About Seals- Chapter Three some seals

Mackintosh

About Compression Packing Seals
- q& H# V: H, D/ QCompression packing seals, or gland seals, include compression packing, gaskets, mechanical seals, gasket material and other fluid sealing devices. Sealing is accomplished by tightening the gland, so that the packing is compressed onto the surface to be sealed.  This is a broad category covering seals that are manufactured in a wide array of shapes, sizes, and constructions, from many materials.
1 `) |& Z! f5 p* U7 C6 pWhen selecting compression packing materials, it is important to understand a number of specific parameters that can affect performance:  the size of the cross section that must be sealed, the media and its specific qualities, the type of equipment (i.e., pumps, valves, mixers, etc.), the temperature and pressure of the media being sealed, and shaft or rubbing speed if the machinery is a pump or rotates.
0 B4 n- R1 w- DThe following are descriptions of basic compression packing seals constructions.
Braid-over-braid construction, also called jacket-over-jacket, is fabricated by braiding one or more covers over a center core of braided, twisted or homogeneous materials. This design can be braided round, then calendared square. The weave is soft and dense because of individual jacket-over-jacket construction and can carry a high amount of lubricant.
Braid-over-core design is produced by round braiding one or more jackets of yarn, rovings, ribbons or other forms of materials over a core, that may be extruded, twisted, wrapped or knitted.
8 G3 Z$ D" _! u% k2 ?+ \; DLaminated compression packing seals are formed from asbestos and rubber, or duck and rubber, which is laminated into slab form, and then fabricated into spirals, coils or rings.
+ B! p% O" Z0 t: ^6 p' sIn multi-braid or lattice seals, strands are diagonally braided for greater strength and longer life. Each strand passes diagonally through the body of the packing at an angle of approximately 45°. The design is resistant to permeation leakage and would not be damaged entirely even if the surface gets worn.
Plaited compression packing is constructed from interwoven yarns with pockets formed between each plait to retain lubricant.
Square-braid seals have a square cross section structure, which is soft and absorbent, and can carry a high amount of lubricant. The design is especially suited for high-speed rotary service, particularly at low pressure. Note: square braid is also called plaited by some manufacturers.
Twisted compression packing seals are of a very simple design.  Yarns or metallic strands are twisted around each other to obtain the desired size. One packing size can be used for various size stuffing boxes. This is primarily a general utility or emergency type of packing where packing space is small.
Conical and Telescopic Screw Covers - Conical and telescopic screw covers are used to protect ball, acme and lead screws from contaminants.
3 T; U! X& o1 }$ hAbout Diaphragm Seals
Diaphragm seals (also called chemical seals, instrument isolators, and gauge protectors) are isolation devices used to separate pressure instruments from the process media, while allowing the instruments to sense the process pressure. A diaphragm, or bladder, together with a fill fluid, transmits pressure from the process medium to the pressure element assembly of the instrument. Diaphragm seals may be attached directly to the instrument, or remotely, using a capillary line. These devices are intended to keep the process medium out of the pressure element assembly and to prevent damage from corrosion or clogging.  Diaphragm seals are also used to maintain the sanitary requirements of the process medium and to reduce the process temperature to which the pressure element is exposed. Diaphragm seals function using a flexible diaphragm that is attached between the upper and lower housing of the seal. The upper housing is filled with a fluid and attached to the pressure instrument. The lower housing is in contact with the process medium and has the connection to the process system. The process media pressure is transmitted via the diaphragm to the fill fluid in the upper housing and subsequently to the pressure instrument.
There are three common diaphragm seal designs, although custom and specialized varieties are also available.
Welded diaphragm seals are welded (or bonded) directly to the upper housing, permitting removal of the upper housing without a loss of fill fluid.
Clamped diaphragm seals are clamped between the upper and lower housing.  This style of diaphragm may be removed and replaced easily, as it is not welded into place.
9 \7 Z+ e" H  L; A, H- UCapsule diaphragm seals are internally sealed devices with the fill fluid within the capsule in the upper housing. The main advantage of this type of seal is that the upper housing can be removed without loss of the fill fluid, because the fill fluid is contained within the seal.
