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The Steam and Condensate Loop

An Introduction to Controls Module 5.1

5.1.2

Block 5 Basic Control Theory

An Introduction to Controls

The subject of automatic controls is enormous, covering the control of variables such astemperature, pressure, flow, level, and speed.

The objective of this Block is to provide an introduction to automatic controls. This too can bedivided into two parts:

o The control of Heating, Ventilating and Air Conditioning systems (commonly known as HVAC);and

o Process control.

Both are immense subjects, the latter ranging from the control of a simple domestic cooker to acomplete production system or process, as may be found in a large petrochemical complex.

The Controls Engineer needs to have various skills at his command - knowledge of mechanicalengineering, electrical engineering, electronics and pneumatic systems, a working understandingof HVAC design and process applications and, increasingly today, an understanding of computersand digital communications.

The intention of this Block is to provide a basic insight into the practical and theoretical facets ofautomatic control, to which other skills can be added in the future, not to transform an individualinto a Controls Engineer

This Block is confined to the control of processes that utilise the following fluids: steam, water,compressed air and hot oils.

Control is generally achieved by varying fluid flow using actuated valves. For the fluids mentionedabove, the usual requirement is to measure and respond to changes in temperature, pressure,level, humidity and flowrate. Almost always, the response to changes in these physical propertiesmust be within a given time. The combined manipulation of the valve and its actuator with time,and the close control of the measured variable, will be explained later in this Block.

The control of fluids is not confined to valves. Some process streams are manipulated by theaction of variable speed pumps or fans.

The need for automatic controls

There are three major reasons why process plant or buildings require automatic controls:

ooooo Safety - The plant or process must be safe to operate.The more complex or dangerous the plant or process, the greater is the need for automaticcontrols and safeguard protocol.

ooooo Stability - The plant or processes should work steadily, predictably and repeatably, withoutfluctuations or unplanned shutdowns.

ooooo Accuracy - This is a primary requirement in factories and buildings to prevent spoilage,increase quality and production rates, and maintain comfort. These are the fundamentalsof economic efficiency.

Other desirable benefits such as economy, speed, and reliability are also important, but it isagainst the three major parameters of safety, stability and accuracy that each control applicationwill be measured.

Automatic control terminologySpecific terms are used within the controls industry, primarily to avoid confusion. The samewords and phrases come together in all aspects of controls, and when used correctly, their meaningis universal.

The simple manual system described in Example 5.1.1 and illustrated in Figure 5.1.1 is used tointroduce some standard terms used in control engineering.

The Steam and Condensate Loop 5.1.3

An Introduction to Controls Module 5.1Block 5 Basic Control Theory

Example 5.1.1 A simple analogy of a control systemIn the process example shown (Figure5.1.1), the operator manually varies the flow of water byopening or closing an inlet valve to ensure that:o The water level is not too high; or it will run to waste via the overflow.o The water level is not too low; or it will not cover the bottom of the tank.

The outcome of this is that the water runs out of the tank at a rate within a required range. If thewater runs out at too high or too low a rate, the process it is feeding cannot operate properly.

At an initial stage, the outlet valve in the discharge pipe is fixed at a certain position.

The operator has marked three lines on the side of the tank to enable him to manipulate thewater supply via the inlet valve. The 3 levels represent:

1. The lowest allowable water level to ensure the bottom of the tank is covered.

2. The highest allowable water level to ensure there is no discharge through the overflow.

3. The ideal level between 1 and 2.

Fig. 5.1.1 Manual control of a simple process

Inlet valve

Visual indicator2

3

1

Overflow

Discharge valve(fixed position)

Final product

Water

The Example (Figure 5.1.1) demonstrates that:1. The operator is aiming to maintain the water in the vessel between levels 1 and 2. The water

level is called the Controlled condition.

2. The controlled condition is achieved by controlling the flow of water through the valve in theinlet pipe. The flow is known as the Manipulated Variable, and the valve is referred to as theControlled Device.

3. The water itself is known as the Control Agent.

4. By controlling the flow of water into the tank, the level of water in the tank is altered. Thechange in water level is known as the Controlled Variable.

5. Once the water is in the tank it is known as the Controlled Medium.

6. The level of water trying to be maintained on the visual indicator is known as the Set Value(also known as the Set Point).

7. The water level can be maintained at any point between 1 and 2 on the visual indicator andstill meet the control parameters such that the bottom of the tank is covered and there is nooverflow. Any value within this range is known as the Desired Value.

