Mechanical Seal For Pumps

Prepared By: Engr. Muhammad Shahbaz

Mechanical Seal• Everywhere where pumps with rotating shafts are used, a shaft seal is involved.

The shaft seal forms a barrier between what is inside the pump and the atmosphere. 

• A pump with a through-shaft is not completely sealed. It is a challenge to the entire pump industry to minimize leakage.

Stuffing box

• A braided stuffing box packing is the simplest type of shaft seal. The packing is placed between the shaft and the pump housing as shown

 • In the stuffing box housing a soft

packing ring is axially compressed until it makes contact with the shaft. After the soft packing has been exposed to wear the stuffing box must be further compressed to prevent excessive leakage.

•  Vibrations and misalignment will cause this seal type to leak.

Stuffing box housing

Soft packing

Shaft

Lip seal

A universal lip seal type is a rubber ring sliding against the shaft. This type of seal is primarily used in connection with a low differential pressure and low operating speed.

Lip seal

Mechanical shaft seal

A mechanical shaft seal consists of two main components 1. Rotating part 2. Stationary part

The rotating part is axially pressed against the stationary part

The clearance between the stationary and rotating part of the seal must be small in order to reduce leakage  Stationary part Rotating Part

Mechanical shaft seal with two axial seal faces

• The best possible way of making a seal with a minimum of clearance and thus a minimum amount of leakage is by pressing two axial surfaces against each other

• These axial surfaces can be obtained with a stepped shaft, running against a flat surface on the pump housing

 • The shaft and pump housing

must be highly wear resistant and well aligned

Pump housing

Seal faces

Stepped shaft

Atmosphere

Mechanical shaft seal with rotating seal ring and stationary seat

• A more practical solution is obtained by fitting a rotating seal ring on the shaft and a stationary seal ring (seat) in the pump housing.

• The tiny space between the seal faces is called the seal gap. 

• This design allows the use of a wide selection of materials for the rotating seal ring and stationary seat.

Stationary seat

Seal gap

Rotating seal ring

Secondary seals

• Secondary seals consist of rubber parts such as O-rings or bellows, used to avoid leakage between the shaft and the rotating seal ring as well as between the stationary seat and the pump housing.

• To minimize leakage, the rotating seal ring must be pressed against the seat. Therefore the rotating seal ring must be able to move axially on the shaft.

• To obtain axial flexibility, the secondary seal must either be a bellows or an O-ring sliding on the shaft.

• The secondary seal that seals between the rotating seal ring and the shaft rotates together with the shaft.

• The secondary seal that seals between seat and pump housing is static.

O-ring, stationary

O-ring, rotating

Spring

• The rotating spring presses the rotating seal ring against the seat and the rotating O-ring along the shaft

Spring

Torque transmission element

• A torque transmission element ensures that the rotating seal ring rotates together with the shaft

Reasons for using Mechanical seal:• To minimize leakage• To prevent toxic fluids escaping

to atmosphere • To reduce power loss

Torque transmission element

Operating principle• The rotating part of the seal is fixed on the pump shaft

and rotates in the liquid during pump operation. • The compression of the rubber bellows between the shaft

and one of the two torque transmission rings fixes the rotating part to the shaft.

 • The spring transfers the torque between the torque

transmission rings . The rotating seal ring is mounted together with the rubber bellows . The torque transmission ring compresses the rubber bellows to the rotating seal ring . The rubber bellows prevents leakage between the shaft and rotating seal ring and ensures axial flexibility despite contamination and deposits.

 • In a rubber bellows seal, axial flexibility is obtained by

elastic deformation of the bellows. However in an O-ring seal, the O-ring slides along the shaft.

 • The compression force from the spring keeps the two

seal faces together during pump standstill and operation

• This flexibility also keeps the seal faces together, despite axial movements of the shaft, surface wear, and shaft run-out. Stationary part Rotating Part

Pump housing

Stationary secondary rubber seal

Stationary seat

Rotating seal ring

Torque transmission Ring

Spring

Torque transmission ring

Rubber bellows (rotating secondary seal)

Shaft

Lubricating film in sealing gap

• The stationary part of the seal is fixed in the pump housing . It consists of a stationary seat and a stationary secondary rubber seal .

•  The secondary seal prevents leakage between the stationary seat and the pump housing .

