tribology of sealing

INTRODUCTION:

                           Tribology ,the science and technology of contacting surfaces involving friction ,wear and lubrication is extremely  important in nearly all mechanical components . A major focus of the field is on friction ,its consequences ,especially wear and its reduction through lubrication and material surface engineering .

                                    The improper solution of tribological problems is responsible for huge economical losses in our society ,including shortened component lives, excessive equipment down time ,and large expenditure of  of energy. It is particularly important that engineers use appropriate means to reduce friction and wear in mechanical system through proper selection of bearings,lubricants ,and materials for all contacting surfaces .

 BRIEF HISTORY OF TRIBOLOGY:

               The word “TRIBOLOGY “ coined in 1966 is derived from the Greek word tribos  meaning ‘rubbing ’so the literal translation would be the science of rubbing . it is only word tribology relatively new ,because interst in  constituent part of tribology is older than recovered history .

                  Leonardo introduced for the first time the concept of  coefficient  of friction as ratio of friction force to normal load .

                   In 1966 amonts found that the friction force is directly proportional to the normal load and is independent of the apparent area of contact .

NEED OF TRIBOLOGY:

                Now a days one third of our global energy consumption is consumed wastefully in friction . energy resources  are on the way ending. So we need ,by making use of best tribological practices to reduce this energy consumption , which wastes in friction .

                 According to some estimates , losses  resulting from ignorance of tribology amouts  in the U.S. to about 6% of its gross national product or about 200Billion Dollars

Per year .

                  According to jost UK could save approximately 500million pounds per year and the US could save in excess of  16 billion   per year by better tribological practices .

                

 The purpose of research in  tribology is understandably the minimization and elimination of losses resulting from the friction and  wear at all levels of technology where the rubbing of surfaces are involved .

      Research in tribology leads  to greater plant efficiency , better performance , fewer break    down and significant  savings .

VARIOUS APPLICATIONS OF TRIBOLOGY:

                 There are various applications of tribology , which are listed as below 

       1.        Automotive transportation and space industries .

2.        Design of internal combustion engine .

3.        Design of air cushion vehicles .

4.        Design of antiskid braking system .

5.        Design of pneumatic and steel -rimmed tyres .

6.        Design of hydrodynamic bearing .

7.        Design of seal .

8.        In biomechanics which regards with lubrication of human joints .

9.        In artificial heart valves .

10.      In total replacement of artificial knee joints .

INTRODUCTION TO SEALING:

                  Leakage from  any type of system is major problem  leading to loss of efficiency , increased power usage ,temperature rise ,environmental damage and safely hazards .

                 Passage of fluid between mating parts of machines and between other mechanical elements isprevented or minimized by the seal . commonly a cap exists between parts formed by inherent  roughness or misfit of parts where leakage must be prevented by seal . one may also have necessity gaps between parts that have relative motion ,but seal is still needed .

                  The fluid to be sealed can be any liquid or gas . most machines operates with fluid and must contain fluid or exclude fluid, most mechanical devices or machines requires a multiplicity of seal.

                  The seals or sealing devices are necessary to prevent both the loss of lubricant from the bearing  and entry of contamination (often either hard particles or water ) from external environment.

FUNDAMENTALS OF SEALING:

                  Sealing can be accomplished  by causing the gap between two surfaces to becomes small but defined by the geometric relationship between the parts themselves.

In this case one has fixed clearance seal, one may also force two materials into contact with each other, and the materials may be either  sliding relative to each other or static.

                  There are two broad classes of surfaces -guided material pairs as follows.

      1.   First and most common involve use of an elestomeric, plastic or soft material against the hard material. In this case soft material deforms to confirm to the details of the shape of the harder  surface and will usually seal off completely in the static case and nearly completely in dynamic  case. E,g. a rubber gasket on metal

      2.   The second class where is one mates a hard but wearable material to a hard material. Since  both materials are relatively hard if one material develops a roughness or grooves, the seal will leak. e.g. a mechanical face seal.

CLASSIFICATION OF SEAL:

                 Seals are categorized as follows.

