1. A
vailability and
reliability are of paramount
importance when it comes to
boil-off gas (BOG) compressors operating
in demanding cryogenic environments. With
process gas temperatures as low as -160°C, and LNG
carriers moored and ready to start unloading, there is no
room for unplanned events. Combined with the ongoing
need for ease of maintenance and low operational costs,
the latest technology can bring all of these components
together into a successful formula.
Engineered
Iulian Mischie, Howden
Thomassen Compressors BV,
introduces an innovative
technology that provides
enhanced availability and
reliability for horizontal
reciprocating compressors
in LNG applications.
TO FLOATTO FLOAT

2. LNGINDUSTRY REPRINTED FROM APRIL 2016
BOG in LNG terminals
LNG is stored as a boiling cryogenic fluid and does
not change as it is cooled by evaporation. BOG is
generated in LNG tanks and liquid-filled lines as a result
of heat influx from the environment. In order to avoid
environmental impact and loss of revenue to the owner’s
operations, BOG is generally not flared, but collected and
either recondensed or compressed for use as fuel gas or
grid send-out. Compression of the BOG is also required
prior to recondensation, as there is no cold energy source
available to condense the BOG at atmospheric pressure.
The amount of BOG that can be recondensed depends
on the amount of LNG send-out, because the LNG is
used as the cold energy source to condense the BOG. If
there is insufficient LNG send-out, the vapour must be
compressed to pipeline pressure.
During ship unloading, the quantity of BOG generated
in the tanks and piping increases significantly. This
additional vapour consists of: volume displaced in the
tanks by the incoming LNG; vapour resulting from the
release of energy input by the ship’s pumps; flash vapour
due to the pressure difference between the ship and the
storage tanks; and vaporisation from heat leakage
through the unloading arms and transfer lines. An increase
by a factor of 4 can be expected during unloading.
Technology considerations
for BOG compressors
In comparison to refinery applications, BOG compressors
do not run at constant operating conditions, because the
amount of BOG can differ significantly over time. The flow
is dependent on the amount of liquid in the tank, as well
as heat ingress. Furthermore, the flow increases heavily
during ship unloading. Therefore, it is required that BOG
compressors are equipped with provisions for capacity
control to be able to handle these variable boil-off
conditions.
Due to the low operating temperatures, lubrication of
compressor cylinders by means of mineral oils or
synthetic lubricants is not possible. Oil-free compressor
cylinders are mandatory for BOG compressors, due to the
oil behaviour at cryogenic temperatures. For instance, oil
could stick to the chillers and clog the piping system
during and after recondensation. Research has shown that
operating in the hydrocarbon mist (typically found in a
BOG compressor) can increase the wear by a factor of
400 – 500%. Therefore, rider rings, which carry the
weight of the piston and piston rod in horizontal
reciprocating compressors (Figure 1), and are always one
of the most vulnerable abrasion points in oil-free
compressors, may have to be replaced four to five times
more often than in conventional, non-lubricated
applications. Furthermore, the wear of the conventional
rider rings in oil-free compressors is unpredictable,
making a linear wear forecast virtually impossible in
scheduling maintenance stops by using a condition-based
maintenance approach. Failing to replace a worn down
rider ring will inevitably lead to unscheduled downtime
and costly damages to the piston and cylinder liner.
Eliminating wear
In order to increase reliability and the mean time between
maintenance (MTBM) for non-lubricated reciprocating
compressors, the wear mechanism needs to be eliminated
altogether by using the process gas readily available
in the compressor cylinder (Figures 2 – 5). Howden
Thomassen’s Free Floating Piston (FFPTM
) technology
creates a hydrostatic gas bearing between the rider
ring and cylinder liner. The small but continuous flow
of process gas from the gap between the rider ring and
cylinder liner creates an extremely thin gas cushion. The
piston is lifted and the rider ring no longer touches the
cylinder liner, thus eliminating wear altogether. Although
Figure 1. Howden Thomassen’s six-cylinder C-85.6
compressor.
Figure 2. Free Floating Piston (FFPTM
) check valves.

