modes of control

7/27/2019 Modes of Control

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Modes of control

An automatic temperature control might consist of a valve, actuator, controller and sensor detecting the space temperature in a room. The control system is said

to be 'in balance' when the space temperature sensor does not register more or less temperature than that required by the control system. What happens to the

control valve when the space sensor registers a change in temperature (a

temperature deviation) depends on the type of control system used. The

relationship between the movement of the valve and the change of temperature

in the controlled medium is known as the mode of control or control action.

There are two basic modes of control:

On/Off - The valve is either fully open or fully closed, with nointermediate state.

Continuous - The valve can move between fully open or fully closed, or be held at any intermediate position.

Variations of both these modes exist, which will now be examined in greater detail.

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On/off controlOccasionally known as two-step or two-position control, this is the most basic

control mode. Considering the tank of water shown in Figure 5.2.1, theobjective is to heat the water in the tank using the energy given off a simple

steam coil. In the flow pipe to the coil, a two port valve and actuator is fitted,complete with a thermostat, placed in the water in the tank.

Fig. 5.2.1On/off temperature control of water in a tank

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The thermostat is set to 60°C, which is the required temperature of the water in

the tank. Logic dictates that if the switching point were actually at 60°C the

system would never operate properly, because the valve would not know

whether to be open or closed at 60°C. From then on it could open and shut

rapidly, causing wear.

For this reason, the thermostat would have an upper and lower switching point.

This is essential to prevent over-rapid cycling. In this case the upper switching

point might be 61°C (the point at which the thermostat tells the valve to shut)

and the lower switching point might be 59°C (the point when the valve is told to

open). Thus there is an in-built switching difference in the thermostat of ±1°Cabout the 60°C set point.

This 2°C (±1°C) is known as the switching differential. (This will vary between

thermostats). A diagram of the switching action of the thermostat would look

like the graph shown in Figure 5.2.2. The temperature of the tank contents will

fall to 59°C before the valve is asked to open and will rise to 61°C before the

valve is instructed to close.

Fig. 5.2.2 On/off switching

action of the thermostat Figure 5.2.2 shows straight switching lines but the effect on heat transfer from

coil to water will not be immediate. It will take time for the steam in the coil to

affect the temperature of the water in the tank. Not only that, but the water inthe tank will rise above the 61°C upper limit and fall below the 59°C lower

limit. This can be explained by cross referencing Figures 5.2.2 and 5.2.3. First

however it is necessary to describe what is happening.

At point A (59°C, Figure 5.2.3) the thermostat switches on, directing the valve

wide open. It takes time for the transfer of heat from the coil to affect the water temperature, as shown by the graph of the water temperature in Figure 5.2.3. At

point B (61°C) the thermostat switches off and allows the valve to shut.

However the coil is still full of steam, which continues to condense and give up

its heat. Hence the water temperature continues to rise above the upper

switching temperature, and 'overshoots' at C, before eventually falling.



Fig. 5.2.3 Tank

temperature versus time

From this point onwards, the water temperature in the tank continues to falluntil, at point D (59°C), the thermostat tells the valve to open. Steam is admitted

through the coil but again, it takes time to have an effect and the water

temperature continues to fall for a while, reaching its trough of undershoot at

point E.

The difference between the peak and the trough is known as the operating

differential. The switching differential of the thermostat depends on the type of

thermostat used. The operating differential depends on the characteristics of the

application such as the tank, its contents, the heat transfer characteristics of thecoil, the rate at which heat is transferred to the thermostat, and so on.

Essentially, with on/off control, there are upper and lower switching limits, and

the valve is either fully open or fully closed - there is no intermediate state.

However, controllers are available that provide a proportioning time control, inwhich it is possible to alter the ratio of the 'on' time to the 'off' time to control

the controlled condition. This proportioning action occurs within a selected

bandwidth around the set point; the set point being the bandwidth mid point.

If the controlled condition is outside the bandwidth, the output signal from thecontroller is either fully on or fully off, acting as an on/off device. If the

controlled condition is within the bandwidth, the controller output is turned onand off relative to the deviation between the value of the controlled condition

and the set point.

