Hydrocarbon Processing - August 2021 - GP-22

SPECIAL FOCUS: SMALL-SCALE LNG
numerous processes along the hydrocarbon value chain.
While SSLNG can expand the scope of LNG availability
and the breadth of usage, it is important to understand that
smaller-scale projects may feature different compression technology
design and operating requirements compared to larger
facilities. At the same time, the technologies used in larger
LNG plants (including the compressors that run the refrigeration
cycle) often cannot be efficiently scaled down.
As is the case across all LNG compressor applications, however,
flexibility, reliability and availability are central considerations
in the design of mixed-refrigerant compressor trains in
SSLNG plants, such as the one installed in the Zhengtai Yida
LNG plant (FIG. 2). As a result, the compressor must be designed
in such a way that its aerodynamics, rotor dynamics, lube
oil, seal support and control systems can precisely meet these
parameters and requirements.
In terms of rotation speeds, the discrepancies in inline compressors
between the optimal and actual speeds at each stage reduce
efficiency for the whole machine. As a result, more power
is needed to drive the compressor, resulting in greater energy
usage than for an IGC. In contrast, all IGC stages are optimized
to the optimum individual pinion speed by selecting the right
teeth (gear ratio between the bull gear and pinion), thereby promoting
overall efficiency.
Overall, therefore, compared to the more flexible IG compressors,
inline compressors require more time to build, deliver
and install. Once delivered, the SSLNG facility can begin operations
sooner, increasing the ROI. As production facilities
increasingly focus on rapid equipment deployment and flexible
operation practices, the basic design (fixed speed and casing)
of inline compressors makes it difficult to realize adjustments
when production requirements change.
FIG. 2. The mixed refrigerant compressor inside the Zhengtai Yida
LNG plant.
Small footprint and low maintenance. With its inherent
compactness, an SMR compressor may be fitted on a comparatively
small skid. In the case of the Zhengtai Yida LNG plant, for
example, the IGC measures 11 m (36 ft) long by 6.4 m (21 ft)
wide with the motor. Comparable inline compressors for this
application and plant size can have nearly twice the footprint.
In addition, IGCs weigh about 25% less than inline compressors.
The IGC's compact footprint helps reduce CAPEX while
increasing efficiency and offering flexible process configuration
and control benefits. In addition to the compressor core and
motor, the skid can accommodate instrumentation and peripherals,
such as lube oil, seal support and control systems.
Additionally, the gearbox casing allows for easy access and
effective " health monitoring " of gears, bearings, vibration and
temperature probes. This compares favorably with the lengthier
and more difficult process of removing the many bolts connecting
the housing of an inline compressor, and then realigning it
after reattaching the bolts.
This aspect is particularly important because SSLNG installations,
including the Zhengtai Yida LNG plant, are often
located in remote areas, where the logistics of aftermarket
support pose a challenge. In such scenarios, onsite repair and
maintenance are easier to conduct with IGCs than with inline
compressors because of the onsite maintainability and smaller
dimensions of the IGC compressor and its components.
Minimizing refrigerant leakage. The seal system design is critical
in a mixed-refrigerant IGC and was of particular importance
in the Zhengtai Yida LNG plant. Generally, standard machine
design calls for a single dry gas seal to be used. However, optionally,
tandem dry face seals may be used. The Zhengtai Yida LNG
plant's single dry gas seal configuration provides a guaranteed
seal leakage rate of 10 Nm3
/hr for the entire machine (FIG. 3).
A typical IGC for an SMR is supplied with three or four imFIG.
3. An installed dry gas seal, mounted at the back of the impeller.
22 JULY/AUGUST 2021 | GasProcessingNews.com
pellers and, as a result, three or four sets of seals. Field experience
has shown that the vast majority of seal failures happen
due to seal support system issues. A properly designed seal support
system with clean, dry seal gas supply should be designed
and tested prior to commissioning.
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Hydrocarbon Processing - August 2021

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