shaft sealingshaft sealing

• To connect the motor to the impeller, the shaft To connect the motor to the impeller, the shaft has to pass through an aperture in the casing.has to pass through an aperture in the casing.

• To allow the shaft to rotate freely in the casing To allow the shaft to rotate freely in the casing aperture there needs to be a gap, but this gap aperture there needs to be a gap, but this gap needs to be closed off to stop air from being needs to be closed off to stop air from being drawn in from atmosphere or liquid from leaking drawn in from atmosphere or liquid from leaking out during operation.out during operation.

• There are two common methods. There are two common methods. • PackingPacking• Mechanical sealMechanical seal• The role of the pump, its speed and the type of The role of the pump, its speed and the type of

liquid being pumped all play a part in deciding liquid being pumped all play a part in deciding which application works best.which application works best.

PackingPacking

PackingPacking

• A stuffing box with a soft packing material is the traditional A stuffing box with a soft packing material is the traditional seal for pumps. Normally made from soft impregnated seal for pumps. Normally made from soft impregnated cotton, which takes the form of a length of square cross-cotton, which takes the form of a length of square cross-section wound spirally onto a tube. This enables the correct section wound spirally onto a tube. This enables the correct length, to suit the external diameter of the shaft, to be length, to suit the external diameter of the shaft, to be manually cut to the correct size.manually cut to the correct size.

• The stuffing box is then repeatedly filled with sections until The stuffing box is then repeatedly filled with sections until almost full, the gland can then be tightened down to almost full, the gland can then be tightened down to provide the axial compressive force. This in turn provides provide the axial compressive force. This in turn provides the necessary radial compressive force required to seal the the necessary radial compressive force required to seal the gap due to the sloping bottom face of the aperture.gap due to the sloping bottom face of the aperture.

• If the force is insufficient the stuffing box will leak, if the If the force is insufficient the stuffing box will leak, if the force is too great, the additional friction, and consequently force is too great, the additional friction, and consequently heat generated by the rotating shaft can damage the soft heat generated by the rotating shaft can damage the soft packing and/or shaft.packing and/or shaft.

Mechanical SealsMechanical Seals• To allow a rotating shaft to emerge from a

high pressure cavity to a low pressure cavity without significant leakage, a mechanical seal required.

• Mechanical seal is used instead of the stuffing box and gland packing to provide the seal around rotating shaft.

Advantages of Mechanical seal Advantages of Mechanical seal over gland packing:over gland packing:

• The power absorbed is lower and is The power absorbed is lower and is constant, whereas a gland constant, whereas a gland excessively tightened causes a excessively tightened causes a considerable increase in power considerable increase in power absorbed.absorbed.

• maintenance costs are reduced, the maintenance costs are reduced, the rotary seal operating for long periods rotary seal operating for long periods without wear or attention.without wear or attention.

Construction of Mechanical Construction of Mechanical SealSeal

• Consist of two componentsConsist of two components1.1. Rotating ElementRotating Element2.2. Stationary ElementStationary Element

• Rotating element is installed on the shaft Rotating element is installed on the shaft and stationary element in the housingand stationary element in the housing

• Rotating element consists of collar, Rotating element consists of collar, setscrew, spring and rotating ringsetscrew, spring and rotating ring

• Stationary element consists of gland plate, Stationary element consists of gland plate, stationary ringstationary ring

• Gland plate is a metal plate installed in the Gland plate is a metal plate installed in the staffing box, in which the stationary ring is staffing box, in which the stationary ring is attachedattached

Construction of Mechanical Construction of Mechanical SealSeal• The stationary seal surfaces faces the rotation The stationary seal surfaces faces the rotation

seal surfaces. These two surfaces provide almost seal surfaces. These two surfaces provide almost perfect sealing, thus prevent the leakageperfect sealing, thus prevent the leakage

• The two surfaces are made perfectly even by The two surfaces are made perfectly even by lappinglapping

• Other component of mechanical seals are “O” Other component of mechanical seals are “O” ring, bellows and gasketring, bellows and gasket

• The “O” ring and the gasket act as a seal The “O” ring and the gasket act as a seal between the gland plate and the stationary between the gland plate and the stationary element, thus keep the fluid from leaking around element, thus keep the fluid from leaking around the stationary elementthe stationary element

• The bellows forms seal between the rotating The bellows forms seal between the rotating element and pump shaft, thus keep the fluid from element and pump shaft, thus keep the fluid from leaking below the rotating elementleaking below the rotating element

Material of Mechanical seal Material of Mechanical seal componentscomponents

• The components of the mechanical seal The components of the mechanical seal are made of various materials for various are made of various materials for various operating conditions.operating conditions.

