What is mechanical seal? Types of mechanical seal

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What is Mechanical seal?

Mechanical seal is used to prevent leakages between rotating shafts and oil/gas field vessels. Mechanical seal is installed between two separate parts, one is rotating parts and other one is stationary parts. Rotating part installed into  the pump shaft and stationary element is installed inside the casing 

Mechanical seals are designed to handle better sealing capability and packing can also be used for sealing. These seals are commonly used in centrifugal pumps attached to the drive shaft inside the impeller housing. 

Pats of mechanical seal

• Stationary seat.
• Spring 
• Retainer
• Bellows 
• Rotating seat 

Types of mechanical seal 

There are many types of mechanical seal and its depend on the application of uses 

Internal Seal: If the seal installed inside the stuffing box of the pump is called internal seal. This seal is very difficult  to install into the pump. 

External seal: External seal is installed outside of the casing. External seal is easy to install and replace. These seals are commonly used in  low temperature, low pressure and low speed applications. 
Balanced seal: In the balance mechanical seal fluid closing force act to the diameter of the seal face equally is called balanced seal. Balanced seals are easy to install and also to remove. 

Unbalanced seal: The seal which are commonly used in cavitation area affected by fluid, available vibrations and shaft misalignment area is called unbalanced seal. This seals generates more heats and installed where keeps low stuffing box pressure. 

Pusher Type: Pusher seals are commonly used in industrial applications according to the category of product. Pusher seals helps to block the fluid path between the shaft and rotating seal face. 

Non pusher type : A couple of bellows which made with flexible material connected with collar to the one  end and other end uses rotating seal. In the chemical industries where abrasive fluids is transferred, there non pusher types seals are used.  

Cartridge type: Cartridge seals are usually pre-assembled  and it is self-contained part which is installed to the pump shaft and no need any extra fittings for installation.  

Main difference between balance & Unbalanced seal

Balanced seal	Unbalanced seal
Generates less heat	Generates more heat
Seal faces gets low loads	Seal faces gets more loads
Less heat generates to the seal faces.	More heat generates to the seal faces.
Poor lubricity is used	Good lubricity is required.
Where high stuffing box pressure require balanced seal is used.	Where low stuffing box pressure require Unbalanced seal is used.

Common reasons for Mechanical seal failings

• Excessive heat is another reason of seal failure. 
• If face is open and motion restricted. 
• Sometimes fluid supplied to the seal that is known as flush to protect the seal faces. If the flush contain abrasive material which may damage the mechanical seal. 

Advantages

1. Invisible sealing system with no leakages
2. Schedule maintenance is not required.
3. Energy savings and cost efficient
4. Reduce product losses 
5. Long lifetime 

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