Pv Value in a Seal Questions and Answers

Pv Value in a Seal chiefs and Answers principal 1 (4 marks)list 4 important parameters ( non the equation parameters) that atomic number 18 determined by the Pv value in a postage stamp.The suitability of material combinations for the mould faces.The amount of heat generated at the faceIt in any case follows that at a given rotational speedAn increase in conk out diam since implies an increase in the Pv value. assessor Feedback scruple 2 (4 marks)The contact property of the interface is influenced by which component in a postage stamp unit? What is the need for seal off counterbalance?The fluid mash in the axial direction is the only atmospheric pres sealed component influencing the contact property of the interface.The fluid pressure in the axial direction is the only pressure component influencing the contact property of the interface. Thus, only this component is considered for seal balance and further employ to compute the resulting contact vehemence acting on t he face.tax assessor Feedback suspense 3(8 marks)Given the following data, calculate the face area, sp butt against pressure and balance ratios (external and internal) of a seal, for both externally acting and internally acting pressures. Is this balance or Unbalanced and why?Seal face contact outer diameter, D = 61.8 mmEffective seal balance diameter, B = 52.6 mm share face contact inner diameter, d = 49.2 mmSpring force at working length, F =188 NCalculate the face area of the seal=(D_02-D_12 )/4=(-68.2-2--49.2-2 )/4=1098.46 -mm-2A=0.001098- m-2Calculate the spring pressureP_sp=F_sp/A=188/0.00109846=171148.7 -N/-_(m2 )=P_sp 0.17115 MPaCalculate the balance ratio for the external loadingB=(D_02-D_b2)/(D_02-D_t2 )=(-61.8-2--52.6-2)/(-61.8-2--49.2-2 )=1052.48/1398.6B=0.753 (BCalculate the balance ration for the internal loadingB=(D_b2-D_t2)/(D_02-D_t2 )=(-52.6-2--49.2-2)/(-61.8-2--49.2-2 )=346.12/1398.6B=0.25 (BAssessor FeedbackQuestion 4 (4 marks)From the following single mechanica l seals, identify the 7 seal off components and separate mechanical components, briefly describing their functions?1)Mating Ring.2)Primary Sealing Ring.3) O-ring.4) O-Ring.5) Disc.6) Spring.7) Retainer. peregrine is to be kept from escaping where the lancet ex flows through the housing, especially as the radiation rotates. A ring, dampen 1, with an O-ring, severalise 4, is sealed against the housing of the container. It is called the mating ring. another(prenominal) ring, part 2, with an O-ring, part 3, is mounted onto the shaft. It is called the original sealing ring. The contacting faces of these rings are lapped flat, within light bands. Initial contact among the faces is maintained by a spring, part 6, which pushes them together. The spring reacts against a retainer, part 7Assessor FeedbackQuestion 5 (4 marks)What difference do you notice amongst the two mechanical seals shown below. Name them and line their application.Shows of an outside balanced seal. The shaft pack ing 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.Inside balanced seal illustrates a conventional inside seal that has been balanced. Notice that a bill in the shaft has allowed the sealing face of the mating ring to be moved radially inward without decreasing the width of the face itself. The primary sealing ring remains mounted on the original shaft diameter, which means that the closing force remains unchanged. Because we be in possession of successfully exposed more of the primary sealing ring face to hydraulic pressure working to open the seal, the trope is considered balanced.Assessor FeedbackQuestion 6 (4 marks)Name the primary sealing elements in a single mechanical seal.1) Prim ary sealing surfaces.2) Secondary sealing surfaces.3) a means of actuation.4) a means of drive.Assessor FeedbackQuestion 7 (4 marks) bandage its accepted that solid alumina ceramic seal rings go out be considerably more expensive than ceramic coated steel rings, why should they be the preferred choice between the two grammatical cases of seal ring?When handling corrosive fluids at moderate pressures and velocitiesalumina ceramics may be utilise as thin coatings or solid rings (preferred)For highly corrosive fluidshigh purity ceramic alumina stub be used.Alumina ceramics haveRelatively low tensile strengthPoor resistance to thermal shockLow thermal conductivityHigh elasticity modulusAssessor FeedbackQuestion 8 (4 marks)Which propelling secondary sealing element is the preferred type to handle axial movement? Give reasons for your answer. What type of damage does a paladin type secondary seal do to pump shafts/sleeves? Explain how this mechanism works.The pusher type of mechani cal seals move axially along the rotating shaft or the sleeve to maintain the contact with the faces of the seal. This feature of these seals wait ons compensate for the wearing that may occur at the seal face, and wobbling due to misalignment. The pusher types of mechanical seals are used commonly, are less expensive and are easily available in the market in wide range of sizes and designs. The only disadvantage of these seals is that they tend to hang up and sometimes there is fretting of the shaft. Pusher seals utilize a dynamic secondary seal which moves axially with the major seal face. which is either an O-ring, storm or other type of equipment, across the shaft as a means of compensation for face wear and/or shaft movement. For high temperature (up to vitamin D deg F) or aggressive chemical substances a Teflon wedge ring may be used. Since Teflon is plastic and does not rebound