With the controlled condition being at set point, the ratio of 'on' time to 'off'

time is 1:1, that is, the 'on' time equals the 'off' time. If the controlled condition

is below the set point, the 'on' time will be longer than the 'off' time, whilst if



above the set point, the 'off' time will be longer, relative to the deviation within

the bandwidth.

The main advantages of on/off control are that it is simple and very low cost.

This is why it is frequently found on domestic type applications such as centralheating boilers and heater fans.

Its major disadvantage is that the operating differential might fall outside the

control tolerance required by the process. For example, on a food production

line, where the taste and repeatability of taste is determined by precise

temperature control, on/off control could well be unsuitable.

By contrast, in the case of space heating there are often large storage capacities

(a large area to heat or cool that will respond to temperature change slowly) and

slight variation in the desired value is acceptable. In many cases on/off control

is quite appropriate for this type of application.

If on/off control is unsuitable because more accurate temperature control isrequired, the next option is continuous control.

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Continuous control

Continuous control is often called modulating control. It means that the valve iscapable of moving continually to change the degree of valve opening or closing.

It does not just move to either fully open or fully closed, as with on-off control.

There are three basic control actions that are often applied to continuouscontrol:

Proportional (P)

Integral (I)

Derivative (D)

It is also necessary to consider these in combination such as P + I, P + D, P + I

+ D. Although it is possible to combine the different actions, and all help to

produce the required response, it is important to remember that both the integral

and derivative actions are usually corrective functions of a basic proportional

control action.

The three control actions are considered below.

Proportional control







This is the most basic of the continuous control modes and is usually referred to

by use of the letter 'P'. The principle aim of proportional control is to control the

process as the conditions change.

This section shows that:

The larger the proportional band, the more stable the control, but the

greater the offset.

The narrower the proportional band, the less stable the process, but thesmaller the offset.

The aim, therefore, should be to introduce the smallest acceptable proportional band that will always keep the process stable with the minimum offset.

In explaining proportional control, several new terms must be introduced.

To define these, a simple analogy can be considered - a cold water tank is

supplied with water via a float operated control valve and with a globe valve onthe outlet pipe valve 'V', as shown in Figure 5.2.4. Both valves are the same size

and have the same flow capacity and flow characteristic. The desired water level

in the tank is at point B (equivalent to the set point of a level controller).

It can be assumed that, with valve 'V' half open, (50% load) there is just the

right flowrate of water entering via the float operated valve to provide thedesired flow out through the discharge pipe, and to maintain the water level in

the tank at point at B.

The questions these people ask about steam are markedly different.

Fig. 5.2.4

Valve 50% open The system can be said to be in balance (the flowrate of water entering and

leaving the tank is the same); under control, in a stable condition (the level is

not varying) and at precisely the desired water level (B ); giving the required

outflow.



With the valve 'V' closed, the level of water in the tank rises to point A and the

float operated valve cuts off the water supply (see Figure 5.2.5 below).

The system is still under control and stable but control is above level B. Thedifference between level B and the actual controlled level, A, is related to the

proportional band of the control system.

Once again, if valve 'V' is half opened to give 50% load, the water level in thetank will return to the desired level, point B.

Fig. 5.2.5 Valve closed This means the system is simpler and less expensive than, for example, a high

temperature hot water system. The high efficiency of steam plant means it is

compact and makes maximum use of space, something which is often at a premium within plant.

Furthermore, upgrading an existing steam system with the latest boilers and

controls typically represents 50% of the cost of removing it and replacing itwith a decentralised gas fired system.

Q. How will the operating and maintenance costs of a steam system affect

overhead costs ?

Centralised boiler plant is highly efficient and can use low interruptible tariff

fuel rates. The boiler can even be fuelled by waste, or form part of a state-of-

the-art Combined Heat and Power plant.

Steam equipment typically enjoys a long life - figures of thirty years or more of

low maintenance life are quite usual.

Modern steam plant, from the boiler house to the steam using plant and back

again, can be fully automated. This dramatically cuts the cost of manning the



plant.