A.        STATIC SEAL:

1.      Gap to be sealed is generally very small.

2.      Accommodates  imperfect surfaces, both roughness and out of flatness.

3.      Subject to very small relative motion due to pressure and thermal cycle loading.

4.      Allows for assembly/disassembly

  

         Static seals can be categorized as follows.

               1. Gaskets  

          ·         Single or composite compliant material.

          ·         Metal encased.

          ·         Wrapped and spiral wound.

          ·          Solid metal.

2   Self -energized elastomeric rings

·             cross section (O-ring)Circular

·             Rectulanger cross section

 3.  Chemical compound or liquid sealants as a gasket.

§    Rubbers

§       Plastics

 B.                DYNAMIC SEAL:

1.      Gap to be sealed is much larger and exists of necessity to permit relative motions.

2.      Relatively large relative motions between surfaces to be sealed.

3.      Motion may be continuous (rotation) in one direction or large reciprocating or                                                   

         amount of motion may be limited.

4.      Seal must not constrain motion.

       Dynamic Seals can be categorized as follows

a.    Rotating or oscillatating shaft.

         1.         Fixed clearance seal.

            ·            Labyrinth

            ·            Clearance or bushing

            ·            Visco seal

            ·             Ferro fluid seal

       2.  Surface guided seal

           ·               Cylindrical surface

           ·                Circumferential seal

                                   ·                 Packing

                                   ·                  Lip seal

                                   ·                  Elastomeric ring

                

                       3.     Annular surfaces (radial face)

                          ·              Mechanical face seal

                    ·              Lip seal

                          ·              Elastomeric ring

b.     Reciprocating

                     1.   Fixed  clearance seal

                          ·               Bushing seal

                          ·              Floating -ring seal

                          ·              Clearance or bushing

                    2.     Surface guided seal

                                   ·              Elastomeric rings

                                   ·               Solid cross section

                                   ·               U-cups, V-rings, Chevron  rings, Split piston rings

                  

                   

                      3.      Limited travel seal

                                    ·             Bellows

                                    ·             Diaphragm

        

    Now we discuss some types of seals in brief as follows

           1.    Gasket

                  Within category of static seal, gasket comprise greatest fraction. The sealing principle common to gasket is that the material is clamped between two surfaces being sealed. Clamping force is large enough to deform the gasket material and hold it in tight contact even when pressure attempts  to open the gap between the surfaces.

                    A simple single material gasket clamped between two surfaces by bolts to prevent leakage is as shown in figure 1. using compliant material gasket can seal even though the sealing surface are not  flat.

                     As shown in the figure 2 gasket need not cover the entire face being sealed. A gasket can be trapped in a groove and loaded by projection on opposite surface as shown in figure 3. Composite material gasket or metal gasket may be contained in groove as shown in figure 4. Gaskets are made in wide varity of ways . A spiral - wound metal /fiber composite, metal or plastic clad, solid metal with sealing projection, and a solid fiber or rubber material are shown in figure 5.

                      Gasket can be made of relatively low - stiffness material such as rubber or cork  for application at low pressure and where the surfaces are not very flat.

                       For highest pressure and loads a gasket may be retained in groove and made either of very strong material or even metal as shown in figure 4. For higher pressure and loads, we must utilize various composite materials and metal encased materials in figure 5.

       2.  Elastomeric or self energized seal:

                               These seal can  seal pressure to 20 Mpa or even higher as shown in fig. The two metal parts are clamped tightly together and they are not supported by elastomer. As pressure increases, the rubber is pushed into the corner through which leakage would otherwise flow .an elsastomer acts much like a fluid so that effect of pressure on one side is to casue equal pressure on all sides .Thus the elstomer pushes tightly against the metal walls and forms a seal. The limitation of this type of seal is that rubber will flow or extrude out of the clearance when pressure is high enough.  

                                O-rings are probably the most widely used type of hydraulic and pneumatics seal in general use for both static and dynamic perposes.

     Advantages

·                           They are inexpensive

·                           They are easy to install

·                           They are reliable in service

for satisfactory performance it is important that the groove in which the ring sites should be correct dimensions, that the sealing surfaces have appropriate surface finishes and that the material of which the ring is made does not react adversly with seal fluid.