like elastomer, it has to be pushed by spring force into the wedge shaped opening to maintain a seal on the shaft. Non-Pusher type or Bellow seals have no dynamic secondary seal under the movable seal ring. Check Which dynamic secondary sealing element is for axial movement. Operation does not cause shaft wear pusher type seals can handle bi-directional shaft rotation, deep pressure, temperature and speed excursions.Assuming a spring- miserly Teflon wedge can also be used as a dynamic shaft seal behind the rotating primary seal ring. The spring and process pressures keep the wedge in contact with the shaft. Like chevrons and U-cups, wedges can only seal in one direction. Because of the tendency for Teflon to cold flow, almost all wedges need to be loaded by one or more springs along with the process pressure. Wedge seals often cause shaft damage by fretting.Assessor FeedbackQuestion 9 (4 marks)What is the distinct advantage offered by a bellows seal? How can the design be improved to accommodate higher(prenominal) pressures? Which bellows material offers the stovepipe corr oding resistance?One of the most frequent and serious problems valves face is gland leakage, results in wasted and increased plant downtime. Apart from the high embody of energy losses, secreter leakages can also cause serious environmental, ecological and health hazards to plant workers and personnel. Leakage of huffy material can also consist to a fire hazard, explosion, or damage to equipment by corrosive material. Air entering the pipeline could produce inflammable explosive or poisonous mixtures. Gland packed valves often demand continual maintenance in accessibility creating particular difficulties. The bellows comply to conditions at high temperatures and are capable of withstanding oer 10,000 cycles without failure. Bellow Seals are also known as Zero Leak Valves or Emission Free ValvesThe multi-ply bellow design is preferred for handling higher pressure fluids (generally two or three plies of the alloy wall). A two ply bellow can increase its pressure rating by 80% to 100% as compared to a single ply bellow of the same thickness. Alternatively, if a single ply bellow of a thickness equivalent to a pressure rating of a two ply bellow is used, the stroke length is reduced. Thus, a multi-ply bellow design offers a distinct advantage all over a single ply bellow.The stainless steel bellow material AISI 316Ti which contains Titanium to withstand high temperatures. Alternatively, Inconel 600 or Inconel 625 improve fatigue strength and corrosion resistance as compared with stainless steel bellows. Similarly, Hastalloy C-276 offers greater corrosion resistance and fatigue strength than Inconel 625. Fatigue resistance can be improved by using a multiply bellows system and reducing the stroke length this can significantly increase the bellow service life.Assessor FeedbackQuestion 10 (6 marks)Do a comparative analysis of inboard and outboard mounting of seals.Inside-mounted seal is the most common in the industry and the most energy-efficient when compared with other sealing method actings, such as packing and seal less equipment. They are used in all industries with respect to fluid types and the seals property ranges, pressure speed, diameter and temperature.Inside seals are mounted within the equipment seal chamber the advantages of this design includeThe seal can be cooled by the pumped fluid in an enlarged dead-ended chamber, by a product bypass flush or by a clean external flush.Depending on the seal chamber design, the set action of the seal assembly may help keep debris away from the seal faces.With proper hydraulic balancing, the product pressure helps keep the seal faces closed.Catastrophic leakage is usually avoided during seal failure. Leakage can be restricted by the stationary elements in the gland.Inside seals are available in many materials and designs.Environmental controls are easily included in the design.Centrifugal forces tend to reduce leakage.Outside seals are mounted external of the equipment housing the adva ntages of this design includeOutside-mounted seals can be used when the radial or axial space in the chamber is not adequate or access is not available for an inside seal installation.Installation may be easier than with an inside seal. However, most equipment designs still require some disassembly.Less expensive materials may be used since many components may not be exposed to the pumped product.The seal can be observed and monitored for seal face wear.Adjustments can be made without equipment disassembly.The seal can often be backed-off for cleaning.Assessor FeedbackQuestion 11 (8 marks)Explain the difference between static and dynamic seals?0-rings are used for effective sealing in applications. Mostly, they are used to help repair or correct any manufacturing or installation defects seen in glands. 