Hydrocarbon Processing - August 2021 - 28

Valves, Pumps and Turbomachinery
TABLE 2. Mechanical seal configuration definition per API 682: Arrangement 3
Configuration
Face-to-back
(FIG. 1A)
Definition
One mating ring is
mounted between
the two flexible
elements, and one
flexible element is
mounted between
the two mating rings
Back-to-back
(FIG. 1B)
Flexible elements
are mounted
between
stationary rings
Pumping liquid Barrier liquid
On the outer
diameter (OD)
of the inner seal
On the inner
diameter (ID)
of the inner seal
and OD of the
outer seal
On the ID of
the inner and
outer seals
On the OD of
the inner and
outer seals
External seal flush
(e.g., Plan 32*)
Required
Operating expenditure
(OPEX)
High, due to the
use of Plan 32;
calculate cost/yr
before ordering
Abrasive
service
Highly
effective
Remark
API 682 prefers
FB- Clause 7.3.3.2.1.
Thermal movement/
misalignment of
the seal gland will
impact the outer seal
performance.
Not required
-
Not suitable If specified by
purchaser, Clause
7.3.3.2.1. Thermal
movement/
misalignment of
the seal gland will
impact the outer seal
performance.
Face-to-face
(FIG. 1C)
Both mating rings
are mounted
between the
flexible elements
On the ID of
the inner and
outer seals
On the OD of
the inner and
outer seals
Not required
-
Not suitable If specified by
purchaser, Clause
7.3.3.2.1. Thermal
movement/
misalignment of
the seal gland will
impact the outer seal
performance.
* Plan 32: A clean and cool seal flush is injected into the seal chamber from an external source. Piping Plan 32 is used in services containing solids or contaminants, for which a suitable
cleaner or cooler external flush will improve the seal environment. It is also used to reduce flashing or air intrusion (in vacuum services) across the seal faces by providing a flush
that has a lower vapor pressure or that will raise the seal chamber pressure to an acceptable level. Refer to API 682 for more explanation.
TABLE 3. Inner seal cooling effectiveness: Arrangement 3
Configuration
Inner seal cooling effectiveness
Face-to-back
(FIG. 1A)
Back-to-back
(FIG. 1B)
Cooling of the inner seal is done by an external
clean source (e.g., Plan 32). This significantly impacts
OPEX, as the volume of seal flush is normally high.
The barrier liquid return port is far from the inner seal
faces' interfaces. Normally, the location of the barrier
liquid in/out ports are near the outer seal ring. This
causes the inner seal faces to not receive enough
cooling where high heat generation is created with
SiC-SiC hard rings at the inner seal.
Face-to-face
(FIG. 1C)
The barrier liquid return port is close to the inner seal
faces' interfaces. Normally, the location of the barrier
liquid in/out ports are near both seal rings. This causes
both the inner and outer seal faces to not receive
enough cooling. Using a face-to-face arrangement,
the barrier fluid does not have to flow and travel as
far to reach the sealing interface of the mating rings.
This will improve the efficiency of the seal face cooling,
resulting in a significant reduction of heat generation.
TABLE 4. Flexible element (i.e., bellows) location: Arrangement 3
Configuration
Location
Face-to-back
(FIG. 1A)
Back-to-back
(FIG. 1B)
Face-to-face
(FIG. 1C)
Face-to-back configuration has rotary bellows in hot service. The rotary bellows
have risk of fatigue failure due to cyclic loading when running in hot services.
Back-to-back configuration has rotary bellows in hot service. The rotary bellows
have risk of fatigue failure due to cyclic loading when running in hot services.
Face-to-face configuration has stationary bellows in hot service. Using
stationary bellows will help for hot service pumping where risk of thermal
distortion is high. This is also the best option for absorbing misalignment, as the
bellows are fixed and only need to move once to adjust to pump misalignment
and not with shaft every rotation. This will help reduce cyclic loading and
prevent fatigue failure of the bellows, especially in hot and dirty services.
28 AUGUST 2021 | HydrocarbonProcessing.com
should be kept cool during hot standby
conditions, as well.
Hot services application
Yes; however, the cost
of Plan 32 is high
No, due to poor
inner seal cooling
Yes; however, it is
not recommended
in abrasive
and dirty services
Risk of coking the outer seal. In
the presence of high temperatures, hydrocarbons
and barrier liquid can cause
a chemical breakdown or oxidation of
the hydrocarbon. Coke is a black, hard,
grainy material buildup on the atmospheric
side of the seal face. This buildup
comprises thermally decomposed hydrocarbons
that is converted to a varnish,
lacquer or abrasive sludge. Heavy coke
buildup can decrease the unit loading on
the seal faces and cause excessive leakage.
Coking will change the balance ratio of
the mechanical seal and also can cause
hang-ups of the mechanical seal. Coking
is normally found on the atmospheric
side of the seal faces.
To reduce risks of coking, the following
operations are recommended:
* Reduce seal cavity temperature by
using a proper plan and seal design.
* Remove the oxygen from the
atmospheric side of the seal faces
by adding a quench (e.g., API Plan
62) with dry steam or nitrogen.
Dry steam removes the oxygen
and will wash away residue;
however, a nitrogen quench only
removes oxygen but will not
wash away residue. Do not use

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Hydrocarbon Processing - August 2021

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