Pa versus fa cv

Steam Turbine Controls Consulting

JOSEPH F. BYRD, JR., INC. PO Box 52 Dunn, NC 28335 PH: (910) 658-1291 [email protected]

The phrase Partial Arc (PA) is used to describe sequential control valve operation, after operational experience determined that improved stress control was needed during start up. The advantage of using a segmented nozzle was to increase the velocity through a segment to improve first stage efficiency at flows less than rated.

This advantage results in the disadvantages that were the driving factor to devolve a FA startup method.


JOSEPH F. BYRD, JR., INC.

At low loads, steam flow through one segment will result in unequal heating around the nozzle casing. Also, the high velocity through the one segment would be applied to the rotating bucket for 90 degrees, and then the bucket would be relaxed for 270 degrees rotation, thus setting up severe cyclic stresses on the buckets and wheel. Both generating stresses that can crack casings and break buckets.



Full Arc (FA), using Main Stop Valve By-Pass control was developed for initial loading. Steam is throttled at the MSVBP to control flow, the control valves are full open, thus, admitting steam to all nozzle segments. The advantages of FA-MSVBP are;

1) the equal heating of the nozzle ring segments, 2) The flow required for initial loading is distributed over a larger area to reduce the velocity (and

erosion) through the nozzle partitions. 3) the flow through the full nozzle segment provides a continues force to the rotating buckets,



However, there are disadvantages. There is severe erosion to the MSVBP due to the high throttle velocities and valve design. Recall, the SV was not original intended to control flow, the internal by pass valve was added to control casing and rotor stresses during startup. The SVBP was sized to only allow loading up to a maximum of 35% flow, thus flow control has to be transferred to the CVs for increased loading. Some turbines are designed not to utilize sequential CV operation. When all CVs open together this is can be refer to as One Admission or FA. These units may not have segmented nozzles. A more sophisticated control method was developed to vary the CV cams between PA and FA Thus the disadvantages of the SVBP could be eliminated with the FA CV mode used for start up and the mode changed to PA for higher load operations.

An advantage of selective admission mode is the first stage metal temperature change from low to high load is less in FA than PA, thus a more rapid load change can be made in FA with the same ramp rate.

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Pa versus fa cv

Engineering

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