Tuesday, June 30, 2020

The Ultimate Guide To Mechanical seal part 1

Mechanical seal

Mechanical seal is a mechanical device which is used to prevent the (leakage) liquid coming from inside casing to outside though between static and rotating parts. Mechanical seals are used in centrifugal pumps. There are many types of mechanical seals used in the many equipment’s. It is classified on the basis of design, working, and arrangement.

The Seal face is the most important part of the seal. Because the face material is the most expensive part of the seal and that creates a seal between static and a rotating part of the equipment. The description of types of the mechanical seal below.

Types of seal

There are two types of seal
  • Static seal
  • Rotating seal or dynamics seal

Static seal

It is used only to prevent leakage between static parts of the equipment and piping flanges. It's not used for  moving or rotating part of equipment. It is  used in flange joints, equipment manway, reactor piping flanges, columns piping flanges, pump casings and others.

Dynamic seal

Dynamic type seal is used to prevent the leakage between static and rotating parts of the equipment. Which rotates to each other. The application of this type of seal is used in pumps, blowers, compressors, turbines, valves, and others.

Types of dynamics seal

  • Gland packing
  • Mechanical seal
  • Labyrinth seal
  • Oil seal
  • Water seal
  • Lip seal
  • Hydrodynamics, etc.

Classification-of-mechanical-seal

Classification of mechanical seal

There are many type of mechanical seals

Classification is based on the arrangement of the seal
  1. Single seal
  2. Dual Mechanical Seal
Single seal

Two types of single seal
  1. Inside mounted single seal
  2. Outside mounted single seal
Inside mounted
The pressure of the liquid acts on the outer diameter of the seal's face.
Outside mounted
Pressure of liquid acts on the inside diameter of the seal part.

Double mechanical seal

If the primary seal fails then the secondary seal is used as a back up seal.
The 0 ring is also used as a secondary seal.

Two types of dual mechanical seal
  • Dual pressurize seal
  • Pressurize seal
  1. Back-to-back arrangements
  2. Face-to-face arrangement
  3. tandem arrangement

Dual pressurized mechanical seal
The dual pressurized seal requires nitrogen or other compatible inert gas and also requires a reservoir pot or nitrogen header line for pressurization. The nitrogen header minimum pressure is required at the level of pressure required to seal it.

Dual non pressurized mechanical seal

In a series arrangement or tandem arrangement. The unpressurized dual seal does not require nitrogen and other inert gas Pressure between the seal face area is higher than the seal chamber pressure is a minimum of 30 PSI.
The dual seal arrangement required an external seal flushing plan. Its pressure approx around the atmospheric pressure of the liquid, the face lubrication is also done by the external sources liquid. Unpressurized seals are used for low pressure, hazardous liquids.

Double mechanical seals are used for many features
  • Inappropriate NPSHA
  • Cavitation by air aspirations
  • On very expensive services
  • For explosive services
  • With toxic and vaporized liquid
  • For volatile liquids that have a tendency to vaporize, convert the gaseous form


Balance and unbalance 


Balance type

If the ratio between closing area to opening area of the seal is less than 1 then it is considered as balance seal. Balance type is less than the closing forces require. Reduces power consumption, It is used for pressure up to 3000 PSI and is mostly used for volatile liquids.

Unbalance

If the ratio between closing area to opening area of seal is greater than 1 then it is considered as an unbalanced seal. In Unbalance seal closing forces require a very high, very low leakage rate and that is used for up to 200 PSI services. but it is not used for volatile liquids.

Cartridge seal and split seal

Cartridge

The cartridge seal is a complete preassemble of seals, flanges and other parts. it is easy to install and does not measure required during the installation. because it is Spring load is fixed during the assembly and already tested by the technician with the help of air and equipment but it is more costly than other types of seal.

