Steam separating apparatus

A liquid-vapor separator for two-phase fluids in general and specifically a steam-water separator in a steam drum of a steam generator includes axial or radial spinner blades to create a centrifugal motion which causes liquid to be forced outward against the outer wall and the vapor to be concentrated in the center. Conical extraction skimmers systematically extract and discharge the liquid outwardly and downwardly through the side walls such that it impinges on an optional discharge screen surrounding the skimmers. The vapor flows out the top through a central opening and enters a secondary separator packed with crimped wire mesh encased in a perforated enclosure. The conical extraction skimmers include an outwardly protruding rim portion which forms an enlarged annular chamber between the rim and the underlying conical extraction skimmer. This forms a converging-diverging flow path out between skimmer sections and an outwardly extending ring forms a tortuous path. Inwardly extending extraction lips of varying size on the top of each skimmer enhance the liquid extraction.

BACKGROUND OF THE INVENTION 
This invention relates to the field of separating vapors from liquids in 
two-phase mixtures such as separating steam from water in a boiler or 
natural gas from liquid hydrocarbons. More particularly, the invention 
relates to steam drum internals for separating steam from water. 
Steam generated in a subcritical pressure drum type boiler is intimately 
mixed with large and variable amounts of circulating boiler water. Before 
the steam leaves the boiler and enters the superheater, practically all of 
this associated water must be separated from the steam. This separation 
must be done within a limited space in the steam drum, within a matter of 
seconds and under a variety of velocity, pressure and other operating 
condition. The pressure drop across the steam and water separators must be 
kept to a minimum so as not to affect the boiler circulation or water 
level controls. Despite many theoretical analyses of steam and water 
separation and a great number of hypotheses to explain these phenomena, 
steam and water separation in boilers retains many aspects of an art and 
has thus far defied complete mathematical representation. 
Nearly all of the liquid and solid impurities in the steam and water 
mixture must be separated from the steam before it is suitable for use. 
Any unseparated liquid in the steam contains dissolved and suspended salts 
which appear as a solids impurity in the steam when the moisture is 
evaporated in the superheater or it is directed to a turbine or other 
steam driven apparatus. 
The drum of a subcritical pressure boiler serves several functions, the 
first being that of collecting the mixture of water and steam discharged 
from the boiler circuits. Also, the drum houses equipment to separate the 
steam from the water and then purify the steam after it has been 
separated. The drum internals in subcritical pressure boilers not only 
separate water from steam but also direct the flow of water and steam to 
establish an optimum distribution of fluids in the boiler during all loads 
of boiler operation. The internals may consist of baffles which change the 
direction of flow of a steam and water mixture, impellers and separators 
which use a spinning action for removing water from steam or moisture 
coalescers such as screen and corrugated plate final dryers. These devices 
are used singly or in consort to separate and purify the steam and remove 
impurities from the steam leaving the boiler drum. 
The space required to accommodate steam separating and purifying equipment 
determines drum size. Drum diameter and length should be sufficient to 
provide accessibility for installation and inspection and for processing 
the maximum flows of water and steam. Providing sufficient drum diameter 
and length to provide this accessibility while still maintaining a drum of 
a reasonable size is a significant challenge to the designer of drum 
internals. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a vapor 
separating apparatus which will afford efficient and effective separation 
and drying within a minimum space and with a relatively low pressure loss. 
More particularly, the present invention relates to a primary separator 
including means for imparting centrifugal motion to the mixture and 
skimmers to capture liquid thrown to the outside and direct the liquid 
downwardly and out the sides. The arrangement of skimmers provides a 
converging and diverging tortuous path to separate and coalesce liquid 
droplets. A discharge screen may be included to reduce the velocity of the 
discharged liquid, release vapor from the surface of the droplets and 
minimize disruption of the liquid pool. Also, a secondary separator may 
follow the primary separator. The invention is particularly applicable to 
the separation of steam from water and separating and purifying other 
vapors from two-phase mixtures such as liquid and gaseous hydrocarbons.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although the invention is applicable to the separation of various 
liquid-vapor mixtures as previously stated, the invention will be 
described with particular reference to steam drums and the separation of 
water and steam. 