When specifying diaphragm seals, there are a number of important points to consider.  These include the type of process fluid and how it will react with the seal materials, the vapor pressure point of the fill, the flexibility of the diaphragm, and that the fill fluid is chemically compatible with the process fluid.  Fill fluids must be chemically compatible with the process fluid to avoid potentially hazardous conditions in the remote possibility of a diaphragm rupture.
! \$ t7 D* L- R+ `* B( h7 `3 GVacuum, compound and low range pressure devices require a highly transmissive, flexible diaphragm to give accurate results.  An important consideration is the vapor pressure point of the diaphragm seals’ fill for a vacuum application. If the combination of pressure and temperature reach the vapor pressure point accuracy will suffer.
About Die Cutting Services
Die cutting services use a variety of die cutting methods to fashion materials into predefined shapes or sizes.  Several different types of die cutting are available, including flat and rotary laser die cutting, blade type rotary die cutting, steel rule die cutting, and ultrasonic die cutting.  The method used can vary, depending on such factors as material, and end product configuration.  Laser, or laser die cutting is performed on computer controlled, 3-axis laser cutting machines.  Laser cutters can be used to cut materials that cannot be cut using conventional steel rule die cutting, such as steel to over .500".  Laser cutters are available in flat and rotary configurations.  Flat laser cutters are used to cut flat sheets of material and rotary laser cutters cut material from roll stock.  Rotary die cutters cut and score soft to semi-rigid material by forcing it between the blades on a cylindrical die and a hard cylindrical anvil.  Rotary die cutters are primarily used to cut material from a web (continuous roll), although they can be used to cut material in sheet form.  Steel rule die cutters score and cut material into various shapes, as determined by the steel dies.  Examples of products that are die cut are flat gaskets, puzzle pieces, and a presentation folder that has a pocket (the pocket is scored, folded and glued).  Steel rule die cutters can cut sheet material or material from roll stock.  Ultrasonic die cutters use vibrations applied at ultrasonic frequencies to cut or seal a material.  The vibrations cause heat and pressure to be generated, which causes sealing of thermoplastic materials and cutting, depending on the blade configuration.
Many different materials can be processed by suppliers of die cutting services for a wide variety of applications.  Materials include felt, fabrics, fiber or cork or paper, metals and alloys, plastics, rubber, foam and sponge, composites, electrical insulating materials, and EMI/RFI shielded foils and laminates.  Examples of products manufactured by die cutting include gaskets and seals, shims, templates, inserts, washers, and puzzle pieces.
Typical services offered by suppliers of die cutting services include CAD / CAM support, design assistance, prototype, short run production, high volume production, and inspection or quality control.  A supplier that has CAD or CAM support has the computer technology to receive solid model files electronically and use those files for part creation.  A supplier with design assistance capabilities can assist with concepts, manufacturing costs, manufacturing techniques and material considerations. Supplier may also be able to assist in upgrading or redesigning, re-evaluating or modernizing existing products to increase performance and/or reduce manufacturing costs.  Prototypes include a prototype or part that will be used in actual functional testing.  Short run production capabilities provide low volumes of product.  Suppliers with high volume production capabilities can provide high volumes of product.  Inspection or quality control services include quality inspection reports or statistical process control data.  Certifications and quality requirements often met by suppliers of die cutting services include ISO 9001, ISO 9002, and QS 9000.
) n& s3 }2 p, ]+ s* W" J6 K  }+ _About Dynamic Seals
! C" H1 X' v3 b+ z4 }8 ODynamic seals includes oil seals, hydraulic & pneumatic seals, exclusion seals, labyrinth seals, piston rings, and bearing isolators. This is a general search form. More detailed search forms are available. Dynamic seals are used to create a barrier between a rotating and a stationary surface. They function to retain or separate lubricants or fluids, keep out contamination, and contain pressure. For proper installation, the seal lip should point towards the medium being contained.
