Examiner s Report March 2013

Transcription

1 Introduction Examiner s Report March 2013 Level 3 Diploma Paper 3: Fire Engineering Science. Candidates were required to answer six questions from a choice of ten questions. The most popular questions with candidates were question 1 (nozzles function, effectiveness, calculation of jet reaction), question 3 (calculations with reference to pumping out a swimming pool) and question 4 (chemical, potential and kinetic energy; water power, brake power and water hammer). The questions where candidates performed best were questions 3, 4 and 5: candidates performed least well on question 7. Feedback on candidate performance in each of the examination questions follows. Question 1 a) Describe the function of a nozzle. b) Outline the supply factors which affect the maximum height that can be achieved by a firefighting jet. c) If the pressure of a 25 millimetre nozzle is 6 bar, calculate the jet reaction at the branch. d) Which criteria should be used to select an effective nozzle for firefighting? This was one of the most popular questions on the paper and most candidates gained good marks from the calculation element in part c). Successful candidates knew the underlying theory and understood the practical applications. Unfortunately some candidates confused supply factors with the criteria for selecting an effective nozzle and many others provided detailed information about nozzle design, which gained them no marks because it was not required by the question.

2 Question 2 a) Within an electrical installation state where and why a miniature circuit breaker (MCB) is used. b) Explain the operating principles of a MCB. c) Within an electrical installation state where and why a residual current device (RCD) is used. d) Explain the operating principles of a RCD. e) Outline the main functional difference between a miniature circuit breaker and a residual current device. A minority of candidates attempted this question and for the most part they achieved limited success with it. Generally the operating principles of the MCB and the RCD were not well understood and it was evident that some candidates resorted to guesswork in order to provide an answer at all. However, the successful candidates correctly identified the areas protected by each device and showed a good understanding of what is an important and fundamental topic. Question 3 An Olympic swimming pool has developed a leak and needs pumping out quickly for emergency repairs prior to the commencement of some high profile events. The fire service has been asked to assist. The pool is 50 metres x 25 metres and varies in depth uniformly from 1.5 metres at the shallow end to 3 metres at the deep end and one third of the water has already leaked out. The water has now stopped leaking. a) Calculate how much water remains and needs to be pumped out. b) How long will it take to pump out the remaining water using two pumps; one pump working at 5 bar discharging with 2 x 25 millimetre nozzles and the other pump working at 3 bar discharging with 1 x 20 millimetre nozzle and 1 x 12.5 millimetre nozzle? (12 marks) This was one of the most popular questions and those candidates attempting it earned the highest average mark for any question on the paper. In fact many

3 candidates gained full marks by identifying the appropriate formulae and working out the calculations accurately as a result. Marks were lost where candidates failed to note that two nozzles were in use by the first pump or arrived at an incorrect figure for the volume of water remaining in the pool and this affected their subsequent calculations. Question 4 a) Define the following terms: i. Chemical energy (3 marks) Potential energy (3 marks) i Kinetic energy (3 marks) b) A pump is delivering 2250 litres per minute at a pressure of 4 bar. i. What is the water power? What will be the brake power required to drive the pump at an efficiency of 70%? c) i. Give two examples of the causes of water hammer. How can water hammer be avoided? (3 marks) The calculation elements of questions in Science exam papers are generally answered well although the knowledge based sections of this question caused some candidates to struggle. While potential energy and kinetic energy were well understood, candidates showed little comprehension of chemical energy and few gained the three marks set aside for part c) on water hammer. Question 5 a) Define each of the following terms. (Note. For numbers v., include the full term represented by the acronym as well as the definition.) i. Hydrocarbon (2 marks) LPG. (2 marks) i LNG (2 marks) iv. UVCE. (3 marks)

4 v. BLEVE (3 marks) b) With the aid of a diagram explain briefly how a BLEVE takes place. c) Which firefighting actions can be applied to prevent a BLEVE or to reduce the risk of one occurring? (3 marks) Many candidates were able to identify correctly the acronyms and define the terms they represented. However there was much confusion on the part of some candidates who described a boilover instead of a BLEVE and wrongly identified what LNG stood for or ignored UVCE altogether. For the record, LNG is liquefied natural gas and UVCE is an unconfined vapour cloud explosion. Question 6 a) What is meant by the following terms? i. Polymer (3 marks) Thermosetting plastics (3 marks) b) Plastics often have materials mixed into them. What is the reason for adding the following? i. Inert fillers (2 marks) Fire retardants (2 marks) i Plasticisers (2 marks) iv. Stabilisers (2 marks) c) What are the main fire hazards with regard to plastics? (6 marks) Although most candidates attempting this question answered part c) confidently and detailed the fire hazards associated with plastics, they had trouble defining the terms in part a) and explaining why materials would be mixed with plastics. For the record, a brief answer to part b) appears below:- Inert fillers such as china clay, wood flour and carbon black, or, in laminated plastics, sheets of paper or fillers. These are substances used to reduce the manufacturing costs, make easier handling and make colouring easier. Fire retardants, to make them more difficult to ignite. Plasticisers, mixed with some thermoplastics to make them more pliable. Stabilisers to inhibit degradation due to atmospheric oxidisation, attack by sunlight or decomposition under conditions of mild heating.

5 Question 7 a) i. Identify one group of elements that can act as oxidising agents and list four members of this group. (6 marks) Write a balanced chemical equation for a member of this group reacting with potassium. b) Organic oxidising agents are especially hazardous. i. Identify the chemical structure of organic peroxides. Describe the particular hazards of organic peroxides. Very few candidates attempted this question and it is evident that chemistry questions are generally poorly answered with regard to this exam paper. The nature of the structure of peroxides and the particular hazards of organic peroxides were not understood. Where candidates did attract marks it was in identifying halogens in part a) and in writing a balanced equation. Question 8 Water at 15 C is applied to cool a fire in a building at the rate of 600 litres per minute. Residual water at 90 C flows from the building at 500 litres per minute: the water not flowing out has been converted to steam. Calculate the percentage of the total heat removed from the fire by this water owing to its temperature being raised to 90 C. (20 marks) (The specific heat capacity of water is 4180 J kgˉ1 Kˉ1. The specific latent heat of vaporisation of water is 2260 kj kg -1 ) This question was much less popular and less well answered than is usual for calculation questions. Although some candidates correctly identified the formulae and understood the processes involved, many others failed to gain marks because they did not state the basic formulae and did not understand that raising 600 litres of water to a temperature of 90 0 C formed the basis of the calculation.

6 Question 9 a) Define the terms density and relative density and write an annotated symbolic formula for the definition of density. b) Explain in general terms the significance of the density of a material in firefighting operations. c) With reference to relative density, describe the hazards of a release of the following:- i. Methane. Petrol vapours. Part a) was generally well answered with many candidates gaining high marks by providing clear and accurate definitions. In part b) however many candidates failed to identify all states of matter, concentrating exclusively on liquids. Examiners were concerned to note the number of candidates who confused the properties of petrol vapour and methane in describing the hazards associated with their release. Question 10 a) Name and state the Gas Law which governs the statement that the pressure gauge on a breathing apparatus set shows how much air there is in the cylinder. b) Name and state the Gas Law which explains why smoke and fire gases collect at ceiling level in enclosed rooms. c) Name the third Gas Law which makes up the General Gas Law (also known as the Combined Gas Law) and write the General Gas Law as a mathematical expression. d) Explain the terms critical temperature and critical pressure. The key to answering this question well was in identifying and stating the appropriate gas laws and applying them to the contexts specified. Although the first three sections of this question were generally answered well, few candidates understood the significance of critical temperature, vapour and true gases.