Siemens Synco 700 Series - Page 68

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Building Technologies Boiler Sequence Controller RMK770 CE1P3132en
HVAC Products 5 General functions, fundamentals 23.04.2009
P-band Xp = 2 × Tu / Tg × Δx / Δy × 100% ˜ 2 × Tu / Tg × Ksmax
Integral action time Tn = 3 × Tu
Change of valve position Δy = 40%
Change of flow temperature
Δx = 18 K
Tu = 6 s
Tg = 18 s
P-band Xp = 2 × 6 s / 18 s × 18 K / 40% × 100% = 30 K
Integral action time Tn = 3 × 6 s = 18 s
The maximum system gain Ksmax can be estimated from the differential of maximum
flow temperature upstream of the mixing valve and the minimum return temperature, for
example. The value of Ksmax may have to be increased to give consideration to a
nonlinear valve characteristic. Tvmax = 80 °C and Trmin = 20 °C => Ksmax = 60 K.
To obtain a reliable step response, it is important during the measurement to keep the
temperature upstream of the valve and the return temperature (mixing) as constant as
possible.
During the measurement, the boiler and return temperatures should reflect winter
conditions at relatively low outside temperatures.
On actual plant, it is not always possible to get a reliable step response.
Without a step response, or in the case of unsatisfactory control action after entry of the
calculated parameters, the on/off pulses after a setpoint step give hints for setting the
parameters.
A distinction is to be made between 2 cases:
A
B
3131D12
Setpoint
Open pulse
Close pulse
Open pulse
Close pulse
Flow
temp.
A The control pulses are too long:
Measure the effective valve running time (0…100% stroke) and enter. If the pulses
are still too long, increase P-band Xp
B Several successive relatively short on or off pulses: Increase integral action time Tn
A
B
3131D13
Open pulse
Close pulse
Open pulse
Close pulse
Setpoint
Flow
temp.
A Difference between the first pulse and the following pulses is small:
Measure the effective actuator running time (0…100% stroke) and enter it. If the
control behavior does not considerably improve: Increase P-band Xp
B Long starting pulse followed by many short pulses: Decrease integral action time Tn
Setting rules
Example
Maximum system
gain Ksmax
Note
Setting without step
response
The flow temperature
fluctuates about the
setpoint
Flow temperature
approaches the
setpoint only slowly