Instrument & Control

Question 1

Draw and explain the theory behind the workings of a thermocouple

Answer 1

Two different wire materials connected via a junction. As the temperatures increase a current will flow around the circuit.

Question 2

Use a diagram to explain how the theory is applied to produce a practical thermocouple temperature measuring device?

Answer 2

A third metal, copper is added to connect to a voltmeter and a junction where a set temperature can be referred to. The EMF is proportional to the difference between the hot junction and the reference of the cold junction.

Question 3

How are thermocouples manufactured to cater for different temperature ranges?

Answer 3

By using different combinations of metals

Question 4

Give two problems associated with thermocouples

Answer 4

Low accuracy

Prone to wiring faults

Question 5

Explain the principle of a PT100?

Answer 5

The resistance will change with the change of temperature

Question 6

What does PT100 stand for?

Answer 6

Platinum (PT)

Produces a resistance of 100 ohms at 0 degrees

Question 7

Why do PT100s typically have 3 or 4 wires to connect them to the temperature indicating device?

Answer 7

The are connect in bridge Circuits as the device requires signal conditioning and the resistance of the leads need to be compensated. A 4th wire is used over long distances

Question 8

With aid of a diagram, explain how a strain gauge based sensor can be used to measure differential pressure?

Answer 8

Strain gauges are glided to a diaphragm, as the diaphragm moves, distort Simon is translated into an increase in resistance. The resistance is then measured and a proportional signal is sent to the pressure controller.

Question 9

State 3 examples of signal conditioning

Answer 9

Amplification
Analogue to digital conversion
Digital to analogue conversion

Question 10

State the standard signal ranges used for electrical and pneumatic signals?

Answer 10

4mA to 20mA

0.2 to 1 bar

Question 11

With signal ranges, explain why the ranges are termed ‘live zero’?

Answer 11

Both start with a non zero value to distinguish between 0% and an error

Question 12

An actuator is used to control the flow of oil into a boiler burner. What would you consider when deciding whether to fit a fail safe or fail set actuator in this situation?

Answer 12

For a deep sea vessel I’d fit a fail set to shut of the fuel from filling the boiler with fuel and vapour.

Question 13

Explain with an aid of a sketch how the diagram motor transducers a varying air pressure signal into a mechanical movement?

Answer 13

A diaphragm actuator has a rubber membrane supported by a metal disk in a pressure tight chamber. Operating air is entered above or below the diaphragm against a spring which moves the steam up or down.

Question 14

Describe with an aid of a sketch how the process fluid pressures acting within the valve body are balanced in order to reduce the load on the diaphragm?

Answer 14

A single seat plug requires a large diaphragm force to open against the fluid. Whereas a double seat plug has the fluid trying to open and close causing an equal pressure. Therefore a smaller diaphragm force can be used.

Question 15

Explain the gain characteristics of a linear control valve?

Answer 15

The valve travel and flow rate are linear related. Therefore an equal amount of opening equals an amount of flow.

Question 16

Explain the gain characteristics of a equal percentage control valve?

Answer 16

When an equal amount of valve travel produces an equal amount of flow

Question 17

Explain the gain characteristics of a Quick Opening control valve?

Answer 17

The control valve is fitted with a flat disk. It allows for maximum flow rate with minimum valve opening

Question 18

Explain why double seated valves are used in control valves?

Answer 18

The double seated valve plug has the force of the fluid acting on both sides. This creates an equal pressure so a smaller diaphragm can be used and less operating force.

Question 19

Outline why a valve positioner maybe used

Answer 19

Valve positioners are used when the stem has high friction or the controller is some distance away causing time lags.

Question 20

State 4 circumstances why a valve positioner maybe used?

Answer 20

To minimise the effects of friction and hysteresis
Increase speed of response
To allow signal reversing
Allow valve characteristics to be changed by adjusting the rotating cam in the feedback system

Question 21

State the principle used by a positioner in control technology?

Answer 21

The positioner compares the requested valve position with the actual valve position through the feedback linkage. They have there own air supply to remove error.