• Collar, Spring, Bellows and Gland plate are Collar, Spring, Bellows and Gland plate are made of SS, brass, or other alloys of high made of SS, brass, or other alloys of high resistance to corrosionresistance to corrosion

• Seal surface is made of tungsten, ceramics Seal surface is made of tungsten, ceramics or various classes of carbon that have high or various classes of carbon that have high resistance to wearresistance to wear

What is necessary for a seal to What is necessary for a seal to workwork

• Face Contact: Face Contact: rotating ring must be free to rotating ring must be free to wobble axially to compensate for wobble axially to compensate for misalignment and to move axially down misalignment and to move axially down into the sleeve to make up face wearinto the sleeve to make up face wear

• Flatness:Flatness: Seal faces have flatness of 2 to 3 Seal faces have flatness of 2 to 3 helium light bands (1 LB = 0.0000116”)helium light bands (1 LB = 0.0000116”)

• Face Lubrication: Face Lubrication: A stable lubricating film A stable lubricating film must be maintained between the faces at must be maintained between the faces at all times. Dry running of a pump seal only all times. Dry running of a pump seal only a few seconds may destroy ita few seconds may destroy it

Leak PathsLeak Paths

• Between seal flange and pump faceBetween seal flange and pump face

• Behind the stationary seal faceBehind the stationary seal face

• Under the rotating seal faceUnder the rotating seal face

• Between the seal mating facesBetween the seal mating faces

Balanced Mechanical SealBalanced Mechanical Seal

• When we are designing the When we are designing the mechanical seal with sleeve on a mechanical seal with sleeve on a shaft or making a stepped shaft, we shaft or making a stepped shaft, we balance the forces which are acting balance the forces which are acting on the sliding surfaces. We call this on the sliding surfaces. We call this the balanced mechanical sealthe balanced mechanical seal

Types of sealTypes of seal

• Simple mechanical seal –Simple mechanical seal –single spring-unbalanced single spring-unbalanced for general purposesfor general purposes

• Single seal (one set of Single seal (one set of seal faces) mounted seal faces) mounted inside a stuffing box. This inside a stuffing box. This seal is typically lubricated seal is typically lubricated by the pump fluidby the pump fluid

1.1. Single Single Seal:Seal:

Types of sealTypes of seal

• Multi spring mechanical seal unbalancedMulti spring mechanical seal unbalanced

• O ring or NR ring O ring or NR ring stationary seatstationary seat

• For hard working For hard working conditions and conditions and chemicalchemical

aggressive media aggressive media with raw materialswith raw materials

• Single spring Single spring mechanical seal-mechanical seal-balancedbalanced

Types of sealTypes of seal

Types of sealTypes of seal

• O ring or NR ring O ring or NR ring stationary seatstationary seat

• For hard working For hard working conditions and conditions and chemicalchemical

aggressive media with aggressive media with raw materialsraw materials

• Multi spring Multi spring mechanical seal-mechanical seal-balancedbalanced

• Data from Data from BB, , ll1k1k size size

Balanced mechanical seal with Balanced mechanical seal with multiple rotating springmultiple rotating spring

Balanced mechanical seal with Balanced mechanical seal with Single rotating springSingle rotating spring

Unbalanced Mechanical seal Unbalanced Mechanical seal with multi springwith multi spring

Bellows Mechanical SealBellows Mechanical Seal

• Metal bellows mechanical seal has a very wide Metal bellows mechanical seal has a very wide field of application, but because of high price it is field of application, but because of high price it is used for extreme conditions, like very high used for extreme conditions, like very high temperature, aggressive and dirty media.temperature, aggressive and dirty media.

• Main advantage is that there is no dynamic “O” Main advantage is that there is no dynamic “O” ring or elastomer part. All elastomer parts have a ring or elastomer part. All elastomer parts have a temperature range limit, and when it is exceeded, temperature range limit, and when it is exceeded, mechanical seal fails.mechanical seal fails.

