4th Class Power Engineering instrumentation and controls practice questions (4A).

Five free practice questions on instrumentation and controls: unit I of paper A (4A) of the 4th Class Power Engineer certificate exam, written to the SOPEEC syllabus. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · Describe the various means by which control signals are transmitted and the function of transducers

   A controller has a 4–20 mA output, and the diaphragm actuator on a fuel-oil control valve needs a 20–100 kPa air signal. Which device converts the controller output for the actuator?

   1. A pressure transmitter
   2. A solenoid valve
   3. A volume booster
   4. An I/P transducer
   Show the answer

   D. Correct. A current-to-pressure (I/P) transducer converts the 4–20 mA controller output into a proportional 20–100 kPa air signal (4 mA gives 20 kPa, 20 mA gives 100 kPa), and the diaphragm actuator positions the valve from that air pressure.

   Why not the others

   A. A pressure transmitter senses a process pressure and sends a signal out: it works the other way round. The device needed takes the controller's current and gives a matching air pressure, an I/P (current-to-pressure) transducer.

   B. A solenoid valve switches air fully on or fully off from an electrical signal. A modulating valve needs an air pressure that varies smoothly with the 4–20 mA signal, which an I/P transducer gives.

   C. A volume booster passes a larger air flow at the pressure of the air signal it receives, to stroke a large actuator faster. It takes an air signal in, not a current, so it cannot convert the 4–20 mA output.

2. Procedural · Describe the types of flow sensing and measuring devices

   An orifice plate in a boiler's steam line gives a differential pressure of 50.0 kPa at a flow of 24 t/h. What differential pressure will it give at 18 t/h, to the nearest 0.1 kPa?

   1. 3.1 kPa
   2. 28.1 kPa
   3. 37.5 kPa
   4. 43.3 kPa
   Show the answer

   B. Correct. Flow through an orifice is proportional to the square root of the differential pressure, so DP varies with flow squared: ΔP = 50.0 × (18 ÷ 24)² = 50.0 × 0.5625 = 28.1 kPa. At three-quarters of the flow the DP is only about 56 % of its full value.

   Why not the others

   A. This squares the drop in flow instead of the flow ratio: 50.0 × (6 ÷ 24)² = 50.0 × 0.0625 = 3.1 kPa. The DP follows the square of the flow itself: 50.0 × (18 ÷ 24)² = 28.1 kPa.

   C. This treats DP as proportional to flow: 50.0 × 18 ÷ 24 = 37.5 kPa. Flow varies with the square root of DP, so DP varies with flow squared: 50.0 × 0.75² = 28.1 kPa.

   D. This applies the square root the wrong way: 50.0 × √0.75 = 43.3 kPa. Flow is proportional to √ΔP, so ΔP is proportional to flow squared: 50.0 × 0.75² = 28.1 kPa.

3. Procedural · Describe the types of temperature sensing and measuring devices

   A temperature sensor is needed in the furnace outlet gas of a refuse-fired boiler, where the gas reaches about 1 100 °C. Which sensor suits this duty?

   1. A platinum resistance detector (RTD)
   2. A bimetallic thermometer
   3. A sheathed thermocouple
   4. A thermistor
   Show the answer

   C. Correct. A thermocouple produces a small voltage from the junction of two dissimilar metals, set by the temperature difference between its hot and reference junctions. Base-metal and noble-metal types cover furnace gas temperatures, with a sheath and protection tube shielding the wires.

   Why not the others

   A. Platinum RTDs are very accurate, but standard industrial RTDs are built for temperatures well below 1 100 °C; the element and its insulation would drift or fail there. A sheathed thermocouple in a protection tube is used.

   B. A bimetallic thermometer bends a strip of two metals with different expansion rates to move a pointer. It is a local dial instrument for moderate temperatures, far below 1 100 °C.

   D. A thermistor is a semiconductor whose resistance changes sharply with temperature, useful over narrow, low ranges. It cannot survive furnace-gas temperatures near 1 100 °C.

4. Recall · Discuss how programmable controllers work and how they act as sequencers for equipment

   In each scan, a PLC works through its main tasks in which order?

   1. Solve the program, read inputs, update outputs
   2. Read inputs, update outputs, solve the program
   3. Update outputs, solve the program, read inputs
   4. Read inputs, solve the program, update outputs
   Show the answer

   D. Correct. The PLC copies all input states into memory (the input image), solves the program from that snapshot, then writes the results to the outputs; it also does housekeeping and communications, and repeats. A change at an input is acted on in the next scan.

   Why not the others

   A. Solving the program before reading the inputs would act on the previous scan's input states. The PLC reads all inputs first, solves the program from that snapshot, then updates the outputs.

   B. Outputs are written only after the program has been solved, so that they carry this scan's results. The order is read inputs, solve the program, update outputs.

   C. Solving the program before reading the inputs, and writing outputs before solving, would both act on stale data. The cycle is read inputs, solve the program, update outputs.

5. Recall · Describe computers and how they operate within control systems

   In a distributed control system (DCS), the loops are spread over several controllers linked by a data highway rather than held in one central computer. The main benefit is that

   1. one failed controller affects only its own loops
   2. the loops no longer need to be tuned by anyone
   3. the data highway carries all the field 4–20 mA signals
   4. every controller runs the whole plant's logic
   Show the answer

   A. Correct. Spreading the loops over several controllers means a single controller failure affects only the loops it handles, instead of the whole plant as with one central computer. Critical controllers are often also duplicated with a standby.

   Why not the others

   B. Every loop in a DCS still needs tuning for its process; distributing the control changes where it runs, not how it is tuned.

   C. The data highway carries digital messages between controllers and operator stations. Field 4–20 mA signals are wired to the I/O of the controller that handles that loop.

   D. In a DCS each controller runs its own share of the loops; that division is what limits the effect of one failure. One controller holding everything would be centralized control.

02Answer key

03What unit I covers

How a plant is measured and controlled: control loops and their parts, pneumatic and electronic signals, pressure, level, flow, temperature, humidity and flue gas measurement, transmitters, recorders, controllers and valve positioners, PID action, PLCs and HMI alarms, cascade control, distributed control and plant computers, and relay control circuits such as start-stop seal-in, interlocks and hand-off-auto selectors.

The SOPEEC 4th Class syllabus names this unit “Energy Plant Instrumentation and Controls” (ABSA numbers it A9) and lists six objectives under it. SOPEEC does not publish how many exam questions come from each unit, and neither do the ABSA, Technical Safety BC, TSASK or TSSA pages we checked. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Scale signals from the live zero

A 4 to 20 mA or 20 to 100 kPa signal starts above zero, so a dead loop can be told apart from a true zero reading. Subtract the live zero, divide by the span, then apply that fraction to the instrument's range.

Orifice DP goes with flow squared

Flow through an orifice plate is proportional to the square root of the differential pressure, so the DP varies with the flow squared. At three-quarters of full flow the DP is only about 56 % of its full value.

Match controller action to the valve

With an air-to-open fuel valve, falling steam pressure must raise the controller output, which is reverse action. Set to direct action, the controller closes the valve as pressure falls and drives the loop away from set point.

Stop wins in a seal-in circuit

In a three-wire start-stop circuit the normally closed Stop button sits in series ahead of Start and the seal-in contact, so Stop always takes priority. A motor whose coil drops out stays stopped until Start is pressed again.

05The other units

Paper 4A has twelve units, A to L. Each has its own page of free questions:

The 4th Class Power Engineering practice exam (4A) page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.