Question 22

Outline the immediate action to be taken to restore automatic control upon failure of a positioner

Answer 22

Switch to manual control
Regain control
Replace parts
Ensure system is steady state
Switch back to automatic

Question 23

Outline 4 advantages of pneumatic control valves

Answer 23

Robust
Cheap to repair
Fast acting
Can be used in hazardous areas

Question 24

Outline 5 disadvantages of pneumatic control valves

Answer 24

Large
Require positioners and boosters
Require clean/dry air
Long lengths of piping
Requires complicated fail set

Question 25

Sketch a block diagram of a closed loop control circuit and describe how it works?

Answer 25

A monitoring element measures the output. It sends the information back via the feedback to the comparing element. Here it compares with the desired value. An error is set to the controlling element where this decides how to correct. The correcting element then corrects the error as it heads back to the monitoring element. The process relates to keep the desired value.

Question 26

Define the term measured value

Answer 26

This is the actual value of the process variable as measured by the relevant sensor

Question 27

Define the term desired value

Answer 27

This is the value set by the operator

Question 28

Define the term deviation

Answer 28

This is the difference between the set value and the actual valve. Also known as the error.

Question 29

Describe with the aid of a sketch, the role of an electronic proportional controller for the operation of a control valve. Give a full answer as you can taking into account it’s operation and the type of control afforded

Answer 29

The regulate a process, such as, flow, temperature, pressure and level. A PID controller with Integral and Deviation turned off will create a proportional action. The system will have a set point ( desired value). The monitoring element will measure the output and feedback the information to the controller where it is then compared to the set point. If there is a difference an error will be created which with then go to the controlling element which will creat a plan to fix. This is then transferred to the correcting element which eliminates the error. The monitoring element then measures the output again and feedbacks the new information.

Question 30

Explain the meaning of a two step controller

Answer 30

This is known as a on/off control. This works by the controller switching the energy in and out of the process. Allow energy to be transferred only when needed.

Question 31

Explain the meaning of proportional control action

Answer 31

This produces an output that is directly proportional to the error between the set point and measured value

Question 32

Explain the meaning of integral control action

Answer 32

Steadily increases the corrective action as long as an error exists.

Question 33

State with reason, a typical ship board application for two step control action

Answer 33

For non critical processes where accuracy is not required. Such as air Compressors and hot water.

Question 34

State with reason, a typical ship board application for proportional plus integral action

Answer 34

Engine jacket water cooling. The integral removes the offset but because reaction time isn’t important, differential control is not required

Question 35

Use a diagram to explain how two step control can be used to regulate the temperature of a water heater

Answer 35

A two step is an on/off system, allowing energy only when needed. The monitoring element measures the temperature of the water and feedbacks the information to the controller which compares to the desired value. The difference between the actual value and the desired will decide if the the process is needed.

Question 36

Show how a water heater system will respond to an increase in demand from the heater

Answer 36

The system will turn on and off to keep the water at the desired value with in the systems deadband. An increase in demand will cause a lot of new cold water enter then tank, dropping the temperature within the tank. The heater will now be on for a longer period to get back to the desired value.

Question 37

Use a diagram to explain the term deadband and explain the results if the deadband is too wide or too narrow.

Answer 37

A deadband is when the temperature is not stable along the set point. Therefore a two step controller will turn on and off to allow it to fluctuate around the set point. If the deadband is to narrow then the system will cut on and off regularly causing instability. If the deadband is to wide the stability of the system increase but we lose out on accuracy of the system.

Question 38

List the instrumentation necessary to convert a simple manually converted flow control system into an automatic closed loop system giving a brief overview of each item listed.

Answer 38

Measuring element - monitors the output of the process Comparing element- compares the difference between the actual value and the desired value. Then creates an error to correct Controlling element - receives and acts apron the error signal Correcting element - corrects the error by the method approved by the controlling element

Question 39

Sketch a basic pneumatic amplifier, label the main parts

Answer 39

``` Air supply Oriffice Output pressure Nozzle Flapper ```

Question 40

State two examples where pneumatic amplifiers maybe used?

Answer 40

- sensor control systems where a small signal change from a sensor must be amplified to provide a proportional control pressure - multiply a signal from one controller to operate several actuators in sequence

Question 41

Sketch a pneumatic transmitter of the flapper/ nozzle type incorporating feedback, labelling all components

Answer 41

``` Air supply Nozzle Flapper Actual value Desired value Gain adjustments ```

Question 42

Explain what PLC’s are, highlighting any of their advantages?

