Training on GET-InstrumentationApril 2014
Document Title:*Type as the subject of emailCode: PI
Brief Introduction to Distillation Control:-
All distillation columns have to be carefully operated in order
to achieve the required production rates and product quality. The 3
main objectives of column control can be stated as:
To set stable conditions for column operation.
To regulate conditions in the column so that the product(s)
always meet the required specifications.
To achieve the above objective most efficiently, e.g. by
maximising product yield, minimising energy consumption, etc.
Process variables like temperatures, pressures, flow rates,
levels and compositions must be monitored and controlled in all
distillation processes. These process variables within a
distillation system affect one another, whereby a change in one
process variable will result in changes in other process variables.
Thus, in column control one should be looking at the whole column
and not focusing on any particular sections only.
Each column has a control system that consists of several
control loops. The loops adjust process variables as needed to
compensate for changes due to disturbances during plant
operation.
Each of the process variables has its own control loop, which
typically consists of a sensor and transmitter, controller and
control valve. See the Figure below. Each control loop keeps track
of the associated process variable. An adjustment is made to a
process variable by varying the opening of its control valve. The
stream flow rate is therefore adjusted and a desirable variable is
being controlled.Block diagram for control loop:-
The sensor measures the process variable from the plant (i.e.
plant data) and the transmitter sends the information to the DCS
(Distributed Control System) controller located in the control
room.The controller checks if the process variable agrees with the
set point. If not, it will send corrective signal to the control
valve that will make adjustment in the plant so as to match the
process variable to the set point. This goes on continuously,
essentially in a loop - hence the term "control loop".Distillation
Control Philosophy:-Some of the general guidelines are noted
below:Column pressure normally controlled at a constant value.Feed
flow rate often set by the level controller on a preceding
column.Feed flow rate is independently controlled if fed from
storage tank or surge tank.Feed temperature controlled by a feed
preheater. Prior to preheater, feed may be heated by bottom product
via feed/bottom exchanger.Top temperature usually controlled by
varying the reflux.Bottom temperature controlled by varying the
steam to reboiler.Differential pressure control used in packed
columns to monitor packing condition, also used in tray columns to
indicate foaming.The compositions controlled by regulating the
reflux flow and boiled-up (reboiler vapour).Pressure is often
considered the prime distillation control variable, as it affects
temperature, condensation, vaporisation, compositions, volatilities
and almost any process that takes place inside the column. Column
pressure control is frequently integrated with the condenser
control system.Reboiler and condensers are integral part of a
distillation system. They regulate the energy inflow and outflow in
a distillation column.
Distillation Control - Material & Energy Balance
A distillation column is controlled by regulating its material
balance and the energy balance.In essence, a material balance means
that the sum of the products leaving the column must be equal
(approximately) to the feed entering the column; and an energy
balance means that the heat input to the column must equal
(approximately) to heat removed from the system.When a column is in
material and energy balance, there is no accumulation or generation
of material or heat within the column, i.e. the column is
"stable".The control system is dynamic, i.e. if a process variable
changes, the control system reacts by adjusting the affected
process variables until the system returns to normal condition.
Sample plant pictureThe term "steady state operation" describes the
condition in a column when the process variables are changing in
small amounts within prescribed limits.When a column is in
steady-state operation, the changes to the column's material
balance and energy balance variables are minimal and are handled by
the control system. As mentioned in the start of this chapter, one
of the objectives of control is to maintain the products within the
required specifications, or simply "specs". A "spec" is a value, or
a range of values, for a physical property or a set of physical
properties that is required for a product or products. A sample of
typical properties of interest in petroleum refining is shown in
Appendix A.
Product specifications are set by the demands of downstream
processes and by the marketplace. Products must meet certain
quality standards. For a product to be saleable, it must comply
with certain pre-determined quality.Products are routinely tested
to ensure that the specifications are met. Testing can be done by
direct composition measurement or by indirect measurement,
according to prescribed standards, such as ASTM.Direct composition
measurements are analysis that allows personnel to directly observe
the percentages of components in a product. An example is the
process chromatograph. It provides a direct read-out of the
component percentages. The readings of the chromatograph can be
compared against the specifications to see if any adjustments are
needed to ensure that the product meets specifications.Indirect
composition measurements are analysis in which one measured
property is used as an indicator of another property. One common
indirect measurement is the boiling temperature. For example, since
the boiling points of the components in a feed mixture are known,
the components in a product can be indirectly identified by their
boiling points. When the product is tested, its composition can be
indirectly measured by recording the temperatures at which the
different components in the product boil.If the composition of a
product is outside of the normal limits for that product, the
product is referred to as "off-specification".Exceeding product
specifications or producing better quality product than is required
is known as product giveaway.
