MECHANICAL SEALS

Mechanical Shaft Seals

1. Operating Principles & Fundamentals
Since the development of the basic mechanical seal introduction of new and innovative seal technologies has enabled mechanical seal installation on virtually any fluid handling application. To sort through which seal design will provide optimum performance a thorough understanding of mechanical seal principles and fundamentals is mandatory.

A. Pusher & Non-Pusher Seal Designs: Pusher seals, while generally less expensive than non-pusher seal designs, will have a tendency to "hang-up" on the pump shaft when handling fluids which coke or crystallize as the secondary sealing member which must accommodate for travel as the seal faces wear is unable.

B. Seal Driving & Spring Compression: The rotary portion of a mechanical seal is either positive or friction drive. Incorporating an improper driving arrangement on a given application will result in premature and catastrophic failure.

C. Balanced & Unbalanced Seals: This difference in seal design will make the difference in seal performance. An unbalanced mechanical seal seeing high pressures has the fluid film between the seal faces reduced due to high hydraulic face loading resulting in overheating, rapid face wear, and premature seal failure.

D. Inside & Outside Seal Mounting: While inside mechanical seals are a preferred method outside seals can be used when fitting a pump with a shallow stuffing box which cannot dimensionally accommodate an inside seal.

Mechanical seals as per construction:

} single coil spring seal

} multi coil spring seal

} metal bellows seal

} rubber/ Teflon below seal

Seal categories:

}Category 1 :
seals are intended for use in non-ISO 13709 pump seal chambers, their application is limited to seal chamber temperatures from – 40 °C to 260 °C and absolute pressures up to 22 bar (315 psi).

}Category 2 :
Dimensional requirements of ISO 13709. Their application is limited to seal chamber temperatures from – 40 °C to 400 °C and absolute pressures up to 42 bar (615 psi).

}Category 3 :

provides the most rigorously tested and documented seal design. It is required that the entire seal cartridge is qualification tested as an assembly in the required fluid. They meet the seal chamber envelope requirements of ISO 13709 (or equal). Their application is limited to seal chamber temperatures from – 40 °C to 400 °C and absolute pressures up to (42 bar) (615 psi).

Seal types

>Type A :
seal is a balanced, inside-mounted, cartridge design, pusher seal with multiple springs and in which the flexible element normally rotates. Secondary sealing elements are elastomeric O-rings.

>Type B :
seal is a balanced, inside-mounted, cartridge design, non-pusher (metal bellows) seal in which the flexible element normally rotates. Secondary sealing elements are elastomeric O-rings.

>Type C :
seal is a balanced, inside-mounted, cartridge-design non-pusher (metal bellows) seal in which the flexible element is normally stationary. Secondary sealing elements are flexible graphite.

Seal arrangements

*Arrangement 1:
Seal configurations having one seal per cartridge assembly;

*Arrangement 2:
Seal configuration having two seals per cartridge assembly, with the space between the seals at a pressure less than the seal chamber pressure.

*Arrangement 3:
Seal configurations having two seals per cartridge assembly, utilizing an externally supplied barrier fluid at a pressure greater than the seal chamber pressure.

Seal orientations

-Arrangement 2 and Arrangement 3 seals can be in the following three orientations:

1. face-to-back (TANDEM OR FACING IN THE SAME DIRECTION) :

dual seal configuration in which one mating ring is mounted between the two flexible
elements and one flexible element is mounted between the two mating rings;

2. back-to-back (FACING IN OPPOSITE DIRECTION) :

dual seal configuration in which both of the flexible elements are mounted between the
mating rings; and

3.face-to-face (FACING TOWARDS EACH OTHER) :

dual seal configuration in which both of the mating rings are mounted between the flexible elements.

FLUSHING

-Why flushing is required ?

* To remove heat from seal faces generated due to friction between faces.
* To provide clean water for seal face lubrication
* To contains the leakage with-in a confined space and act as leakage carrier to safe location

- DIFFERENT METHODS OF BRINGING FLUID TO STUFFING BOX


1. DISCHARGE RECIRCULATION

2. SUCTION RECIRCULATION

3. FLUSHING

4. BARRIER/BUFFER FLUID

DUAL SEAL ARRANGEMENT

- TO PREVENT A COSTLY PRODUCT FROM LEAKING

- TO PREVENT DANGEROUS PRODUCT FROM LEAKING

- TO PREVENT A POLLUTENT FROM ESCAPING TO THE ATM.

- AS A BACKUP SEAL

FLUSHING PLANS

1. Plan 01 :
internal porting is used to direct flow to the seal chamber from an area behind the impeller near the discharge.

2. Plan 02 :
dead-ended seal chamber with no flush fluid circulation

3. Plan 11 :
product is routed from the pump discharge to the seal chamber .

4. Plan 13 :

Suction recirculation

5. Plan 14 :
combination of a Plan 11, recirculation from pump discharge, and Plan 13, recirculation to pump suction.

6. Plan 21 :

Recirculation from discharge thru orifice & cooler

7. Plan 22 :
Plan 21 with strainer

8. Plan 23 :

equipped with an internal circulating device that circulates seal chamber fluid through a cooler and back to the seal chamber

9. Plan 31 :
product is routed from the discharge of the pump into a cyclone separator.

10. Plan 32 :
the flushing product is brought from an external source to the seal.

11. Plan 52 :
Unpressurized buffer fluid circulation thru reservoir.fluid is circulated by pumpingring used with Arrangement 2 seals.

12. Plan 53A, Plan 53B, Plan 53C :
Pressurized barrier fluid circulation thru reservoir/blader/piston accumulator.Arrangement 3 pressurized dual seal systems

13. Plan 54 :
pressurized dual-seal systems with inner seal leakage into the pumped product

14. Plan 62 :

a quench stream is brought from an external source to the atmospheric side of the seal faces

15. Plan 71 :

Arrangement 2, unpressurized dual seals, which utilize a dry containment seal and where no buffer gas is supplied but the provision to supply a buffer gas is desired.

16. Plan 72 :
Arrangement 2 unpressurized dual seals. unpressurized buffer gas control.

17. Plan 74 :
Arrangement 3, dual pressurized seals, where the barrier medium is a gas. Pressurized barrier gas control system. The most common barrier gas is plant nitrogen

18. Plan 75 :
Arrangement 2, unpressurized dual seals, which also utilize a dry containment seal and where the leakage from the inner seal may condense.

19. Plan 76 :
Arrangement 2, unpressurized dual seals, which also utilize a dry containment seal and where leakage from the inner seal will not condense. Vent from containment seal cavity to vapor recovery.