Page 215 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

with forced flow (not induced flow), it becomes a good seal for
resolving some operational and design problems in the pumping system
like air aspiration, inadequate NPSHa, and operation away from the
pump’s BEP. Let’s look at support systems.

Support systems for dual seals

Double seals require some type of support system. The reason is simple.
With two seals mounted onto the same shaft, one seal is the principal or
primary seal and the other becomes the secondary or back up seal
already installed. If the primary seal is performing its function and
sealing the pumped liquid, the secondary seal would be running dry,
overheat, burn and self-destruct. Then when the crucial moment
comes, we won’t have a second seal to assume the functions, which was
the original reason to consider a dual seal.

So the support system serves to lubricate and cool the faces of the
secondary seal while the first is performing its functions. The pressure
applied to the support system governs the optimum life of the dual
arrangement, and it also governs what happens when one of the two
seals fails.
There are three distinct support systems for double seals. They are
often referred to as ‘barrier tanks’. The term barrier tank was initially
applied to the thermal convection tank, although the term today refers
to any of the three support systems. Each support system has different
attributes.

The thermal convection tank

The Thermal Convection Tank (Figure 13-22).
Can be pressurized or un-pressurized.

Conducts a re-circulated flow by thermal convection. When enough
heat is generated inside the mechanical seal, it expands initiating a
flow into the tank where it cools and contracts and is brought again
into the seal.
The tank is sealed and welded, and meets the boiler code for
pressure vessels.
Has a specific location in relation to the seal to optimize the
convective flow.