Appendix A: Examples of Typical Petroleum Cut or Fraction
PropertiesNOTE: "Cut" is the refinery term of a fraction obtained
direct from a fractionating unit. Several cuts can be blended for
the manufacture of a certain product. A "fraction" is a portion of
petroleum separated from other portions in the fractionation of
petroleum products. It is often characterised by a particular
boiling range.Initial Boiling Point (IBP)The temperature at which
the first drop of distillate appears after commencement of
distillation in the standard ASTM laboratory apparatusFinal Boiling
Point (FBP)The maximum temperature observed on the distillation
thermometer when a standard ASTM distillation is carried
out.Boiling RangePetroleum products (which are mixtures of many
compounds, each having a different boiling point) do not have a
simple boiling point but have a boiling range instead, i.e. the
temperature range from bubble point to dew point.API GravityIn the
U.S. an arbitrary scale known as the API degree is used for
reporting the gravity of a petroleum product. The degree API is
related to the specific gravity scale (15oC / 15oC) by the
formula:ViscosityThe dynamic viscosity of a liquid is a measure of
its resistance to flow. The kinematic viscosity is equal to the
dynamic viscosity divided by the density of the liquid.
Cloud PointThe temperature at which a fuel, when cooled, begins
to congeal and present a cloudy appearance owing to the formation
of minute crystals of wax.Flash PointThe lowest temperature under
closely specified conditions at which a combustible material will
give off sufficient vapour to form an inflammable mixture with air
in a standardised vessel. Flash point tests are used to assess the
volatilities of petroleum products.Freezing PointThe temperature at
which crystals first appear when a liquid is cooled under specified
conditions. It is an important characteristic of aviation
fuels.Pour PointThe temperature below which an oil tends to
solidify and will no longer flow freely.Reid Vapour Pressure
(RVP)The pressure caused by the vaporised part of a liquid and the
enclosed air and water vapour, as measured under standardised
conditions in standardised apparatus: the result is given in psi at
100 oF, although normally reported simply as "RVP in lb". RVP is
not the same as the true vapour pressure of the liquid, but gives
some indication of the volatility of a liquid, e.g. gasoline.Octane
NumberThe octane number of a fuel is a number equal to the
percentage by volume of iso-octane in a mixture of iso-octane and
normal heptane having the same resistance to detonation as the fuel
under consideration in a special test engine. It is a measure of
the "anti-knock" value of a gasoline and the higher the octane
number the higher the anti-knock quality of the
gasoline.("Anti-knock" is an adjective signifying the resistance to
detonation (pinking) in spark-ignited internal combustion
engines).Smoke PointThe maximum height of flame measured in
millimetres (mm) at which a kerosene will burn without smoking when
tested in a standard lamp for this purpose.
The following controls are briefly discussed in this
Section:
Reboiler and Steam Control Condenser and Pressure Control
Analyser Control Temperature Control Feed Preheat Control
Reboiler Control
This is required to provide good response to column
disturbances, and to protect the column from disturbances occurring
in the heating medium. The reboiler boil-up is regulated either:
(1) to achieve desired product purity, or (2) to maintain a
constant boil-up rate.
In a typical reboiler control (see Figure below), the control
valve is located in the reboiler steam inlet line.
Typical reboiler control - steam flow
For inlet steam controlled reboiler, the heat transfer rate is
regulated by varying the steam control valve opening, thereby
changing the steam condensing pressure and temperature. When an
additional boil-up is required, the valve opens and raises the
reboiler pressure, which increases the temperature, and in turn
increases the boil-up rate. This scheme has the disadvantage of
non-linear relationship between pressure and boil-up, and is
affected by fouling in the reboiler.An alternative is to control
the condensate flow, i.e. by putting the control valve on the
condensate line (see Figure below). The main disadvantage is that
this scheme has poorer dynamic response than the previous scheme.