Sophisticated energy monitoring equipment will ensure that the plant remains

energy efficient and has a low manning requirement.

All these factors in combination mean that a steam system enjoys a low lifetimecost.

Q. If a steam system is installed, how can the most use be made of it ?

Steam has a range of uses. It can be used for space heating of large areas, for

complex processes and for sterilisation purposes.

Using a hospital as an example, steam is ideal because it can be generated

centrally at high pressure, distributed over long distances and then reduced in

pressure at the point of use. This means that a single high pressure boiler can

suit the needs of all applications around the hospital, for example, heating of

wards, air humidification, cooking of food in large quantities and sterilisation of

equipment.

It is not as easy to cater for all these needs with a water system.

Q. What if needs change in the future ?

Steam systems are flexible and easy to add to. They can grow with the company

and be altered to meet changing business objectives.

Q. What does using steam say about the company ?

The use of steam is environmentally responsible. Companies continue to choosesteam because it is generated with high levels of fuel efficiency. Environmental

controls are increasingly stringent, even to the extent that organisations have to

consider the costs and methods of disposing of plant before it is installed. All

these issues are considered during the design and manufacture of steam plant.



Fig. 5.2.6 Valve

open The system is under control and stable, but there is an offset; the deviation in

level between points B and C. Figure 5.2.7 combines the three conditions used

in this example.

The difference in levels between points A and C is known as the ProportionalBand or P-band, since this is the change in level (or temperature in the case of a

temperature control) for the control valve to move from fully open to fully

closed.

One recognised symbol for Proportional Band is Xp.

The analogy illustrates several basic and important points relating to

proportional control:

The control valve is moved in proportion to the error in the water level

(or the temperature deviation, in the case of a temperature control) from

the set point.

The set point can only be maintained for one specific load condition.

Whilst stable control will be achieved between points A and C, any loadcausing a difference in level to that of B will always provide an offset.

Fig. 5.2.7 Proportional band Note: By altering the fulcrum position, the system Proportional Band changes.

Nearer the float gives a narrower P-band, whilst nearer the valve gives a wider

P-band. Figure 5.2.8 illustrates why this is so. Different fulcrum positions



require different changes in water level to move the valve from fully open to

fully closed. In both cases, It can be seen that level B represents the 50% load

level, A represents the 0% load level, and C represents the 100% load level. It

can also be seen how the offset is greater at any same load with the wider

proportional band.

Fig. 5.2.8 Demonstrating the relationship between P-band and offset The examples depicted in Figures 5.2.4 through to 5.2.8 describe proportional

band as the level (or perhaps temperature or pressure etc.) change required to

move the valve from fully open to fully closed. This is convenient for mechanical systems, but a more general (and more correct) definition of

proportional band is the percentage change in measured value required to give a

100% change in output. It is therefore usually expressed in percentage termsrather than in engineering units such as degrees centigrade.

For electrical and pneumatic controllers, the set value is at the middle of the proportional band. The effect of changing the P-band for an electrical or

pneumatic system can be described with a slightly different example, by using a

temperature control.

The space temperature of a building is controlled by a water (radiator type)

heating system using a proportional action control by a valve driven with anelectrical actuator, and an electronic controller and room temperature sensor.

The control selected has a proportional band (P-band or Xp) of 6% of the

controller input span of 0° - 100°C, and the desired internal space temperature is

18°C. Under certain load conditions, the valve is 50% open and the required

internal temperature is correct at 18°C.

A fall in outside temperature occurs, resulting in an increase in the rate of heat

loss from the building. Consequently, the internal temperature will decrease.

This will be detected by the room temperature sensor, which will signal the

valve to move to a more open position allowing hotter water to pass through the



room radiators.

The valve is instructed to open by an amount proportional to the drop in room

temperature. In simplistic terms, if the room temperature falls by 1°C, the valve

may open by 10%; if the room temperature falls by 2°C, the valve will open by20%.