• Metal bellows mechanical seals work in high Metal bellows mechanical seals work in high temperature, high aggressive and high viscosity temperature, high aggressive and high viscosity media without failure.Because of metal bellows media without failure.Because of metal bellows design, seal could work with media containing design, seal could work with media containing solidssolids

Types of sealTypes of seal

• Mechanical seals from extremly hard Mechanical seals from extremly hard working conditions, chemical aggressive working conditions, chemical aggressive and flammable media with row materialsand flammable media with row materials

• Non spring constructionsNon spring constructions• Electrometric bellows constructions Electrometric bellows constructions

without metals or O ring at rotary unitwithout metals or O ring at rotary unit• Stationary unit with O or NR ring sealStationary unit with O or NR ring seal• Balanced or unbalanced Balanced or unbalanced • See next slideSee next slide

Single sealSingle seal

• Single seal (one set of seal faces ) Single seal (one set of seal faces ) mounted inside a stuffing box.mounted inside a stuffing box.

• Lubricated by the pump fluidLubricated by the pump fluid

• Single seal will weep a small amount Single seal will weep a small amount of the pumped fluid, they are not of the pumped fluid, they are not acceptable for toxic or dangerous acceptable for toxic or dangerous liquidsliquids

Dual seal (Un pressurized) Dual seal (Un pressurized) (tandem Seal)(tandem Seal)• Two seals works independently of each otherTwo seals works independently of each other• Uses un-pressurized “buffer” fluidUses un-pressurized “buffer” fluid• Primary seal in process fluid, works and Primary seal in process fluid, works and

flushed like a single seal.flushed like a single seal.• Failure of the primary seal results in buffer Failure of the primary seal results in buffer

fluid contaminationfluid contamination• Secondary seal is in the “buffer” fluidSecondary seal is in the “buffer” fluid• Secondary seal act as a short term backup Secondary seal act as a short term backup

until maintenance can be performed.until maintenance can be performed.• Failure results in loss of buffer fluid to the Failure results in loss of buffer fluid to the

environmentenvironment

Area of ApplicationArea of Application

• Process fluid leakage to the Process fluid leakage to the atmosphere is controlled or atmosphere is controlled or restrictedrestricted

• Process fluid that cannot be Process fluid that cannot be contaminated by the buffer fluidcontaminated by the buffer fluid

• Process fluid must be able to provide Process fluid must be able to provide lubrication to the primary seal lubrication to the primary seal

Double Seals or pressurized Double Seals or pressurized dual sealsdual seals• Two seals mounted within a seal chamber Two seals mounted within a seal chamber

with a pressurized barrier fluid between with a pressurized barrier fluid between them. The higher pressure barrier fluid them. The higher pressure barrier fluid lubricates seals and weep between both.lubricates seals and weep between both.

• Two seals working as one within a seal Two seals working as one within a seal chamberchamber

• Barrier fluid pressurized between two sealsBarrier fluid pressurized between two seals• No product gets between the faces or No product gets between the faces or

weeps to the atmosphereweeps to the atmosphere• Back to back, face to face and in-line Back to back, face to face and in-line

configuration are availableconfiguration are available

Areas of ApplicationAreas of Application

• Fluids that are dirty or content abrasivesFluids that are dirty or content abrasives

• Any liquid that does not provide adequate Any liquid that does not provide adequate face lubrication or change state.face lubrication or change state.

• Fluids or vapors that are hazardous to the Fluids or vapors that are hazardous to the environmentenvironment

Note: Barrier fluid pressure must always be Note: Barrier fluid pressure must always be kept higher than the product to provide kept higher than the product to provide lubrication to both primary and secondary lubrication to both primary and secondary seal faces. It must be compatible with seal faces. It must be compatible with process fluidprocess fluid

DOUBLE MECHANICAL DOUBLE MECHANICAL SEALSSEALS

• Double mechanical seals are in forms Double mechanical seals are in forms of balanced-balanced, balanced-of balanced-balanced, balanced-unbalanced or double unbalanced .unbalanced or double unbalanced .

