In this section we will look at the control system
controllers are systems that accept a measured value as an input compare it to
some desired value and then output a control signal the control signal is
connected to a valve or switching mechanism which will move the measured
value toward the desired level for instance imagine a temperature
controller with a heater regulated by controller input to the controller would
be a thermal detector like an RTD or thermocouple suppose the controller
calls for no heat ie zero control variable output with no heat added the
system would stay at a certain temperature call it t1 on the other hand
if the controller calls for full heat the system will eventually stabilize at
a higher temperature call it t4 for every intermediate control output there
is some stable temperature connect temperature t1 to t4 with a line that
illustrates the way the system responds to heat being added usually we will not
know what the shape of this curve is suppose we want to control the
temperature to a setpoint indicated by the horizontal green line labeled
setpoint we could design our controller so it would call for full heat if the
temperature is well below the setpoint no heat if it is well above the setpoint
and a proportionate amount in between as indicated by the dark blue line the
darkened bands shown is the proportional band suppose then we position the
proportional band evenly about the setpoint temperature since before
starting we don't know what the shape of the black line will be the system will
eventually stabilize at temperature t2 since that is the point at which the
system responds line intersects the controller output since t2 is above our
setpoint there is an offset the difference between temperatures t2
and the set point proportional controllers are simple but they are
likely to result in an offset proportional integral control is the
same as proportional control except an integral adjustment gradually reduces
the offset to zero also since the viscosity of a liquid is
decreased by the addition of heat or a solvent the system response line slopes
down with increasing control output since it is assumed that a viscosity P I
controller health puts is used to either heat the liquid or add solvent the
output has an inverse relationship between the control output and the
measured value ie the blue control output line slopes up to the right
this illustration shows an initial setting with P I viscosity control the
black line represents the system response to either adding heat or
solvent the graph illustrates the system in its initial State with an offset
once the system enters the band of proportional control the PI controller
periodically tests to determine whether the actual viscosity is above or below
the setpoint if it is below the proportional band is very slowly
adjusted upward at a rate that is slow compared to the response time of the
system eventually the offset is reduced to zero at which point the system
continues to dither above and below the setpoint by a small amount
if the proportional band width is reduced the response of the controller
will be faster and in general the offset will be smaller if the proportional band
is set to small the system will oscillate as a rule of thumb said the
proportional band as small as possible without inducing oscillations
to visualize the oscillation process imagine that the proportional band is
set to zero with temperature based control if the temperature is below the
setpoint the heat will be on full full heat application will continue until the
thermal detector reaches the setpoint temperature at that point the heat
application will go full off but by then the system will be overheated and will
overshoot the setpoint when the temperature detector cools to the
setpoint the heat will switch on full but by then
much of the system will have cooled below the setpoint the result is
oscillation proportional integral derivative control is similar to PI
control except a derivative action increases the reaction time of the
control system the smaller the derivative action the faster the control
system will be within certain limits the viscosity controller positions the steam
valve of the fuel oil heater directly or indirectly by adjusting the setpoint of
a separate slave controller the slave controller is of the PID type the
feedback signal to the slave controller is the mean tuber metal temperature of
the fuel oil heaters
the viscosity sensor can be mounted anyplace between the fuel oil heater and
Emme inlet by specification of nominal transport time delay note that the
actual time delay will be adjusted according to fuel oil flow the delay
will increase rapidly at low fuel oil flow and may then cause oscillations at
low load it may prove necessary to stabilize the control by reducing the
steam supply to the fuel oil heaters
here we have a schematic illustration that shows how the visco therm automatic
viscosity controller is built into the fuel oil system a differential pressure
transmitter is used which linearly converts the differential oil pressure
to a control air pressure varying between three and 15 psi this controller
air pressure which varies linearly with the viscosity measured flows to a
central adjustment or control panel this control system consists of the
following main parts control or adjustment panel viscosity reader
shutoff valve on the visco therm visco therm sensor electric motor fuel line
two fuel pump shutoff valve in fuel bypass line fine filter fuel oil pre
heater differential pressure transmitter steam control valve shutoff valves valve
for manual control compressed air supply line combined air filter and pressure
reducer steam supply fuel oil inlet outlet for condensate combined air
filter and pressure reducer the housing contains a stainless steel gear pump to
which a stainless steel capillary tube is connected the gear pump is driven by
an electric motor and continuously draws a constant amount of oil from the
chamber and forces it through the capillary tube as the flow through the
capillary tube is laminar the differential pressure across the
capillary tube is directly proportional to the viscosity of the oil with this
viscosity sensor the differential pressure is transferred to an indicator
and the viscosity controller the indicator scale is graded as required in
Redwood seconds or centistokes a temperature correction scale is provided
under the viscosity scale to make the adjustment of the supply of steam
thermal oil or electrical oil preheater easier possible pressure pulsations in
the fuel line can be dampened by means of the hand operated valves mount
under the indicator
the viscosity sensor consists furthermore of a housing in which is
mounted a capillary tube assembly as the measuring element together with a gear
pump an electric motor with reduction gear drives the pump in such a way that
the volume of oil flowing through the capillary tubes is constant as the flow
through the capillary assembly is laminar the pressure differential across
the capillary is directly proportional to the viscosity of the fuel oil
each capillary tube assembly consists of a glass measuring capillary in a
resilient stainless steel housing and a stainless steel damping capillary to
compensate for undue pressure waves in the fuel lines the use of a glass
capillary reduces the possibility of clogging by cold heavy fuel oil a
thermometer displays the actual fuel temperature to pressure taps are
provided to connect the differential pressure developed across the
capillaries to the differential pressure transmitter the plus tap is connected to
the measuring capillary and the minus tap is connected to the damping
capillary a magnetic coupling between viscosity sensor motor and gear pump
provides a leak free power transmission and prevents overloading the electric
motor the viscosity control station receives a zero point two to one point
zero bar or three to 15 psi signal from the differential pressure transmitter
and compares this signal with the desired setpoint viscosity value any
deviation between the actual viscosity and the desired viscosity results in a
corrective air signal to the control valve to control the amount of steam
thermal oil or electric power to the fuel heater