• It also prevents the seat from rotating in the pump housing .

 

• The pumped medium to be sealed is generally in contact with the outer edge of the rotating seal ring . When the shaft starts to rotate, the pressure difference between the pumped medium in the pump housing and the atmosphere forces the medium to penetrate the sealing gap between the two flat rotating surfaces. The lubricating film is generated.

Pumped medium

Rotating seal ring, pumped medium side

Rotating seal ring, atmospheric side

Atmosphere

Types of seals

There are two types of seam1. Static Seal2. Dynamic Seal

3. STATIC SEALS: Sealing takes place between two parts that don’t move in relation to each other.

• Application: Pipe flanges ,vessel /Tower nozzles, pump casing joint. - Fan /Blower casing joint , Compressor casing joint. - Turbine casing joint, Heat exchanger joints

• Types: Gaskets , O-rings etc.• GASKET: Packing designed to go between two rigid parts in stationary conditions May be in

form of sheet , strip , bulk. Properties: Impermeability, Ability to flow into joints, Corrosion resistance

2. DYNAMIC SEALS: Used for sealing fluid between parts that move in relation with each other. • Application: Centrifugal pump gland, valve gland , bearing housing Turbine/compressor inter

stage and end sealing , Reciprocating compressors cylinder sealing • TYPES: Gland packings, Mechanical contact seals, Labyrinth seal, Oil seal, Oil film seals

Essential elements of a mechanical sealThe basic design of the Mechanical seal consists of the following elements:

1. Flexibly mounted seal face2. Rigidly mounted seal face 3. Compression device4. Secondary seal

Mechanical seal Components • Rotary seal face• Stationary seal face • Springs • Retainer • Sealing /flushing media • O-rings

Essential Requirements for Proper Operation of a Mechanical SealThese are the essential requirements:• Seal faces must be flat and polished.• Seal faces must be installed perpendicular to the shaft.• Spring force must be sufficient to maintain contact of the faces

Effective forces in a Mechanical Seal

These are the forces operating in mechanical seals1. Axial and radial forces2. Closing and opening forces3. Hydrostatic and hydrodynamic forces

Material of Seal Faces• Few materials are suitable for seal faces. To keep leakage as low as possible, the seal gap must

be very small. As a result, the lubricating film is very thin.

• Consequently, the seal face materials must be able to withstand rubbing against each other at high load and speed.

• The best seal face materials have low friction, high hardness, good corrosion resistance and high heat conductivity.

• Examples: Carbon graphite, Aluminum Oxide (Alumina), Tungsten carbide, Silicon carbide, Diamond coatings

Classification of Mechanical Seal

Mechanical seals are classified by arrangement and configuration.

Selection of the best type is not always easy and straight forward as there is usually a compromise between economical and technical factors.

Mechanical seal classification by Configuration i.e. Design

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By Design: Pusher vs. Non-pusher and Balanced vs. Non-balancedPusher vs. Non-pusherPusher seals utilize a dynamic secondary seal which moves axially with the major seal face. Non-pusher seals have a static secondary seal which stays stationary against the shaft or sleeve.

Defined by the secondary seal type: O-ring or polymer wedge versus bellow, rubber or metal.

Balanced Seal:• Reduced closing forces• Reduced power consumption• For pressure up to 3000 psig• Always recommended for volatile liquids

Non- balanced:• High closing forces• Low leakage• For pressure up to 200 psig• Not recommended for volatile liquids

Balance Seal vs. Non Balance Seal

Cartridge seals and split sealsCartridge seals• Seal are pre-assembled with sleeve and flange in one unit.• Easy to install.• No measurements during installation.• Spring load is preset.• May be factory tested with air, water or oil.• More costly as compared to component seal.

Split seals• Seat is axially split.• Does not require disassembly of the pump to install , reduce down time.• Leaks more than a conventional seal.• More costly as compared to conventional seal.

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Factors need to be considered in the application of a mechanical seal

• Pressure & speed (PV limit = Pressure x Velocity).• Temperature.• Fluid properties or characteristics.• Run out of the shaft.• Seal chamber type, available space radial and axial.• Flushing/cooling arrangements, utilities in the plant.• Mode of operation of the pump in the plant: continuous, cyclic, multi-• purpose.• Static versus dynamic pressure.• Test requirements.