Chapter 14 - Seals

Adjust the Gland and Check the Leakage

1. After the last ring is installed. allow the gland to enter the stuffing box by I/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. S. 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.

Install Shaft Into Lip Seal

1. Ensure that the sealing edge does not slide over sharp edges such as key-seats or threads. 2. Use plastic, shim stock, or evcn paper to cover any sharp edges over which the seal must pass. 3. Unless the shaft has a chamfer with a long taper, use a 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. Caution! Do not use a screwdriver to pry the seal onto the shaft because this can damage the sealing edge

Install O-rings

1. Lubricate the a-ring with a light oil before seating. 2. Roll the a-ring into its seat. Stretch it only as much as needed for installation. 3. Do not cut back-up washers. Teflon washers are supplied as spiral wound split washers to permit stretching. Leather back-up washers can be stretched by soaking them in hot water for a prescribed time before installing. Air-dry the washers at about 200°F (93°C) to shrink the leather. Put the flesh side (muscle side) next to the a-ring. 4. Protect the a-ring from cuts and from nicks from sharp metal corners.

Clearance Seals Bushings

A bushing seal is a non-positive seal which is mounted in the stationary housing. The minimal clearance between it and the rotating shaft restricts the amount of leakage from the high to low pressure sides. It is considered the simplest of seals.

V-ring packing (chevron packing)

A circular, one-piece moulded ring with a V-shaped cross section. It is used in sets of three or more rings, depending on the working pressure. It is used as an inside or outside packed seal. Must be used with shaped internal and external adapter rings

Double Lip Seal

A double lip seal consists of two sealing edges. They can have their lips back to back or facing the same direction. These are used where additional sealing is required. Examples are where there is higher housing pressure or as inclusion and exclusion seals in dirty environments.

Labyrinth Seals

A labyrinth seal is a non-positive seal which has a tongue and groove design. A series of internal and external passages form a maze-like path for foreign material to follow. A labyrinth seal can be made to seal axially or radially, depending on application. Radial labyrinth seals require split housings.

Ring Seals

A ring seal is a non-positive seal which is mounted into a groove in the housing. They are made of cork, felt, leather, or synthetic rubber. shows a felt ring seal. Cork, felt and leather ring seals are precision-cut washers with a sealing edge broader than that of the lip seals. Over an extended period, they tend to polish the shaft. Synthetic rubber ring seals are molded into shapes required for the job.

Emergency Lip Seal Repair

A shaft wears under the seal contact point when fme abrasives or dirt are held by the seal. When the amount of wear interferes with proper sealing, the shaft should either be refurbished or replaced. These repairs are time consuming and short-term solutions may be needed. The following are common short-term solutions: • Change the seal to a wider seal to move the location of the lips. • Reverse the seal if the lip has a pronounced offset. • Instal! a spacer behind the new seal of original size to move the lip contact to a new area (Figure A). • Install a thin sleeve over the worn spot. In some cases these sleeves can be purchased (Figure B)

Single Lip Seal

A single lip seal consists of one sealing edge. It is usually installed with the lip inward to include or contain the lubricant in the housing (inclusion seal or seal in), assuming that the atmospheric side is relatively clean. When the atmospheric side is dirty, it can be installed with the lip facing out, to exclude foreign material (exclusion seal or seal out). The small amount of lubricant that leaks out acts as a flushing medium to keep the contact surfaces clean.

O-rings

An O-ring is circular with a round cross section like a doughnut. It is usually made from synthetic rubber. It can be used as a dynamic or a static seal. The O-ring docs not usually make a difference to the relative position of the machine parts. The joint is designed so that the correct compression of the O-ring is accomplished when the two machine surfaces are in contact with each.

Static Seal O-ring

An O-ring used for a static seal has standard dimensions. The groove which holds the O-ring in place has the correct allowance for compression.

Flexible

Are available in a variety of materials such as butyl, acrylic, silicone and polyurethane. These compounds contain curing or setting agents which also allow them to remain flexible. Some of these are true rubbers and others are adhesive. They are all resistant to various environments.

Tapes

Are available in different shapes for a variety of different applications such as edge, sandWich. and lap sealing. Teflon packing in string form iq frequently used to replace flat gaskets. The ends are twisted together three or four times to form a ring gasket. Tapes have various adhering methods such as pressure sensitive, solvent activated. and self-sticking. Teflon tape is commonly used to seal pipe-threaded connections.