In due course, the outside temperature stabilises and the inside temperature

stops falling. In order to provide the additional heat required for the lower

outside temperature, the valve will stabilise in a more open position; but the

actual inside temperature will be slightly lower than 18°C.

Example 5.2.1 and Figure 5.2.9 explain this further, using a P-band of 6°C.

Example 5.2.1 Consider a space heating application with the followingcharacteristics:

1. The required temperature in the building is 18°C.

2. The room temperature is currently 18°C, and the valve is 50% open.

3. The proportional band is set at 6% of 100°C = 6°C, which gives 3°Ceither side of the 18°C set point.

Figure 5.2.9 shows the room temperature and valve relationship:

Fig. 5.2.9 Room

temperature and valve relationship - 6°C proportional band As an example, consider the room temperature falling to 16°C. From the chart it

can be seen that the new valve opening will be approximately 83%.

With proportional control, if the load changes, so too will the offset:



A load of less than 50% will cause the room temperature to be above the

set value.

A load of more than 50% will cause the room temperature to be below theset value.

The deviation between the set temperature on the controller (the set point) and

the actual room temperature is called the 'proportional offset'.

In Example 5.2.1, as long as the load conditions remain the same, the controlwill remain steady at a valve opening of 83.3%; this is called 'sustained offset'.

The effect of adjusting the P-bandIn electronic and pneumatic controllers, the P-band is adjustable. This enables

the user to find a setting suitable for the individual application.

Increasing the P-band - For example, if the previous application had been

programmed with a 12% proportional band equivalent to 12°C, the results can

be seen in Figure 5.2.10. Note that the wider P-band results in a less steep 'gain'

line. For the same change in room temperature the valve movement will be

smaller. The term 'gain' is discussed in a following section.

In this instance, the 2°C fall in room temperature would give a valve opening of about 68% from the chart in Figure 5.2.10.

Fig. 5.2.10 Room

temperature and valve relationship - 12°C Proportional band

Reducing the P-band - Conversely, if the P-band is reduced, the valvemovement per temperature increment is increased. However, reducing the P-



band to zero gives an on/off control. The ideal P-band is as narrow as possiblewithout producing a noticeable oscillation in the actual room temperature.

Example 5.2.2Let the input span of a controller be 100°C.

If the controller is set so that full change in output occurs over a proportional band of 20% the controller gain is:

Gain

The term 'gain' is often used with controllers and is simply the reciprocal of proportional band.

The larger the controller gain, the more the controller output will change for a

given error. For instance for a gain of 1, an error of 10% of scale will change

the controller output by 10% of scale, for a gain of 5, an error of 10% will

change the controller output by 50% of scale, whilst for a gain of 10, an error of

10% will change the output by 100% of scale.

The proportional band in 'degree terms' will depend on the controller input

scale. For instance, for a controller with a 200°C input scale:

An Xp of 20% = 20% of 200°C = 40°CAn Xp of 10% = 10% of 200°C = 20°C

Example 5.2.2Let the input span of a controller be 100°C.

If the controller is set so that full change in output occurs over a proportional band of 20% the controller gain is:

Equally it could be said that the proportional band is 20% of 100°C = 20°C andthe gain is:

Therefore the relationship between P-band and Gain is:

As a reminder:



A wide proportional band (small gain) will provide a less sensitive

response, but a greater stability.

A narrow proportional band (large gain) will provide a more sensitive

response, but there is a practical limit to how narrow the Xp can be set.

Too narrow a proportional band (too much gain) will result in oscillationand unstable control.

For any controller for various P-bands, gain lines can be determined as shown inFigure 5.2.11, where the controller input span is 100°C.

Fig. 5.2.11 Proportional band and gain

Reverse or direct acting control signalA closer look at the figures used so far to describe the effect of proportional

control shows that the output is assumed to be reverse acting. In other words, arise in process temperature causes the control signal to fall and the valve toclose. This is usually the situation on heating controls. This configuration would

not work on a cooling control; here the valve must open with a rise in

temperature. This is termed a direct acting control signal. Figures 5.2.12 and

5.2.13 depict the difference between reverse and direct acting control signals for the same valve action.



