Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
67 Cards in this Set
- Front
- Back
|
Describe |
A) static seals - used between two stationary parts B) dynamic seals - used between a moving and a stationary part C) positive seals - seals which prevent any fluid from leaking from one side of a system to their other side D) non-positive seals - seals which control the amount of such leakage |
|
|
How are two mating surfaces sealed without additional material |
By using a tapered fit or an interference (shrink) fit |
|
|
What are the applications for gaskets |
Gaskets are installed at a joint between stationary surfaces to: -seal against a pressure load -seal against leakage -control position |
|
|
What forces are the gasket and bolts subject to in pressure sealing application |
The gasket is subject to the force of compression exerted by the bolts. This compression force must be greater than: -The internal pressure, which tends to move a gasket sideways -the hydrostatic end force due to the internal pressure which tends to push the joint apart |
|
|
Why is the gaskets thickness important when a replacement is required |
To avoid increasing or decreasing any internal clearance |
|
|
List gasket and flange arrangements |
Flat (full)face Raised face Tongue and groove Spigot and recess |
|
|
Describe how gasket material is made to conform to the flange surfaces |
Is designed to conform to the machined surfaces or to flow slightly to fill small irregularities. The compression of the gasket is greatest at the bolt locations and is least 1/2 way between the bolts |
|
|
What are the prerequisites for gasket materials |
Gasket material should: -withstand the bolt load without being crushed -withstand the necessary temperature and pressure extremes -be compatible with the product being contained |
|
|
Compare the applications for non-metallic gaskets to metallic gaskets |
Non metallic gaskets - are made of paper, cork, natural or synthetic rubber or plastics. Theses are soft and used in relatively low pressure applications compared with metallic gaskets Metallic gaskets - come in a variety of different materials and configurations, they are harder than non-metallic gaskets and are normally used in high pressure and high temperature applications |
|
|
List the materials used in non metallic and metallic gaskets |
Non metallic - paper, cork, natural rubber, synthetic rubber and asbestos Metallic - lead, brass, stainless steel, various alloys such as inconel and hestelloy |
|
|
What o-ring cross-sectional shapes are used in hydraulics |
Multi-lobed, square or rectangular section rings |
|
|
What are the applications for sealants |
Sealants are frequently used with a flat gasket to fill up minor depressions in surfaces that are slightly corroded or pitted. Can be tape, paste or liquid form |
|
|
Briefly describe non hardening sealants, hardening sealants and tapes |
Non hardening sealants - are mastic (resin) materials which are soft and adhesive. They contain plasticizer which allows them to remain soft Hardening sealants A) Flexible-are available in a variety of materials such as butyl, acrylic, silicon, and polyurethane, these compounds often contain curing or settling agents which allow them to remain flexible B) Rigid - rigid sealants are based on epoxies, polyester, acrylic, polyamides or polyvinyl acetates. Because these cure to a rigid state, they tend to crack and results in joints that are difficult to remove Tapes - are available in different shapes for a variety of different applications such as edge, sandwich and lap sealing. Teflon packing in string form is frequently def to replace flat gaskets |
|
|
What are the two main categories of dynamic seals |
Contact and clearance |
|
|
What are the applications for non-positive seal and positive seal packing |
Contact seals -non positive seals controls leaks between stationary and moving parts, the movement can be reciprocating, rotary, helical or swinging rotary found in equipment such as pumps, compressors, presses and blowers Positive seals - prevents any leaks between stationary and moving parts, found in situations such as gate valves |
|
|
Define inside packed and outside packed |
Packing installed in he moving unit is called inside packed Packing installed in a stationary unit is called outside packed |
|
|
Compare hard packing to soft packing |
Hard packing withstands extrusion better than soft packing but it also creates more friction |
|
|
How is leakage controlled with compression packing |
-leakage is controlled by compression -the amount of sealing or leakage is controlled by the amount the packing is compressed -compression packing commonly found in centrifugal pumps and reciprocating water pumps |
|
|
What classes of materials are compression packing made from |
Fabric, metallic and plastic |
|
|
What cross-sectional shapes are compression packing |
Round, square or rectangular |
|
|
What shapes are metallic gaskets |
A) flat B) v or u shaped (light section) C) rectangular, triangular or octagonal (heavy section/) D) round or oval E) spiral wound F) corrugated or embossed G) jacketed with a filler material core (jacket may be corrugated) H) flexi metallic has a metallic inner ring and a non metallic outer ring |
|
|
What makes compression packing and/or die moulded rings self lubricating at startup |
It is impregnated with lubricant such as graphite or teflon so it is self lubricating at startup |
|
|
Describe the construction of a lantern ring |
