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Module 13 Assignment Questions Question 1 (4 marks) List 4 important parameters that are determined by the Pv value in a seal. 1) Type your answer here 2) Assessor Feedback Question 2 (4 marks) The contact property of the interface is influenced by which component in a seal unit? What is the need for seal balance? 3) Type your answer here 4) Assessor Feedback Question 3 (8 marks) Given the following data, calculate the face area, spring pressure and balance ratios (external and internal) of a seal, for both externally acting and internally acting pressures. Is this Balanced or Unbalanced and why?  Seal face contact outer diameter, D = 61.8 mm  Effective seal balance diameter, B = 52.6 mm  Deal face contact inner diameter, d = 49.2 mm  Spring force at working length, F =188 N 5) Type your answer here 6) Assessor Feedback Question 4 (4 marks) From the following single mechanical seals, identify the 7 sealing components and other mechanical components, briefly describing their functions? 7) Type your answer here 8) Assessor Feedback Question 5 (4 marks) What difference do you notice between the two mechanical seals shown below. Name them and describe their application. 9) Type your answer here 10) Assessor Feedback Question 6 (4 marks) Name the primary sealing elements in a single mechanical seal. 11) Type your answer here 12) Assessor Feedback Question 7 (4 marks) While it’s accepted that solid alumina ceramic seal rings will be considerably more expensive than ceramic coated steel rings, why should they be the preferred choice between the two types of seal ring? 13) Type your answer here 14) Assessor Feedback Question 8 (4 marks) Which dynamic secondary sealing element is the preferred type to handle axial movement? Give reasons for your answer. What type of damage does a wedge type secondary seal do to pump shafts/sleeves? Explain how this mechanism works. 15) Type your answer here 16) Assessor Feedback Question 9 (4 marks) What is the distinct advantage offered by a bellows seal? How can the design be improved to accommodate higher pressures? Which bellows material offers the best corrosion resistance? 17) Type your answer here 18) Assessor Feedback Question 10 (4 marks) A few common seal configurations are listed below along with their applications. Indicate whether these combinations can be put into actual practice and if not, why. 1. Metal bellows seal - Cryogenic and hot fluid sealing applications 2. Split seals – Applications alternating between vacuum and pressure 3. Outside mounted seal - High pressure applications 4. Concentric type seal - Mixer applications 19) Type your answer here 20) Assessor Feedback Question 11 (4 marks) Which type of single seal would you not recommend for liquids containing abrasives. Explain your reasons for not using the seal. 21) Type your answer here 22) Assessor Feedback Question 12 (4 marks) Do a comparative analysis of inboard and outboard mounting of seals. 23) Type your answer here 24) Assessor Feedback Question 13 (8 marks) Explain the difference between static and dynamic seals? 25) Type your answer here 26) Assessor Feedback Question 14 (4 marks) Why are mechanical seals preloaded? Name four of the most common types of preloading for pump seals. 27) Type your answer here 28) Assessor Feedback Question 15 (4 marks) The table below contains a list of commonly used seal materials. For the given set of temperatures in degree centigrade, indicate the suitability of these materials by marking S for suitable and LS for limited suitability of short period durations. Type of Material Temperature in º C 50 100 200 300 350 Natural Rubber Nitrile Neoprene Viton Silicone Fluoro-elastomer Question 16 (4 marks) What are the characteristics that make pure carbon-graphite the most favoured material in the chemical industry? 29) Type your answer here 30) Assessor Feedback Question 17 (4 marks) While it’s accepted that solid alumina ceramic seal rings will be considerably more expensive than ceramic coated steel rings, why should they be the preferred choice between the two types of seal ring? 31) Type your answer here 32) Assessor Feedback Question 18 (4 marks) Ignoring temperature limitations, what type of elastomeric material would you suggest for the following applications? 1. Sealing for Vacuum systems 2. Applications involving acids 3. Rotary shaft seals 4. Applications that require high tear and abrasion resistance Question 19 (4 marks) Why is it difficult to achieve a perfectly uniform load distribution across the shaft circumference, when using a single coil spring? What does this condition further lead to? What are the main advantages associated with the use of multi-coil springs? 33) Type your answer here 34) Assessor Feedback Question 20 (4 marks) What is the three-point contact method? Explain in brief, the procedure involved. 35) Type your answer here 36) Assessor Feedback Question 21 (4 marks) Metal impregnations are sometimes added to graphite seal rings in order to “ improve” their properties. Give three areas of concern when selecting these metal additions. 37) Type your answer here 38) Assessor Feedback Question 22 (4 marks) Outline the nature of seal malfunction that may be caused by the following: a. Incorrect pressure differential between inside and outside seal face sets b. Cavitation or vaporization of liquid between sealing faces c. Thermal distress of material d. Large bearing clearances 39) Type your answer here 40) Assessor Feedback Question 23 (4 marks) The leakage rate in a seal while running is found to be constant. Can the reason be attributed to a damaged seal face? State three important causes for each of the below: Intermittent leakage 1. 2. 3. Constant dripping in a seal. 1. 2. 3. Question 24 (6 marks) It is a requirement to identify and select a suitable mechanical seal for a given application and process. What is the nature and type of questions that you would raise, in order to help arrive at the right selection? 41) Type your answer here 42) Assessor Feedback Question 25 (4 marks) Which of the following conditions satisfy the requirements for sealing high-speed applications? Explain in brief: 1. Lower spring load at seal faces 2. Increased hydraulic balance ratio 3. Face combinations such as carbon vs. tungsten carbide 4. Stationary seal designs 43) Type your answer here 44) Assessor Feedback Question 26 (6 marks) A change in pumping fluid temperature adversely affect the seals in the stuffing boxes. a) What are the effects and b) How do you control the temperature in the stuffing box and the seal? 45) Type your answer here 46) Assessor Feedback Question 27 (4 marks) The mechanical shaft seal should be selected according to the operating conditions at the shaft seal location. What important factors must be considered when selecting a mechanical shaft seal? 47) Type your answer here 48) Assessor Feedback Question 1 Seal seats may be equipped with either one or two seal face True False Question 2 Rotary seals are sensitive to any misalignment between the shaft and the face of the stuffing box? True False Question 3 Rotating seal heads are normally preferred for: 1. Large sized seals 2. Smaller seals 3. For speeds above 5000 fpm 4. For speeds below 5000 fpm Question 4 The best choice for applications involving fluids containing abrasives and foreign particles is: 1. Single inside seal 2. Single outside seal 3. Double seal 4. None of the above Question 5 In the double concentric type seal 1. The two seals are located side-by-side 2. The two seals are located one inside the other, but do not share a common stationary face 3. The two seals are located one inside the other, sharing a common stationary face 4. None of the above Question 6 The Pv factor which is the algebraic product of the face contact pressure and the rubbing velocity is used to characterize: 1. Seal leakage rate 2. Ability of a material to resist wear 3. Seal efficiency 4. None of the above Question 7 Balanced seals have lower face contact pressure True False Question 8 A balance ratio corresponding to a value of 0.70 implies: 1. Existence of a balance value of 30% of the total unbalanced condition of the present design 2. Existence of a balance value of 30% of the total balanced condition of the present design 3. Existence of a balance value of 70% of the total unbalanced condition of the present design 4. Existence of a balance value of 70% of the total balanced condition of the present design Question 9 Which of these statements regarding seal preloading is incorrect? 1. Single helical compression spring designs are very sensitive to solids, abrasives and corrosive agents 2. Wave springs tend to have a relatively high load rate 3. Elastomeric preloading is employed in seals operating under low speeds and low pressures 4. None of the above Question 10 In a dynamic seal, the relationship between the seal and the housing is: 1. Static 2. Dynamic 3. Neither static nor dynamic 4. Both static as well as dynamic Question 11 Identify the material that is not used in hard faces: 1. Carbon graphite 2. Tungsten carbide 3. Alumina ceramic 4. None of the above Question 12 Tungsten carbide is the most common material used in metal bellows True False Question 13 Which of these materials can be employed under conditions of severe corrosion? 1. Case hardened steels 2. Glass-filled Teflon 3. Nickel-silver 4. Graphite-plastic

