Method and apparatus for splitting two-phase flow at pipe tees

A method and apparatus for diverting into a branch line a portion of a gas-liquid flow stream from a two-phase flow trunk line. Each of two or more takeoff pipes withdraw a two-phase portion of the trunk line flow stream and transmit it to a single common branch line. The takeoff pipes are oriented to produce branch line flow of predetermined quality. The quality of branch line flow may be monitored and control valves positioned in one or more takeoff pipes and in the branch line to facilitate branch line flow of variable quality and rate.

FIELD OF THE INVENTION 
This invention relates to methods and apparatus for splitting the flow of 
two-phase fluid mixtures being transported in a pipeline. This invention 
more particularly relates to methods and apparatus for diverting 
steam-liquid water flow from a trunk line into a branch line. 
DESCRIPTION OF THE PRIOR ART 
Pipe tees are extensively employed in pipeline networks used to transport 
two-phase fluid mixtures. It has long been known that the quality (the 
ratio of the mass of a gas component to the total mass of both gas and 
liquid components) of a two-phase fluid usually changes during splitting 
of the flow at a pipe tee. In fact, the quality of the various downstream 
branch flow streams in such networks may often be drastically changed, 
causing operation in unintended flow regimes because of liquid buildup or 
causing certain branches to be bypassed or to become inoperative. 
A related problem has been observed in natural gas transmission networks. 
Frequently, liquid hydrocarbon condensates appear at some natural gas 
delivery stations while at other stations, the incoming natural gas 
remains relatively dry. Excessive condensate content can impair the 
operation of gas compressors in such networks, and a variety of complex 
schemes for completely separating gas from liquid have been proposed to 
solve this problem (see e.g., U.S. Pat. No. 4,160,652 (1979) to Martin, et 
al). The requirement of complete phase separation, for example, to allow 
separated liquids to bypass a gas compressor, mandates that such schemes 
be complex and cumbersome. 
These complex schemes are generally unsuitable solutions to the problem 
involved in diverting a portion of a flowing two-phase fluid without 
complete separation into gas and liquid components. Such flow-splitting 
problems have been particularly significant in oil field steam stimulation 
projects involving the transport of wet steam from a common generator for 
injection into two or more wells. In such projects, subterranean 
reservoirs of viscous hydrocarbons are heated by injection of a 
steam-liquid water mixture. The wet steam delivery networks employing 
traditional pipe tees often fail to distribute steam of sufficiently high 
quality to some of the wells served by a common steam generator. Injection 
of low quality steam or hot liquid water may be ineffective because 
sufficient heat cannot thereby be transferred to the reservoir. This is 
because liquid water has a heat content much less than that of gaseous 
steam at the same temperature. Thus, the large extra amount of latent heat 
released when steam is condensed in such a reservoir is unavailable when 
liquid water rather than steam is injected. 
Several factors affect the difference in quality between the flow upsteam 
and downsteam from a pipe tee. The most important of these factors include 
the velocity of flow, the angle between the incoming trunk line and the 
branch line, the orientation of the branch line relative to the horizontal 
plane, and the ratio of the volume of incoming flow to that of flow 
diverted into the branch line. 
The problems of maintaining approximately constant quality when two-phase 
flow is split at a pipe tee have been recognized, but the proposed 
solutions are too restrictive. Frequently, these prior solutions require 
fixed relative flow rates in combination with fixed relative orientation 
of main trunk line and branch line, for example, constraining the flow 
system to the horizontal plane. Thus, it has been suggested in Hong, K. C. 
"Two-Phase Flow Splitting at a Pipe Tee", Journal of Petroleum Technology, 
vol. 30, no. 2, Feb. 1978, pp. 290-296, that where all flow streams are 
horizontal in the vicinity of the tee, and where the incoming stream is 
perpendicular to both outgoing streams, and on the further condition that 
at least 15% of the flow is withdrawn in each downstream branch, the 
quality of flow in the upstream and both downstream branches will be 
roughly equal. 
Alternate proposed solutions for the case in which branch flow is diverted 
vertically upward from a horizontal trunk line have been suggested in 
Fouda, A. D. and Rhodes, E., "Two-Phase Annular Flow Stream Division in a 
Simple Tee", Transactions of the Institution of Chemical Engineers, vol. 
52, 1974, pp. 354-360. The authors there suggest the insertion of a 
vertical baffle in the bottom of the trunk line opposite the branch line, 
or the positioning of a homogenising orifice in the trunk line immediately 
upstream from the branch line. 