Manipulating the inlet valve immediately changes the vapour flow,
giving faster dynamic response. On the other hand, the condensate
outlet valve has no direct effect on vapour flow. The response time
varies with the condensate level in the exchanger.
Alternative reboiler control - condensate side
The other main disadvantage is the sizing of the condensate
valve. If condensate cannot be drained in time, vapour flow may be
restricted as much of the reboiler remains flooded. On the other
hand, too fast of condensate draining (faster than vapor
condensation in the reboiler) as result in loss of liquid seal in
the reboiler and steam will pass into the plant's condensate
recovery system.Some reboiler control features the use of
condensate pot. This is particularly important in fouling or
corrosive services (where leakage is a serious problem). An example
is shown in the Figure below.
Reboiler control with condensate pot
In the system shown, by varying the level control set point, the
tube surface area in the reboiler that is exposed for vapour
condensation can be adjusted, thus changing the available heat
transfer area. The heat transfer rate can therefore be
adjusted.This arrangement also automatically minimise the
condensation (and therefore, tube wall) temperature. A
pressure-balancing line is provided to maintain a steady pressure
and level in the condensate pot.
Condenser and Pressure Control:-The 3 main methods of pressure
and condensation control are:
(1) Vapour flow variation, (2) Flooded condenser, and(3) Cooling
medium flow variation.Vapour Flow VariationThe simplest and direct
method for column producing a vapour product. The pressure
controller regulates the vapour inventory and therefore the column
pressure. See the Figure below.
An important consideration here is the proper piping of the
vapour line to avoid liquid pockets.
Flooded CondenserThis method is used with total condensers
generating liquid product. Part of the condenser surface is flooded
with liquid at all times. The flow of condensate from the condenser
is controlled by varying the flooded area. Increasing the flooded
area (by reducing flow) increases the column pressure (less surface
area for condensation).
Flooded condenser pressure control
Cooling Medium Flow VariationPressure can also be controlled by
adjusting the flow of coolant to the condenser (see Figure below).
Operation using cooling water can cause fouling problems at low
flow condition, when cooling water velocity is low and outlet
temperature is high.
Pressure control - CWS flow
For air-fin condensers, the controller varies the fan speed or
fan pitch to control pressure (see Figure below). This arrangement
is energy-efficient as it minimises fan power consumption, but
requires the use of variable-pitch fan or variable speed motor.
Fin-fan pressure controlOther method: pressure control using
inerts (see Figure below).
Pressure control with inertsWhen column pressure falls, an inert
gas is admitted to raise the column pressure. Or: split-range
pressure control venting excess gas to flare (see Figure
below)Split range pressure controlIn most instances, both vapour
and liquid phase are present in the column overhead. The vapour
contains components that can condense out but are undesirable in
the liquid, i.e. excessive condensation may lead to
off-specification liquid product. In addition, it is also
undesirable to lose liquid product (through insufficient
condensation) to the vapour. It is therefore important to control
the rate of condensation to obtain the desired vapour-liquid
split.
This is usually done by controlling the temperature of the
liquid product just downstream of the condenser. One common scheme
used is shown the Figure below.
Temperature ControlColumn temperature control is perhaps the
most popular way of controlling product compositions. In this case,
the control temperature is used as a substitute to product
composition analysis.Ideally, both top and bottom compositions
should be controlled to maintain each within its specifications.
See the Figure below.
Temperature control and interactionIn practice, simultaneous
composition control of both products suffer from serious "coupling"
(interaction) between the 2 controllers, resulting in column
instability. In the system shown, suppose that there are
concentration changes in the feed conditions that result in lower
column temperature. The top and bottom temperature controllers will
respond by decreasing reflux and increasing boil-up
respectively.
If the actions of the 2 controllers are perfectly matched, and
response is instantaneous, both control temperatures will return to
their set points without interaction.
However, the 2 actions are rarely perfectly matched, and their
dynamics are dissimilar - usually the boil-up response is faster.
The reflux and boil-up will "cycle" as shown in the Figure
above.