       To seal properly, an o-ring must have the proper amount of squeeze or preload, have enough room temperature to thermally expand, not have a bridge to large gap, have rubber hardness suitable to job and be made of suitable rubber . Some practical elastometric materials used for O-ring manufactureare shown in table.

Elastic Type	Properties	Operating Temperature Range
Natural rubber	Suitable for animal and vegetable oil	-60 to +80
Neoprene	Resistant to outdoor weathering and mineral oil	-30 to 150
Butadiene styrene,SBR	Suitable for synthetic animal and vegetable oils	-60 to +90
Butyl rubber	Resistant to phosphate ester and gas permeation	-40 to +90
Silicon rubbers	Resistant to high and low temperature but poor mechnanical strength	-60 to 200

        

3. Ferrofluid seal:

                         The Ferrofluid seal is as shown in figure has foud application in computer disk drives where a true 'positive seal '  is necessory to exclude contaminants from the flying heads  of the disk. The ferrofluid seal operates by retaining a ferrofluid  (a suspension of iron particles in special fluid) within the magnetic flux fluid, as shown.  The fluid creates continuous bridge betwwen the rotating and nonrotating parts at all times and thus creates a positive seal. Each stage of ferrofluid seal is capable of withstanding on the order of 20,000 pa, so although these seals can be staged they are usually limited to low pressure application.

4. Soft Packing Seal:

                         There are many types of soft packing used in manner as shown in above figure. The packing is composed of various types of fibres and is oven in different ways for various purposes. It is often formed into a rectangular cross section so it can be wrapped around a shaft and pushed into a packing gland as shown. As packing nut is tightened the packing deforms and begins to press on shaft. Contact or near contact with the shaft from the seal. If the packing is over tightened the packing material will generate a expensive heat from the friction and burn. If it is too close, leakage will be excessive. At the point where the packing is properly loaded, there is some small leakage, which acts to lubricate between the shaft and the packing material. Soft packing for the continuously rotating shaft is restricted to moderate pressure and speed. For valve stem and other reciprocating application, soft packing can be used at high pressure and temperature.

5. Lip seal

              

                  The lip seal (oil seal) operating on shaft surface represents one of the most common sealing arrangements. The lip seal is made of rubber (or, much less commonly ,a plastics)  or similar material that can be readily deflected inward toward the shaft surface by garter spring. This lip is very lightly loaded, and in operation in oils with rotation, a small liquid film thickness develops between the rubber lip and the shaft. The shape of the cross section determines which way the seal will operate. as shown in the figure the seal will retain oil to the left . Lip seal can tolerate only (1000 Pa maximum). The normal failure mechanism is deterioration (shifting of rubber)  so lip seals have limited speed and temperature of services. Various elastomers are suited for varity of the applications.

6. Mechanical face seal

                  The mechanical  face seal as shown in figure has widely used to seal rotating  and oscillating shafts in pumps  and equipments. The mechanical face seal consist of a self - aligning primary ring , a rigidly mounted mating ring, a secondry seal such as a O-ring that gives a primary ring freedom to self - align without permiting  leakage , springs to provide loading of the seal faces. It is common to have the pressure is on the inside. The flexibility mounted primary ring  may be either the rotating or non-rotating members .

                   Face seal faces are initilly very flat(1 micrometer or greater) so that when they come into contact only a very small leakage gap results . In fact, using suitable material ,such faces lap themselves into conformity so that such aseal can leak as little as a drop of liquid per hour .

SEALING FACE CONDITIONS

For good performance of sealing following face conditions are essentials .

1. FACE FLATNESS:

                   Good face seal performance is dependant on the flatness of the mating sealing surfaces ,which is in between  3 to 5 helium light bands . The same amout of flatness per inch of diaphragm is seldom required on both opposing sealing faces . Initial flatness depends upon the material of the construction , design of the seal , the condition under which seal operates and amount of initial and long term leakage allowed.

2. FACE FINISH

·      Roughness--

                    Microscopic structure of sealing surface material has large influnce on allowable range of roughness . Some material gives higher roughness as compared to convetial material . A surface with highly polished and reflective finish may actually rougher than unpolished dull material finish .