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Question 1
The Parameters determined by the PV Value are:
A. Power Loss
B. Surface Temperature Increase
C. Wear Rate
D. Lifetime

Question 2:
The contact property of the interface is matter of concern for the Primary and Mating Rings, which are typically chosen from dissimilar materials to reduce friction and wear.
The various forces acting on the floating element of the seal are to be considered to balance the pressure difference across the seal, which affects the mean pressure between the seal faces. A proper balancing with the appropriate balance factor is important, as an underbalanced seal might incorporate too high mean pressure not allowing the minimum clearance. Or an overbalanced seal might introduce leakage of high pressure fluids

Question 3:
Face area A = π (61.8²-49.2²)/4 = 1098 mm²
Balance Ratio B = (61.8²– 52.6²) / (61.8²– 49.2²) = 0.75
Spring Pressure Psp= Fsp / A = 188 / (1098 x 10¯⁶) = 171.22 KPa
The system is unbalanced as the balance ratio is less than 1.

Question 4:
Primary Seals:
A. Retainer
B. Spring
C. Disk
D. Primary Sealing Ring
E. Mating Ring
Secondary Seals:
A. “O” Ring
B. “O” Ring

Question 5:
In the First Figure, we see a cross section of a conventional inside unbalanced seal. Most of the stuffing box pressure is exerting a closing force on the primary sealing ring. Only a small portion of the primary sealing ring face is exposed over the top of the mating ring, allowing a proportionately small amount of pressure to work against the primary sealing ring in the opposite direction (in addition to the opening force exerted by the liquid film between the faces). If the closing force becomes great enough, the liquid film between the faces is literally squeezed out. Deprived of the lubrication, if highly loaded, the faces might soon destroy themselves. The solution to the problem is to balance the seal.
The Second Figure demonstrates a cross section of an outside balanced seal. The shaft packing is forced against the retainer, leaving an area under the seal ring exposed to stuffing box pressure. The closing force exerted by the stuffing box pressure, acting against the shoulder of the seal ring, is slightly greater than the opening force exerted by the liquid film between the faces, thereby keeping the faces in contact at all times.

Question 6:
The Primary Sealing Elements in a single mechanical seal are:
A. Retainer
B. Spring
C. Disk
D. Primary Sealing Ring
E. Mating Ring...

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