MECHANICAL SEAL

MECHANICAL SEAL

CONCEPT

BASIC SEAL CONSTRUCTION

  Concept Mechanical seal is Two lapped faces one rotating with shaft another stationary in Gland are brought in contact with each other by means of spring force, the surfaces are separated by Micron thickness of Film of fluid being sealed .Thus the film acts as a Lubricant and reduces the friction and heat generation well within the limits of seal face materials

SEAL SELECTION CRITERIA;-

Seals are selected based on the following

1. Pressure

2. Velocity

3. Media

4. Arrangement –Back to back/tandem

Concept of Double Mechanical Seal

Basic idea going for single to double mechanical seal is momentarily avoid the breakdown, inboard seal breaks the outboard seal comes into action, the leakage of inboard seal is evident when the level of thermosyphon pot keep on rising

Thermosyphon Pot

   As the name itself signifies, this pot acting as a cooling and lubricating media for out board seal, to safe guard the system pot is having pressure gauge, tubular level indicator ,we have to fill the barrier liquid compatible with process up to certain level ,increment and decrement in level indicates leakage of internal and external seal respectively

Pot is provided with cooling coil to cool the Barrier liquid and to be pressurized with nitrogen to the tune of 1 to 1.5 Kg/Sq cm above the stuffing box pressure, for effective thermosyphon action, to be located around 1 to 1.5 meters height then the pump stuffing box

   Normally, plant personnel confuse with amount of Nitrogen Pressure to be maintain above the barrier liquid level in pot, Stuffing Box pressure is defined by OEM of pump in the absence we can safely considered as 1/3 of discharge pressure

FACE COMBINATIONS;-

The face combination selection based on MOC /Chemical properties of fluid it is handling normally the following face combinations employed

Corrosive –GFT Vs Ceramic

Contain solid particles; - TC/Silicon carbide Vs carbon Or TC itself

SEAL BALANCING

Seal is unbalanced because a high proportion of hydraulic load is applied to the fluid film and in balanced seal

For seal to perform, the seal faces must in contact with each other in Dynamic conditions .the heat generation due to interface pressure load must not vaporize the liquid film

STATIONARY SEAL DESIGN

The basic principle of the Mechanical seal contains two main components-one of them to be adjusted in a way on the shaft, the other one stationary, arranged in the casing or cover plate

In customary mechanical seals the spring-loaded component rotates –within the stationary seal design the spring –loaded component is arranged in a stationary way I the casing/cover

OBSERVATIONS

Once the Mechanical seal is opened the following observations is essential to find out the root cause of failure of Mechanical seal

a. If two faces wears excessively;-

  It means more then required compression given to the faces or due to prolonged period pump might have run dry

b. Faces broken

When we open the seal found faces broken condition means some hard particle entrapped between the faces resulting in failure of seal

C. Seal spring clogged

Solid percentage in process fluid is high and hence clogged the seal springs, if we keep on observing the same continuously, which reminded us to change the configuration of seal

  

D .Inner seal corroded

On opening the Mechanical seal found Inner seal heavily corroded indicates the MOC of seal is not compatible with process fluid

E.Outboard damaged

If we found corroded outboard indicates barrier liquid is not compatible with MOC

F.Sleeve condition

Sleeve are used to protect the shaft ,when we found serration or groove on sleeve which signifies hardness of sleeve is less and finishing is required