It is a ring with channels in its inner and outer perimeters. The lantern ring is H shaped, channels are connected by radial holes. The lantern ring distributes sealing liquid under pressure to the packing. It prevents air infiltration and provides lubrication, Collins and cleaning to the area |
|
|
What are the purposes of lantern rings |
-distributes sealing liquid under pressure to the packing, prevents air infiltration and provides lubrication, cooling and cleaning to the area |
|
|
What are the sources of fluid supplied to the lantern rings |
The sealing liquid is commonly delivered from the high pressure section of the pump (internally supplied) -when semi-fluids such as slurry are pumped, an external source of sealing liquid is used (externally supplied) |
|
|
What causes compression packing to deteriorate |
It deteriorates from wear and also dries out |
|
|
How is the pump prepared for compression packing replacement |
A) prepare the pump for replacement work B) remove the old packing and identify its sequence, amount and size C) check the conditions of the lantern ring and stuffing box D) cut the new packing E)install the new packing F) adjust the gland and check the leakage The pump is prepared for replacement work 1) ensure the pump is locked out and drained to relieve any pressure behind the packing 2) if externally supplied sealing liquid is used, shut off the supply and drain the line |
|
|
Describe how to remove the old packing and confirm or calculate the correct materials for replacement |
1) remove the gland 2) remove all the old packing using a packing puller 3) remove the lantern ring if used 4) count the number of rings removed and record the position of the lantern ring if used 5) confirm this info with its stuffing box code 2-L-3-G indicates the packing sequence is: 2 rings of packing, a lantern ring (L), 3 rings of packing, and then the gland (G) ** if the stuffing box code is unavailable, determine the correct amount of packing by measuring the depth of the stuffing box. Then: -subtract the width of the lantern ring is used -divide it by the width of the new packing 6) determine the correct size and type of packing to be used by referring to equip manuals |
|
|
What checks are needed to confirm condition of the stuffing box and lantern ring? |
1) if a lantern ring is used, ensure that the sealing water inlet to the stuffing box is clear 2) check the condition and cleanliness of the stuffing box and shaft or shaft sleeve |
|
|
Describe how to cut the new packing |
1) cut the new packing to size. This is best done on a wooden mandrel with the same diameter as the pump shaft or shaft sleeve. Wrap the packing around it is the same direction as it comes off the roll -cut rings straight with axis of the mandrel (butt cut) this reduces the chance of fraying and sloppy joints -you may also cut rings diagonal. To do this draw two lines, the width of the packing apart. Then cut each ring of packing diagonally between the lines -if it is necessary to cut rigid (rolled, pressed, or die formed) packing, make a diagonal cut (a skived cut) |
|
|
Describe how to install the new packing |
1) wrap mandrel cut packing around the shaft -for rigid packing use an S-twist to place it over the shaft ( do not pull it open) 2) where compatible, lightly lubricate the ID and the OD of the packing 3) install one ring into the stuffing box at a time. If stuffing is slightly bulged, it may be difficult to install. If this is the case, the packing may be slightly flattened with a roller. Ensure this is done on a clean surface 4) use a split bushing or tamping tool to place each ring in position. Ensure that the ends of the packing line up to each other. Then work it around the circumference in both directions. Ensure that each ring of packing is seated at the bottom 5) ensure that the ring joints are staggered approx 90 degrees from each other 6) if a lantern ring is used, ensure that it lines up with the seal water inlet |
|
|
How do metallic gaskets seal |
Metallic gaskets seal by the flow or malleability of the gasket when the bolts compress the two surfaces together. The smoother the surface finish the better the seal |
|
|
Describe the procedure for adjusting the gland to permit the correct leakage for lubrication |
1) after the last ring is installed, allow the gland to enter the stuffing box by 1/8” to 3/16” 2) tighten the gland nuts with a wrench to seat and form the packing to the stuffing box and the shaft 3) ensure that the gland enters the stuffing box square with the shaft 4) undo the nuts and allow the packing to expand 5) finger tighten the nuts again 6) start the pump allowing the packing to leak freely 7) gradually tighten the nuts equally ( a sixth of a turn or one flat) until leakage is reduced to the recommended lubrication level |
|
|
Describe the operating principle of automatic packing |
-uses system pressure to improve its sealing action -does this by forcing the flexible lips of the packing onto its contact surfaces -consists of v-ring, u-ring and o-ring and cup packing |
|
|
Why is V-ring considered balanced and cup packing considered unbalanced |
V-ring, u-ring and o-ring are considered balanced packing as they seal on the ID and the OD -cup packing is considered unbalanced as it seals on one diameter only |
|
|
What are the purposes of internal and external adapters |
-used with v-ring packing -an internal adapter ring should be hard enough to prevent extrusion, yet soft enough to compress slightly under pressure -the external adapter merely shapes the v-ring and is not subject to wear |
|
|
Describe the installation of chevron packing and the various methods of tightening it |
