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The suitability of material combinations for the seal faces.
The amount of heat generated at the face
It also follows that at a given rotational speed
An increase in shaft diameter since implies an increase in the Pv value.
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The fluid pressure in the axial direction is the only pressure 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 used to compute the resulting contact force acting on the face.
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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?
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Fluid is to be kept from escaping where the shaft extends through the housing, especially as the shaft rotates. A ring, part 1, with an O-ring, part 4, is sealed against the housing of the container. It is called the mating ring. Another ring, part 2, with an O-ring, part 3, is mounted onto the shaft. It is called the primary sealing ring. The contacting faces of these rings are lapped flat, within light bands. Initial contact between the faces is maintained by a spring, part 6, which pushes them together. The spring reacts against a retainer, part 7
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What difference do you notice between the two mechanical seals shown below. Name them and describe their application.
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Shows 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.
Inside balanced seal illustrates a conventional inside seal that has been balanced. Notice that a step 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 have successfully exposed more of the primary sealing ring face to hydraulic pressure working to open the seal, the design is considered balanced.
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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.
The pusher type of mechanical 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 helps 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, wedge 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 500 deg F) or aggressive chemicals 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, large pressure, temperature and speed excursions.
Assuming a spring-loaded 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.
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One of the most frequent and serious problems valves face is gland leakage, results in wasted and increased plant downtime. Apart from the high cost of energy losses, Gland leakages can also cause serious environmental, ecological and health hazards to plant workers and personnel. Leakage of sensitive material can also constitute 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 over 10,000 cycles without failure. Bellow Seals are also known as ‘Zero Leak Valves’ or ‘Emission Free Valves’
The multi-ply bellow design is preferred for handling higher pressure fluids (generally two or three plies of the metal 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 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.
Inside-mounted seal is the most common in the industry and the most energy-efficient when compared with other sealing methods, 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.
Catastrophic leakage is usually avoided during seal failure. Leakage can be restricted by the stationary elements in the gland.
Outside seals are mounted external of the equipment housing the advantages of this design include:
Outside-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.
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 material used to manufacture 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.
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 moving. Hence, the interval of maintenance procedures for dynamic 0-rings should be shorter than that of static 0-rings.
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.
Depending on the application it may be necessary to have either a positive or a negative operational clearance in the bearing 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 electric motors, or bearing arrangements for oscillating movement, where a negative operational clearance, i.e. a preload, is needed to enhance the stiffness of the bearing arrangement or to increase running 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, Helical Compression Spring, Wave Washer Springs, Single Belleville Washer Springs, Elastomeric Preloading
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.
Typically, a single-coil spring used for a seal head has a relatively large wire cross section and therefore provides more substance to combat corrosion from the system fluid. But 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.
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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.
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.
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 friction 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.
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Incorrect pressure differential between inside and outside seal face sets
Cavitation or vaporization of liquid between sealing faces
Thermal distress of material
Large bearing clearances
A change in temperature. Many 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 velocity increase the problem
Heat 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 flashing 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.
The leakage rate in a seal while running is found to be constant. Can the reason be attributed to a damaged seal face?
There 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 causing 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 problem
Constant 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 applications
Leaking between the cartridge sleeve and the shaft
Question 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 thorough evaluation of many important factors that include:
Shaft diameter
Rotational rubbing speed in the contact faces
Service temperature
Process fluid pressure
Physical and chemical properties of the various liquid constituents
Mechanical/structural properties of the primary seal components
Environmental control systems
Properties and resistance characteristics of the secondary sealing components.
Pressure.
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Lower spring load at seal faces
Increased hydraulic balance ratio
Face combinations such as carbon vs. tungsten carbide
Stationary seal designs
Number 4. Stationary seal designs:
What are the effects and
How 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 forms:
The liquid can convert to a film between the sliding seal components, restricting their movements. The magnetite that forms in hot water is a good example of this.
b) A balanced mechanical seal and installed at the proper compression, is your best insurance against a significant rise in stuffing box temperature:
Proper 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. Tungsten 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 better than solid carbon/ graphite for removing heat from between the lapped faces.
Sometimes, that is not good enough, so occasionally you’ll have to come up with some additional method of controlling the temperature in the stuffing box area and between the lapped seal faces. A heating /cooling jacket, quench flush or drain connection, dual seal, heat exchanger
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?
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