1 C& ]9 T2 K- ^About Electrical Connector Caps, Plugs and Covers
: p/ l7 q/ z. M  a+ {These caps, plugs, and covers are designed to provide mechanical protection for electrical connectors. They are available in a variety of different shapes for different connector sizes.
About Exclusion Seals
$ Z3 P  {) u! k/ D, f) {Exclusion seals are primarily used to keep dirt and other contaminants from passing into bearings and other sensitive areas.  Some of the devices that fall within the exclusion seals category do not actually have any sealing capabilities.  Instead, they serve to wipe or scrape away debris from moving or reciprocating shafts or rods. This helps to protect the seal and extend its life.
Exclusion seals are available in three main configurations, V-ring, wipers, and scrapers.
V-ring seals are all rubber seals used primarily to exclude dirt and other contaminants. V-rings stretch over a shaft and make a seal against the side of a housing or other machine component.  Wipers are flexible lip exclusion seals. They are used to wipe contaminants from a shaft. Some wipers also act as seals.  Scraper exclusion seals have a scraping edge that maintains contact with the rod regardless of any rod deflection or motion. The lip of the scraper is shaped with a semi-rigid leading edge that removes any contamination from the sealing area. Scrapers have no real sealing capabilities.
6 M# R5 |9 r% D7 L5 v  |) M3 d. AWhen selecting between available exclusions seals, it is important to be aware of both the operating dimensions needed for the seal, and the performance stress to which it will be subjected.  Important specifications to consider include the I.D. and O.D. of the exclusion seal and the O.D. of the housing bore to which it will be applied.  Other important considerations include the axial cross section and radial cross section of the seal, and finally the gland width into which the exclusion seal will be mounted.
! s' d+ I+ N0 p7 r0 d% G0 N- V0 b8 `Service limits to consider when working with exclusion seals include the temperature range through which the seals will need to perform, and any fluids with which they may come in contact.  Both of these situations bear consideration, as they will help to determine the best material construction for the exclusion seals.  Additional concern should be paid to the pressures, vibration or shaft speed, to which the exclusion seals will be subjected, and whether they will be needed in vacuum rated applications.  
' S; u7 Z& ?4 N% [, i, SExpansion Joints - Expansion joints and couplings connect sections of pipe and provide allowance for movement due to service load, shock, or thermal cycles.
About Gaskets and Gasketing
" B% Q( F2 k& C1 n+ y" F0 x8 u6 k) uGaskets are used to prevent fluid or gas leaks by providing a barrier between two mating surfaces. The gasket must be able to maintain a seal under pressure and temperature changes. A number of different gaskets are available: pre-cut compression (flat or extruded style), formed-in-place, and cured-in-place. Some gaskets and gasketing can also be used to shield against electromagnetic and radio frequency interference (EMI/RFI).
Common types of gaskets and gasketing include sheet or die-cut, gasket strip, sanitary gasket, spiral wound, spring finger, tape, window gasket, cured-in-place (CIP), and formed-in-place (FIP).  In a sheet or die cut gasket the gasket material is a flat sheet that can be cut prior to assembly or is already die-cut from flat gasket material.  Gasket sheets are frequently rolled when purchase in long lengths.  In a gasket strip the gasket material is in a strip configuration, such as weather-stripping material.  Gasket strips are manufactured in many different cross sections.  Some more common cross-sections are round (solid and hollow), square or rectangular, channel, C-fold, D-section, L-section, P-section, knife-edge, and wedge.  Sanitary gaskets are used in processing pipeline systems for the food, dairy, beverage, pharmaceutical and biotech industries.  Sanitary gaskets are available in many different configurations, such as clamp, bevel seat, flanged, PTFE envelope, schedule 5, proprietary sanitary fittings (Cherry-Burrell I and Q line, John Perry, etc.), and others.  Spiral wound gaskets are constructed with alternating layers of a soft filler material and a formed metal wire.  They may include a centering ring, an inner ring or both.  The outer centering ring acts as a compression limiter and the inner ring adds radial strength.  They are used for a variety of applications due to their resiliency and strength.  Spring finger gaskets are usually made of beryllium copper or other EMI shielding material and are available in strips.  They have spring finger wiping and low closure properties.  Spring finger gaskets are available in a variety of cross sections.  Tape gaskets are of flat or tape construction.  Window gaskets are used to seal a glass or plastic window in a sheet metal opening.  They are manufactured in a variety of shapes and styles, most having an "H" cross-section.  Window gaskets may be have one or two-piece construction.  Cured-in-Place (CIP) gaskets are applied in precision beads to one of the mating surfaces as a liquid.  The liquid is then cured by ultraviolet (UV) light to form an elastomeric material that is adhered to the surface.  Sealing occurs as the elastomer compresses during assembly of the mating parts.  Formed-in-Place (FIP) gaskets are applied to one mating surface as a liquid sealant before assembly.  After the mating parts are assembled, the gasket spreads and cures between the two surfaces, filling all gaps and surface irregularities.
