The LNG industry has long been characterised by high CAPEX costs. Indeed, since the industry’s early days more than seven decades ago, the requirement to make sizeable investments has been a central tenet to the growth and development of LNG processing. Over the last half decade, however, there has been a larger focus on reducing CAPEX and OPEX. In addition to heat exchangers, various compressors (refrigeration, feed gas, boil-off gas, end flash gas, etc.), constitute a major part of the CAPEX and OPEX in large scale LNG plants. Driven by the desire of LNG plant
users for lower costs, there have been advancements in the compression technology that is deployed throughout LNG plants. At the same time, any compression solution must be able to function without compromising the performance, and the reliability/availability and efficiency guarantee of an LNG plant.
A result of these market dynamics can be seen in highpressure boil-off gas(BOG) compressor technology, which has reduced CAPEX while also meeting performance objectives.
Generally, BOG compressors are essential to any LNG plant’s function and performance, whether it is large, medium or small scale. In fact, in some ways, the high-pressure BOG system is the most critical component of the entire LNG process. This is because of the inherent nature of LNG at atmospheric pressure, which begins to boil at approximately -165˚C and convert to gas. Therefore, this presents the central challenge when dealing with LNG: once the gas starts to evaporate, the pressure in the system consequently increases.
It then becomes imperative that the pressure is reduced, otherwise it will eventually cause serious plant safety and environmental issues.

So, in order to maintain the pressure in LNG tanks(where the LNG is stored at atmospheric pressure) at a required range, some measures need to be taken in order to handle the BOG. Some of the environmentally friendly options for handling BOG include either reliquefying it and putting it into the tank; or to use it as a fuel, which requires advanced, reliable, and efficient compression technology.
Traditionally, BOG compressors are used on two parts of an LNG value chain – one is inside liquefaction plants, which can serve as export terminals, and the second is at the receiving terminal of regasification plants, which are usually served by either between-bearings centrifugal compressors or with reciprocating compressors to carry out BOG duty.
In their quest for lower CAPEX and OPEX, both LNG plant operators and process licensors have showed a willingness to accept a different technological approach, ultimately paving the way for a high-pressure compression solution.

An experience-enabled response to market needs

Responding to a market in need of more CAPEX and OPEX-friendly solutions for LNG facilities, Atlas Copco Gas and Process moved to address this desire with its high pressure BOG compressor. This six-stage integrally geared turbo compressor has both cryogenic and warm compressor stages on a single gearbox and skid, which reduces the equipment’s overall footprint.
It is important to note that this high-pressure BOG compressor is not an entirely new solution, but it is instead a combination of proven technology used in different applications in the industry. Integrally geared compressor technology offers a different design approach to tackling a known challenge: the handling of BOG reliably and efficiently.
Experiences from the field in three key areas were factored into this solution. First is that Atlas Copco Gas and Process used its several decades of experience dealing with BOG(in low-pressure processes, to discharge approximately 3 - 5 bar), as well as other processes which operate at cryogenic temperatures(such as refrigeration).
Second is its experience with fuel gas boosters for gas turbine power generation. In this, high-pressure natural gas is delivered to the gas turbine combustor at constant pressure – something that requires very high reliability and availability due to continuous power production requirement by the grids.
Thirdly, the Atlas Copco Gas and Process team drew on its experience from other specific product solutions, notably its Compander, which combines compressor and cryogenic expander stages on one single gearbox and skid.
The team also has more than five decades of experience in one of the highly critical components of the rotating machinery: the seal and seal-support system. All three application areas mentioned earlier require extensive expertise in designing and building seal and seal-support systems that can operate in cryogenic temperature or high-pressure conditions. In 1975, Atlas Copco became the first in the world to implement Type 28 dry gas seals in the integrally geared compressors.
Companders used for reliquefaction duty on LNG vessels(and small scale onshore LNG plants) process nitrogen – the working fluid in the Brayton cycle. The most critical element in the process is the seal type used. In the Compander, both dry gas and carbon ring seals have been used in the past, while for fuel gas boosters specially designed dry gas seals have been employed. Now, this combination of dry gas and carbon ring seals has been successfully transferred and applied to the high-pressure BOG compressor, and it has been greeted with a high degree of market acceptance.
Experience working with high-pressure compression is also rooted in the company’s expertise in building fuel gas boosters for natural gas driven power plants. In these applications, the suction pressure into the compressor is high and the discharge pressure is even higher – something that is very much comparable to a pipeline compressor. This experience was also applied to the high-pressure BOG compressor concept.

