Single-shaft turbocompressors for Boil Off Gas (BOG) recovery
at LNG re-gasification terminals
Sven Brink
Siemens Power Generation Industrial Applications
Today the demand for safe, efficient and cost-effective systems for handling BoilOff Gas (BOG) at LNG receiving terminals is greater than ever before. And without
any doubt at all, this demand will continue to grow significantly.
This paper will describe a little of how Siemens has developed equipment to cope
with this challenge – equipment that fully meets the LNG industry’s most stringent
needs
for
safety,
reliability,
efficiency,
whilst
being
cost-effective
and
environmentally-friendly.
Until recently, flows at LNG receiving terminals were low enough to be a good fit
for reciprocating-type units – and indeed they still are at many existing terminals.
But for today’s much higher BOG volume flows in many new and future terminals,
an alternative type of compressor is often required.
The LNG market is changing rapidly. More than 200 LNG carriers are now in
service, compared with only about 75 just ten years ago. Tanker sizes are
increasing. Today vessels are under construction which are capable of carrying as
much as 265,000 cubic meters.
Inevitably, this means that the maximum BOG flows are increasing during LNG
carrier Unloading Mode and with ever-increasing LNG send-out the ratio between
Holding Mode (BOG volume evaporating from storage tanks during static
conditions minus LNG send-out volume) and LNG tanker unloading is increasing
too. And with this constellation - different optimized compressor BOG-handling
concept becoming more and more attractive - rather than installing a multiple
© Siemens AG 2006. All rights reserved.
number of reciprocating compressors, very often the BOG duty can be handled by
single shaft turbocompressors.
Siemens is able to apply considerable experience in the field of single-shaft
turbocompressors for BOG compression to the changing demands of LNG regasification terminals. By using the advanced designs and technologies already
thoroughly proven in BOG single-shaft turbocompressors installed for base load
applications at LNG export terminals, Siemens has adopted the special
compressor design features to have them available also for BOG duties at LNG
receiving terminals.
At the core of this capability is the groundbreaking development work that has
gone into our single-shaft turbocompressors. In their LNG receiving-terminal role,
these machines need to operate with complete reliability whilst experiencing
suction flow temperatures down to -164º C. They also experience very high
discharge temperatures during start-up and run-in – temperatures possibly as high
as 120º C..
© Siemens AG 2006. All rights reserved.
Notable gains in efficiency and reliability have been made throughout the singleshaft turbocompressors, but two areas in particular stand out in terms of providing
really significant benefits to customers.
The first area of unique technical innovation that makes Siemens single-shaft
turbocompressors ideal for LNG receiving terminal applications is their dry gas
seal technology. This solution was introduced in 1991, dramatically changing the
technology. for the BOG single shaft turbocompressor.
To eliminate or to minimize thermal tension and stress across the enormous range
of operating temperatures involved, these dry gas seals are encapsulated in
heated seal carriers that act as thermal shields. It is essential to keep the seal
areas at an almost constant temperature and to keep rapid temperature
fluctuations away from the seal area if at all possible operating modes, including
start-up and stop, continuous running and stand-still. This constant temperature is
achieved by oil-heated seal carriers, supplied by the existing lube-oil supply.
© Siemens AG 2006. All rights reserved.
Siemens
pioneered
this
concept
and
all
of
the
Siemens
single-shaft
turbocompressors for BOG applications are fitted with dry gas seals equipped with
oil-heated carriers.
The biggest change happened in 1993 when Siemens introduced infinitely
adjustable in let guide vanes (IGV) for BOG single-shaft turbocompressor
performance control
© Siemens AG 2006. All rights reserved.
IGVs work in a completely different way to a suction throttle or a discharge throttle.
Depending on their setting angle, a swirl is induced onto the flow stream directly in
front of the first impeller. Depending on the selected degree and extent of this
swirl, performance and control of the turbocompressor can be optimized at all
operating conditions. The IGVs are located inside the sealed compressor casing,
so there is no danger of atmospheric air entering the process or of dropping below
saturation conditions.
In practice, the system brings major advantages compared with conventional
control via a discharge throttle. Firstly, it gives high and smooth flexibility for
different suction conditions and mass flows. It also ensures outstandingly large
turndown. Maximum starting torque can be kept very low and it minimizes the
primary power installation required. As a result, the IGV system contributes
powerfully to the exceptionally high reliability of these turbocompressors.
The described innovations like IGV concept and Dry Gas Seals capsuled in heated
seal carriers ensure that the single-shaft turbocompressors can handle different
flows of BOG and that the packaged unit can run under start / stop conditions for
an almost unlimited number of starts and stops per year without affecting their
overall reliability.
Most of the major inspection work required can be undertaken while the
compressor is running. A planned complete overhaul of the turbocompressor
(which could involve changing the rotor, dry gas seals, labyrinth seals and so on)
© Siemens AG 2006. All rights reserved.
can be achieved in a few working days. Minor inspections can, of course, also be
completed while the turbocompressor is running. This exceptionally high reliability
and availability means no spare unit is required as a stand-by.
Key advantages
So, to summarize, what are the key advantages of single-shaft turbocompressors
for BOG duties at LNG receiving terminals?
First, without doubt, is their very high reliability and availability.
They also ensure lower operating and personnel costs and they need the absolute
minimum of surveillance and maintenance costs. No wear parts means no periodic
shutdown for overhauls is needed.
The compactness of a single-shaft turbocompressor package compared with
reciprocating-type units allows considerably lower foundation costs and, naturally,
much less space is required. This means that the single shaft turbocompressor
concept is particularly ideal for offshore installations where space, weight, vibration
and reliability are of utmost importance.
From the point of view of installation, the turbocompressor comes packaged, fully
piped and wired and shop-tested with no need for pulsation bottles or for interconnecting piping to be erected at site.
© Siemens AG 2006. All rights reserved.
No atmospheric emissions are produced.
Start-up from warm and cold conditions alike is optimized, while no overheating
can occur either during start-up or at part-load operation.
Conclusion
The key factors that make single-shaft centrifugal-type turbocompressors ideal for
BOG applications in LNG receiving terminals are the development of dry gas seals
for shaft sealing purposes and of variable inlet guide vane technology. The
optimized performance ensured by these technologies makes single shaft turbocompressors the equipment of choice for operators of all LNG-receiving terminals
built in recent years.
Their reliability, availability and clear through-life economic advantages ensure
their benefit to LNG terminal operators will become even more pronounced as the
industry continues to experience extensive worldwide expansion.
From operational, environmental and economic standpoints, Siemens single-shaft
centrifugal turbocompressors for BOG compression have established a clear
market lead. They will continue to enhance the profitability of more and more LNG
customers worldwide for decades to come.
© Siemens AG 2006. All rights reserved.