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What You Need to Know About Steam Separators

When asked to size various devices such as valves, regulators, etc., to control steam, an assumption is made that the steam is “dry saturated steam.”  Dry saturated steam is steam that has been completely evaporated so that it contains no droplets of water, but is not superheated.

In actual practice, steam carries tiny droplets of water with it and should not be described as dry saturated steam.  These droplets determine steam “quality.”  Steam quality is described by a “dryness fraction.”  Dryness fraction is the proportion of completely dry steam in an amount of steam.  This is important since the water droplets carry no latent heat.

All steam saturation tables are based on dry steam — 100% quality steam.  To find the actual enthalpy of a pound of steam, the dryness fraction must be known and applied to the tables.

For example:  The enthalpy of dry steam at 100 psig is shown as 1190.6 Btu/Lb on a saturation table.  This is made up of 309 BTU’s of sensible heat and 881.6 Btu’s of latent heat.  If the steam is actually “wet” steam that is 95% dry, the dryness fraction is expressed as .95.  Each pound of this wet steam will contain its full amount of sensible heat of 309 Btu’s, but there will be only 95% of the latent heat, or 881.6 x .95 = 837.52 Btu’s.  This results in 44.1 Btu/Lb less total heat in each pound of steam at 100 psig.  This may be significant when dealing with large amounts of steam.

Most steam boiler manufacturers design their boilers to provide 99.5% quality steam.  In other words, 0.5% of the volume of steam exiting the boiler is water droplets.  Due to operating the boiler at less than design pressure, poor piping, failed traps, etc., the steam quality in saturated systems will deteriorate and can become very wet.  Most saturated (not superheated) steam systems operate with wet steam.  This is easily seen.  Dry steam is a transparent gas.  The visible white cloud effect of steam is due to water droplets in the steam reflecting light.

Besides robbing a steam system of heat, wet steam is very detrimental to all the components that make up a steam system.  Valves, regulators, heat exchangers, even the piping is adversely affected by wet steam.

Seldom is the quality of steam given when asked to size a steam device for a system.  Most customers do not know what dryness fraction means, much less supply a dryness fraction. Most of the time, the steam systems we deal with will be of a size where the heat output won’t be affected enough to worry about the quality of the steam.  We can use the dry saturated steam tables for sizing, unless specifically told otherwise.

Because of the detrimental effect that wet steam has on the system components, steam separators should be used, especially ahead of pressure reducing stations, control valves, and any process equipment that uses dry saturated steam.

Separators should be quoted and recommended on all jobs using saturated steam, since the steam quality will not be 99.5%, but more like 95% or worse, much worse.  The added cost of a separator will be recovered by the extending the life of any downstream devices.

Separators come in all sizes, vertical or horizontal styles, and made of steel or cast iron.  They have no moving parts and require no maintenance.  Removal of the water droplets in the steam is accomplished using a series of baffles that the wet steam passes over.  The water droplets collect on the baffles and fall out by gravity to a drain that is piped to a trap system.

Separator manufacturers, such as Sarco, print sizing charts and instructions on how to select a separator.  Separators are sized based on line size, velocity, and pressure drop.  Ideally, the separator will have no more pressure drop than the equivalent length of pipe it is replacing, but often the selection will be the best compromise between line size, velocity, and pressure drop.

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