8. Assume the level is strictly maintained at any point between 1 and 2. This is the water level atsteady state conditions, referred to as the Control Value or Actual Value.

Note: With reference to (7) and (8) above, the ideal level of water to be maintained was atpoint 3. But if the actual level is at any point between 1 and 2, then that is still satisfactory.The difference between the Set Point and the Actual Value is known as Deviation.

9. If the inlet valve is closed to a new position, the water level will drop and the deviation willchange. A sustained deviation is known as Offset.



5.1.4


Elements of automatic control

Controller(Brain)

Actuator(Arm muscle)

Sensor(Eye)

Process(Tank)

Controlled device(Valve)

Outputsignal

Inputsignal

Desiredvalue

Controlled conditionManipulated variable

Fig. 5.1.2 Elements of automatic control

Example 5.1.2 Elements of automatic controlo The operators eye detects movement of the water level against the marked scale indicator.

His eye could be thought of as a Sensor.

o The eye (sensor) signals this information back to the brain, which notices a deviation. Thebrain could be thought of as a Controller.

o The brain (controller) acts to send a signal to the arm muscle and hand, which could bethought of as an Actuator.

o The arm muscle and hand (actuator) turn the valve, which could be thought of as a ControlledDevice.

It is worth repeating these points in a slightly different way to reinforce Example 5.1.2:

In simple terms the operators aim in Example 5.1.1 is to hold the water within the tank at apre-defined level. Level 3 can be considered to be his target or Set Point.

The operator physically manipulates the level by adjusting the inlet valve (the control device).Within this operation it is necessary to take the operators competence and concentration intoaccount. Because of this, it is unlikely that the water level will be exactly at Level 3 at all times.Generally, it will be at a point above or below Level 3. The position or level at any particularmoment is termed the Control Value or Actual Value.

The amount of error or difference between the Set Point and the Actual Value is termed deviation.When a deviation is constant, or steady state, it is termed Sustained Deviation or Offset.

Although the operator is manipulating the water level, the final aim is to generate a properoutcome, in this case, a required flow of water from the tank.

Assessing safety, stability and accuracy

It can be assumed that a process typical of that in Example 5.1.1 contains neither valuable norharmful ingredients. Therefore, overflow or water starvation will be safe, but not economic orproductive.

In terms of stability, the operator would be able to handle this process providing he pays full andconstant attention.

Accuracy is not a feature of this process because the operator can only respond to a visible andrecognisable error.



Summary of terminology

Set pointThe value set on the scale of the control system in order to obtain the required condition.If the controller was set at 60C for a particular application: 60C would be termed as the set point.

Desired value The required value that should be sustained under ideal conditions.Control value The value of the control condition actually maintained under steady state conditions.Deviation The difference between the set point and the control value.Offset Sustained deviation.Sensor The element that responds directly to the magnitude of the controlled condition.

Controlled medium The medium being controlled by the system. The controlled medium in Figure 5.1.1 is thewater in the tank.

Controlled conditionThe physical condition of the controlled medium.The controlled condition in Figure 5.1.1 is the water level.

Controller A device which accepts the signal from the sensor and sends a corrective (or controlling)signal to the actuator.

Actuator The element that adjusts the controlled device in response to a signal from the controller.

Controlled deviceThe final controlling element in a control system, such as a control valve or a variablespeed pump.

There are many other terms used in Automatic Controls; these will be explained later in thisBlock.

Elements of a temperature control system

Example 5.1.1 depicted a simple manual level control system. This can be compared with asimple temperature control example as shown in Example 5.1.3 (manually controlled) and Figure5.1.3. All the previous factors and definitions apply.

Example 5.1.3 Depicting a simple manual temperature control systemThe task is to admit sufficient steam (the heating medium) to heat the incoming water from atemperature of T1; ensuring that hot water leaves the tank at a required temperature of T2.

Fig. 5.1.3 Simple manual temperature control

Hot water to process (T2)

Steam

Steam trap set

Cold water(T1)

Thermometer

Closed vesselfull of water

Coil heat exchanger

Alarm

Thermometer



5.1.6


Assessing safety, stability and accuracyWhilst manual operation could probably control the water level in Example 5.1.1, the manualcontrol of temperature is inherently more difficult in Example 5.1.3 for various reasons.

