Type of mechanical seal :- Basic learning guide

this topic will cover basic, type of mechanical seal, seal component and about seal faces. A mechanical seal is the most common and important component of any rotary equipment like a pump, compressor etc.also we can say that seal is a heart of any equipment which is used in the hazardous operation. nowadays a mechanical seal is a very essential component of equipment for reliable service.

What is to be sealed by a mechanical seal?

mechanical seal use for pressure acting on stuffing box to be sealed.

this pressure depends on impeller design. stuffing box calculation as below,

1.  Open impeller   = suction pressure + 25% differential pressure
2.  Close impeller  =  suction pressure + 10% differential pressure
3.  double suction  = suction pressure = stuffing box pressure

Sealing are done by two methods: -

• by using gland packing
• by using a mechanical seal

before the invention of the mechanical seal, gland packing is the standard method for the sealing system.

as the day by day operation system become more complex as the pump has to handle verity of toxic and hazardous fluid mechanical seal come into action.

Disadvantage of packing:-

1. Power consumption is high with compare to the mechanical seal.
2. No automatic wear compensation as there is no compression unit.
3. Damage of sleeve or shaft due to fretting.
4. leakage is very high. it varies with compression on packings.

Why mechanical seals are used?

• Used for toxic and hazardous liquids
• Reliable running condition as well as operation
• Less friction loss compare to gland packing
• Zero leakage
• high efficiency

Parts of mechanical seal 

• Seal Faces
• Compression unit or Seal Flexible member
• Shaft Packing
• Other Component like a sleeve, snap ring, retainer, gland plate 

Seal Faces-

The most common seal face in an industry is carbon face rotating on a ceramic face as a stationary face. but nowadays mostly Silicon Carbide (SIC) & Tungsten Carbide (TC) material seal face are used instead of a ceramic face.

Seal face is machined very high accuracy for a high degree of flatness. seal face flatness is measured as a light band. normally it should be 3 helium light band.

Compression Component or Flexible Member -

we know that there is always some axial and radial movement of a shaft in any rotating equipment.which will require some flexibility to provide a closing force between seal faces.

this flexibility is provided by a flexible member.there is normally three type of component used as a flexible member.

• single coil spring
• multi-coil spring
• bellow

although the main closing force is provided by stuffing box pressure. the main function of seal or bellow is to compensate the movement of the shaft and keep seal faces in a closed condition to prevent leakage of fluid during running as well as in static condition.

Mechanical seal leakage path:-

There is Five path of leakage in a mechanical seal as mention in the above figure.for prevention of this leakage path various elastomers and gasket is used.

Five paths of leakage as below.

1. Gland plate to a stuffing box attachment which is prevented by gland plate gasket
2. leak path at which O ring located between stationary face to gland plate sitting.or stationary face O ring or packing.
3. leak path at which O ring located between rotary unit to the shaft sleeve 
4. O ring located between Shaft sleeve to the shaft that is also potential leak path
5. between stationary face to rotary face.

Type of Mechanical Seal:-

• Single coil mechanical seal
• Multi-coil mechanical seal
• Balanced mechanical seal
• Unbalanced mechanical seal
• Pusher mechanical seal
• Non-pusher mechanical seal
• Single mechanical seal
• Double mechanical seal

MOC of elastomer which is used in mechanical seal:-

• Neoprene
• Viton
• Glass/asbestos filled Teflon
• Grafoil
• Buna-N
• Buna-S etc.

QUESTION:-

WHICH TYPE OF MECHANICAL SEAL PREFER FOR HOTOIL??WHY??

This question for your friends. Give me answer on the comment box.

Detail discussion on Mechanical seal type and mechanical seal specification will cover in another session.

Thanks