Compressor Station Fuel Gas Superheating Using Lube Oil Waste Heat

Abstract

Compressor station gas turbine engines require protection from fuel gas liquid drop-out caused by the Joule-Thomson effect when natural gas is let down from transportation line pressure to the burner supply pressure. Indeed, gas turbine manufacturers specify a minimum gas superheat, which requires fuel gas heating at pipeline temperatures experienced in Northern Europe. Conventionally, fuel gas superheating is achieved through the use of either electric or gas fired water bath heaters, which require maintenance, and an external heat source. Meanwhile, waste heat from the turbo-compressor lube oil system is released to atmosphere, typically by air-cooled heat exchangers. Hence, there is an obvious opportunity to protect the gas turbine engine, whilst reducing the amount of heat rejected to the environment. Mechanical integrity is a key operational requirement when combining fuel gas superheating with lube oil cooling in a single heat exchanger. Fuel gas at high pressure must not enter the low pressure lube oil system. High integrity Printed Circuit Heat Exchangers (PCHEs) are ideally suited to this application, as they are diffusion bonded and fully welded heat exchangers. Used extensively in offshore high pressure gas compression trains in the North Sea, PCHEs have demonstrated that they are low maintenance items that are ideal for use in remote unmanned applications, such as those required by gas compression stations. PCHEs are highly compact, reducing space and structural requirements. This allows the exchanger to be installed underneath the compressor, minimizing the visual impact of the heat exchanger. In addition, safety and pressure relief requirements are significantly reduced, a PCHEs do not have a failure mode analogous to tube rupture in shell and tube heat exchangers. National Grid Transco have realized the opportunities of PCHEs and operated them successfully over many years in many of their compression stations throughout the United Kingdom.