·      Reflectivity--

                     Reflectivity depends upon the smoothness of the surface . Too smooth surface degrades initialperformance of seal .

·      Scratches--

                     In some cases scratches are beneficials to the performance of the seal .Apparent magnitude and direction of the scratches are deceptive and are depedant  upon the reflectivity of the material , intensity of the light rays illuminating surfaces in relation to the viewer .

·      Chips--

                     Chips result from the material breakdown occurring at an edge of sealing face during normal part manufacturing and handling  procedures . Magnitude of  chip is usually defined by amount it intrudes into sealing surface of seal .

MATERIAL OF CONSTRUCTION 

                         For  construction of seal following materials are commonly used .

                         

        1.Sealing face material

· Synthetic carbon and grphite             

· plastic

2. Sealing counterface material

·Ceramic

·Iron

·Ni-resist

·Stellite

·Tungsten carbide

·Bronze

·Stainless steel

·Tool steel

3.Elastmers for secondry seal

·Ethylene propylene

·Chorousulphonated polyethylene

·Fluroelastmers

·Silicon elstomers

·Flurosilicons

·Nitrile

·Carboxylated nitrile

·Chloprene

·Isobutylene isoprene

·Polyacrylic

·Epichlorohydrin

·Polyurethene

4.Metal componants

·Cadmium

·cadmium and chromate

·Zinc

·Zinc and chromate

·Black oxide

HANDLING AND INSTALLATION OF SEAL

HANDLING OF SEAL

           

1. Rough handling of the seal can scrath and chip the sealing faces and perhaps destroy their flatness .

2. Unequal thickness of seal can cause misalignment or distortion of critical part.

3. Sharp corners can cut secondry seals.

4. Sealing face surfaces may also be contaminated with dirt and foreign matter if care is not taken .

INSTALLATION OF SEAL

                          1. If seal is likely to run dry during initial startup period , the sealing faces may be lubricated with fluid to be  sealed .

                          2.  If initial startup period should long then sealing phases should be prelubricated with oil.

                          3.  Do not use wiping material which will leave any fibres on or around sealing faces

                          4.  Shafts and bores should be lightly lubricated prior to installing seal components to   

                               ensures proper sealing

5.     Before starting up mechanism with newly installed seal , a static pressure check be    

      applied .

 SEAL MULFUNCTION:-                                                                                                                                                                           

                            

§  Slight initial leakage, which decrease.

§  Initial leakage which continues.

§  Seal emits chips.

§  Seal leaks and ices.

§  Black powder shows up out side seal faces.

§  Short seal life.

      INFLUENCE OF EXTERNAL FACTORS ON PERFORMANCE OF SEAL:-

 1.The temperature operating condition

                    The temperature operating condition for seal are determined by temperature of fluid being sealed and amount of heat generated by friction.

                    In operating condition, friction temperature can exceed the fluid temperature by 80⁰ C   to 100⁰ C.

                     Due to increase in temperature , lubricating film developed brealk up and it affets the performance of seal.

2. sliding speed :    

            Increased sliding speed leads to growth in temperature in contact zone. At high               

            sliding speed , the material used has good heat and wear resistance .

3. Pressure and nature of fluid being sealed.

                    Growing pressure of the fluid being sealed increases radial forces , restrict the supply lubricant to the contact region , enlarges the part of the contact area where dry rubbing take place , and raises friction force and contact temperature . These condition results in weakened tightness and shorter seal life.

ENVIRONMENTAL CONTROL:

                    Apart from of the temperature of sealed fluid , considerable heat is generated at seal interface when running and as temperature increase it necessary   to consider use of more expensive seal material . Due to increase in temperature of fluid in the area around seal , seal life is reduced  .

                    To control temperature , cool product fluid is circulated usually from pump discharge line to seal cavity , ensuring the continuous flow of the fluid over the seal and back into the pump body . typical values of fluid circulation  rate are shown in table as bellow.