0-rings can be used in static as well as dynamic applications. The make of both these 0-rings will be different owing to the difference in application requirements.The following are s ome major differences between static and dynamic 0-ringsThe material used to devise dynamic 0-rings should be tougher than that used to manufacture static 0-rings.This is because the dynamic 0-ring will have to undergo movement while the application is functioning.In dynamic applications, the surface finish and material of the gland should be such that it doesnt abrade the 0-ring during movement otherwise, the 0-ring could tear this is irrelevant in static applications, where the 0-ring doesnt move.0-rings used in dynamic applications are likely to wear at a faster rate as compared to static 0-rings. This is because dynamic 0-rings are constantly travel. Hence, the interval of maintenance procedures for dynamic 0-rings should be shorter than that of static 0-rings.All 0-rings should be powerful lubricated at all times.Dyna mic 0-rings require more lubrication than static ones.Depending on the application, static or dynamic seals will be used. O-rings are useful in a varied number of industries and applications. They are effective across a wide range of temperatures and pressures.However, using the right 0-ring for each application will help you increase the reliability and service life of the 0-ring.You can consult with your 0-ring manufacturer on the trump fit for your applicationAssessor FeedbackQuestion 12 (4 marks)why are mechanical seals preloaded? Name four of the most common types of preloading for pump seals.Depending on the application it may be necessary to have either a positive or a negative operational clearance in the posture arrangement. In the majority of applications, the operational clearance should be positive, i.e. when in operation, the bearing should have a residual clearance, however slight.However, in many cases, machine tool spindle bearings, pinion bearings in automotive axle drives, bearing arrangements of small voltaic motors, or bearing arrangements for oscillating movement, where a negative operational clearance, i.e. a pre load, is needed to enhance the stiffness of the bearing arrangement or to increase speed accuracy. The application of a preload, e.g. by springs, is also recommended where bearings are to operate without load or under very light load and at high speeds. In these cases, the preload serves to provide a minimum load on the bearing and prevent bearing damage as a result of sliding movements of the rolling elements, voluted Compression Spring, Wave Washer Springs, Single Belleville Washer Springs, Elastomeric PreloadingAssessor FeedbackQuestion 13 (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. face of MaterialTemperature in C50100200300350Natural RubberSSLSNitrileSSSLSNeopreneSSSLSVitonSSSSSSiliconeSSSSSFluoro-elastomerSSSSSQuestion 14 (4 marks)What are the characteristics that make pure carbon- graphite the most favoured material in the chemical industry? strongly anisotropic, Greasy feel, Readily marks, High thermal conductivity, High tensile strength, Low thermal expansion coefficient, Good resistance to thermal shock.Assessor FeedbackQuestion 15 (4 marks)Ignoring temperature limitations, what type of elastomeric material would you suggest for the following applications?Sealing for Vacuum systemsBuna-N, silicone, Fluorocarbon, and PerfluorocarbonApplications Vacuum systemsPolyurethaneRotary shaft sealsFluorocarbon, PolyacrylateApplications that require high tear and start resistanceNatural RubberQuestion 16 (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?Typically, a single-coil spring used for a seal head has a relatively large wire cross s ection and therefore provides more substance to combat corrosion from the system fluid. only if it is rigid in construction because of which there is difficulty in achieving a perfectly uniform load distribution across the shaft circumference.This may in turn lead to distortion of the primary seal ring face and this is particularly critical at high rotational shaft speeds.This arrangement consists of a series of small coil springs uniformly distributed along the circumference of the seal cartridge. The chief advantage with this design is that the possibility of distortion of the primary seal ring face is minimized and this is particularly true at higher rotational speeds of the shaft. The use of multiple-coil springs makes seal design independent of seal diameter sizes. Deterioration arising from chemical corrosion can be greatly minimized by the use of stainless steel for spring manufacture.Assessor FeedbackQuestion 17 (4 marks)What is the three-point contact method? Explain in br ief, the procedure involved.Three-point contact method where three setscrews positioned at 120 from each other ensure squareness of the rotating face, by deforming the sleeve to the shaft OD. Another set of three setscrews also located 120 apart and positioned between the earlier set of screws, enable locking of the sleeve to the shaft.Assessor FeedbackQuestion 18 (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 surface additions.Both the metal filler and the carbon have to be compatible to the sealed liquid.The metal will heat up and expand more rapidly than the carbon.The coefficient of skirmish of the metal will be higher than the parent carbon and the seal face will run hotter than if it were made just from pure carbon.Assessor FeedbackQuestion 19 (4 marks)Outline the nature of seal malfunction that may be caused by the followingIncorrect pressure differential betwee n inside and outside seal face setsCavitation or vaporization of liquid between sealing facesThermal distress of materialLarge bearing clearancesA change in temperature. some products solidify at temperature extremes, the product is taking a pressure drop across the seal faces and solidifying. The inner face of a back to back double seal application is not positively locked in position. A snap ring must be installed to prevent the inboard stationary face from moving towards the rotating face when the high pressure barrier fluid pressure is lost or overcome by system pressure.Erosion / Corrosion. An accelerated attack caused by a combination of corrosion and