If the flow of water varies, conditions will tend to change rapidly due to the large amount of heatheld in the steam. The operators response in changing the position of the steam valve maysimply not be quick enough. Even after the valve is closed, the coil will still contain a quantity ofresidual steam, which will continue to give up its heat by condensing.

Anticipating changeExperience will help but in general the operator will not be able to anticipate change. He mustobserve change before making a decision and performing an action.

This and other factors, such as the inconvenience and cost of a human operator permanently onduty, potential operator error, variations in process needs, accuracy, rapid changes in conditionsand the involvement of several processes, all lead to the need for automatic controls.

With regards to safety, an audible alarm has been introduced in Example 5.1.3 to warn ofovertemperature - another reason for automatic controls.

Automatic controlA controlled condition might be temperature, pressure, humidity, level, or flow. This means thatthe measuring element could be a temperature sensor, a pressure transducer or transmitter, alevel detector, a humidity sensor or a flow sensor.

The manipulated variable could be steam, water, air, electricity, oil or gas, whilst the controlleddevice could be a valve, damper, pump or fan.

For the purposes of demonstrating the basic principles, this Module will concentrate on valves asthe controlled device and temperature as the controlled condition, with temperature sensors asthe measuring element.

Components of an automatic controlFigure 5.1.4 illustrates the component parts of a basic control system. The sensor signals to thecontroller. The controller, which may take signals from more than one sensor, determines whethera change is required in the manipulated variable, based on these signal(s). It then commands theactuator to move the valve to a different position; more open or more closed depending on therequirement.

Fig. 5.1.4 Components of an automatic control

Sensor Controller Actuator

Valve

Controllers are generally classified by the sources of energy that power them, electrical, pneumatic,hydraulic or mechanical.

An actuator can be thought of as a motor. Actuators are also classified by the sources of energythat power them, in the same way as controllers.



Valves are classified by the action they use to effect an opening or closing of the flow orifice, andby their body configurations, for example whether they consist of a sliding spindle or have arotary movement.

If the system elements are combined with the system parts (or devices) the relationship betweenWhat needs to be done? with How does it do it?, can be seen.

Some of the terms used may not yet be familiar. However, in the following parts of Block 5, allthe individual components and items shown on the previous drawing will be addressed.

Fig. 5.1.5 Typical mix of process control devices with system elements

Set pointControl knob / remotepotentiometer

Controller

Measuringelement

Controlledelement

ProcessControlled

device

Measured variablePressure / temperature signal

Temperature /pressure /humidity sensor

Controlled condition

Vat, heat exchanger, steriliser2-port / 3-port valve

Pneumatic /electric /

SA actuator

Manipulated variableCompressed air (0.2 to 1.0 bar)

Electric current 4 to 20 mA

Proportional (P)Proportional + Integral (P+I)

Proportional + Integral + Derivative(P+I+D)

Manipulatedvariable



5.1.8


Answers

Questions

1. Air temperature in a room is controlled at 25C. If the actual temperature varies fromthis, what term is used to define the difference?

a| Offset b| Deviation c| Sustained deviation d| Desired value 2. A pneumatic temperature control is used on the steam supply to a non-storage heat

exchanger that heats water serving an office heating system. What is referred to asthe manipulated variable?

a| The water being heated b| The steam supply c| The air signal from the controller to the valve actuator d| The temperature of the air being heated 3. If an automatic control is to be selected and sized, what is the most important aspect to

consider?

a| Safety in the event of a power failure b| Accuracy of control c| Stability of control d| All of them 4. Define control value?

a| The value set on the scale of the control system in order to obtain the required condition b| The quantity or condition of the controlled medium c| The flow or pressure of the steam (or fluid) being manipulated d| The value of the controlled condition actually maintained under steady state conditions 5. An electronic controller sends a signal to an electric actuator fitted to a valve on the

steam supply to a coil in a tank of water. In control terms, how is the water described?

a| Control agent b| Manipulated variable c| Controlled medium d| Controlled variable 6. With reference to Question 5, the controller is set to maintain the water temperature at

80oC, but at a particular time it is 70oC. In control terms how is the temperature of 80oCdescribed?

a| Controlled condition b| Control value c| Set value d| Control point

1: b 2: b, 3: d, 4: d, 5: a, 6: c

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Introduction to Steam Distribution Module 10.1

10.1.2

Block 10 Steam Distribution

Introduction to Steam Distribution

The steam distribution system is the essential link between the steam generator and the steamuser.