- U* K: r  O+ ?) iCommon materials of construction for gaskets and gasketing include beryllium copper, butyl, copper, cork, EDPM, fluoroelastomer (FKM-Viton), foam, graphite, neoprene, nitrile (NBR – Buna-N), non-asbestos, paper, PTFE, polyurethane or urethane, RTV silicone, silicone rubber, and steel.  Important specifications to consider when searching for gaskets and gasketing include maximum operating pressure, operating temperature, and EMI or RFI shielding.
Hermetic Seals - Hermetic seals provide a vacuum or leak tight seal such as a brazed glass-to-metal seal.
6 O- L3 P5 N& k! O, oAbout Hydraulic Seals and Pneumatic Seals
9 Z) K6 q7 ^0 L( B" `/ |8 nHydraulic seals and pneumatic seals include piston seals, rod seals, U-cups, vee, cup, W, piston, and flange packings. Hydraulic seals and pneumatic seals are designed for the reciprocating motion that is common in hydraulic and pneumatic applications, such as cylinders.  Hydraulic seals are designed for high-pressure dynamic applications such as hydraulic cylinders. Pneumatic seals are used in pneumatic cylinders and valves and generally are designed for lower operating pressures than hydraulic seals. Pneumatic applications also typically demand higher operating speeds and lower friction seals than hydraulic applications.  These seals may be used for rotary and reciprocating motion.  Some hydraulic seals and pneumatic seals are composite. Composite seals are two- or multi-part seals manufactured as an integral unit. A typical composite seal consists of an integral PTFE ring and elastomer ring, providing the properties of an elastomeric ring with a rigid, low friction (PTFE) working face. These seals can have a variety of different cross sections. Some forms of composite seals are also referred to as polyseals, wedge-action seals, crown seals and tri-seals (3 part seals).  
! q* W" f0 {; s! |  Z" zCommon sealing orientation and directions for hydraulic seals and pneumatic seals include internal or rod seal, external or piston seal, symmetric seal, and axial seal.  Rod seals are radial seals.  The seal is press-fit into a housing bore with the sealing lip contacting the shaft. Also referred to as a shaft seal.  Piston seals are radial seals.  The seal is fit onto a shaft with the sealing lip contacting the housing bore. V-rings are considered external lip seals.  Symmetric seals are symmetrical and works equally well as a rod or a piston seal.  An axial seal seals axially against a housing or machine component.  Sealing direction pertains primarily to hydraulic and pneumatic seals that are used in applications with axial motion, such as cylinders and pistons.  The action can be single or double.  Single acting, or unidirectional seals, offer an effective seal in one axial direction only.  In order to seal in both directions for a reciprocating motion, more than one seal must be used.  Double acting, or bi-directional seals, are effective when sealing in both directions.