The interaction can be avoided by controlling only 1 of the 2
product compositions.
On-line analyser can be used together with temperature control
to control product composition. The principal control action is
rapidly performed by the temperature controller, while the analyser
slowly adjusts the temperature set point to prevent
off-specification product purity. A set up is shown in the Figure
below.
Temperature-analyser control
In the above set-up, delayed analyser response is acceptable, as
its time lags become a secondary consideration. The fast
temperature controller action renders this control method less
sensitive to upsets and step changes in an analyser-only control
system.Another advantage is that, should the analyser become
inoperative, the temperature controller will maintain automatic
control of the process.Feed Preheat ControlFeed preheat is usually
practised for heat recovery or to attain the desired vapour and
liquid traffic above and below the feed tray. The objective of the
preheat control system is to supply the column with a feed of
consistent specific enthalpy. With a single-phase feed, this
becomes a constant feed temperature control; with a partially
vaporised feed, a constant fractional vaporisation is required.As
an example, consider case (a) as shown in the Figure below whereby
the feed is a cold liquid. In this case, all the liquid feed will
go to the stripping section. In addition, because the feed is cold,
it will also condense some of the rising vapour.
As a result, the amount of liquid flow in the stripping section
is much larger than the liquid flow in the rectifying section. The
vapour flow in the rectifying section is lower than the vapour flow
in the stripping section because of the condensation into the
liquid.The following Figures showed 2 other feed conditions: case
(b) for saturated liquid (left) and case (c) for vapour-liquid
mixture (right): And the following Figures showed 2 other feed
conditions: case (d) for saturated vapour (left) and case (e) for
superheated vapour (right): Sub cooled feed or superheated feed can
be controlled (see Figure below) by preheating (left) or de
superheating (right) the feed prior to column entry:
A superheated bottom feed can be cooled by injecting a quench
stream as shown in the Figure below.
An Example of Distillation Column Control:-A typical
distillation column has a combination of different control loops.
The control system of a particular column is designed to meet that
column's particular process requirements. An example is shown in
the Figure below.
There are several control loops associated with the distillation
column:Temperature:1. Overhead condensation (Fin-fan)2. Overhead
column (Reflux)3. Feed preheat 4. Column bottom (Reboiler
steam)Pressure:1. Overhead accumulator (Off gas)Level:1. Overhead
accumulator (Distillate product)2. Column bottom (Bottoms
product)Flow:1. Column feed
In this distillation column, the material balance (MB) loops
consisted of the following:
Feed flow control loop (which sets the throughput, i.e.
production rate)Bottom level control loop (which controls the
column level)Accumulator level control loop (which regulates the
product flow by regulating the overhead accumulator level)Off gas
pressure control loop (which controls the column pressure)
The energy balance (EB) control loops are the following:
Boiler temperature control loop (which control the column bottom
temperature by controlling the steam input to the reboiler)Feed
preheater temperature control loop (which controls the feed inlet
temperature)Overhead condenser temperature control loop (which
regulates amount of cooling in the column)External reflux
temperature control loop (which controls the temperature at the top
of the column)In this example, the main influence on the heat input
to the column is the steam flow to the reboiler. Heat also enters
the system via the preheater. Heat balance is achieved when the
heat input from the reboiler and preheater is removed by the
condenser.(Note that there is also a balance between the energy in
the feed stream and product streams, but this balance does not have
much effect on the overall energy balance)In this type of control
system, the material balance control loops react to the changes in
the column's energy balance.For example, a change in the reboiler
steam flow will lead to a series of changes in the column; and the
column's control system react to this change in order to maintain
the material balance and energy balance. Sample plant pictureAn
increased steam flow to the reboiler means an increase in heat
input which will result in increased vaporisation in the reboiler
and an increased bottom temperature. There will be an increased
vapour flow and temperature throughout the column. The liquid level
in the bottom of the column decreases as more liquid is being
boiled-off, and the bottom product rate decreases. Hence, a change
in the EB leads to a change in the MB.Increased vapour flow to the
top will cause a higher temperature at the top of the column, and
the temperature (reflux) controller will increase the reflux flow
back to the column. Increased reflux flow will condense the
additional vapour in the column.The larger amount of vapour also
requires additional cooling in the overhead system and this is
handled by the temperature control that increases the fan speed of
the overhead condenser. This will increase the heat removal and
tends to restore the EB. Increased condensation leads to increased
liquid flow into the overhead accumulator (reflux drum). The
accumulator level controller responds by increasing the outflow of
top product. This increased outflow of materials from the top will
offset the decreased in outflow from the bottom, hence the MB is
restored.