Shaft Diameter (mm)	Minimum Flow -1/min
20	0.35
40	0.9
60	2
80	3.5
100	5

                          Where further sophistication is necessary , a cooler may be added to the circulation line . other auxiliary  devices such as filter or cyclone separators may also be included to remove solids form circulation flow and ensure a cleaner fluid  in seal area . For better performance , a completely separate , cool flushing fluid may be supplied  to seal . e.g.

1. Cool diesel oil is suppied to high temperature heavy oil pump .

2. Many pumps fitted with water jacket around the seal housing and this can be supplied with cooling water.

                   

                   If fluid handled becomes very viscous or solidifies at ambient temperature , steps must be taken to ensure that it remains fluid in an around the seal by providing  some form of heating , steam for this purpose can be provided to water jacket around the seal . Similarly, if leakage is likely  to solidify around the seal it must be prevented . Leaking hydrocarbon is prone to carbonize around the seal but the steam quench supply to the atmospheric  side of seal will soft  and wash away such deposits  in many cases  .

                     If fluid to be sealed contains solid particles , then environmental control must be directed to ensure that fluid in seal area is as free from solids as possible  . The solution to above problem  is

1. To fit cyclone separator and filters to circulation lines.

2. Provision of separate supply of the clean fluid to seal chamber .

                   LUBRICATIONS:

                   Hydrodynamic lubrication plays an important part in lubrication of seal . In some cases hydrostatic lubrication is also considered . During starting and stoping boundry lubrication take place . Rubber dynamic seal are used with oil and there are few problems  related with it .But mechanical seal and soft packing can be used with any imaginable fluid and there fore there is wide rane of materials in current use foe sealing interface .

                      These materials are not only good boundary lubricants but also be resistant to chemical or electrochemical attack. Some examples of elstomers used in seal and there compatibility, resistance are shown in table as bellow.

                   

Polymer type	Max temp	Min temp	Mineral oil compatibility	Water compatibility	Wear resistance	Comp. set resistance
Natural	80	-60	Poor	Good	Good	Good
Nitrile	130	-45	Good	Good	Fair	Good
Polycyclic	150	-15	Good	Poor	Fair	Poor
Polyurethane	100	-50	Fair	Fair	Good	Fair
Silicon	200	-60	Fair	Good	Poor	Good
Fluorocarbon	175	-15	Good	Fair	Fair	Good

                         HEAT TRANSFER:

                             There  are two sources of heat  , one internal from the friction at the sliding interface , and other external from the process fluid or the environment .

                               The heat generated is conducted away through  each of two sealing surfaces and taken out in a heat sink . There are two important factors taken in account when considering heat transfer process .

                 1. The fluid in sliding interface will be at higher temperature than the process fluid and must not be allowed to reach its boiling point at lowest pressure to which it is exposed  usually atmospheric pressure

                 2. Resultant temperature  distribution should not be such as to produced adverse thermal stresses which might upset sealing face alignment of seal .

 The thermal stresses can also give rise to another problem which localized on scaling surfaces thermal razing. the appearance of surfaces in this condition is of fine network of more or less hairline cracks which produces surface damage in several cases .

There are several factors affecting thermal crazing as fallow.

1.Tensile strength(6ts)

2.Thermal conductivity (λ)

3. Youngs modulus(E)

4.Thermal expansion(α)

5. Relative velocity (V)

6.Sliding  friction (F) 

               All above parameters are combined to give parameters B that measures

resistances to crazing.   

B=бts λ / E α F V         

        

Heat transfer processes are important in minimizing lubricant film temperature and both bulk surface thermal stresses the former interfacing with full film lubrication modes and the latter causing crazing and premature failure.

CASE STUDY:-

      

Seals for rotary regenerative heat exchangers:-

           Automotive gas turbine as shown in figure.it has small size,economical for manufracture and operates at low compression.For good efficiency of turbine,it must be provided with heat exchanger which recovers considerable heat of exhaust gases by heating the compressed air in the engine immediately prior to combustion chamber the efficiency of heat exchanger system is critical factor in overall efficiency of engine and it depends much on  seal.

                                 

Heat Exchanger System:-

There are two fundamental types of heat exchangers as follows .

