Hydrocarbon Processing - August 2021 - 27

Special Focus Valves, Pumps and Turbomachinery
S. ZARDYNEZHAD, TurboTech Consulting Corp.,
Calgary, Alberta, Canada
Dual-mechanical seal arrangement
for hot pump service
Selecting mechanical seals for hot services
in an oil refinery or petrochemical
plant is always a challenging task-and
it is a task that needs careful attention,
since the wrong seal selection can have
a significant impact on pump reliability
and availability. Hot pumping services include
liquid temperatures above 175°C
(347°F); however, this article includes lessons
learned based on liquid temperatures
of more than 200°C (392°F). This work is
based on the author's experience in dual
mechanical seal arrangement selection for
large-scale hydrocarbon processing plants.
Mechanical seal categories, types
and arrangements. The seal configurations
covered by the API 682 standard1
are classified into three arrangements
(i.e., 1, 2 and 3). The definitions of these
arrangements are detailed in TABLE 1. Arrangement
2 and Arrangement 3 seals are
classified in three orientations: face-toback
(FB), back-to-back (BB) and faceto-face
(FF); however, Figure 3 in API
682 shows only an FB configuration for
Arrangement 2. FIG. 1 shows three configurations
based on Arrangement 3 as
per API 682.
Arrangement 2, which is used in API
Plan 52 (unpressurized), is not a suitable
plan for hot pumping services due to the
risk of leakage in the primary seal. In addition,
this arrangement has low cooling
efficiency, and, since it is connected to
the plant's flare system, the back pressure
from the flare system may cause
an issue for the pump mechanical seal
during operation. This article includes
several lessons learned using Arrangement
3 (pressurized) for hot pumping
services. The following will examine the
differences between the three arrangements
shown in FIG. 1.
Comparison of configurations. Comparisons
of the three different configurations
in FIG. 1 are detailed in TABLES 2, 3,
4 and 5.
The installed population of Arrangement
3 seals arranged in series (3CW-FB)
is relatively small vs. other configurations
(3CW-FF and 3CW-BB) as per API 682.
However, recently, by new modification,
the FB design is more popular. Both BB
and FF configurations (3CW-BB and
3CW-FF) potentially offer more compact
designs, and can provide higher levels of
performance in clean services. However,
the FB series configuration is preferred
because any abrasive contamination is
centrifuged out and, therefore, has less effect
on the inner seal. In the event of loss
of barrier fluid pressure, the seal will behave
like an Arrangement 2 seal.
Cost of using API Plan 32, Arrangement
3 FB configuration. The following
will show how to calculate the cost
of using Plan 32 in an FB configuration.
Assume that light vacuum gasoil (LVGO)
is selected as the seal flush-i.e., Plan 32
for a bearing pump with two sets of dual
mechanical seals. If the minimum flush
liquid is specified as 20 l/min for each
mechanical seal in the seal drawing, then
each pump needs 40 l/min. If the pump
runs continuously for 300 d/yr, then the
pump needs 40 l/min × 60 min × 24 hr
× 300 d. This equals 17,280,000 l or approximately
109,000 bbl/yr.
Assuming that the cost of LVGO is
equal to $30/bbl, then using LVGO as
Plan 32 would cost $3.27 MM/yr for
each pump. The same amount should be
considered for the hot standby pump, because
the inner seal of the standby pump
FIG. 1. Mechanical seal Arrangement 3
per API 682.1
TABLE 1. Mechanical seal arrangement defi nitions per API 682.1
Arrangement Defi nition
1
2
3
Seal confi gurations having one seal per
cartridge assembly
Seal confi guration having two seals per cartridge
assembly, with the space between the seals at a
pressure less than the seal chamber pressure
Seal confi guration having two seals per cartridge
assembly, utilizing an externally supplied barrier fl uid
at a pressure greater than the seal chamber pressure
API plan
Diff erent API plans
can be used
52 (unpressurized)
53 and 54 (pressurized)
Hydrocarbon Processing | AUGUST 2021 27
Hydrocarbon Processing - August 2021

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