Referring specifically to FIG. 1 of the drawings, the drum 12 is a 
conventional steam drum configuration with an elongated cylindrical shape 
and disposed with its axis parallel to the horizontal. The drum 12 is 
penetrated by riser pipes 14 which receive the steam/water mixture from 
the steam generator and discharge this mixture into the annular space 16 
between the drum liner or baffle 18 and the drum 12. Although the riser 
pipes 14 have been illustrated as being distributed rather uniformly 
around the annular space 16, the actual sections of the drum penetrated by 
the risers is a variable that depends on the drum operating pressure, the 
type of furnace circulation and the mass loading of steam and water into 
the drum. The baffle 18 is closed off at the bottom ends by the baffle 
portions 20 and the baffle includes the horizontal ledge portions 22. This 
baffle 18 including its portions 20 and 22 extends the full length of the 
drum thereby providing the enclosed annular space 16. 
Mounted on the baffle ledge portions 22 are a plurality of steam separating 
units 24 in two horizontally extending rows on either side of the axis of 
the drum. Although two rows have been illustrated, there may be more than 
two. Each row would contain as many separators as desired and would be 
dependent on the drum size and capacity. The steam separators 24 are 
mounted over apertures 25 in the baffle ledge portions 22 thereby 
directing the flow of the steam-water mixture from the pipes 14 into the 
annular space 16 and then up through the apertures 25 in the baffle ledge 
portions 22 and into the interior of the separators 24. 
Referring now to FIG. 2, the base of the separator is a cast steel support 
ring 26 with the circular spinner housing 28 welded on top. Contained 
within the housing 28 is a core 30 which is a cylindrical member having 
domed top and bottom ends. The bottom domed end has a hole for pressure 
equalization. Located in the annular space between the core 30 and the 
housing 28 are the spinner blades 32. These spinner blades 32 are welded 
to the core 30 and to the housing 28 to form a unitary spinner unit. The 
profiles and discharge angles of the spinner blades 32 can be optimized to 
enhance the centrifugal motion of the steam/water mixture. The profile and 
discharge angle of the blades depends on the composition and 
thermophysical properties of the liquid-vapor mixture being separated. 
FIG. 3 and 4 illustrate an alternate form of spinner that could be used in 
the separator. Although the details may vary for any specific situation, 
the shapes illustrated for the spinner housing and blade configuration are 
only by way of example and the invention is not limited to any specific 
dimensions or angles. The centrifugal motion imparted to the mixture 
causes liquid to be forced against the outer wall of the separator and the 
vapor to move to the center. 
Attached to the top of the spinner housing 28 is a conical diffuser section 
34 which has a larger diameter at the top than the bottom. This diffuser 
section reduces the momentum of the separated mixture as it travels upward 
because of the increased cross-sectional flow area. This loss of momentum 
further enhances the separation. The diameter and height of the diffuser 
section are optimized depending upon the mixture being separated and the 
separation of hydrocarbons or other vapor-liquid mixtures may require a 
different diameter and height. 
Located above the diffuser section 34 are a series of formed liquid 
extraction skimmers 36. These extraction skimmers are generally conical in 
shape with the major central portion of the upper surface being open to 
form the aperture 37 and the remaining extraction lips 38 to 46. These 
extraction lips 38 to 46 are in the form of annular inwardly extending 
portions which protrude into the two-phase flow and tend to trap the 
liquid droplets as they flow up along the inside surface of the separator. 
The size of these extraction lips and the amount that they intrude into 
the flow path increases in the direction of flow as seen in FIG. 2. This 
tends to spread the liquid extraction over the full height of the primary 
separator and assure that the maximum amount of liquid has been extracted. 
The number of extraction skimmers varies depending on the mass flow rate 
being processed, on the ratio of liquid and vapor present and on the 
thermophysical properties of the mixture being separated. The drawing 
shows six skimmers for illustrative purposes only. 