This Module will look at methods of carrying steam from a central source to the point of use. Thecentral source might be a boiler house or the discharge from a co-generation plant. The boilersmay burn primary fuel, or be waste heat boilers using exhaust gases from high temperatureprocesses, engines or even incinerators. Whatever the source, an efficient steam distributionsystem is essential if steam of the right quality and pressure is to be supplied, in the right quantity,to the steam using equipment. Installation and maintenance of the steam system are importantissues, and must be considered at the design stage.

Steam system basicsFrom the outset, an understanding of the basic steam circuit, or steam and condensate loop isrequired see Figure 10.1.1. As steam condenses in a process, flow is induced in the supplypipe. Condensate has a very small volume compared to the steam, and this causes a pressuredrop, which causes the steam to flow through the pipes.

The steam generated in the boiler must be conveyed through pipework to the point where itsheat energy is required. Initially there will be one or more main pipes, or steam mains, whichcarry steam from the boiler in the general direction of the steam using plant. Smaller branchpipes can then carry the steam to the individual pieces of equipment.

When the boiler main isolating valve (commonly called the crown valve) is opened, steamimmediately passes from the boiler into and along the steam mains to the points at lower pressure.The pipework is initially cooler than the steam, so heat is transferred from the steam to the pipe.The air surrounding the pipes is also cooler than the steam, so the pipework will begin to transferheat to the air.

Steam on contact with the cooler pipes will begin to condense immediately. On start-up of thesystem, the condensing rate will be at its maximum, as this is the time where there is maximumtemperature difference between the steam and the pipework. This condensing rate is commonlycalled the starting load. Once the pipework has warmed up, the temperature difference betweenthe steam and pipework is minimal, but some condensation will occur as the pipework stillcontinues to transfer heat to the surrounding air. This condensing rate is commonly called therunning load.

Fig. 10.1.1 A typical basic steam circuit

Steam

Steam

Ste

am

Steam

Cond

ensate

Cond

ensate

Condensate

Make-up

water

Feedpump Feedtank

Pan Pan

Space

heating

system

Condensate

Process

vessel


Introduction to Steam Distribution Module 10.1Block 10 Steam Distribution

The resulting condensation (condensate) falls to the bottom of the pipe and is carried along bythe steam flow and assisted by gravity, due to the gradient in the steam main that should bearranged to fall in the direction of steam flow. The condensate will then have to be drained fromvarious strategic points in the steam main.

When the valve on the steam pipe serving an item of steam using plant is opened, steam flowingfrom the distribution system enters the plant and again comes into contact with cooler surfaces.The steam then transfers its energy in warming up the equipment and product (starting load),and, when up to temperature, continues to transfer heat to the process (running load).

There is now a continuous supply of steam from the boiler to satisfy the connected load and tomaintain this supply more steam must be generated. In order to do this, more water (and fuel toheat this water) is supplied to the boiler to make up for that water which has previously beenevaporated into steam.

The condensate formed in both the steam distribution pipework and in the process equipmentis a convenient supply of useable hot boiler feedwater. Although it is important to remove thiscondensate from the steam space, it is a valuable commodity and should not be allowed torun to waste. Returning all condensate to the boiler feedtank closes the basic steam loop, andshould be practised wherever practical. The return of condensate to the boiler is discussedfurther in Block 13, Condensate Removal, and Block 14,Condensate Management.

The working pressureThe distribution pressure of steam is influenced by a number of factors, but is limited by:

o The maximum safe working pressure of the boiler.

o The minimum pressure required at the plant.

As steam passes through the distribution pipework, it will inevitably lose pressure due to:

o Frictional resistance within the pipework (detailed in Module 10.2).

o Condensation within the pipework as heat is transferred to the environment.

Therefore allowance should be made for this pressure loss when deciding upon the initialdistribution pressure.

A kilogram of steam at a higher pressure occupies less volume than at a lower pressure. It followsthat, if steam is generated in the boiler at a high pressure and also distributed at a high pressure,the size of the distribution mains will be smaller than that for a low-pressure system for the sameheat load. Figure 10.1.2 illustrates this point.

Generating and distributing steam at higher pressure offers three important advantages:

o The thermal storage capacity of the boiler is increased, helping it to cope more efficiently withfluctuating loads, minimising the risk of producing wet and dirty steam.

o Smaller bore steam mains are required, resulting in lower capital cost, for materials such aspipes, flanges, supports, insulation and labour.

o Smaller bore steam mains cost less to insulate.