       1.Static recuperator

       2.Rotary regenerator

Out of two rotary regenerator heat exchangers is more efficient than recuperator they are less bulky and have application in automotive .

 The use of high temperature rotary generator has been impelled for many years by sealing problems caused by differential heating of disc which results in thermal distortion

           Most companies on development contract from US Energy Research And Development Administration remove wear problem that persists if seats are to be efficient

            The heat exchanger disc or course in American Usage  are usually about 75 mm thick and diameter is upto 700 mm thickness is dertermined by thermal properties of the material while diameter depends on engine power

            The surface finish of heat exchanger disc has significant factor both in reducing friction and leakage.The surface could either be ground or lapped .

Seal Duty and Design:-

             The perpose of the seal is to be define exist path of exhaust gases to atmosphere out of heat exchanger at atmospheric pressure . It  is essential that there will be no leakage of cold pressurized air but practical 3% leakage can be accepted.

            A hot side seal is shown in fig. the seals are narrow strips, about 10 mm wide formed such that heat exchanger surface is divided into 40%cold inlet air and 60% hot exhaust gas. the seal surface is attached by engine by st speed bellows and loaded against heat exchanger disc by entire gas pressure. The pressure on seal is originally upto 275 KN/m² . But engine development reduced this to 70KN/m² . The highest temperature of operation of high side seal is taken as 800 deg. C and on cold side seal  it is 250 deg. C.

Hot Side Seal

       Development of hot side seal :

 Material used for hot side seal  is Nickel Oxide because its thermal expression is similar to steel. The properties of  Nickel Oxide are increased by calcium fluoride.

Effect of density on wear rate:

 As density of pellet increased the wear rate decreases. The graph is shown in figure. Effect of graph size on wear is shown in figure.

Friction in Ni –O based seals:

Nickel Oxide additives has little effect on friction but Cobalt,Chromium and Zinc reduce friction. Coefficient of friction is measured during wear test and typical results are presented in the graph as shown below.

     Lead, Copper and Nickel Oxide additives has little effect on friction but cobalt, chromium and zinc reduce friction.

                                             

Cold side seal:

Material used for the cold side seal is graphite. It is expensive to manufacture. Nowadays search is made by various companies for alternative of graphite. The effect of wear and friction on cold side seal material at 200 deg. C and at 140 KN/m² is shown below

Seal material	Wearμh-1	Coefficient of friction μ
Boron nitride (ρ=1.76-1.80)	17.5-25 x 103
Boron nitride (ρ=2.10)	5-10 x 103
Carbon graphite	6 - 15	0.2 – 0.3
Carbon graphite(brownze filled)	0.3 – 1.25	0.1 – 0.3
Fine grained eletrographide	0.03 – 0.05	0.05 -0.08
Oxidation inhebited eletrographite	0.005	0.05 – 0.08
Ekonol 100% hot pressed	8.5x 103
+5% PTEE	20	0.06 -0.07
+5% FEP	15- 30	0.10 -0.15
+10% Graphite	65
+10% Graphite+18% FEp	1.5	0.06 -0.07
Polyamide + 15% graphite	0.2	0.06- 0.08
+40% graphite	0.8	0.1
+15% graphite	0.5	0.04 – 0.06
PTFE + 40 % bronze and graphite	4	0.16
+22% glass	4	0.16
+28%carbon	5	0.16
+29% bronze	45	0.15
Polyamide + 15%graphite +MoS2	3.5	0.12
Polypherylese Sulphide + 30%graphite	1.8	0.16
PTFE + Mica	3-5	0.05
Glass cloth: Polyamide bonded	45	0.06

CONCLUSION

     Tribology has been playing most important role in industry work. In America and foreign countries researches on tribology has been done to minimize the losses resulting in our society including shortened component lives, excessive equipment down time and large expenditure of energy. 

        Leakage from any type of system is major problem leading to loss of efficiency, increased power uses, temperature rise enviormental damage and safely hazard. 

               The passage of fluid between mating part of machine and between other mechanical elements is prevented or minimized by seal. 

               Therefore it has been concluded that seals reduce friction between mating part of various machine part.