The liquid which is removed by each extraction lip then flow down and out 
between adjacent skimmers. The extraction skimmers are shaped as shown in 
FIG. 2 to form a converging-diverging path as well as a tortuous path for 
the extracted liquid-vapor mixture as it flows out between the adjacent 
skimmers. Each skimmer 36 has an outwardly protruding rim portion 48 
which, in cooperation with the underlying skimmer, forms an annular flow 
chamber 50. Extending into these annular flow chambers 50 are the annular 
ribs or rings 52 attached around the underlying skimmer. The rib 52 for 
the bottom skimmer is attached to the diffuser section 34. This rib 52 
thus forms a tortuous flow path for the liquid. Also, this shape provides 
a throat or narrow portion 54 between adjacent extraction skimmers. As the 
fluid flows towards this throat as indicated by the arrow 56, there is a 
convergence. Then, as the fluid flows down through the chamber 50 around 
the rib 52 and out the bottom opening 58, there is a divergence. The 
combination of the tortuous path and the convergence-divergence promotes 
contact and coalescence of any residual steam bubbles in the liquid. The 
coalesced bubbles are thus more likely to disengage from the liquid pool 
in the drum rather than be carried down with recirculated water. The 
converging-diverging tortuous path also reduces the velocity of the liquid 
exiting from between the skimmers which diminishes the steam carry under 
into the water pool in the bottom of the drum. Each extraction skimmer 36 
is supported by a plurality of support bars 59 spaced around and attached 
to each skimmer and supported on the bottom annular ring 61. 
Surrounding the extraction skimmers 36 is an optional water discharge 
screen 64. This discharge screen may be formed from a variety of materials 
such as woven wire mesh, unwoven wire mats or perforated plates. The 
discharge screen 64 is mounted between the brackets 66 spaced around the 
unit. The lower brackets are attached to the spinner housing 28 and the 
upper brackets are attached to the secondary separator to be described 
hereinafter. The use of the discharge screen is optional and will 
primarily depend upon whether or not there is sufficient space in the 
drum. 
The liquid droplets flowing downward and outward from the extraction 
skimmers 36 tend to be thrown against this water discharge screen 64. The 
discharge screen confines the water streams and further reduces their 
velocity. This minimizes disruption of the liquid pool in the bottom of 
the drum and reduces re-entrainment of vapor into the liquid water. 
Furthermore, it minimizes the entrainment of vapor in the liquid water 
since vapor tends to be released upon impact with the discharge screen. 
Attached on top of the uppermost extraction skimmer 36 is a cap or cover 60 
which has a central opening 62. This forms an annular barrier to the 
upward flow of water along the wall while providing a central opening for 
the upward flow of vapor. 
The vapor which has thus far been separated from the liquid flows up 
through the opening 62 in the cap or cover 60 and enters the secondary 
separator section 68. This secondary separator section has a perforated 
cylindrical container 70 and a round perforated cover 72. Stacked inside 
the container 70 are layers of crimped wire mesh 74 with each layer being 
oriented with respect to the adjacent layer such that the crimps are 
perpendicular as shown in FIG. 2. The bottom layers of wire mesh (two 
layers 76 and 78 are illustrated) are open in the center and "donut" 
shaped. This forms an open central area 80. This central vapor inlet area 
enhances the vapor flow distribution into the secondary separator. The 
residual liquid that is carried into the secondary separator is coalesced 
and deposited on the packed mesh and flows down through the mesh where it 
is discharged primarily from the bottom periphery of the secondary 
separator. The vapor flows up and out the perforations in the container 70 
and cover 72. 
The vapor (steam) coming from the tops of the separators 24 enters the 
vapor space around and above the separators and flows upward toward the 
steam outlet 82 shown in FIG. 1. Between the separators 24 and the steam 
outlet 82 is the final steam dryer generally designated as 84. Final steam 
dryers are conventional and any desired design may be used in conjunction 
with the separators of the present invention. Therefore, the specific 
details of the final steam dryer have not been described. 
Referring to FIGS. 3 and 4, a portion of a separator is illustrated with a 
modified spinner design. The base of the separator is a cast steel support 
ring 26 with the circular spinner housing 28 welded on top. This spinner 
housing has a narrow throat section 86 and an enlarged section 88. Mounted 
within the enlarged section over the throat section are the profiled 
radial spinner blades 90. The configuration of these spinner blades is 
shown in FIG. 4. The tops of the spinner blades are covered by the plate 
92. It can be seen that the liquid-vapor mixture that enters up through 
the throat section 86 will be forced radially outward with a spinning 
motion. This will produce a centrifugal motion as the mixture moves 
upwardly. This centrifugal motion causes the liquid to be forced out 
against and up the wall and the vapor to move to and up through the center 
of the separator. From that point, the separator is constructed and 
operates in the same way as the first embodiment. 
While certain embodiments of the present invention have been shown and 
described, these are intended to be by way of example and are not intended 
to limit the scope of the invention as claimed.