Inside integrally geared design

A closer look at integrally geared compressor technology is pivotal to understanding the fundamental design of the high-pressure BOG compressor. The conventional centrifugal between-bearings compressors that are traditionally employed in liquefaction applications feature either a separate gearbox or a variable frequency drive to achieve the necessary rotational speed. In contrast, the integrally geared technology that is used for the high-pressure BOG compressors utilises high speed pinions in combination with low speed bull gear in the gearbox to achieve the necessary speeds. Along with the gearbox and compressor core, the lube oil system, seal-support system, and all the compression stages are built on one single skid.
However, whether a compression solution is integrally geared or not, designing the key components of a BOG compressor core does not come without challenges, many of which are dictated by physics. In this regard, a key design characteristic is to ensure that the cryogenic stage and the warmer stages of BOG compressors do not impact each other.
On the one side there is a cryogenic temperature of approximately -155˚C to -165˚C; on the other side(just several centimetres away) is a temperature of approximately 90˚C or 100˚C(inside the gearbox).
Underpinned by decades of experience, engineers know how important it is to manage the thermal stresses. Subsequently, when designing the rotor, shaft, and impeller connections, it is vital to provide adequate measures to maintain the correct cryogenic and warm stage temperatures.
Even though every high-pressure BOG compressor is custom designed for each specific installation, insulation is also a vital factor in ensuring cold stage and warm stage integrity.
The basis of every centrifugal compressor is that it needs to provide kinetic energy, and it does this by increasing the velocity of the gas via rotating impellers. On the one hand, the importance of the impeller system is the same for both between-bearings and integrally geared compressors. On the other, because between-bearings machines have impellers mounted on the same rotor, velocity is, to a high degree, determined by the rotational speed of the impellers – which are somewhat restricted due to impeller tip speed limitations.
In contrast, the advantage of integrally geared compressors is that each impeller set is optimised by matching the impeller geometry with the optimal speeds. This ultimately provides greater velocity, resulting in higher head development(pressure ratio) with fewer impellers. Between-bearings machines, for example, run at a maximum of 8000 ‑ 10 000 rpm. An integrally geared machine runs anywhere between 10 000 - 30 000 rpm.

Reliability, efficiency, and availability

In discussing the high-pressure BOG compressor, it is worth returning to the earlier question of reliability and a compressor’s ability to achieve the intended LNG plant performance. When assessing different compression technologies for LNG plants, this factor is of essential importance.
Reliability is defined as the probability that an item can perform a required function under given conditions for a given time interval. Availability is the ability of a system or product to be in such a state to perform a required function under given conditions at a given time interval. From an engineering perspective, reliability is seen as something like the measure of a product’s probability to operate without failure. Reliability, however, is also closely related to quality, and a compressor needs to function reliably and demonstrate a high-quality level. Measured according to industry standards and derived from field feedback, the level of reliability for the high-pressure BOG compressor can be greater than 98%.
While reliability is concerned with the probability of delivering the intended performance of the machine, efficiency and availability of high-pressure BOG compressor technology focus on ensuring that it delivers the intended performance at any given instance during a given time interval (applications include shiploading, unloading, or the hold mode of plants operating in either rich gas or lean gas scenarios). Availability refers to the time the machine is available and if it delivers the intended performance as per the design. In essence, this measure of availability is defined as running time vs downtime. Logically, if the machine is shut down, this affects its availability. Seals and impellers, as discussed earlier, impact availability performance, as do the gearbox and bearings. High vibration, for example, can be the result of errors in rotodynamic designs and/or manufacturing, assuming the process is set and stable. Such a scenario would eventually lead to a diminution of the gearbox, seal failure, and then shutdown. This underpins the necessity to custom design gears and bearings for each individual high-pressure BOG compressor installation and to take into consideration specific process conditions at the LNG plant.
The use of dry gas seals in high-pressure BOG compressors reduces gas leakage compared to carbon ring and labyrinth seals. This adds to both the machine’s efficiency and availability. Used ostensibly in the warm stages, though sometimes also in the cryogenic stages, high-pressure BOG compressors’ dry gas seals are advancements on those used for decades on cryogenic temperatures – as well as other high-pressure applications. This includes fuel gas boosters, which are required to run for five years non-stop.

Conclusion

High-pressure BOG compression addresses a general trend and desire in the LNG industry: leaner and more CAPEX- and OPEX-friendly solutions are in high demand. At the same time, they must meet the test of the highest of plant performance and reliability requirements. The example described in this article also serves as an example of how proven technologies and design concepts can be leveraged to develop different previously unknown approaches to BOG handling.

■ Contact: Atlas Copco Korea +82(0)31-620-0722
www.atlascopco.com