Rigid

Are based on epoxies, polyesters, acrylics, polyamides. or polyvinyl acetates (PVA). Because these cure to a rigid state. they tend to crack and result in joints that are difficult to remove. Some of these sealants can be used to join as well as seal.

Non-hardening sealants

Are mastic (resin) materials which are soft and adhesive. They contain plasticizer which allows them to remain soft. They are usually brushed, extruded, or thumbed into place

Back Up Washers

Back-up washers are often made of leather, Teflon or moulded nylon. Their purpose is to: • Eliminate extrusion of the a-ring • Permit wider clearance between moving parts • Form a seal across a clearance gap • Act as a dirt wiper Note: When back-up washers are used with O-rings, the grooves are required to be wider. Do not use a back-up washer and O-ring in a groove designed for O-rings only.

Application of (Labyrinth Seal)

Because their surfaces are not in contact, _________ _____ are used in high speed applications. They are also very effective where the environment includes abrasive contaminants or semi-solid liquids. This seal is usually lubricated and the lubricant fills the passages and forms a seal to trap contaminants. As the seal is re-lubricated, the lubricant forces trapped contaminants out.

Installing New Gaskets

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 lubricant on the contact surfaces. This helps the gasket material to flow between the rigid mating parts and makes it easier to remove.

Cutting New Packing

Best done on a wodden mandrel with the same diameter as the pump shaft. Wrap the packing around in the same direction as it comes off the roll.

Metallic O-rings

Commonly used where extreme conditions of temperature, pressure, or corrosion exist. These O-rings are formed to the required shape, butt welded, and ground smooth. They may be open or closed. They compress slightly when installed, forming an effective seal.

Primary Seal Mechanical Seal

Composed of two lapped (polished) faces in contact. One face is stationary and the other rotates. The mating faces are at right angles to the shaft. A spring loaded device keeps these faces together. The contact faces are made of various combinations of materials such as carbon-graphite, ceramic, tungsten carbide, stellite, and stainless steel..

Sealants

Compound often used in zero or low-pressure applications. They are found in tape. paste or liquid form. These are frequently used with a flat gasket to fill up minor depressions in surfaces that are slightly corroded or pitted. They are classified as non-hardening, hardening-flexible, hardening-rigid, or tape.

V -ring and U-ring packing and O-rings

Considered balanced packing as they seal on both the ID and OD. Pressure is balanced at both diameters.

Non-Positive Seals

Control the amount of leakage from one side to the other.

Butt Cutt

Cut rings straight with the axis of the mandrel. This reduces the change of fraying and sloppy joints.

Flat Diaphragm Seal

Designed for a small amount of movement. It is cut from a flat membrane and the elasticity in the material is enough to handle the movement.

O-Ring Squeeze

Diameter squeeze is necessary to maintain seal. Too much squeeze can rapidly wear out the _________. Generally Squeeze is equal to 10% of the ring diameter, exact amount shown in suppliers tables.

Diagonal Cut (Skived Cut)

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.

Mechanical Seals

Extremely efficient axial seaL It is much more effective than any other dynamic seals in preventing leakage. The leakage is so minimal that the naked eye cannot detect it. Any fluid that leaks past tends to evaporate immediately.

Material Used for Lip Seals

Felt, leather or synthetic rubber. Consists of a lightweight pressed metal housing with felt or leather secured into it or synthetic rubber bonded onto it. For the lip to maintain contact with the mating surface, it relies on the elasticity of the material used or on a garter spring behind this material.

O-Rings Under Pressure

Figure A shows the action of an a-ring under pressure up to and over the rated maximum. Figure B shows the action of the same a-ring with backup washers under pressures up to 20,680 kPa (3000 psi).

Gasket Compression

Gaskets fill the small irregularities in the faces. The pressure is lowest between the bolts and highest at the bolts. The thickness of the material of the joint also determines the number of bolts required. Thin flanges require more bolts to hold an even pressure than heavy flanges of the same configuration. Thin flanges have a tendency to distort under bolting forces while heavy flanges do not.

Natural Rubber

Has excellent mechanical properties and resists water and air. Its upper temperature limit is 121 "C (250°F).

Cup packing

Has only one lip and is considered an unbalanced packing. At low pressures, the cup may seal at the lip, but it normally does not seal until the heel diameter has been expanded to the diameter of the cylinder wall. The assembly must be tight enough to prevent leakage along the piston shaft. It is standard to fasten the cup against the piston by means of a tightly clamped plate. U-ring packing often replaces cup packing because it requires less maintenance time. This packing with double-acting pistons requires a rigid separator between the two cups. In some assemblies the separator is made of synthetic or laminated bearing material and acts as a guide as well as a separator. When installing new cups, there should be a gap between the follower and the back-up plate Material can be leather, synthetic rubber or elastomers, or impregnated fabric, depending on air or oil used and the maximum pressure.

