Page 212 - Know and Understand Centrifugal Pumps

Mechanical Seals

w Air aspiration.
w Entrained gas bubbles.
w Turbulence.
w Dry-running the pump.
w Pumps in intermittent service.

w When operating just one pump in a parallel pump system.
3. On costly liquids.
4. With explosive liquids.
5. With toxic liquids.

6. With volatile liquids (tendency to gas or vaporize).
7. As the already installed spare seal.
Many plants use dual cartridge seals because:
1. They can be tested before accepting the seal into stock from the
vendor.
2. They can be vacuum- and pressure-tested before installation.
3. In easy applications, these seals reduce maintenance costs by
offering the life of two seals with only one installation.

The tandem dual seal

This dual seal has both the rotary units facing in the same direction.
This type of seal is recommended for very high pressures. The support
system, and thus the area between the two seals, would be pressurized
at ‘/2 the actual seal chamber pressure inside the pump (Figure 13-19,
next page).
For example, if the application is actually 800 psi, you would pressurize
the barrier tank support system and the area between the two seals at
400 psi. This way the inboard seal would seal 400 psi (800 - 400 = 400
psi) and the outboard seal would also seal 400 psi (400 - 0 = 400 psi).
Each seal independently could seal maybe 500 psi, but not 800 psi.
Together, the two seals in this tandem arrangement can seal 800 psi and
higher depending on the barrier tank pressure.
Some purists hold back from calling this type seal a true ‘double or dual
seal’. This is because if one seal in this tandem arrangement fails, the
other will immediately fail too.

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