8 Z2 A7 `8 \: b' m0 RImportant dimensions to consider when specifying hydraulic seals and pneumatic seals include shaft outer diameter or seal inner diameter, housing bore diameter or seal outer diameter, axial cross section or thickness, and radial cross section.  Important service limits parameters to consider include maximum operating speed, maximum operating pressure, vacuum rating, and operating temperature.  Rubber sealing element or lip material choices for hydraulic and pneumatic seals include ethylene acrylic, EDPM, fluoroelastomer, fluorosilicone, nitrile, nylon or polyamide, polychloroprene, polyetheretherkeytone, polyoxymethylene, polytetrafluoroethylene, polyurethane or urethane, and natural rubber.  Metallic choices include sintered bronze, cast iron, and stainless steel.  Other materials include felt and leather.  Many seal manufacturers use their own proprietary material.  Consult with manufacturer for proprietary material specifications.  Common features for hydraulic seals and pneumatic seals include spring loaded, integral wiper, and split seal.
Inflatable Seals - Inflatable seals are elastomeric tubes designed to round out with the introduction of a medium (usually air) to form an tight barrier between a mounting and striking surface.
' n" |3 [* \8 w; W' P8 R* xAbout Mechanical Seals
; p/ Y  L6 e; ~. i, L" e  qMechanical seals are designed to prevent leakage between a rotating shaft and its housing under conditions of extreme pressure, shaft speed and temperature.  Mechanical seals can be single acting or double acting.  Single (acting) mechanical seals have one sealing gap. The lubrication film required by the sliding seal faces is provided by the medium to be sealed.  The lubrication film required by the seal faces in double (acting) mechanical seals is provided by a higher pressure buffer medium (sealant liquid) that is compatible with the pumped product. The sealant liquid is at a higher-pressure so that any leakage across the seal faces will be the sealant liquid into the pumped product. This buffer serves to separate the product and the atmosphere.  Seal design choices include pusher, metal bellows, and elastomeric bellows.  A pusher mechanical seal utilizes a dynamic secondary seal or o-ring that is responsible for sealing the fluid path between the pump shaft and the inside diameter of the rotating seal face. The secondary seals move axially along a shaft or sleeve to maintain contact at the seal faces, compensating for seal face wear and for any seal wobble due to misalignment.  Metal bellows design is a non-pusher seal design. The secondary seal in a non-pusher design does not have to move along the shaft or sleeve to maintain seal face contact. The bellows itself provides the necessary spring loading for seal face contact. Metal bellows provide effective sealing in a wide range of temperatures and use no elastomers.  An elastomer bellows seals is a non-pusher seal design in which a single spring coil fits over the shaft and bellows.
& ]$ l, O8 f$ P& TOther important design parameters to consider for mechanical seals include spring configuration, shaft mounting, and seal configuration.  Spring configuration can be single or multi.  Single springs are sometimes called "monocoil" or "single coil" design. This type of seal uses a large spring cross section that resist corrosion. Its chief limitations are its tendency to distort at high surface speeds, the large axial and radial space it requires and the need to stock a different size spring for each seal size.  Multiple small springs are not as susceptible to distortion at high speeds as are single coil springs and they consequently exert an even closing pressure on the seal ring at all times.  Shaft mounting choices include cartridge unit, noncartridge, split seal (fully split), and semi-split seal.  The seal can be tandem, face-to-face, back-to-back, or concentric.
. S; \/ u# d! v9 J, A7 c# n, Y! v( s9 [( }Common applications for mechanical seals include pump, agitators or mixers, marine stern tube (propeller shaft), gas seal (spiral groove seal), and cryogenic seal.  The seal can be internally or externally mounted.  Important shaft size and service limits to consider when searching for mechanical seals include nominal shaft diameter, shaft speed, alternate shaft or rubbing speed, operating pressure, and operating temperature.  Common features for mechanical seals include balanced or unbalanced construction, dependent on direction or rotation or independent of direction of rotation, capability to handle slurries, and encased spring element.  The direction of the shaft rotation is important to consider.  This is the direction of a shaft's rotation as seen from the drive. Mechanical seals that are dependant on the direction of rotation are those that transmit torque using a conical spring or those that are equipped with a pumping screw.  The direction can be clockwise or counter-clockwise.