Answer 42

A Programmable Logic Controller PLC is a digital electronic device that uses programmable memory to store instructions and to implement specific functions, such as sequences and timings. The program can be modified with features added and removed without requiring devices making it a flexible system.

Question 43

Explain a cascade control system

Answer 43

When two parameters are monitored on the same system. This is also known as a master and slave controller

Question 44

Explain a split range control system

Answer 44

Where there is one measurement but more than one manipulated variable

Question 45

Explain the term distance velocity lag

Answer 45

The time delay due to the distance between the disturbance and the monitoring element

Question 46

Explain the term Transfer lag

Answer 46

Time taken for energy to be transferred from one medium to another

Question 47

For a control system fitted with a P + I controller:- Describe how the proportional band and integral time can be adjusted, with the system in operation, to give slight under-damping and without offset.

Answer 47

* Turn off integral and derivative control. * Increase proportional gain until the measured variable oscillates. * Reduce the proportional gain until the measured variable stabilises. * Adjust integral control until the measured variable just oscillates. * Back off the proportional gain to stabilise the measured variable.

Question 48

Describe how Ziegler & Nichols tuning could be applied to a P+I Controller

Answer 48

When the system is just oscillating, record the gain setting (KU) and measure the time period of the oscillation (TU) and use these values along with the appropriate Ziegler & Nichols table to obtain the required dial settings for KP and TI.

Question 49

Draw a sketch of a Quarter Decay system response giving one advantage and one disadvantage with this type of response.

Answer 49

Removes error quickly | Poor stability

Question 50

Describe why we need to tune a system

Answer 50

Every system is slightly different, therefore it is unique and requires to be able to tune different amounts of gain and integral action

Question 51

Explain with the aid of sketches, the difference between under damped, over damped and critically damped system performance.

Answer 51

Underdamped will respond very quickly but create a large overshoot which will take a long time to settle out. Critical damped, after change it will return to steady state in the quickest time possible without any overshoot. Over damped will be slower than critical damped to return to steady state but again will not over shoot but takes a long time to get back to steady state

Question 52

What test could you carry out to verify the performance curve of a P+I system?

Answer 52

While monitoring the system, stop a pump or closes a valve and monitor how quickly the system responds to the change

Question 53

Explain how you would tune a P+I controller using: The Trial and Error Method.

Answer 53

1. Turn off integral action (set to max) and derivative action (set to min). 2. Decrease gain to a low value (0.5) and then increase in stages, until process becomes unstable. 3. Reduce gain until process just stabilises. 4. With the system stable, slowly increase the integral action until the offset is eliminated and the system just starts to become unstable. 5. Decrease the gain slightly until stability is regained.

Question 54

Explain how you would tune a P+I controller using: The Ziegler and Nichols Ultimate Cycle Tuning Method.

Answer 54

1. Turn off integral action (set to max) and derivative action (set to min). 2. Decrease gain to a low value (0.5) and then increase in stages, until process becomes unstable. Note down the gain setting (KU) and the oscillation cycle time (TU). 3. Use appropriate Z & N chart along with the recorded values, to determine the optimum settings for P or P+I as required.

Question 55

List 7 Diesel Engine process variables

Answer 55

``` Output Speed Load Torque Fuel Flow LO Pressure JW Temperature Bearing Temperature Cylinder Pressure ```

Question 56

Define Steady State

Answer 56

The energy supplied to the system is equal to the energy used and output by that system

Question 57

Define Process Stability

Answer 57

Ability of the process to achieve steady state after a change in operating conditions

Question 58

Examples of Open Loop Control System

Answer 58

Washing Machine Cycle Toaster Microwave

Question 59

Advantages and Disadvantages of an Open loop system

Answer 59

Simple and cheap to set up | highly inaccurate

Question 60

Advantages and Disadvantages of an Closed loop system

Answer 60

Potentially unstable Complicated and expensive to set up Very accurate

Question 61

Types of measuring sensors

Answer 61

``` Pressure Temperature Level Flow Displacement Speed Torque Viscosity ```

Question 62

Draw and describe a measuring sensing system

Answer 62

A measuring system will include a sensor, transducer, conditioner, transmitter and a display. This senses the condition of the process and the information can be used or interpreted.

Question 63

Define Discrete in measuring systems

Answer 63

A discrete system produces single on/off pulses or individual steps.

