Method and apparatus for eliminating unequal phase splitting at piping junctions

The separation of wet steam into its liquid and vapor phases, prior to encountering a pipe junction, is facilitated. The liquid phase is then recombined by aspiration to effectively behave as a single phase vapor of homogeneous density at the point where the flow stream is divided by the junction.

BACKGROUND OF THE INVENTION 
1. The Field of the Invention 
The present invention relates to a method and apparatus for substantially 
eliminating unequal phase splitting of wet steam at piping junctions and, 
in particular, to a system which separates the liquid and vapor phases 
upstream of the junction and then recombines them at the inlet of a piping 
junction. 
2. The Prior Art 
There is a need for a simple method and apparatus to control phase 
splitting which occurs at piping junctions in wet steam distribution 
systems, particularly at impacting T-junctions. Such an apparatus, if 
simplified, would be particularly useful in controlling steam quality and 
thereby the amount of heat which is available since more heat is 
transmitted by the vapor phase of the steam than by the liquid phase. 
Generally, as pressurized wet steam flow flows through a piping system, 
there is a tendency for the steam to separate into its vapor and liquid 
phases. The separation occurs with the heavier and slower liquid phase 
becoming annular and adhering to the piping walls while the lighter and 
faster gaseous phase moves axially through the piping. This results in 
steam of unequal quality coming off, for example, the arms of an impacting 
T-junction. 
It is important, therefore, as a matter of economic practicality that a 
means be instituted in the steam pipeline to prevent unwanted phase 
separation and promote homogeneity of the steam, particularly where it 
comes into and out of piping junctions. 
Phase splitting is a phenomena of two phase vapor (or gas) and liquid flow 
that occurs at all piping junctions such as impact T's, branch T's, Y's, 
crosses, manifolds, etc. In standard junctions the liquid and vapor phases 
do not diverge in equal mass proportions except in junctions with 
symmetrical flow, such as in impact T's where the vapor mass rates are 
equal in each junction outlet (a vapor extraction ratio of 0.5). This is 
important, for example, in steamflood distribution systems, used for 
enhanced oil recovery, where it is desirable to deliver nearly equal steam 
quality throughout the entire distribution system. Steam quality is a 
measure of the proportion of the total mass that is vapor. The vapor 
extraction ratio is defined below. 
Vapor Extraction Ratio is, 
##EQU1## 
where, F.sub.g3 =Vapor Extraction Ratio 
M.sub.v1 =Inlet mass rate of the vapor phase 
M.sub.v2 =Outlet branch 2 mass rate of vapor phase 
M.sub.v3 =Outlet branch 3 mass rate of vapor phase 
Numerous studies investigating phase splitting have been conducted and 
various devices to equalize or control phase splitting have been tried. 
However, only a few of these ideas have been implemented in the design of 
new steam distribution systems and none have become standard practice 
throughout the industry. Still fewer of these methods are commonly 
encountered as "fixes", to minimize or control phase splitting, in 
distribution systems which were built before phase splitting was widely 
understood. The method disclosed here meets the criteria required of a 
"fix" in that it requires no operator action, creates minimal pressure 
drop, and is both inexpensive and effective. 
An example of where the present invention would be particularly useful is 
secondary recovery of hydrocarbons from marginal fields or heavy oil 
reserves that require a degree of stimulation to achieve satisfactory flow 
of crude petroleum. In such operations steam is sent through a patterned 
array of injection wells to heat the formation being treated and drive the 
hydrocarbons towards a production well. The steam quality will directly 
affect the formation heatup effect and thus the efficiency of the recovery 
operation. The vapor phase of the steam will have the most heat and 
therefore have the greatest effect on the formation. Thus it is desirable 
to have steam of uniform quality injected into all portions of the 
formation. 
SUMMARY OF THE INVENTION 
The present invention substantially eliminates unequal phase splitting at 
piping junctions. The present invention provides means to facilitate 
separation of the liquid and vapor phases and then acts as a mixer 
creating a homogeneous mixture as the liquid phase is aspirated and 
dispersed into the vapor stream. The present invention introduces very 
little additional pressure drop into the system.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
The invention 10 has a phase separator 12, here shown as a large diameter 
section of pipe sufficient to allow separation of the liquid and vapor 
phases, connected between a flow pipe 14 and outlet pipe 16 leading to an 
inlet arm 18 of a horizontal impact T junction 20. The outlet pipe 16 
contains a constriction 22, such as an orifice plate, fixedly mounted 
upstream of the junction 20. The phase separator 12 contains a sump 24 
with a small diameter pipe 26, such as 1/2" tubing, connected between an 
outlet 28 on sump 24 to an inlet 30 in the outlet pipe 16 immediately 
downstream of the constriction 22. 