Concentrations of the top and bottom product streams are
affected as well - higher bottom temperature will results in more
heavy components being vaporised from the bottoms product.
This can be illustrated using a multi-component separation of 8
products: C1, C2, C3, C4, C5, C6, C7 and C8+. The main separation
is between 2 key components: the light key (C4) and heavy key (C5).
This is shown in the Figure below.
If the bottom temperature is too high, more of the heavy key
(HK) will be vapourised from the bottom product. The vapour thus
had become heavier due to the presence of the HK. The final boiling
point (FBP) of the top product will be higher but the initial
boiling point (IBP) did not change.
On the other hand, the IBP of the bottoms product will be
higher, because the bottoms product has been depleted of the HK and
become heavier. The FBP of the bottoms product is not affected by
the bottom temperature increase.
Other possible disturbancesThis example illustrated just one of
the many disturbances that can upset the smooth operation of a
distillation column. Besides the reboiler example, which could be
due to controller malfunctioning, other disturbances, can also
occur. The following list is not exhaustive, but only serves as a
reference of what possible events that can disrupt the smooth
operation of a plant.
Reboiler and other heat exchangers: fouling of heat transfer
surfaces, tube leaks, etcCharge heater: loss of fuel gas and/or
fuel oil (e.g. due to low fuel gas pressure trip)Overhead
condenser: loss of cooling water or loss of power supply (for
air-fin coolers)Pumps: overload trip, loss of power, cavitations,
etcControl valves failure: e.g. loss of instrument air, jammed
valve, faulty positioners, etc.Faulty instruments: wrong signals
transmitted false alarms, etc.Feed changes: feed rate, lower
boiling components, contaminations, etc.Tower internals: e.g.
flooding, weeping, channelling, etc.
Drum Level Control SystemsDrum Level Control Systems are used
extensively throughout the process industries and the Utilitiesto
control the level of boiling water contained in boiler drums on
process plant and help provide aconstant supply of steam.
If the level is too high, flooding of steam purification
equipment can occur.If the level is too low, reduction in
efficiency of the treatment and recirculation function.Pressure can
also build to dangerous levels.
A drum level control system tightly controls the level whatever
the disturbances, level change, increase/decrease of steam demand,
feedwater flow variations.In the process industries, boiling water
to make steam is a very important procedure.The control of water
level is a major function in this process and it is achieved
through a water steam interface established in a cylindrical vessel
called the drum which is usually lying on its side and located near
the top of the boiler.Maintaining the correct water level in the
drum is critical for many reasons. A water level that is too high
causes flooding of the steam purification equipment; resulting in
the carry over of water and impurities into the steam system. A
water level that is too low results in a reduction in efficiency of
the treatment and recirculation function. It can even result in
tube failure due to overheating from lack of cooling water on the
boiling surfaces. Normally drum level is expected to be held within
2 to 5cm of the set-point with some tolerance for temporary load
changes.
Components Affecting Drum Water Level:-
Under boiling conditions, steam supporting field products such
as bubbles exist below the water/steam level interface. These
bubbles have volume and therefore displace water to create a
misrepresentation of the true water level in the drum. Another
effect upon drum level is pressure in the drum. Because steam
bubblescompress under pressure (if the drum pressure changes due to
load demands), the steam bubbles expand or contract respective to
these pressure changes. A higher steam demand will cause the drum
pressure to drop, and the steam bubbles to expand to give the
appearance of a water level higher than it truly is. This
fictitioushigher water level causes the feed water input to be shut
down at a time when more water is really required. A surge in water
level as a result of the drum pressure decreasing is called
'swell'. A water level decrease due to drum pressure increase is
called 'shrink'.
Level Control Strategies:
Figure 1 depicts three types of drum level control strategies
with typical applications for each. While single-element drum level
control is acceptable for steady boiler load conditions; as load
changes become more frequent, unpredictable, or severe; this type
of level control cannot respond quickly enough to compensate.