Selection of Mechanical SealSelection of Mechanical Seal

• All of the seal materials must be chemically All of the seal materials must be chemically compatible with any fluids /cleaners or steam compatible with any fluids /cleaners or steam

• The seal faces must stay together. If they The seal faces must stay together. If they open, the seal will leak and allow solids to open, the seal will leak and allow solids to penetrate between the faces. The solids will penetrate between the faces. The solids will eventually destroy the lapped surfaces. eventually destroy the lapped surfaces.

• Good seal life is defined as running the Good seal life is defined as running the mechanical seal until the carbon face is worn mechanical seal until the carbon face is worn away away

• Any other condition is called a seal failure Any other condition is called a seal failure and is always correctable and is always correctable

Selecting Materials - The Selecting Materials - The Faces.Faces.• Reaction bonded Silicone Carbide has excellent wear Reaction bonded Silicone Carbide has excellent wear

characteristics, but contains up to 17% free silica which can characteristics, but contains up to 17% free silica which can be attacked by many chemicals including caustic. Alpha be attacked by many chemicals including caustic. Alpha sintered Silicone Carbide is also available and is Silica free. sintered Silicone Carbide is also available and is Silica free.

• 85% ceramic should never be recommended as a hard seal 85% ceramic should never be recommended as a hard seal face because it can break with as little as a 100 degree face because it can break with as little as a 100 degree Fahrenheit (55 C) temperature difference. 99.5% would be Fahrenheit (55 C) temperature difference. 99.5% would be a much better choice. a much better choice.

• Plating or coating a seal face will not give it corrosion Plating or coating a seal face will not give it corrosion resistance. Coatings are used for wear resistance and low resistance. Coatings are used for wear resistance and low friction. To get corrosion resistance the outer coating must friction. To get corrosion resistance the outer coating must be at least 1/8" (3 mm) thick. If the base material is not be at least 1/8" (3 mm) thick. If the base material is not corrosion resistant to the pumping fluid and any cleaners or corrosion resistant to the pumping fluid and any cleaners or solvents used in the lines the corrosive will go through the solvents used in the lines the corrosive will go through the coating and attack the base, causing the plating to come off coating and attack the base, causing the plating to come off in sheets. in sheets.

Points to rememberPoints to remember• The best way to cool a seal is to use the jacketed The best way to cool a seal is to use the jacketed

stuffing box that came as a part of the pump. This stuffing box that came as a part of the pump. This jacket will not only cool down the seal area, but will jacket will not only cool down the seal area, but will provide the necessary cooling to the shaft so that it provide the necessary cooling to the shaft so that it will not transmit stuffing box heat back to the will not transmit stuffing box heat back to the bearings. bearings.

• Dual seals should be balanced in both directions to Dual seals should be balanced in both directions to prevent failure when barrier fluid pressure changes. prevent failure when barrier fluid pressure changes. The practice of using "one direction" seal balance is The practice of using "one direction" seal balance is commonly employed by most seal companies and commonly employed by most seal companies and should be avoided for both safety and reliability. should be avoided for both safety and reliability.

• Avoid using flushing fluid as a coolant in stationary Avoid using flushing fluid as a coolant in stationary mechanical seals. The coolant will be directed to only mechanical seals. The coolant will be directed to only one side of the seal and since a stationary seal does one side of the seal and since a stationary seal does not rotate the sliding components, the differential not rotate the sliding components, the differential temperature can cause the faces to go out of flat. In temperature can cause the faces to go out of flat. In the case of stationary bellows seals, it could cause a the case of stationary bellows seals, it could cause a bellows rupture. bellows rupture.

• Do not use rotating, "Back to Back" double Do not use rotating, "Back to Back" double seals in dirt or slurry service. The solids will seals in dirt or slurry service. The solids will prevent the inner seal from moving forward prevent the inner seal from moving forward as the faces wear and if the barrier fluid as the faces wear and if the barrier fluid pressure is lost, solids can open and pressure is lost, solids can open and penetrate between the inner seal faces. penetrate between the inner seal faces.

• Be sure to vent vertical pumps back to the Be sure to vent vertical pumps back to the suction side of the pump. Air trapped in the suction side of the pump. Air trapped in the stuffing box can cause the seal faces to run stuffing box can cause the seal faces to run hot and in some instances destroy the hot and in some instances destroy the elastomer. elastomer.