Fig. 5.2.12 Reverse acting signal

Fig. 5.2.13 Direct acting signal On mechanical controllers (such as a pneumatic controller) it is usual to be able

to invert the output signal of the controller by rotating the proportional controldial. Thus, the magnitude of the proportional band and the direction of the

control action can be determined from the same dial.

On electronic controllers, reverse acting (RA) or direct acting (DA) is selectedthrough the keypad.

Gain line offset or proportional effectFrom the explanation of proportional control, it should be clear that there is a

control offset or a deviation of the actual value from the set value whenever the

load varies from 50%.

To further illustrate this, consider Example 5.2.1 with a 12°C P-band, where an

offset of 2°C was expected. If the offset cannot be tolerated by the application,

then it must be eliminated.

This could be achieved by relocating (or resetting) the set point to a higher

value. This provides the same valve opening after manual reset but at a roomtemperature of 18°C not 16°C.



Fig. 5.2.14 Gain

line offset

Manual resetThe offset can be removed either manually or automatically. The effect of

manual reset can be seen in Figure 5.2.14, and the value is adjusted manually by

applying an offset to the set point of 2°C.

It should be clear from Figure 5.2.14 and the above text that the effect is the

same as increasing the set value by 2°C. The same valve opening of 66.7% nowcoincides with the room temperature at 18°C.

The effects of manual reset are demonstrated in Figure 5.2.15.



Fig. 5.2.15 Effect of manual reset

Integral control - automatic reset action'Manual reset' is usually unsatisfactory in process plant where each load change

will require a reset action. It is also quite common for an operator to beconfused by the differences between:

Set value - What is on the dial.

Actual value - What the process value is. Required value - The perfect process condition.



Such problems are overcome by the reset action being contained within the

mechanism of an automatic controller.

Such a controller is primarily a proportional controller. It then has a reset

function added, which is called 'integral action'. Automatic reset uses anelectronic or pneumatic integration routine to perform the reset function. Themost commonly used term for automatic reset is integral action, which is given

the letter I.

The function of integral action is to eliminate offset by continuously and

automatically modifying the controller output in accordance with the controldeviation integrated over time. The Integral Action Time (IAT) is defined as the

time taken for the controller output to change due to the integral action to equal

the output change due to the proportional action. Integral action gives a steadily

increasing corrective action as long as an error continues to exist. Such

corrective action will increase with time and must therefore, at some time, be

sufficient to eliminate the steady state error altogether, providing sufficient time

elapses before another change occurs. The controller allows the integral time to

be adjusted to suit the plant dynamic behaviour.

Proportional plus integral (P + I) becomes the terminology for a controller

incorporating these features.

The integral action on a controller is often restricted to within the proportional band. A typical P + I response is shown in Figure 5.2.16, for a step change inload.

Fig. 5.2.16

P+I Function after a step change in load The IAT is adjustable within the controller:

If it is too short, over-reaction and instability will result.



If it is too long, reset action will be very slow to take effect.

IAT is represented in time units. On some controllers the adjustable parameter

for the integral action is termed 'repeats per minute', which is the number of

times per minute that the integral action output changes by the proportionaloutput change.

Repeats per minute = 1/(IAT in minutes)

IAT = Infinity - Means no integral action IAT = 0 - Means infinite integral action

It is important to check the controller manual to see how integral action isdesignated.

Overshoot and 'wind up'With P+ I controllers (and with P controllers), overshoot is likely to occur when

there are time lags on the system.

A typical example of this is after a sudden change in load. Consider a processapplication where a process heat exchanger is designed to maintain water at a

fixed temperature.

The set point is 80°C, the P-band is set at 5°C (±2.5°C), and the load suddenly

changes such that the returning water temperature falls almost instantaneously

to 60°C.

Figure 5.2.16 shows the effect of this sudden (step change) in load on the actual

water temperature. The measured value changes almost instantaneously from a

steady 80°C to a value of 60°C.

