential valve opening curve defines the relationship between load reference and each individual valve's position. As load reference rises from zero, the first valve opens from closed to fully open; only once it's essentially fully open does the second valve begin its own opening sequence, and so on through however many governor valves the turbine has. At any given load point, this pattern keeps most valves either fully open or fully closed, with typically just one valve "in transition" at a time.
This sequencing is the direct control-system implementation of the nozzle governing efficiency advantage introduced in Module 1.1: steam passing through a fully open valve experiences essentially no throttling loss, while a valve sitting at a partial position — a "valve point," per Module 1.4 — does introduce real throttling loss. Sequential opening minimizes how much time the system spends with any valve sitting at a lossy partial position.
The whole point of sequential valve opening is minimizing time spent at valve points. Fully open or fully closed valves are efficient; partially open valves are where throttling loss (Module 1.4) actually occurs.
Valve Overlap — Smoothing the Handoff
Valve overlap is a deliberate design refinement: rather than one valve reaching 100% open and only then having the next valve begin to open, the next valve is designed to begin opening slightly before the previous valve completes its travel, creating a small transition zone. Without this overlap, load response could momentarily stall right at the exact handoff point between two valves — overlap ensures continuous, incremental control authority is available across the entire load range, including through each transition.
Overlap is a narrow transition zone, not extended simultaneous modulation of two valves. Most of the load range still has exactly one valve actively in motion at any moment — overlap only matters in the brief handoff window between valves.
Partial Arc vs. Full Arc Admission
At low-to-moderate loads with only some governor valves open, steam enters the turbine through only part of the nozzle ring's full circumference — partial arc admission. This is exactly what makes nozzle governing's efficiency benefit real: steam flowing through the open valves' corresponding nozzle arc experiences essentially no throttling, even though the rest of the nozzle ring isn't receiving flow at that moment.
At or near full load, all governor valves are open and steam enters through the entire nozzle ring circumference — full arc admission. Beyond simply representing maximum flow, full arc admission at high load also tends to improve stage efficiency and reduce certain vibration-inducing forces on the rotor compared to partial arc operation, since flow (and the resulting force on the rotor) is distributed evenly around the full circumference rather than concentrated in one section.