The phase separator 12 serves to promote the complete separation of the 
liquid and vapor components of the wet steam by substantially reducing the 
velocity of the steam and allowing gravity to cause the actual separation 
of the two phases. The vapor phase moves axially through the large 
diameter section exiting through outlet pipe 16 and constriction 22, which 
is placed in close proximity to, but upstream of, the inlet 18 of the 
horizontal junction 20. The Figures depict an impact T junction, though 
other types of junction are possible, provided they are laterally 
symmetrical and offer no preferential path for the steam to follow (no 
"path of least resistance" as it were). The liquid phase of the separated 
steam leaves the large diameter portion of separator 12 dropping 
vertically downward, under the influence of gravity, to sump 24 and 
subsequently flows through small diameter pipe 26 to the inlet 28 where 
the small diameter pipe 26 passes through the wall of the outlet pipe 18 
immediately downstream of the flow constriction 22. The flow of high 
velocity steam vapor through the constriction 22 causes a low pressure 
region to occur immediately downstream of the constriction 22 (vena 
contracta) at the same point where the small diameter pipe 26 enters the 
outlet flow pipe 16. The existence of this low pressure region forces the 
liquid phase to be drawn forcibly into the high velocity vapor stream 
causing the liquid to form tiny droplets which become entrained in the 
flowing vapor. The liquid droplets are then carried with the vapor as it 
enters the horizontal impact T junction 20. Because the tiny droplets are 
finely dispersed in the vapor stream, the two phase steam behaves as a 
single phase fluid of homogeneous density. The "fog" or "spray" flow 
stream is then divided into two separate flow streams by the T junction 
20. The two resulting steam streams, because the liquid and vapor phases 
are thoroughly mixed, leave the arms 32, 34 of T junction 20 at 
substantially equal steam qualities. 
The shape and location of the end of the small diameter pipe 26 has been 
found to be of importance in optimizing the performance of the present 
invention. Experimentation has shown that the most effective location for 
the small pipe 26 is to have its open end located substantially aligned 
with the center of the constriction 22. Further, the performance of the 
device has been found to be best if the shape of the open end of the pipe 
26 conforms to the diagram in FIG. 3. 
The present invention substantially eliminates the effect of phase 
splitting at impact T piping junctions with minimal pressure loss to the 
steam. The invention can also be applied to junctions with more than 2 
outlets, provided that the multiple outlets are configured in such a way 
that there exists no "preferential path" for the steam to exit the 
junction. The invention performs this function by first separating the 
steam into its liquid and vapor components and conducting each component 
separately to a location immediately upstream of an impact T or other 
laterally symmetrical junction where they are recombined in such a fashion 
to cause substantial mixing of the two components to occur. The remixed 
steam then behaves as a single phase vapor of homogeneous density at the 
point where the flow stream is divided into the exit branches of the 
junction. The mixing of the two components is accomplished through the 
application of the same principle as that used to cause the atomization of 
liquids into a spray in a perfume atomizer, namely, aspiration. 
The shape and location of the ends of the small diameter pipes 32, 34 has 
been found to be of importance in optimizing the performance of the 
present invention. Experimentation has shown that the most effective 
location for the ends of the pipes is substantially aligned with the axis 
of the respective constriction 22, 24. Further, the performance of the 
device has been found to be best if the shape of the open ends of the 
pipes 32, 34 is as shown in FIG. 3. 
The square root of the pressure drop created by the constriction is 
directly proportional to mass rate of vapor. The mass flow rate of liquid 
entering each outlet branch through the liquid bypass lines is also a 
function of the square root of the pressure drop created by the flow 
constrictions. Thus the flow rate of liquid entering each outlet branch is 
directly proportional to the vapor flow rate in each branch. The result is 
a system which is self compensating with respect to changes in the vapor 
extraction ratio. 
Any means of separating the liquid and vapor phases upstream of the 
junction can be used. However large diameter piping, which reduces the 
velocity sufficiently to achieve stratified flow, can be used effectively, 
at low cost, without the requirements of a coded pressure vessel. Once 
separated the liquid phase is directed through relatively small diameter 
junction bypass piping to each junction outlet downstream of the outlet 
flow constriction. 
The vapor phase flows predominately axially through the piping junction. 
The present invention is not limited to impact or branch T junctions but 
could also be used with a manifold having any number of outlets, as long 
as a junction bypass and a flow constriction is provided for each outlet. 
Since only single phase vapor passes through the junction, phase splitting 
cannot occur within the junction. 
The sizing of the separator, the vapor flow constriction, and the junction 
bypass line define the range of flow conditions over which the device 
works effectively. The sizing of the vapor flow constriction relative to 
the diameter of the junction bypass pipe must allow the vapor phase to 
flow predominately through the junction while permitting all of the liquid 
to flow through the junction bypass. Undersizing the bypass pipe would 
cause the separator to fill with liquid while oversizing could result in 
significant vapor liquid phase splitting in the sump. To overcome this 
limitation, a level control could be used to insure that only the liquid 
phase flows in the junction bypass piping and thus eliminating the 
potential for oversizing the bypass. However, excellent performance has 
been obtained without resorting to level control or means other than 
sizing to limit vapor flow in the junction bypass. 
The present invention may be subject to many modifications and changes, 
which will be apparent to one skilled in the art, without departing from 
the spirit or essential characteristics thereof. The present embodiment is 
therefore to be considered in all respects as illustrative and not 
restrictive of the scope of the invention as defined by the appended 
claims.