Question 64

Define Analogue in measuring systems

Answer 64

An analogue system produces an infinitely variable signal between the limits of the working range.

Question 65

Define Digital in measuring systems

Answer 65

A digital system produces a combination of on/off signals (binary code) that replicates a stepped analogue response

Question 66

Methods of measurement sensing changes

Answer 66

``` Mechanical movement Elastic Distortion Expansion/ Contraction Change in thermal Properties Change in electrical properties ```

Question 67

What calibration record would an instrument typically contain

Answer 67

Description of the instrument and identification number Serial Number Fitting date Date of last calibration Result of last calibration Details of calibration procedure and acceptable error Details of repairs and adjustments made

Question 68

What two ways can pressure be measured

Answer 68

Absolute pressure relative to a perfect vacuum (Abs) | Pressure relative to atmospheric pressure (Gauge)

Question 69

Draw 3 types of diaphragms

Answer 69

Simple flat diaphragms Corrugated diaphragms Corrugated bellows

Question 70

Explain with a sketch a strain gauge

Answer 70

As the pressure changes across the diaphragm, it will deflect causing it to expand and contract. this will change the length of the sensors, which then changes the resistance properties.

Question 71

Explain with a sketch a Piezo-Electric pressure sensor

Answer 71

It uses rock crystals which when they are compressed produce an electrical discharge. This is due to the crystals moving across each other. The larger pressure means a greater electrical discharge. The sensor can measure rapid changed in pressure.

Question 72

Whats a PT100s temperature range

Answer 72

-200 to +600 degrees C

Question 73

Draw and Explain, how an orifice plate measure flow?

Answer 73

It restricts the flow of liquid through a pipe. This changes the liquids pressure and velocity which is measured by sensors before and after.

Question 74

What has to be thought of when fitting an orifice plate?

Answer 74

It must be 12 pipe diameters of pipe length down stream of obstructions and bends. Then it must have 5 pipe diameters of pipe length after the plate to reduce erosion.

Question 75

Draw and explain how a turbine flow meter works?

Answer 75

The fluid spins the rotor. As this spins the magnet sends a signal to the detector. These frequencies are then related to a flow rate.

Question 76

Draw and explain a Coriolis Flow meter

Answer 76

A fluid flows through the vibrating tube the liquid causes the tube to twist. The positions pickups can the measure the amount of twist which is relative to flow.

Question 77

Draw and explain a Viscosity sensing device

Answer 77

A gear pump pushes the liquid through a capillary where it measures the pressure drop which relate to the viscosity via a D.P Cell. The D.P Cell sends a signal to the heater to adjust the viscosity.

Question 78

Draw and explain hydrostatic pressure level sensor

Answer 78

It compares the hydrostatic pressure of the liquid at the bottom of the tank and the atmospheric pressure at the top of the tank.

Question 79

Draw and explain an electro optical sensor

Answer 79

A light shines onto a disk with holes. As the light passes the holes it creates light pulses which are received by a photo transistor. This then senses the movement.

Question 80

Sketch the 4 different actuator valves

Answer 80

Percentage V Port Percentage Parabolic Linear Quick Opening

Question 81

What is fail safe

Answer 81

This will fail fully open or fully shut depending on the system

Question 82

What is fail set

Answer 82

This will fail in the position it was operating in when failed

Question 83

Draw and table some of the ladder logic symbols

Answer 83

Relay Coil Normally Open Contact Normally Closed Contact

Question 84

Describe timer blocks in PLC systems

Answer 84

Can be used to set time delays or time operation times

Question 85

Describe counter blocks in PLC systems

Answer 85

Used to count the number of times a pump has started for maintenance logs

Question 86

Describe specialised blocks in PLC systems

Answer 86

Carryout maths functions or PID Controls

Question 87

Draw and Explain a boiler water level control system

Answer 87

As the level detector changes this informs the P+I controller which is also told the desired value. This control tells the I/P controller to open the feed water valve more to top the boiler water up. The level detects the change and the process is repeated.

Question 88

Draw and Explain a ME Cooling water temperature control system

Answer 88

JCW leaves the ME while a P+I controller has a thermocouple temperature sensor on the inlet to ME. This tells the P+I controller to either cool the JCW via a three way valve to the SW cooler or bypass the cooler if still cool enough.