By the nature of the integration process, the generation of integral control action

must lag behind the proportional control action, introducing a delay and more

dead time to the response. This could have serious consequences in practice,

because it means that the initial control response, which in a proportionalsystem would be instantaneous and fast acting, is now subjected to a delay andresponds slowly. This may cause the actual value to run out of control and the

system to oscillate. These oscillations may increase or decrease depending on

the relative values of the controller gain and the integral action. If applying

integral action it is important to make sure, that it is necessary and if so, that the

correct amount of integral action is applied.

Integral control can also aggravate other situations. If the error is large for a

long period, for example after a large step change or the system being shutdown, the value of the integral can become excessively large and cause



overshoot or undershoot that takes a long time to recover. To avoid this

problem, which is often called 'integral wind-up', sophisticated controllers will

inhibit integral action until the system gets fairly close to equilibrium.

To remedy these situations it is useful to measure the rate at which the actualtemperature is changing; in other words, to measure the rate of change of thesignal. Another type of control mode is used to measure how fast the measuredvalue changes, and this is termed Rate Action or Derivative Action.

Derivative control - rate actionA Derivative action (referred to by the letter D) measures and responds to the

rate of change of process signal, and adjusts the output of the controller to

minimise overshoot.

If applied properly on systems with time lags, derivative action will minimisethe deviation from the set point when there is a change in the process condition.

It is interesting to note that derivative action will only apply itself when there is

a change in process signal. If the value is steady, whatever the offset, then

derivative action does not occur.

One useful function of the derivative function is that overshoot can be

minimised especially on fast changes in load. However, derivative action is not

easy to apply properly; if not enough is used, little benefit is achieved, and

applying too much can cause more problems than it solves.

D action is again adjustable within the controller, and referred to as TD in timeunits:

T D = 0 - Means no D action.

T D = Infinity - Means infinite D action.

P + D controllers can be obtained, but proportional offset will probably be

experienced. It is worth remembering that the main disadvantage with a P

control is the presence of offset. To overcome and remove offset, 'I' action is

introduced. The frequent existence of time lags in the control loop explains the

need for the third action D. The result is a P + I + D controller which, if properly tuned, can in most processes give a rapid and stable response, with nooffset and without overshoot.

PID controllersP and I and D are referred to as 'terms' and thus a P + I + D controller is often

referred to as a three term controller.



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Summary of modes of control

A three-term controller contains three modes of control:

Proportional (P) action with adjustable gain to obtain stability.

Reset (Integral) (I) action to compensate for offset due to load changes.

Rate (Derivative) (D) action to speed up valve movement when rapidload changes take place.

The various characteristics can be summarised, as shown in Figure 5.2.17.







Fig. 5.2.17 Summary of control modes and responses Finally, the controls engineer must try to avoid the danger of using

unnecessarily complicated controls for a specific application. The least

complicated control action, which will provide the degree of control required,should always be selected.

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Further terminology

Time constant








This is defined as: 'The time taken for a controller output to change by 63.2% of

its total due to a step (or sudden) change in process load'.

In reality, the explanation is more involved because the time constant is really

the time taken for a signal or output to achieve its final value from its initialvalue, had the original rate of increase been maintained. This concept isdepicted in Figure 5.12.18.

Fig. 5.2.18 Time

constant Example 5.2.2 A practical appreciation of the time constant

Consider two tanks of water, tank A at a temperature of 25°C, and tank B at

75°C. A sensor is placed in tank A and allowed to reach equilibrium

temperature. It is then quickly transferred to tank B. The temperature difference between the two tanks is 50°C, and 63.2% of this temperature span can be

calculated as shown below:

63.2% of 50°C = 31.6°C

The initial datum temperature was 25°C, consequently the time constant for this

simple example is the time required for the sensor to reach 56.6°C, as shown

below:

25°C + 31.6°C = 56.6°C

Hunting Often referred to as instability, cycling or oscillation. Hunting produces a



continuously changing deviation from the normal operating point. This can be

caused by:

Hunting

The proportional band being too narrow.

The integral time being too short.

The derivative time being too long.

A combination of these.

Long time constants or dead times in the control system or the processitself.