V-ring packing (chevron packing) -is a circular, one piece moulded ring with a v-shaped cross-section. It is used in sets of three or more rings, depending on working pressure -must be used with internal and external adapters to prevent spreading -when v-rings must be cut stagger the cuts 90 degrees -at initial install tighten rings just enough to prevent leakage |
|
|
Describe the construction and installation of u-ring and cup packing |
-u cup (u ring) packing is circular, one piece moulded ring with a u-cross section made from synthetic elastomers -this endless design is meant to be used singly per seat -u-ring packing can be used as an inside or outside packed seal -used in systems less than 1500 psi, pressures above this require the addition of an anti-extrusion washer (support ring) |
|
|
Describe how o-rings and their grooves and backup washers control leakage in both low and high pressure conditions |
-diametral squeeze is necessary to maintain a seal -too much squeeze rapidly wears out rings -the squeeze is equal to 10% of ring diameter -standard o-rings are rated to 1500 psi with properly designed grooves -up to 3000 psi using anti extrusion rings or back up washers |
|
|
Describe the o-ring installation procedure |
1) lubricate the o-ring with a light oil before seating 2) roll the o-ring into its seat 3) do not cut backup washers 4) protect the o-ring from cuts or nicks from sharp metal corners |
|
|
Are piston rings a positive seal |
They are not a positive seal because they leak slightly |
|
|
What are the advantages and disadvantages of floating packing compared to synthetic seals |
-advantages -creates less friction -can take high pressure -is less affected by temp extremes Disadvantages Are that floating packing: -conforms less to the mating surfaces -has a longer run-in period |
|
|
Why are ring gaskets preferred over full face gaskets on flanges |
Ring style covers are inside bolt circle. Full face cover entries flange area even under bolts Ring style is preferred because it takes less time to make and because it is easily installed because you do not have to remove all the bolts for install |
|
|
Describe the construction and principles of operation of a lip seal |
-used in low pressure ares 2 to 10 psi -primarily used on rotating shafts which are partially or totally immersed in lubricant -a hydrodynamic film of lubricant is formed at the point of contact to prevent friction and wear -the film should be 0.025mm (0.001”) thick (if thicker leakage may occur, thinner - friction and heat occur rapidly wearing seal down |
|
|
Describe the applications for a single lip seal |
-consists of one sealing edge -usually installed with the lip inward to include or contain the lubricant in the housing, assuming the atmospheric side is relatively clean -when atmosphere is dirty, it can be installed with lip facing out to exclude foreign material -the small amount of lubricant that leaks out acts as a flushing medium to keep the contact surfaces clean |
|
|
What are the applications for a double lip seal |
-consists of two sealing edges -can have their lips back to back, or facing the same direction -these are used where additional sealing is required -used where there is higher housing pressure or as inclusion and exclusion seals in dirty environments |
|
|
Describe the procedure for installing a lip seal in the housing and sliding a shaft into it |
1) lubricate the shaft, the housing and the sealing edges 2) determine whether the seal is to be used as an inclusion or exclusion seal 3) remove any sharp edges around the bore of the housing 4) aligh the seal with the bore 5) press the seal into the housing squarely and smoothly 6) if the seal is to rest below the machine face, ensure that the pressing device is slightly smaller than the bore of the housing **shaft into seal 1) ensure that the sealing edge does not slide over sharp edges such as keyseats or threads 2) use plastic, shim stock or even paper to cover any sharp edges 3) unless the shaft has a chamfer with a long taper, used proper leader or light gauge shim stock. When using shim stock, make sure the seal twists away from the sharp edge of the shim stock |
|
|
What emergency repairs can be made to correct a leaking seal caused by wear on the shaft |
Common short term solutions -change to a wider seal to move the location of the lips -reverse the seal if the lip has a pronounced offset -install a spacer behind the new seal of original size to move the lip contact to a new area -install a thin sleeve over the worn spot |
|
|
Describe the construction of and applications for ring seals |
-a ring seal is a non-positive seal which is mounted into a groove in the housing -made of cork, felt, leather or synthetic rubber -they are precision cut washers with a broader sealing edge than lip seals -over an extended period they tend to polish the shaft |
|
|
Describe the purpose and operation of a wiper |
-a wiper (scraper) is a non-positive seal mounted on the stationary housing. It flexes against axially moving parts. Wipers expel foreign material away from the seals, packing, or bearings. -commonly found on hydraulic and pneumatic cylinders in dusty or dirty locations |
|
|
What types of seals are the following and what are their applications |
A) boots and bellows seals - are positive seals which have one end secured to the housing and the other to the moving part. Boots are used for short movements and bellows for long ones B) diaphragm seals - is a positive seal which has a flexible membrane which spans between the stationary and moving parts. It is clamped between the two stationary housing halves and is clamped to the moving part with plates flat diaphragm - is designed for small amounts of movement A rolling diaphragm seal is a moulded membrane and rolls on the two mating surfaces as it moves |
