In steam turbines, there are two main modes of admission controls. The two modes of operation are known as partial-arc and full-arc, each referring to the way the steam flow enters the steam turbine to produces work.
Partial-arc operation results from installing an arc rather that an array of nozzles. The nozzle block can be partitioned to support partial-arc operation. For example, the block of nozzles is divided in 4 section at the nozzle block, so depending on the requirements, ¼ of the array of nozzles could be engaged whilst the remaining sections are shut down. The control system could potentially support shuffling through the different section of the nozzle block, only limited by the robustness of the control system operating the isolation and governor valves. 
One of the most recited problems in partial-arc operations is that it subjects the steam turbine component to high thermal stresses. However, if planned correctly with the relevant analysis and information about the turbine thermal stresses limitations and bands of operation, this mode of operation results in improvement of heat rate, specifically operating at partial loads.
Thermal stresses are caused by the uneven flow entering the first stage of the steam turbine, heating up the metal. The area going away from the entry point reduces in temperature as it spins, circling back to the steam turbine stream and heating up again.
Figure 1: Steam Turbines Contour plots of instantaneous static pressure at walls of inlet pipes, nozzle boxes, nozzle blades and rotor blades. 
It is savvy to invest in a reliable system which can adequately switch between the full and partial-arc operation back and forth at any load preferably minimizing subjecting the unit to thermal stresses. The unit must not be operated in partial-arc if it was not designed specifically for this mode as it can cause damage to the blading resulting in high cost repairs.  
It is difficult to establish the best mode of operation without considering any operating regimes and external factor. However, there is a best-fit mode depending on the operating requirements for any unit. The partial-arc mode is not available during start-up because it does not provide a uniform temperature profile to the steam turbine, something that it is very important before reaching synchronization. On the other hand, whilst dropping load, the full-arc mode does not achieve as much drop in temperature at the first stage as the partial-arc mode, which saves time when the unit enters maintenance, for example. 
In summary, having both modes of operation and a control system that efficiently switches between them provide a wide range of operation management specifically during start-up and shut-down, resulting in higher efficiencies and time reduction.
|||R. Couchman, K. Robbins and P. Schofield, “GE Steam Turbine Design Philosophy and Technology Programs,” GE Company, Schenectady, NY, 1991.|
|||Polish Academy of Sciences, “The Szewalski Institute of Fluid-Flow Machinery Polish Academy of Sciences,” 04 06 2017. [Online]. Available: https://www.imp.gda.pl/en/research-centres/c5-centre-of-heat-and-power-engineering/o5z1/research/partial-admission-turbines/. [Accessed 29 01 2018].|
|||C. Robert L and J. Lloyd H, “Full Arc/Partial Arc Admission Using Control Valves”. United States of America Patent US3403891, 1 October 1968.|
|||Global Asset Protection Services LLC, “Steam Turbine Principles,” GAPS Guidelines, 2015.|
|||Florida Center for Instructional Technology, “Educational Technology Clearinghouse,” University of South Florida, 2004. [Online]. Available: https://etc.usf.edu/clipart/77700/77750/77750_dl_trbine.htm. [Accessed 2018].|