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Availability 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.

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…..

This article was originally published in the April 2016 issue of LNG Industry. To read the full version of this article, sign in or register for a free trial subscription.

Written by Iulian Mischie, Howden Thomassen Compressors BV. Edited by

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