Heat affects a seal several Heat affects a seal several ways: ways: • The faces can be attacked. Plated faces can have The faces can be attacked. Plated faces can have

the hard coating crack off and filled carbons can the hard coating crack off and filled carbons can have the binder melted out in high heat. have the binder melted out in high heat.

• The elastomer (rubber part) has a temperature The elastomer (rubber part) has a temperature limit determined by the compound used. limit determined by the compound used.

• The corrosion rate of all liquids increases with The corrosion rate of all liquids increases with temperature. temperature.

• Thermal expansion can cause seal face loads to Thermal expansion can cause seal face loads to alter and seal face flatness to change. alter and seal face flatness to change.

• Many products will change from a liquid to a solid Many products will change from a liquid to a solid or gas at high temperature. If this should occur or gas at high temperature. If this should occur between the seal faces, they can be blown open. between the seal faces, they can be blown open.

TechnicalTechnical• Seals lapped to less than three helium light bands Seals lapped to less than three helium light bands

( 0.000034") inches or 1,0 microns) should not show ( 0.000034") inches or 1,0 microns) should not show visible leakage. Visible leakage occurs at about 5 light visible leakage. Visible leakage occurs at about 5 light bands. bands.

• A typical mechanical seal face load would be 30 psi. A typical mechanical seal face load would be 30 psi. (0,2 N/mm2) when the carbon is new and 10 psi. (0,07 (0,2 N/mm2) when the carbon is new and 10 psi. (0,07 N/mm2) when the carbon is fully worn away. You must N/mm2) when the carbon is fully worn away. You must never guess as to how much to compress a never guess as to how much to compress a mechanical seal. Either take the information from the mechanical seal. Either take the information from the seal print or calculate the correct length from the seal print or calculate the correct length from the above information. above information.

• Both rotating and stationary metal bellows seals Both rotating and stationary metal bellows seals require vibration damping. Elastomer seals do not require vibration damping. Elastomer seals do not experience this vibration problem because the experience this vibration problem because the elastomer touching the shaft is a natural vibration elastomer touching the shaft is a natural vibration damper. Vibration can be either harmonic or caused damper. Vibration can be either harmonic or caused by poor lubricating fluids (slip stick) by poor lubricating fluids (slip stick)

TechnicalTechnical• Balanced mechanical seals consume about one sixth Balanced mechanical seals consume about one sixth

the horsepower of packing. Packing a pump would be the horsepower of packing. Packing a pump would be like running your automobile with the emergency like running your automobile with the emergency brake engaged. The car would run, but the fuel brake engaged. The car would run, but the fuel consumption would be high. consumption would be high.

• Single spring seals are wound in either a right or left Single spring seals are wound in either a right or left handed direction. Check to see if your seal will a handed direction. Check to see if your seal will a problem in keeping the faces together because of the problem in keeping the faces together because of the spring winding. spring winding.

• Do not relap the carbon face unless it's an Do not relap the carbon face unless it's an emergency. Seal face opening is a common seal emergency. Seal face opening is a common seal failure. When the faces open solid particles imbed failure. When the faces open solid particles imbed them selves into the carbon face and will be driven in them selves into the carbon face and will be driven in even further during the lapping process. If you must even further during the lapping process. If you must relap in an emergency never use lapping powder, as relap in an emergency never use lapping powder, as the abrasive particles will imbed into the soft carbon. the abrasive particles will imbed into the soft carbon.

TechnicalTechnical• Avoid oil as a barrier or buffer fluid between Avoid oil as a barrier or buffer fluid between

two mechanical seals. Most petroleum base two mechanical seals. Most petroleum base and other oils have a low specific heat (0.2 - and other oils have a low specific heat (0.2 - 0.4) and combined with poor conductivity (0.5 0.4) and combined with poor conductivity (0.5 of water) makes them a poor choice compared of water) makes them a poor choice compared to fresh water. If oil is mandatory, a clean heat to fresh water. If oil is mandatory, a clean heat transfer oil would be your best choice. transfer oil would be your best choice.

• Oil on the seal faces can cause the faces to Oil on the seal faces can cause the faces to stick together during long periods of non stick together during long periods of non running. If you do not intend to run the running. If you do not intend to run the equipment soon remove any oil that might be equipment soon remove any oil that might be on the seal faces during the assembly on the seal faces during the assembly procedure. procedure.