In Figure 5.2.19 the heat exchanger is oversized for the application. Accuratetemperature control will be difficult to achieve and may result in a large

proportional band in an attempt to achieve stability.

If the system load suddenly increases, the two port valve will open wider, filling

the heat exchanger with high temperature steam. The heat transfer rate increases

extremely quickly causing the water system temperature to overshoot. The rapidincrease in water temperature is picked up by the sensor and directs the two port

valve to close quickly. This causes the water temperature to fall, and the two

port valve to open again. This cycle is repeated, the cycling only ceasing when

the PID terms are adjusted. The following example (Example 5.2.3) gives anidea of the effects of a hunting steam system.

Fig. 5.2.19 Hunting

Example 5.2.3 The effect of hunting on the system in Figure 5.2.19

Consider the steam to water heat exchanger system in Figure 5.2.19. Under minimum load conditions, the size of the heat exchanger is such that it heats the



constant flowrate secondary water from 60°C to 65°C with a steam temperature

of 70°C. The controller has a set point of 65°C and a P-band of 10°C.

Consider a sudden increase in the secondary load, such that the returning water

temperature almost immediately drops by 40°C. The temperature of the water flowing out of the heat exchanger will also drop by 40°C to 25°C. The sensor detects this and, as this temperature is below the P-band, it directs the

pneumatically actuated steam valve to open fully.

The steam temperature is observed to increase from 70°C to 140°C almost

instantaneously. What is the effect on the secondary water temperature and thestability of the control system?

As demonstrated in Tutorial 13.2 (The heat load, heat exchanger and steam load

relationship), the heat exchanger temperature design constant, TDC, can becalculated from the observed operating conditions and Equation 13.2.2:

Equation 13.2.2 Where:

TDC = Temperature Design Constant

T s = Steam temperature

T 1 = Secondary fluid inlet temperature

T 2 = Secondary fluid outlet temperature

In this example, the observed conditions (at minimum load) are as follows:

When the steam temperature rises to 140°C, it is possible to predict the outlettemperature from Equation 13.2.5:



Equation 13.2.5 Where:

T s = 140°CT 1 = 60°C - 40°C = 20°C temperature

TDC = 2

The heat exchanger outlet temperature is 80°C, which is now above the P-band,and the sensor now signals the controller to shut down the steam valve.

The steam temperature falls rapidly, causing the outlet water temperature to fall;and the steam valve opens yet again. The system cycles around these

temperatures until the control parameters are changed. These symptoms are

referred to as 'hunting'. The control valve and its controller are hunting to find a

stable condition. In practice, other factors will add to the uncertainty of the

situation, such as the system size and reaction to temperature change and the

position of the sensor.

Hunting of this type can cause premature wear of system components, in

particular valves and actuators, and gives poor control.

Example 5.2.3 is not typical of a practical application. In reality, correct design

and sizing of the control system and steam heated heat exchanger would not bea problem.

Lag

Lag is a delay in response and will exist in both the control system and in the process or system under control.

Consider a small room warmed by a heater, which is controlled by a room space

thermostat. A large window is opened admitting large amounts of cold air. The

room temperature will fall but there will be a delay while the mass of the sensor

cools down to the new temperature - this is known as control lag. The delay

time is also referred to as dead time.

Having then asked for more heat from the room heater, it will be some time



before this takes effect and warms up the room to the point where the thermostatis satisfied. This is known as system lag or thermal lag.

RangeabilityThis relates to the control valve and is the ratio between the maximum

controllable flow and the minimum controllable flow, between which thecharacteristics of the valve (linear, equal percentage, quick opening) will be

maintained. With most control valves, at some point before the fully closed

position is reached, there is no longer a defined control over flow in accordance

with the valve characteristics. Reputable manufacturers will providerangeability figures for their valves.

Turndown ratioTurndown ratio is the ratio between the maximum flow and the minimum

controllable flow. It will be substantially less than the valve's rangeability if the

valve is oversized.

Although the definition relates only to the valve, it is a function of the completecontrol system.


modes of control

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