Forces Acting on a Gasket

Hydrostatic force and internal pressure.

Installation of Lip Seal Into Housing

I. Lubricate the shaft. the housing. and the sealing edges. Leather seals should have the leather saturated with oil before they are installed. 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. Align the seal with the bore. 5. Press the seal into the housing squarely and smoothly. Ensure that the pressing device contacts the seal on its outer rim only, not to the unsupported inner section. 6. If the seal is to rest below the machine face. ensure that the pressing device is slightly smaller then the bore of the housing

Installing Lantern Ring

If a lantern ring is used, ensure that it lines up with the seal water inlet.

Mechanical Seal Maintenance

If either face is damaged it is best to replace the complete seal than to try and adjust a seal face or mix seal parts. Seals may be sent to the manufacturer for factory overhaul. If the proper equipment is available, the seal can be hand lapped in the following procedure: I. Charge the lapping (polishing) plate with a diamond lapping paste 2. Lightly press the seal facc against the lapping plate and move it in a figure eight motion. 3. Continue this until the surface appears flat. 4. Wash the face with solvent. 5. Check the surface with a monochromatic light and optical flats. 6. Repeat the procedure if necessary. Caution! Mechanical seals are precision products. Handle them carefully. Do not touch their mating faces.

Inside Packing

Installed in the moving unit. Ex. Cylinder piston (Piston rings?) and air valve spools.

Outside Packing

Installed in the stationary unit Ex. Stuffing box.

Secondary Seal Mechanical Seal

Is static and is between the contact face and the shaft. and between the contact face and !be housing. The seal between these parts can be done in a variety of different ways, but the most common is by means of a-rings.

Asbestos

Is used today only for special applications such as in high temperatures and stearn turbines.

Gasket Thickness

It is critical to replace the gasket with one of the same thickness.

Lip Seals

Low pressure (2-10 psi) primarily used on rotating shafts that are partially or totally immersed in lubricant. A hydrodynamic film of lubricant is formed at the point of contact to prevent friction and wear. This film of lubricant should be about 0.025 mm (0.001 ") thick. If it is any thicker, leakage may occur. If it is much thinner, friction and heat occur, rapidly wearing the seal down.

V-Ring Packing Spring Loaded External Adapter Rings

Maintain constant pressure. In this case, the gland is tightened down to the housing. This leaves no chance of overloading the packing set (provided it has the correct number of rings and has been properly installed). Figure a shows one continuous spring supporting the external adapter ring. Figure 2b shows a series of small springs supporting the external adapter ring. These packing sets used on double-acting pistons should face away from each other. The internal adapter ring should be supported by a solid backing, not by the opposing series of V-rings. In this case the adapter ring is often a non-metallic material and acts as a bearing to guide the piston. Materials in the V-ring packing sets can all be the same or be dissimilar. Dissimilar materials consist of hard and soft rings arranged in a specific sequence. The soft rings do most of the sealing while the hard rings act mainly as spacers and bearing rings. When installing a set of mixed rings (for a special order), always keep the rings in the same sequence as they were supplied.

Dynamic Seals (Contact)

Make contact with the two surfaces to be sealed. Examples of contact seals are packing. and lip, diaphragm. and mechanical seals (piston rings).

Metallic Gaskets

Metallic gaskets come in a variety of different materials and configurations. They are harder than non-metallic gaskets and are normally used ill high pressure and high-temperature applications. A few of the various materials are lead, brass, and stainless steel (of various alloys such as Inconel and Hastelloy). Their temperature limit ranges from 100°C (212°F) for lead to 1094"C (2000°F) for Inconel™. See suppliers' charts for specifications.

Sealing Action in Metallic Gaskets

Metallic gaskets seal by the flow or malleability of the gasket when the bolts compress the two surfaces together. The surface finish of the gasket and the sealing surface is very important. The smoother the surface fmish the better the seal.

Moulded Diaphragm

Moulded membrane and rolls on the two mating surfaces as it moves.

O-rings in Hydraulic Equipment

Multi-lobed, square, or rectangular section rings are often used as gaskets in hydraulic equipment. The material is the same as that used in standard O-rings.