|
|
What is the efficiency of a mechanical seal |
-extremely efficient axial seal -much more effective than any other dynamic seal -leakage is 50mm that the naked eye cannot detect it |
|
|
Describe the operating principles of mechanical seals |
-a mechanical seal has a primary and secondary sealing -the primary seal is dynamic and is composed of two lapped faces in contact. One is stationary and the other rotates. The mating surfaces are at right angles to the shaft -a Pringle’s loaded device keeps the mating surfaces together -the secondary seal is static and is in between the contact face and the shaft and between the contact face and the housing, most commonly used is o-ring
|
|
|
Describe the procedures used to make new gaskets |
1) get the OD, ID, bolt circle Durante’s and any other info from drawings or actual equip 2) draw the gasket shape onto the gasket material or: 1) smear the part with marking ink or graphite 2) lay gasket stock on the material to get an impression Or: 1) hold the gasket stock against the face 2) draw the outline & bolt holes |
|
|
How is flushing achieved with mechanical seals |
-the mechanical seal must always run in fluid -the flushing ports feed the fluid into the product side of the seal -the fluid can be used directly form the product discharge or from an external source -a cyclone strainer may have to be used to remove solids or a heat exchanger to remove excess heat |
|
|
What are the advantages of flushing vs not flushing |
-maintain a consistent operating temp -prohibiting vaporization of volatile fluids around the contact faces -prevent crystallization of fluids near the contact faces -prevent solids from accumulating around the seal -prevent the product from entering the stuffing box (if the flushing fluid is at at higher pressure than the system) |
|
|
How is quenching achieved with mechanical seals |
-is done through quenching ports on the atmospheric side of the seal -fluid comes form an external source and must be compatible with the seal faces, the product being seals and the atmosphere -steam is commonly used -a throttling bushing is used to keep the fluid at the contact face |
|
|
What are the advantages of quenching vs not quenching |
Advantages of quenching: -prevent leakage of corrosive or toxic fluids into the atmosphere -prevent crystallization around the contact face -maintain a constant operating temp and minimize any heat transfer along the shaft |
|
|
Is it good mechanics to mix old and new seal parts for installation |
It is best to replace the complete seal |
|
|
How are repairs made to seal faces |
-by sending to manufacturer for overhaul -or by hand lapping: 1) charge the lapping/polishing plate with a diamond lapping plate 2) lightly press the seal face against the lapping plate and move it in a figure 8 motion 3) continue this until the surface appears flat 4) what’s the face with solvent 5) check the surface with a mono-chromatic light and optical flats 6) repeat the procedure if necessary |
|
|
How should mechanical seals be handled |
Handle them carefully Do not touch their mating faces |
|
|
Describe the following clearance seals |
A) bushings - non-positive seal mounted in a stationary housing -min clearance between it and the rotating shaft, restricting the amount of leaking from high to low pressure sides -considered the simplest of seals B) annulus seals - non-positive seal, with a series of concentric or helical grooves in the housing bore -in a concentrically grooved seal, lubricant fills the grooves and prevents entry of contaminants -in a helically grooved, the oil is returned to bearings by this action C) slinger (flinger) seals - non-positive seal which has a cupped washer-like flange mounted to the rotating shaft -placed just outside the seal housing to assist the sealing of a ring seal -as any lubricant passes the ring seal, the lubricant is flung out by centrifugal force -prevents contaminants from entering the seal D) labyrinth seals - non-positive seals with a tongue and groove design -a series of internal and external passages from a maze like path for foreign material to follow -can be made to seal axially or radially -labyrinth seals used in high speed applications -effective where the environment includes abrasive contaminants or semi solid liquids |
|
|
What should be done before installing a new gasket |
1) remove all old gasket stock 2) check the metal faces for irregularities 3) smooth the faces where necessary 4) smear a light film of lubrication on the contact surfaces |
|
|
What bolt tightening procedures should be complied with while installing a new gasket |
-the bolts are first pulled snug around the casing, then tightened gradually working across the body It is more efficient to take two or more passes around the bolt sequence than to try to reach full tension the first time around |
|
|
What type of sealing applications do o-rings perform |
Dynamic or static seals |
|
|
How much does an o-ring change the position of a part |
Does not usually make a difference |
|
|
What materials are o-rings made from |
Chosen to resist chemicals and suit the temps O-rings made of materials other than synthetic rubber have manufacturers marking on them Metallic o-rings - are commonly used where extreme conditions of temp, pressure or corrosion exist |
