York YST Operation Manual page 75

FORM 160.67-O1
ISSUE DATE: 4/1/2015
MSSp2 – Capacity ratchet Mode prv
reselection
Screen: Capacity Controls
Range: 0-5 %
This is only used if MSSP7-Speed Control Mode is set
at Variable and should rarely need adjustment. While
the Capacity Ratchet Selector is in the PRV mode, the
logic waits until either the capacity control or anti-
surge signal has changed the PRV control signal more
than this value before it re-evaluates whether the mode
control needs to be changed to Speed (if loading)or Hot
Gas (if unloading). However, if the PRV control signal
is at 100% when loading or at MV% when unloading,
the mode is changed immediately.
MSSp3 – Capacity ratchet Mode Spd
reselection
Screen: Capacity Controls
Range: 0-5 %
This is only used if MSSP7-Speed Control Mode is set
at Variable and should rarely need adjustment. While
the Capacity Ratchet Selector is in the Speed mode,
the logic waits until either the capacity control or an-
ti-surge signal has changed the Speed Setpoint signal
more than this value before it re-evaluates whether the
mode control needs to be changed to PRV. However, if
the Speed Setpoint signal is at MS% when unloading,
the mode is changed immediately to PRV.
MSSp7 – Speed Control Mode
Screen: Capacity Controls
Range: Variable/Fixed
This determines if the capacity controls will reduce the
speed setpoint at part load conditions. For maximum
efficiency, the Variable speed mode should be used.
The Fixed speed mode would likely only be used in
plants where the condensing water is always at a high
temperature so that speed reduction would not be pos-
sible due to the high system pressure differential.
MSSp8 – power limiting Mode
Screen: Capacity Controls
Range: Horsepower/Position/None Default: Horsepower
JOHNSON CONTROLS
Default: 3 %
Default: 3 %
Default: Variable
SECTION 2 - OPTIVIEW CONTROL CENTER INTRODUCTION
This override controller is used to unload the chiller to
prevent the turbine from producing enough horsepow-
er to overload the compressor bearings. It would usu-
ally only be activated during system pulldown or other
off design (high load) conditions. The Position mode is
only used when the Turbine First Stage Steam Pressure
Transmitter is not present. In this case, the only avail-
able means to prevent the turbine from producing too
much power is to monitor the Governor control signal
at normal full load conditions and then enter this per-
cent as the setpoint for the position-limiting controller.
rrSp2 – hot gas valve ramp rate
Screen: Capacity Controls
Range: 0.1-10 %/Sec
This is used to provide a controlled loading rate after
the chiller has been ramped up to the minimum rated
speed and the turbine stabilization delay has elapsed.
The default value matches the approximate stroke rate
of the hot gas valve actuator since the chiller will ca-
pacity will only increase as fast as the valve opens. In
the variable speed mode, this rate may be increased to
initiate a Capacity Ratchet Mode change to PRV before
the Hot Gas Valve has actually closed fully. This will
allow the vanes to begin opening and pull the system
down to design conditions sooner.
rrSp8 – prv ramp rate
Screen: Capacity Controls
Range: 0.1-10 %/Sec
This is used to provide a controlled loading rate after
the chiller has been ramped up to the minimum rated
speed and the turbine stabilization delay has elapsed.
The default value matches the approximate stroke rate
of the PRV actuator since the chiller capacity will only
increase as fast as the vanes open. In the variable speed
mode, this rate may be increased to initiate a Capacity
Ratchet Mode change to Speed before the vanes have
actually opened fully. This will allow the speed to be-
gin increasing and pull the system down to design con-
ditions sooner.
Default: 5.0 %/Sec
Default: 5.0 %/Sec
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