Gasket Surfaces

Must be parallel and regular whether they are smooth or grooved.

Wipers (Scrapers)

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. They are commonly found on hydraulic and pneumatic cylinders in dusty or dirty locations. They are also used on guides and slides on which equipment rides. Their lip shape allows lubricant from the equipment to remain on the rod or guide but restricts any entry from the atmospheric side.

Slinger (Flinger) Seals

Non-positive seal which has a cupped washer-like flange mounted to the rotating shaft. It is 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. This prevents contaminants from entering the seal.

Clearance Seals Annulus Seals

Non-positive seal with a series of concentric or helical grooves in the housing bore. A drain hole is placed at the bottom of the seal to make the seal effective. In a concentrically grooved seal. lubricant fills the grooves to prevent the entry of contaminants. When helical grooves are used. the oil is returned to the bearings by this action.

Packing Compression

Note also that inside the stuffing box, the individual rings of packing do not compress equally. The rings closest to the gland compress the most.

Hardness of packing

Packing must withstand temperature, pressure, various fluids, and shaft misalignment and movement The hardness of packing or a seal helps to determine its action under pressure and movement. Hard packing withstands extrusion better than soft packing, but it also creates more friction. Where clearances are excessive, hard packing, special packing, or packing aids should be used.

Diaphragm Seals

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. This seal does not allow any fluid to transfer from one chamber to another.

Boots and Bellows

Positive seals which have one end secured to the housing and the other to the moving part. Boots are used for short movements (strokes) and bellows for long ones. They are made of rubber or some other flexible material. They replace wipers when it is vital that no atmospheric contaminants reach the working parts (for example, on shock absorbers).

Positive Seals

Prevent any fluid from leaking from one side of a system to the other.

Synthetic Rubber or Plastic Wipers

Protect against dust and fine materials.

Metallic Wipers

Protect against solid abrasive materials.

Quenching Mechanical Seals

Quenching is done through quenching ports on the atmospheric side of the seal. The fluid comes from an external source and must be compatible with the seal faces, the product being sealed, and the atmosphere. Steam is commonly used. A throttling bushing is used to keep the fluid at the contact face.

Bolt tightening procedures

Regardless of the shape of the mating parts or the gasket material, 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.

V-Ring Packing Adjustment

Requires both diameters to contact mating surfaces to obtain initial sealing. An adjustable gland is a common means of ensuring this and to maintain this seal due to wear. At the initial installation, the rings are tightened just enough to prevent leakage. This can be done in two ways: -By operating the unit and tightening the gland until any leaks cease -Or, if the packing is frequently replaced, a spacer can be made the thickness of the necessary gap between the gland and the housing. Tightening the gland to the spacer ensures correct loading. As leaks develop, the gland can be tightened further. Excessive tightening during first installation causes friction and rapid wearing of the packing.

Dynamic Seals (Clearance)

Seal without touching each other. Examples of clearance seals are bushings. and annulus. slinger, and labyrinth seals.

Floating Packing

Seat retained in a groove but not held in a static position. Able to float in the groove and seal via spring tension. Ex. piston rings. Automotive or steam piston rings are often used when rapid travel and frequent cycling are needed. Material in rings can be cast iron or steel, either plain finished or chrome plated. Piston rings are not a positive seal because they leak slightly.

O-Ring Carrier Clearance (O-Rings)

Should be held to a minumum to prevent extrusion into this gap. Standard _________ are rated to 10,340 kPA (1500 psi) with properly designed grooves can go up to 20,680 kPAv(3000 psi) using an anti-extrusion ring or back up washer.

Plastics

Such as acrylic and Teflon (PFFE) are commonly used. Acrylic resists heat, oil, and oxidation but does not perform well at low temperatures. Its upper temperature limit is 232°C (450°F). Teflon is nearly chemically inert and performs well in extremely low temperatures. Its usable temperature range is from -190°C (310°F) to 250°C (480°F).

V-Ring Packing Hardness

The internal adapter ring should be hard enough to prevent extrusion, yet soft enough to compress slightly under pressure. The external adapter ring merely shapes

O-ring Materials (O-rings)

The material used is chosen to resist chemicals and suit the temperature. ______ made of materials other than synthetic rubber have manufacturer's markings on them. Refer to manufacturers' information sheets to ensure that the markings comply with the type of O-ring needed

Flushing Mechanical Seal (Flushing Port)

The mechanical seal must always run in fluid. The _______ _____ feed the fluid into the product side of the seal. The fluid can be used directly from the product discharge or from an external source. If the product discharge is used, this fluid may have to go through a cyclone strainer (separator) to remove any solids. Or it may pass through a heat exchanger to remove any excess heat.