Moreinformation must be included and processed to predetermine the
amount of water to be added to the drum to compensate for load
changes. The addition of elements (flow and transmitter devices)
enables the controller to predict the amount of water added to the
drum to maintain drum level set-point
Single-Element Drum Level Control:
Figure 2 depicts the control scheme for single-element drum
level control. In this configuration, only the water level in the
drum is being measured (hence the term " single element" ) . LT- 1
is an electronic differential pressure transmitter with a high
static pressure range. The high side of the transmitter is
connected to the bottom ofthe drum. Because of the drum's static
pressure, the low side of the transmitter is connected to the top
of the drum above the water/steam interface. This provides a
reference for the transmitter by cancelling the static pressure
effect and allowing only the water hydrostatic head to be measured.
A constant head reservoir is required to maintain a consistent head
in the reference leg of the transmitter. This is often referred to
as a ''wet leg" The output of the electronic DP transmitter is the
process Input for the MOD 30ML Controller, (LC-1), and theoutput is
then compared to a drum level set-point. Any discrepancy between
setpoint and drum level causes an output from the MOD 30ML
controller in compensation. Because controller action is reverse,
as the drum level Increases, a resultant output signal will
decrease to close the feedwater control valve. The output of the
Controller is fed to the feedwater control valve (FCV-1). If the
feedwater valve is pneumatic, an lP (current-to-pressure) converter
is required to change the Controller current output to accommodate
the pneumatic valve.Note that the response from the controller to
the feedwater control valve is reactive; i.e. feedwater is added
only in response to a drop in drum level. This type of control is
acceptable if steam load changes are not dramatic because the
controller can respond well to steady demands. In applications
where steam load changes becomefrequent and unpredictable, a
reactive strategy is better suited. This type of system requires
more field devices for input.
Two-Element Drum Level Control System:-A two-element drum level
control system is capable of providing close adherence of drum
level to its set-point under steady-state conditions as well as
being capableof providing the required tight control during a
transient. Its performance during transient conditions permits its
use on many industrial boiler applications. Suchapplications are
characterized by adequately-sized drums used with load changes of
moderate rate and degree. These characteristics are usually found
in plants with continuous-type processes, and those with mixed
heating and processing demands. Caution should be exercised in its
use on systems without reasonably constantfeedwater pressure. The
term 'two-element' is derived from two variables: steam flow and
drum levelinfluence on the feedwater valve position. It is often
classified as a combination 'feed-forward-feedback' system because
the steam flow demand is fed forward as the primary index of the
feedwater valve position. The drum level signal becomes the
feedback which is used to constantly trim the accuracy of the
feed-forward system and provide final control of the water/steam
interface in the drum. Refer to Figure 3 for the control scheme of
a two-element drum level control. Note the left side of the doted
line is identical to that used in single-element control.Additional
equipment required for two-element drum level control consists of a
steam flow measuring device, a differential pressure transmitter, a
square root extractor, a feedwater flow computer and a feedwater
flow mode transfer station. At first this may appear like a large
investment in order to gain stable drum level control, but asyou
will see this is not necessarily true. How it works:Steam flow is
measured by the steam flow transmitter (FT-1), its signal is fed to
the feedwater flow computer (FC-1) after processing through the
square root extractor (FY-1). As in the single-element level
control, the drum level is measured by the level transmitter (LT-1)
and its signal is transmitted to the drum level controller (LC- 1).
In the drum level controller, the process signal is compared to the
drum level setpoint, where a required corrective output signal to
maintain the drum level is produced. This corrective signal is sent
to the feedwater flow computer. The feedwater flow computer
combines the signal from the two variables, and producesan output
signal to the feedwater control valve (FCV-1). Auto/Manual transfer
of the feedwater control valve is accomplished via FK-1. Nearly all
of the load change work is done by the feed-forward system, for
example,a pound of feedwater change is made for every pound of
steam flow change. The drum level control system is used for
compensation only. It is expected that the drum level will be
maintained very closely to the set-pointvalue. This is true in
spite of the low-to-moderate volume/throughput ratio and a wide
operating range. As a result, integral response (reset) is a
necessary function in the drum level controller. Using one MOD 30ML
Controller, four of the functions in the two-element control scheme
are accomplished: level control (LC-1), square root extraction
(FY-1), feedwater flow computation (FC-1), and feedwater flow mode
transfer (FK-1). TheMOD 30ML Controller is a multi-functional
controller providing level control for LC-1. Utilizing the
linearization block in the ML will provide the required square root
function to obtain a linear signal from the steam flow transmitter.