Split seal

The split seal is available in two parts,that is mostly axially split, the main advantage is that it is easy to install and does not require more time for installation, also reducing the downtime. but it is also more costly than seals.
What factors consider selection of mechanical seal applications?
Here mention the factors for the selection of mechanical seal considerations below.
  1. Temperature and pressure of the service liquid inside the casing.
  2. RPM for driver, equipment and misalignment
  3. Tolerance runs out of shafts in seal chambers
  4. Clearance of the seal chambers radially and axially
  5. Flushing and cooling plan required
  6. purpose barrier and buffer fluid requirement
  7. Availability of power
What are the factor requirements for smooth operation of the mechanical seal?
Factor requirements for smooth operation of mechanical seal.
  • Service fluid at the pump.
  • Seal leakage rate should be minimum
  • The fluid is in the pump seal area appropriate with seal face materials.
  • Effective force in mechanical seal
  • liquid lubricant for mechanical seal face
  • Mechanical seal face material compatibility with service liquid.
  • Power consumption and power losses
What is the purpose of providing seal plans?
The purpose of seal planes is to provide cooling and flushing to seal face. and remove raised temperatures as well. a variety of mechanical seals are available in the industry, mechanical seals are used in many types of equipment: like pump, compressors, liquid ring compressors, blower fans, agitators, valves, rotators and others.
The API standard used is 682 for the pump, there are many types of seal plans used, and Dry gas seal used in centrifugal compressor There are many components of mechanical seal like springs, O rings, Rotary seal's face, static seal's face, retainer rings, seal's face material, below, spring.
What should be the gap between seal face?
The gap between seal faces is very small. When it is running the hydrodynamic gap is created and that gap is approx around 3 to 5 microns.

There are two types of material used for seal face

for seal face one is a soft material used like carbon and Teflon another
One is a hard material that is tungsten carbide, silicon carbide, PTFE and

Where was the mechanical seal used?


The life of a mechanical seal is very long as compared to gland packing its maintenance time is much less. But it is highly expensive and very sensitive, because of the use of mechanical seals to save the downtime of the equipment.

Mechanical seals are used in many applications like pumps, compressors, blowers, liquid ring compressors, agitators, submersibles, pumps and others.

What is the reason for the mechanical seal being used?
  • To prevent and minimize the leakage of service fluid.
  • Prevent the hazards and toxic fluids and gases that come out of casings of equipment.
  • Reduce the maintenance downtime of equipment and power losses.
seal-selection-on-the-based-of-pressure-of-driver

seal-selection-on-the-based-of-speed-of-driver

What is the advantage of a mechanical seal?
The primary advantage of a mechanical seal use any equipment for a good deal safe operation.
  • Reduction in power losses
  • Save the equipment downtime and
  • Reliability of equipment
  • Safe operations.
Why is the Cartridge seal mostly used?
The advantage of cartridge seals is more than compared to other seals. Advantage.
  • Fewer number of skilled workmen required
  • Increase the reliability of equipment
  • Reduce the time for maintenance
  • Reduce the downtime of equipment
  •  It is easy to fix
Why did the mechanical seal fail?
The key reason for seal failure is
  • Lubrication failure and Dry run
  • Contamination in flushing fluid
  • Foreign materials inside the seal face
  • Rubbing seal faces against each other's system
  • High Temperature
  • Pressure Variations in the Seal Chamber and Misalignment
  • Overloading and not Operating at Designs
  • High vibrations and not installed properly.
What type of force is acting on a mechanical seal face?
  • Radial and axial force
  • Seal faces closing and Opening Force
  • Hydrodynamic and hydrostatic types of force.
Manufacturer names of Mechanical Seals.

Top seal manufacturers are John Crane, Flowserve, Eagle Bergmann, Grundfos and SanMar.
What is the type of mechanical seal face materials?
Seal face materials are

Carbide, carbon, graphite, aluminium oxide, tungsten carbide, silicon carbide, diamond coated, glass coated and others.

Selection-of-seal-face-materials

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