Leakage

The packing in stuffing boxes should not be compressed too tightly. A slight leakage is allowed to provide lubrication between packing and shaft. If leakage is completely stopped the resulting friction causes excessive heat to build up. This burns the packing and damages the shaft.

U-ring packing (U-cup)

This type of packing is a circular, one-piece molded ring with a U-cross section usually made from one of the synthetic elastomers, This endless design is meant to be used singly per seat, unlike compression or V-ring packing, Can be used as an inside- or outside-packed seal. Due to its low friction, it is primarily used in systems with pressures of less then 10,340 kPa (1500 psi), Pressures above that require the addition of an anti-extrusion washer (support ring).

Maintain Hydrodynamic Film In Lip Seal

To maintain this hydrodynamic film, the shaft's sutface must be controlled. For speeds up to 8 m/s (26 ftls), the shaft should have a ground finish (16 micro-inches = 0.4 microns). For speeds above this, the shaft should be hardened and ground (4 micro-inches = 0.1 microns). Note: 1 micro-inch (µin) = 0.0254 micron

U-Ring for Double Acting Pistons

Two seperate seats must be used. They should not be placed against each other in the same groove. Two different options for mounting this type of packing are available.

Synthetic Ring Seals

Used in high speed applications and can withstand temperatures as high as 250°C (480°F). Synthetic seals can retain lubricant without absorbing it and resist acids and alkalis.

Felt and Leather Ring Seals

Used in slow speed, light duty applications but where their operating temperatures won't exceed 105°C (220°F). These seals absorb the lubricant and redistributes it as a film between the seal and the shaft.

Cork In Ring Seals

Used in slow speed, light duty applications where its operating temperature does not exceed 65°C (150°F).

O-Rings as Dynamic Seals (O-Ring)

Used on slow turning shafts or reciprocating parts. Sits in a groove whose width is generally 135-150% of it's cross section. This allows for deformation due to squeeze, swelling due to fluid contact and slight rolling. This provides some lubrication during reciprocating motion

V-Ring Packing Cutting

Where possible, packing sets should be installed as endless rings. When this kind of packing must be cut for installation, stagger the joints about 90° on successive packing.

Installing New Packing

Wrap mandrel-cut paeking around the shaft. -For rigid packing, use an S twist to place it over the shaft. Do not pull it open. -Use lube where compatible -Install one ring at a time. -If the stuffing is bulged you may slightly flatten it with a roller. -Use a split bushing or tamping tool to place each ring in position then work it in around the circumference in both directions and ensure it seats at the bottom. -Ensure the ring joints are staggered approximately 90° from each other.

Stuffing Box Code

check starting ponit "2-L-3-G" indicates that the packing sequence is: 2 rings of packing, a lantern ring (L), 3 rings of packing, 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 if used. -Divide it by the width of the new packing.

Synthetic Rubber

includes a variety of materials such as buna-S, buna-N, butyl, neoprene, nitrile, Viton, and silicone. All have specific characteristics and temperature limits. Refer to manufacturers' specifications.

Paper

inexpensive. It is often impregnated with fillers such as wax and seals well against water, oil, and petroleum fuel. Its upper temperature limit is 120°C (23O°f).

Cork

is used in low-load applications. It can be impregnated with an elastomer such as neoprene or nitrile rubber and seals well against water, oil, and solvent. Its usable temperature range is from -30°C (-22°P) to 150"C (300"F) depending on its composition. Refer to manufacturers' specifications.

Disadvantages of Floating Packing Over Synthetic Seals

• Conforms less to the mating surfaces • Has a longer run-in period

Advantages of Floating Packing Over Synthetic Seals

• Creates less friction • Can take high pressure • Is less affected by temperature extremes

Advantages of Fluid Flushing Mechanical Seals

• Maintain a consistent operating temperature • Prohibit any vaporization of volatile fluids around the contact faces • Prevent crystallization of fluids near the contact faces • Prevent solids from accumulating around thc seal • Prevent the product from entering the stuffing box (if the flushing fluid is at a higher pressure then the system).

Advantages of Quenching Mechanical Seals

• Prevent leakage of corrosive or toxic fluids into the atmosphere • Prevent crystallization around the contact face • Maintain a constant operating temperature and minimize any heat transfer along the shaft.