3. REPRINTED FROM APRIL 2016 LNGINDUSTRY
thin, this gas cushion is completely stable. If the gap
between the rider ring and cylinder liner decreases,
the flow of process gas is reduced, increasing the
pressure in the bearing area, which provides more lift
force and increases the gap. This design ensures correct
performance under all operating conditions.
The process gas supply for the resulting hydrostatic
gas bearing is realised by using the interior of the hollow
compressor piston as a pressurised gas buffer. During
compression stroke, the process gas is fed to the
hydrostatic gas bearing by allowing high pressure gas to
flow into the hollow piston through a check valve,
preventing backflow during the expansion and intake
stroke. The low gas flow through the nozzles ensures that
there is not an appreciable loss of efficiency in the
compressor’s performance.
In situations where the gas bearing is temporarily
unavailable, such as during start-up of the compressor
when no gas is actually compressed in the cylinder, the
FFP functions as a conventional piston, running on the
rider ring surface. As soon as the compressor is loaded
and starts compressing gas, the pressure is built up within
a few strokes and the piston is lifted.
Case study
The FFP has proven itself in the field in a wide variety of
applications. One Howden customer requested support
for a non-Howden three-stage reciprocating compressor,
with a first-stage double-acting cylinder and the second
and third stage combined into a tandem cylinder design.
Although the reciprocating compressor was working,
the effective lifetime of the piston and rider rings –
especially on the tandem cylinder – was disappointing.
Maintenance records revealed that the MTBM and mean
time between failure (MTBF) was 4000 – 6000 hr. The
short and unpredictable MTBF/MTBM of the compressor
caused a significant loss of production, due to plant
outages. Furthermore, the customer was confronted with
high direct and indirect operating expenses, such as high
spare parts costs, field personnel, damaged pistons and
cylinder liners. Therefore, the customer’s objective was to
eliminate any unscheduled maintenance and increase the
MTBM to 2 years, or more than 16 000 operating hours.
Solution
Facing a challenging process environment with a highly
corrosive gas and a non-lubricated cylinder, Howden
proposed to apply its FFP technology. Field reverse
engineering activities, as well as intensive discussions
with the customer related to process requirements and
maintenance experience, provided the proper ground for
an innovative and sound design.
After implementation of the technology, the
compressor was running smoothly, with a significant
increase in availability and reliability. The resulting MTBM
exceeded 16 000 hr. The FFP technology can be applied as
retrofit to virtually any horizontal reciprocating compressor,
either in non-lubricated service, or by converting existing
lubricated cylinders to non-lubricated service.
A significant increase in reliability and availability of
BOG compressors can be achieved by leveraging FFP
technology for non-lubricated compressors (Figure 6). The
benefits of FFP technology include:
Figure 3. FFP operation principle for a double acting
compressor cylinder.
Figure 4. Gas nozzles accommodated in the FFP rider rings.
Figure 5. FFP design for cryogenic applications.

4. LNGINDUSTRY REPRINTED FROM APRIL 2016
 Eliminating rider ring wear: by applying FFP
technology, the contact between piston rider rings
and cylinder liners is eliminated during compressor
operation. In this way, unpredictable rider ring wear in
cryogenic, non-lubricated service is avoided.
 High compressor efficiency: the leakage across
the double acting piston is significantly less than
for conventional piston and rider rings designs, or
for labyrinth piston designs, as found in vertical
compressors.
 Maintenance: the pistons require no special skills
or tools for service and maintenance operations,
while offering long MTBM cycles in comparison with
conventional, non-lubricated polytetrafluoroethylene
(PTFE) based piston and rider ring designs.
 Retrofits: the technology can be retrofitted on existing
reciprocating compressors, irrespective of brand
or type. The impact on the compressor from such a
retrofit is minimal, as the existing piston rod, piston
rod packing and compressor valves can generally be
re-used without any modification.
Conclusion
Reliability is paramount for horizontal reciprocating
compressors operating in BOG or send-out service.
As oil-free compressor cylinders are mandatory for
this application, reducing downtime caused by the
unpredictable wear of rider rings is of crucial importance
in increasing the reliability and availability of these
compressors. Getting to the root cause and eliminating
this wear mechanism altogether by using cutting edge gas
bearing technology, Howden Thomassen’s FFP prevents
unscheduled downtime and extends the lifetime of rider
rings significantly.
Figure 6. FFP installation during compressor assembly at
Howden Thomassen facilities.