Problems still exist with all previously suggested approaches for splitting 
two-phase fluid flow, in particular for splitting wet steam flow. On one 
hand, complete phase separation techniques (such as those for diversion of 
liquid condensates around gas compressors in natural gas pipelines) cannot 
feasibly be applied in general because they are very complex, cumbersome, 
and expensive. On the other hand, the simpler prior art solutions that are 
appropriate for a limited class of wet steam splitting applications, are 
much too restrictive in terms of relative flow rates, main trunk line and 
branch line orientation, and ability to adjust branch flow quality. 
SUMMARY OF THE INVENTION 
The present invention is a method and apparatus capable of diverting 
two-phase flow of variable quality into a branch line, without a phase 
separation step, and independently of the relative orientation of trunk 
and branch lines. The method utilizes multiple takeoff pipes, each of 
which withdraws a two-phase portion of the incoming trunk line flow stream 
and delivers that portion to a single common branch line. In the preferred 
embodiment, only two takeoff pipes are employed. Each takeoff pipe is to 
be oriented substantially perpendicularly to the incoming trunk line flow 
stream at its point of intersection with the trunk line. The flow in at 
least one takeoff must be oriented generally downward and the flow in at 
least one other takeoff must be oriented generally upward. 
The quality of flow through a takeoff depends on its orientation relative 
to the horizontal plane. All else being equal, flow through a takeoff 
oriented vertically downward will always be of lower quality than flow 
through a takeoff oriented vertically upward. Thus, branch line flow of a 
wide range of quality may be obtained by orienting the takeoffs generally 
vertically and controlling the relative flow rate through each takeoff. By 
positioning a control valve to regulate flow rate in one or more takeoff 
pipes, the quality of branch line flow may readily be varied. 
The present invention provides a simple method and apparatus for diverting 
two-phase flow at variable quality into a branch line without a 
phase-separation step and without significant energy loss.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention utilizes to advantage the fact that steam of 
differing quality flows through a takeoff pipe depending on its 
orientation relative to the horizontal plane. This phenomenon occurs at 
pipeline branch points where a takeoff pipe extends radially outward from 
the axis of a trunk line (that is, substantially perpendicularly to the 
axis of the truck line). By practicing this invention, steam of a given 
quality flowing in a trunk line can be diverted into a branch line without 
significant energy loss and at a desired quality. The drawing illustrates 
a preferred embodiment of the invention in which two takeoff pipes, one 
oriented generally upward and the other generally downward, divert flow 
radially outward from a trunk line. The two takeoffs need not be oriented 
exactly vertically. In certain cases, where the truck line orientation 
substantially deviates from horizontal, the takeoffs should not be 
oriented exactly vertically. Each takeoff removes a portion of the steam 
having a different quality. The two takeoff streams are the combined to 
form a branch stream having the desired quality. While more than two 
takeoffs may be employed, it is contemplated that for ease of 
manufacturing, a double takeoff apparatus will be preferred for most 
applications. 
Referring to the drawing, the liquid-gas two-phase flow arrives at the 
branch point 9 in incoming trunk pipeline 10. A two-phase portion of the 
incoming trunk flow is withdrawn in each of the two takeoffs, takeoff pipe 
11 and takeoff pipe 12. The two takeoff flow streams are combined to 
produce a single outgoing flow stream in branch pipeline 17. A control 
valve 15 is positioned in branch line 17 for regulating the flow rate 
through branch pipeline 17. 
As illustrated schematically by the drawing, takeoff pipe 11 and takeoff 
pipe 12 are oriented substantially radially outward from trunk line 10 
near their respective intersections with trunk line 10. Takeoff pipe 11 is 
oriented generally upward and takeoff pipe 12 is oriented generally 
downward. It is preferred that takeoffs 11 and 12 be oriented vertically 
upward and downward, respectively, so far as is possible subject to the 
constraint that they both be substantially perpendicular to trunk line 10. 
Vertical diametrically opposed takeoff pipe orientation is preferred since 
it facilitates adjustment of branch flow quality over the widest possible 
range. Vertical takeoff orientation ensures that for all values of flow 
rate in takeoff pipe 12 the two-phase flow through takeoff pipe 12 will be 
of lower quality than incoming two-phase flow in trunk line 10 and for 
most values of flow rate in takeoff pipe 11, the two-phase flow through 
takeoff pipe 11 will be of higher quality than the incoming flow in truck 
line 10. When the two takeoff flow streams are combined to produce a 
single branch flow stream, the quality of that branch flow will be in the 
range defined by the average of the qualities of each takeoff flow stream. 