Fig. 10.1.2 Dry saturated steam - pressure /specific volume relationship



10.1.4


Having distributed at a high pressure, it will be necessary to reduce the steam pressure to eachzone or point of use in the system in order to correspond with the maximum pressure requiredby the application. Local pressure reduction to suit individual plant will also result in drier steamat the point of use. (Module 2.3 provides an explanation of this).

Note: It is sometimes thought that running a steam boiler at a lower pressure than its ratedpressure will save fuel. This logic is based on more fuel being needed to raise steam to a higherpressure.

Whilst there is an element of truth in this logic, it should be remembered that it is the connectedload, and not the boiler output, which determines the rate at which energy is used. The sameamount of energy is used by the load whether the boiler raises steam at 4 bar g, 10 bar g or100 bar g. Standing losses, flue losses, and running losses are increased by operating at higherpressures, but these losses are reduced by insulation and proper condensate return systems.These losses are marginal when compared to the benefits of distributing steam at high pressure.

Pressure reductionThe common method for reducing pressure at the point where steam is to be used is to use apressure reducing valve, similar to the one shown in the pressure reducing station Figure 10.1.3.

A separator is installed upstream of the reducing valve to remove entrained water from incomingwet steam, thereby ensuring high quality steam to pass through the reducing valve. This is discussedin more detail in Module 9.3 and Module 12.5.

Plant downstream of the pressure reducing valve is protected by a safety valve. If the pressurereducing valve fails, the downstream pressure may rise above the maximum allowable workingpressure of the steam using equipment. This, in turn, may permanently damage the equipment,and, more importantly, constitute a danger to personnel.

With a safety valve fitted, any excess pressure is vented through the valve, and will prevent thisfrom happening (safety valves are discussed in Block 9).

Other components included in the pressure reducing valve station are:

o The primary isolating valve - To shut the system down for maintenance.

o The primary pressure gauge - To monitor the integrity of supply.

o The strainer - To keep the system clean.

o The secondary pressure gauge - To set and monitor the downstream pressure.

o The secondary isolating valve - To assist in setting the downstream pressure on no-loadconditions.

Fig. 10.1.3 Typical pressure reducing valve station

Steam

Separator

Strainer

Pressurereducing valve

Safety valve

Steam

CondensateTrap set


Introduction to Steam Distribution Module 10.1Block 10 Steam Distribution

Questions

1. Distributing steam at high pressure, instead of low pressure, will have the followingeffect.

a | Heat losses from the pipes will be less.

b | A lower storage capacity in the high pressure pipes.

c | High pressure small bore steam pipes cost less to install and insulate.

d | The steam pipes will be smaller creating wet steam.

2. A steam pressure reducing valve is fitted to:

a | Prevent the pressure at the plant exceeding its safe working pressure.

b | Help dry the steam supply to the plant.

c | Reduce the flash steam losses as condensate passes through the plant steam traps.

d | Supply the plant with steam at the designed temperature and pressure.

3. The start-up condensate load of a steam main is generally greater than the running loadbecause:

a | The pipework and fittings are cold, so steam is required to heat it up to steamtemperature.

b | The steam space within the pipework has to be charged with steam to thedesired running pressure.

c | The boiler crown valve or stop valve is opened very slowly and initially thereis insufficient pressure to discharge condensate through the steam traps.

d | On initial opening of the crown valve, the steam distribution pressure will be lowand the enthalpy of evaporation of low pressure steam is greater than at high pressureso a greater mass of steam will be condensed.

4. The pressure at which steam is supplied to the plant should be dictated by:

a | The boiler operating pressure.

b | The steam distribution pressure.

c | The maximum allowable safe working pressure of the plant.

d | The plant design pressure and temperature.

5. Which of the following results in pressure losses in distribution pipework?

a | Sizing the pipes on low pressure instead of high pressure.

b | Frictional resistance within and heat loss from the pipe and fittings.

c | Sizing the pipes on start-up load of the plant.

d | Large steam users.

6. The steam pipe after a pressure reducing valve is likely to be:

a | Smaller than the upstream pipe because of the smaller volume of low pressure steam.

b | The same size as the connection to the plant.

c | Larger than the upstream pipe because the volume of the low pressure steamis greater.

d | The same size as the upstream pipe because the flowrate through each pipeis the same.

1: c, 2: d, 3: a, 4: d, 5: b 6: c

Answers



10.1.6


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