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Refinery UtilitiesD. North

1 STEAMSteam is a flexible and widely used source ofenergy for heating and for generating power withina typical refinery. It is used in the process unitsthemselves for heating process streams, for strip-ping and as a process feedstock for certain unitssuch as crackers, reformers and hydrogen plants.It is used in offsite areas and elsewhere for tankheating and trace heating of pipelines.

Steam is normally, although not invariably,distributed at three pressureshigh, medium andlowwhich vary from site to site.

High-pressure (HP) steam, typically at about42 bar g, 400C, is normally generated in centralutility boilers and by waste heat boilers in processareas. It is used for generating power, either aselectrical power or directly in turbine drivers forpumps, compressors and other machinery. Somerefineries use HP steam at 31 bar g, 315335C,while higher pressures, up to 100 bar g, are increa-singly used where there is a need to generateelectrical power. HP steam is also used for processheating duties where high temperatures are needed,as an alternative to using a fired heater.

Medium-pressure (MP) steam, may be at a pres-sure from 8.5 bar g to approximately 20 bar g, typi-cally with 5070C of superheat. It is generated

principally from process waste heat steam gener-ators, exhaust from high-pressure turbines, andby direct pressure letdown and desuperheatingfrom HP steam. MP steam is typically used forprocess heat exchangers and as supply to smallturbine drives. It is also the preferred motive steamsupply to ejectors, used for maintaining vacuum invacuum distillation units, and vacuum condensers.For viscous process fluids which need to be main-tained at a high temperature to ensure fluidity, MPsteam may be used for line tracing and tank heating.

Low-pressure (LP) steam is used for heatingservices, including process exchangers, tankheating, line tracing and miscellaneous services,including hose stations. It is also used generally asthe heating steam for the boiler feedwater (BFW)deaerators, either directly or after letdown to alower pressure. LP steam may be used to drivesmall turbines in intermittent operation in remoteareas where an alternative to electric power isrequired, higher-pressure steam is not availableand the exhaust can discharge to atmosphere.The pressure used is usually between 2 bar gand 5 bar g, and typically 3.03.5 bar g. Lowerpressures tend to become impracticable, since ona site of any size the pressure losses in thedistribution system become large in relation to thenominal pressure. LP steam is generated in process

Edited by Alan G. Lucas. Published on behalf of the Institute of Petroleum. Modern Petroleum Technology 2000 John Wiley & Sons Ltd.

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