Thus, vertical takeoff orientation not only facilitates adjustment of 
branch flow quality over a wide range, given a fixed branch line flow 
rate, but permits flexible adjustment over the widest possible range of 
branch line flow rates. 
The relative flow rates of the takeoff streams can be preset by employing 
takeoff pipes having appropriate relative diameters. These relative 
diameters are estimated by considering that the desired branch line 
quality for a given branch line flow rate is the average of the known 
takeoff flow qualities weighted by the respective takeoff flow rates and 
that the relative flow rates through the takeoffs are proportional to the 
relative cross-sectional areas of the takeoffs. To permit adjustment of 
relative flow rates and takeoff flow qualities, control valves 13 and 14 
are inserted in takeoff pipe 11 and takeoff pipe 12 respectively. However, 
for many applications, only one control valve, in either takeoff pipe 11 
or 12, will provide sufficient relative flow rate and quality variability. 
Since the flow through branch line 17 is the sum of the two takeoff flow 
streams, adjustment of the relative flow rates through the takeoffs will 
vary the quality of the branch flow. 
It is preferred that the quality of branch flow will be continuously 
monitored by insertion of a suitable quality meter 16 in branch line 17. 
One such quality meter is discussed in pending application Ser. No. 
435,817 by Mohammed A. Aggour and Sanjoy Banerjee entitled "Apparatus and 
Method for Determining the Hydrogen Content of a Substance". Means for 
automatically adjusting control valve 13 and control valve 14 so as to 
produce branch line flow of preselected quality may optionally be 
included. Such means, triggered by the output of quality meter 16, may 
include microprocessor-controlled electronic circuitry which comprises 
means for detecting the output of quality meter 16, comparing that output 
with a predetermined reference value representing the desired branch line 
flow quality, and applying a signal when the quality meter output differs 
from the predetermined reference value to activate the flow rate 
adjustment mechanism of one or both of control valves 13 and 14. 
EXAMPLES 
The present invention was tested on a pipeline network embodying the method 
and apparatus thereof. An air-water mixture was diverted from a horizontal 
trunk line into a horizontal branch line having an inner diameter of 2" by 
means of two vertical, diametrically opposed takeoff pipes having relative 
inner diameters in the range 1/2" to 1". In the annular flow regime, 
branch flow having equal quality to that of the trunk flow stream was 
achieved by employing an upward-oriented takeoff 1" in diameter and a 
downward-oriented takeoff 1/2" in diameter. In the slug flow regime, 
branch flow having equal quality to that of the trunk flow was achieved 
using upward and downward takeoffs having diameters 1" and 3/4" 
respectively. 
These results have been extrapolated to oil field steam stimulation project 
conditions. There, typically, wet steam having quality in the range 50% to 
90% is transported through a trunk line at a pressure of 2000 psi and a 
temperature of 636.degree. F. Where horizontal trunk line and branch line 
inner diameters are 16" and 8" respectively, and vertical, diametrically 
opposed takeoffs are employed, identical branch flow and trunk flow 
quality can be achieved using upward and downward takeoff diameters of 8" 
and 3 1/4" respectively. Under similar conditions, such equal quality flow 
splitting can be achieved where trunk line diameter, branch line diameter, 
upward takeoff diameter, and downward takeoff diameters are respectively 
12", 6", 6", and 2 1/2" or 10", 5", 5", 2". To permit adjustment of 
relative takeoff flow rates in oil field steam stimulation applications, a 
high pressure control valve should be positioned in at least one takeoff. 
A suitable control valve may be selected from those models commercially 
available. For example, Gray Tool Company Model D Graygate.RTM. valves 
would be suitable for many oil field applications. 
Thus, the present invention provides a method and apparatus for diverting a 
two-phase fluid flow stream from a trunk line to a branch line, without 
substantial energy loss and with a desired quality. The invention is 
capable of achieving such flow diversion without a phase separation step, 
and is independent of the relative orientation of trunk and branch lines. 
Various modifications and alterations in the practice of this invention 
will be apparent to those skilled in the art without departing from the 
scope and spirit of this invention. Although the invention was described 
in connection with a specific preferred embodiment, it should be 
understood that the invention as claimed should not be unduly limited to 
such specific embodiment.