Hydrocarbon Processing - August 2021 - 29

Valves, Pumps and Turbomachinery
TABLE 5. Advantages and disadvantages of Arrangement 3
Confi guration Advantages
Face-to-back
(FIG. 1A)
* Pumping liquid on the OD of the seal
faces
* Abrasive contamination is centrifuged
and has less eff ect on the inner seal
* High performance in dirty services
* Low risk of solids accumulation on
the faces, and low risk of hangups
* Pressure reversal capability: In the
event of a loss of barrier fl uid
pressure, the seal will behave like
an Arrangement 2 seal
Back-to-back
(FIG. 1B)
* Greater use worldwide
* Higher levels of performance
because of the barrier fl uid cooling
due to fl owing over both inner
and outer seals
* No need for external fl ush
Disadvantages
* Poor cooling of the inner seal
by the barrier liquid
* Restricted seal chamber dimensions,
and the resulting cartridge hardware
construction can aff ect the ability of
the barrier fl uid fl ush to adequately
cool the inner seal
* Inadequate cooling of the inner seal
can result in reduced seal reliability
* Use of external seal fl ush for inner
seal cooling
* Higher OPEX
* Process liquid is on the ID of the
seal faces
* Low performance in dirty services
because any entrained abrasive solids
will be centrifuged into the seal faces
with the potential for damage
* A dead zone underneath the inner
seal creates susceptibility to solids
accumulation
* Poor pressure reversal capability
Face-to-face
(FIG. 1C)
* Greater use worldwide
* Higher levels of performance
because of the barrier fl uid cooling
due to fl owing over both inner
and outer seals
* No need for external fl ush
* Process liquid is on the ID of the
seal faces
* Low performance in dirty services
because any entrained abrasive solids
will be centrifuged into the seal faces
with the potential for damage
* A dead zone underneath the inner
seal creates susceptibility to solids
accumulation
* Poor pressure reversal capability
wet steam, which can cause
further seal problems.
* Select the proper type of barrier
liquid, with a viscosity range and
temperature as per API 682.
Recommendations. The following are
recommended for mechanical seal selection
in hot pumping applications:
* Ask a process engineer to clarify
the condition of the pumping
liquid in terms of coke, particles,
corrosive elements, etc.
* Ask the vendor to design the
seal based on the maximum
temperature and pressure.
* Always use pressurized dual
mechanical seals (i.e., API Plan
53B, 53C or 54).
* Check the seal drawing, and make
sure that both the inner and outer
seals are effectively cooled. It is
recommended to add a flow guide
to promote more turbulent flow
around the inboard faces to keep
the barrier fluid from burning
and coking at the faces.
* Check the seal design and ensure
that it promotes barrier fluid
circulation for enhanced cooling.
Additionally, check that the outer
seal has reduced heat generation.
* Check the risk of coking
formation at the outer seal and
add API Plan 62 where needed.
* Check that the barrier fluid does
not have to travel far to reach the
sealing interface, and that the
barrier liquid inlet and outlet
port center and orientation
are compared to the seal ring
interfaces.
* Utilize stationary bellows to help
the seal handle the pump's thermal
distortion. An FF design with
a stationary bellows assembly
is also best for accommodating
misalignment, as the bellows only
need to move once to adjust to
pump misalignment and not with
every rotation. This will help
reduce misalignment in cyclic
loading, and will also help prevent
the risk of bellows fatigue failure.
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Hydrocarbon Processing - August 2021

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