mechanical wear. Vaporization, liquid turbulence, vane passing syndrome, and suction recirculation are special cases often called cavitation. Solids in the liquid and high f number increase the problemHeat checking is caused by thermal distress of the material resulting in small radial cracks. Scoring may be present or uneven wear with the heat checking on the high spots. Occurs typically with tungsten carbides and silicon carbides Seal drips when stationary and when the shaft is rotating. Seal may pop from photoflash during operation.The tendency for a seal to wedge is enhanced by a rough surface, lack of lubrication and high reciprocating speeds. Wedging is unlikely to occur with small clearance gaps in the range of 0.0550.127mm (0.002-0.005 inch). For clearances greater than 0.25 mm (0.010 inch) the possibility of wedging always exists as the radial clearance increases, the axial clearance increases as well the more room (radial clearance), the more the elements can shift in relation to each other. With a higher clearance there is more tolerance of thermal expansion effects, differential temperature between the inner and outer faces.Assessor FeedbackQuestion 20 (4 marks)The leakage rate in a seal while running is found to be constant. privy the reason be attributed to a damaged seal face?State three important causes for each of the belowIntermittent leakageThere is a leak between the face and the holder that becomes visible only when the unit comes up to operating temperature.A bending or bent shaft is cause the seal outside diameter to contact the inside diameter of the stuffing box, or some other stationary object.The shaft/ sleeve is too large in diameter and it is restricting movement of the seal. Spring loaded dynamic elastomers such as Teflon wedges, U- cups, chevrons and spring loaded O-ring designs are very sensitive to this problemConstant dripping in a seal.There is damage in the O-ring groove. Maybe the O-ring was removed with a sharp metal instrument and this has caused a scratch in the O-ring groove.Leaking between the gland and the stuffing box. This leak path is very visible in most applicationsLeaking between the cartridge sleeve and the shaftQuestion 21 (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?General Selection Considerations The process of mechanical seal selection involves a staring(a) evaluation of many important factors that includeShaft diameterRotational rubbing speed in the contact facesService temperatureProcess fluid pressurePhysical and chemical properties of the various liquid constituentsMechanical/structural properties of the primary seal componentsEnvironmental control systemsProperties and resistance characteristics of the secondary sealing components.Pressure.Assessor FeedbackQuestion 22 (4 marks)Which of the following conditions satisfy the requirements for sealing high-speed applications?Explain in briefLower spring load at seal facesIncreased hydraulic balance ratioFace combinations such as carbon vs. tungsten carbideStationary seal designsNumber 4. Stationary seal designsThe Internal seal configuration, with the seal heads mounted sta tionary and the seal seats rotating with the shaft is advantageous, especially in high-speed applications where dynamic balancing considerations demand minimum mass of the rotating partsAssessor FeedbackQuestion 23 (6 marks)A change in pumping fluid temperature adversely affect the seals in the stuffing boxes.What are the effects andHow do you control the temperature in the stuffing box and the seal?a)Many fluids are adversely affected by a change in their temperature, and when this change takes place, seal failure is almost sure to follow. The failure can take several formsCoated hard faces can heat check (crack).Elastomers can take a compression set and crack at elevated temperatureCold temperatures can cause elastomers to harden.The liquid can crystallize, restricting seal movement and open the faces.The liquid can vaporize between the faces forcing them open.The viscosity of the fluid can change either restricting seal movement, or making the fluid less of a lubricant.The liqui ds corrosion rate will double with an 18 Fahrenheit (10 C) rise in temperature.The liquid can convert to a film between the sliding seal components, restricting their movements. The magnetic iron-ore that forms in hot water is a good example of this.A film can form on the seal faces causing them to separate.Lapped seal faces can distort and go out of flat at elevated or cryogenic temperatures.b) A balanced mechanical seal and installed at the proper compression, is your best insurance against a significant rise in stuffing box temperatureProper face balance. 70/ 30 is the most common to 5000 fpm. (25 Meters per sec.)Low friction face materials. Carbon/graphite vs. a silicon carbide hard face is the best.The correct spring compression to control face loading.Faces with good heat conductivity. west carbide and silicon carbide have excellent thermal conductivity compared to most other hard face materials.A small cross section carbon/ graphite face press fitted into a metal holder is b etter than solid carbon/ graphite for removing heat from between the lapped faces.Sometimes, that is not good enough, so now and then youll have to come up with some additional method of controlling the temperature in the stuffing box area and between the lapped seal faces. A warmth /cooling jacket, quench flush or drain connection, dual seal, heat exchangerAssessor FeedbackQuestion 24 (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?For a specific ap