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Amongst the technologies
developed for oil and gasproduction in the last 20
years, multiphase pumping is one of
the most promising. The significant
reduction of capital investment either
on or offshore together with an
increased production rate has led to a
growing demand for pumping systems
to be installed.
The biggest challenge to be overcome
by a piece of rotating equipment in
multiphase service is the variation inthe process fluid from
100% liquid to
100% gas. This creates very difficult
operating conditions for the pump andeven more difficult
conditions for its
mechanical seals, which are a critical
part of any pump.
Mechanical seals are readily available
for liquid or gas phase applications.
However, in a multi-phase pump the
mechanical seals will see
transient conditions. The transient
nature of the wellhead stream creates
pressure surges, slugs of liquid and gas
locks. A multiphase pump and itsmechanical seals must be able
to
withstand all these rather harsh oper-
ating conditions. Consequently specialmechanical seals have to
be developed.
Types of pump
Multiphase pumps can be subdivided
into two groups, each with benefits
and limitations. These are
rotordynamic and positive
displacement pumps.
Rotordynamic pumps rely on theconcept of helico-axial
hydraulics. A
Mechanical sealing technologyused in multiphase pumping the
benefits
Nikolaus Necker from Burgmann Industries GmbH & CoKG
describes theadvantages you can gain when using mechanical sealing
technology inmultiphase pumping.
f e a t u r e m u l t i p h a s e p u m p s
WORLDPUMPS August 2005 www.worldpumps.com
Figure 1. Pressurized
dual mechanical seal(API 610, plan 53 or
54), type SH, for
multiphase
pumping.
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multistage pump can have up to
15 compression cells on a
single shaft. Each compression
cell comprises a rotating helico-
axial flow impeller and a stationary
diffuser.
The produced effluent can thus
be boosted over long distances
without the need for prior separat-
ion and with no limitation regard-
ing gas volume fractions. This type
of pump is compact, offers mech-
anical simplicity and reliability,
is tolerant of solids and produces high
flow rates.
Positive displacement pumps, or
twin screw pumps as they are alsoknown can already handle
gas
volume fractions up to 70%. In this
respect only some minor improve-
ments have to be made to existing
pump design. The twin rotor screw
pump consists of two contra-
rotating screws.
A certain amount of liquid is
needed to seal the gaps between
the screws and the liner. This kind
of pump has true dry runningcharacteristics, thus can handle
an extreme range of specific
gravity and gas volume fraction.
They have proven their re-
liability in several real multiphase
boosting applications.
The problem task
Unlike sealing systems, which are
exclusively and specifically applied
in liquid or gaseous media,
mech-anical seals for multiphase
pumps operate with continually
chaning gas and liquid flow rates.
Thus the mechanical seal has
to cope with the following
conditions:
unpredictable process medium
composition
density and viscosity variations
temperature variations
erosion effects, mainly by sand high and low operating
pressures
insufficient lubrication and
cooling of the seal faces.
Chemicalparameters
The chemical analysis of the process
medium differs, depending on the oil
field and/or the location. This means
that the crude oil may contain:
wax
salt
water
sour gas (H2S) and / or carbon
dioxide (CO2)
cracked hydrocarbons
solids.
All this affects the operating
performance of the seal and results in
erosion or corrosion of the mechanicalseal components.
Sealing principle
The fluid being sealed decreases in
pressure across the width of the seal
face. The friction at the seal face
results in a temperature increase in the
fluid at the seal face. The decrease in
pressure and the simultaneous
temperature increase of the fluid at the
seal face results in the vaporisation of
the volatile elements within the
sealed fluid.
f e a t u r e m u l t i p h a s e p u m p s
Figure 2. Rotordynamic Pumps rely on the
concept of helico-axial hydraulics.
Figure 3. The Twin Rotor Screw Pump consists of two
contra-rotating screws.
Figure 4. A typical
dual high pressure
seal in a face-to-face arrangement.
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This can result in insufficient
lubrication and cooling of the seal
faces. The gas content in a multi phase
process medium creates an additional
risk of insufficient seal face
lubrication which may result in the
failure of the seal faces.
In an optimum parallel gap the axial
opening and closing forces are
balanced. In this case the contact
force (Fc) is evenly distributed across
the width of the seal face. The
pressure gradient of the sealed fluid
(Fs) is linear.
Depending on the mechanical
seal face stiffness, two other gap
formations can occur.
In an A-gap situation a high contact
force at the outside diameter is
created. The pressure gradient
steepens. This results in an unstable
running condition of the seal.
In a V-gap situation, a low contact
force is created at the inner diameter
of the seal face, the pressure gradient
shallows, the mechanical seal runs in
a stable condition.
Two examples of rotating sealing
solutions follow.
Single mechanical seal(API 610, plan 32)
Single mechanical seals can be used
in twin screw pumps for multiphase
applications. Single seals are
lubricated and cooled by a flush from
an external source (API610-plan
32). The flush has to be continuous
and reliable even during start-up
and shut-down of the pump. A close
clearance throttle bushing in front of
the seal controls the rate of flow of
the flush medium into the pump,
thus isolating the seal from the
process fluid.
Pressurised dualmechanical seal (API610, plan 53 or 54)
Engineered pressurised dual seals
are predominantly used in multi-
phase pumps. The example shows
a typical dual high pressure seal
in a face-to-face arrangement.
This type of seal is commonly
used in areas where the practicality
of using a single seal with
plan 32 external (continuous loss)flush is limited. Dual
seals
are lubricated by an external closed
loop supply system.
This mechanical seal is a special
design employed for high duty sealing
applications. The seal faces are
double balanced; in the event of
barrier fluid pressure loss or a high
pressure transient in the seal chamber
the seal remains closed allowing
pump rundown with little risk of face
damage or product contamination to
the atmospheric side. The identical
stationary seal face assemblies give
optimum running characteristics
under changing operating conditions.
The seal is designed as a cartridge
unit, thus is compact, robust and easy
to maintain.
Corrosion and erosion is avoided by
choosing appropriate seal face andconstructional materials.
Silicon
carbide is typically used for the seal
faces as it has high hardness and
excellent heat transfer properties.
Duplex and Super duplex stainless
steels are used for the metal parts.
Conclusion
Mechanical seals have proven
their ability to seal multiphasemixtures over a wide range
of
operating conditions in hundreds of
applications in extremely harsh
environments around the world.
The mechanical seal types and sliding
materials available today are capable
of meeting any technical demand
defined by multiphase pumping
services. In order to assimilate the
sealing systems with the specific
operating conditions close co-
operation between seal manufacturer,
pump manufacturer and end
user is a basic precondition. Correct
application, installation and
operation are the key factors for a
long meantime-between failure. s
CONTACTNikolaus Necker
Burgmann Industries GmbH & Co. KG
Aeussere Sauerlacher Str. 6-10
82515 Wolfratshausen
Germany
Phone +49 8171 23 1200Fax: +49 8171 23 1214
E-mail:[email protected]
Website: www.burgmann.com
f e a t u r e m u l t i p h a s e p u m p s
parallel-
gap
A-gap
V-gap
gap and contact
force
indifferentFS
FC
pa > pi
k1 = 0,5
0 < k1< 0,5
0,5 < k1
< 1
unstable
stable
Figure 5. Mechanical steel gap stiffness.
mailto:[email protected]:[email protected]:[email protected]
