Training on GET-InstrumentationApril 2014

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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