A math block in the Controller enables feedwater flow computations.
Finally, a feedwater flow transfer station is easily provided for
with an operator-accessible Auto/Manual button on the Controller
display. Once in manual, the controller output is ramped up or down
by an operator using keys on the controller display. Should a
totalized steam flow be required, the MOD 30ML Controller provides
an eight-digit display of the totalized value.FT-1 is an ABB
electronic transmitter providing accuracy of 0.2% and is rugged
enough to handle static pressures up to 6000 PSI.
Three-Element Drum Level Control System:-
In most drum level control applications, the two-element drum
level control will maintain the required water/steam interface
level even under moderate load changes. However, If an unstable
feedwater system exists exhibiting a variable feed header-to-drum
pressure differential, or if large unpredictable steam demandsare
frequent, a three-element drum level control scheme should be
considered. As implied from the previous information, this control
strategy supplies control of feedwater flow in relationship to
steam flow.The performance of the three-element control system
during transient conditions makes it very useful for general
industrial and utility boiler applications. It handles loads
exhibiting wide and rapid rates of change. Plants which exhibit
load characteristics of this type are those with mixed, continuous,
and batch processingdemands. It is also recommended where normal
load characteristics are fairly steady; but upsets can be sudden,
unpredictable and/or a significant portion of the load.How it
works:Figure 4 shows the control scheme for three-element drum
level control. To the left of the dotted line, the instrumentation
is the same as that for the two-element drum level control, with
one exception: the output of the feedwater flow computer now
becomes the set-point of the feedwater flow controller (FIC-2).
Equipment requiredto complete our three-element drum level control
scheme includes an additional flow device (FE-2) and differential
pressure transmitter (FT-2). The area to the left of the dotted
line in figure 4 functions the same as that of a twoelement drum
level control. We can pick up the operation for this scheme where
the output signal of the feedwater flow computer (the combination
of steam flow and drum level) enters the feedwater controller
(FIC-2).This in effect becomes the set-point to this controller.
Feedwater flow Is measured by the transmitter (FT-2). The output
signal of the feedwater flow transmitter is linearized by the
square root extractor, (FY-2). This signal is the process variable
to the feedwater controller and is compared to the output of the
feedwater flow computer(set-point). The feedwater flow controller
produces the necessary corrective signal to maintain feedwater flow
at its set-point by the adjustment of the feedwater control valve
(FCV-1). As in the two-element drum level control scheme, nearly
all of the work necessary to compensate for load change is done by
the feed-forward system (i.e. a pound of feedwater change is made
for every pound of steam flow change). The drum level portion of
the control scheme is used only in a compensating role. Despite
low-tomoderate volume/ throughput ratio and a wide operating range,
it is expected the drum level will be maintained very close to
set-point. Achieving this requires use of the integrating response
and reset in both the drum level and feedwater controllers. This
application may suggest that an additional controller is required
for the feedwater flow controller, however this Is not true. The
MOD 30ML Controller is a multi-loop unit. An easily-configured
feed-forward command in the MOD 30ML means no additional wiring is
required to have the drum level controller and feedwater controller
working together. Feedwater flow computations are effortlessly done
in the maths block of the controller, all square root functions are
performed within. The feedwater flow element (FE-2), is an ABB
WEDGE unit. A reliable, rugged, yetaccurate measuring device that
will be in service for many years. Many models include the option
of mounting the transmitter on the WEDGE itself, thus eliminating
the need for expensive lead lines, valves and flanges. The
feedwater flow transmitter (FT-2), is an ABB electronic
differential pressure transmitter. If the system is appropriately
designed. FT-1. FT-2, and LT-1 may be the same type of transmitter.
This means stocking only one type of transmitter In the case of a
transmitter failure