3 Water Level Control
04.09.2016 Mk8 MM Expansion Features Installation and Set-Up Guide Page 29
3.2.3 Feed Water Control
Traditional feed water control manages water level in the boiler, steam flow and feed water flow
separately, whereas the Autoflame feed water control coordinates the water level, firing rate and
steam pressure simultaneously. This 3-element feed water control has been granted a worldwide
patent. The feed water going to the boiler can be controlled by either pump on/off only, pump on/off
and servomotor, or pump on/off and VSD.
The table below shows the terminals on the MM allocated for the feed water control parts.
Terminal Description
P- 0V supply to top blowdown and feed water servomotors
P+ +12V supply to top blowdown and feed water servomotors
TW Signal from feed water servomotor, indicating position
I+ Current output, 4-20mA to feed water VSD
V+ Voltage output, 0-10V to feed water VSD
IV Common terminals for I+ and V+00
BFW Feed water pump contactor
The table below shows the expansion options relating to feed water control.
Expansion Option Description
2 Feed water control element
10 Pump turn off point
11 Pump turn off percentage
12 Pump turn on percentage
13 Feed water control proportional band
14 Feed water control integral time
15 Feed water control derivative time
16 Feed water servo open angle
20 Burner operation on feed water control fault
The feed water pump can be set so that it turns off and on and at percentages above and below the
control point/high water, via the feed water pump contactor on terminal BFW.
If the feed water control is set with servomotor or VSD, then the 3-term PID control will operate. The
proportional band set in expansion option 13 is the percentage between the control point and 1
st
low
level where corrections are made to the servomotor/VSD to maintain the control point; outside of the p-
band, the servomotor/VSD will remain fully open. The integral time set in expansion option 14 will set
how fast the system responds to feed water changes; for a slower response increase time, and vice
versa for a faster response. The derivative time set in expansion option 165 is used when a quick
response is required but where overshoot is undesired; the water level should not rise too high above
the control point, so the derivative element will need to be enabled.
The feed water servomotor closed position is set by zeroing the potentiometer in Commissioning mode,
however the open position is set in expansion option 16.
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