184
W. Li et al.
Figure 2 shows the change of theoretical and actual head difference with time during
the actual production process, and the difference is proportional to the gas content, so it
can represent the gas content and gas resistance of the pipeline. It can be seen from the
figure that the head difference of each station changes periodically with time, indicating
that the gas migrates by the way of accumulation, discharge, re-accumulation and redischarge. In the continuous undulating pipeline, due to the influence of topography,
gas is likely to exist in both the up-slope and down-slope sections of the pipeline, and
the presence of gas also greatly affects the flow of liquid. According to the momentum
equation, when the section V present (or ), liquid flow will change, at the same time
because of the existence of gas inside the liquid energy loss cannot be recycled, lead to
the section of the pressure loss is bigger.
3 Impact Analysis of Oil Mixing During Commissioning
The mixing process of oil replacement water in large drop pipeline is similar to that of
sequential conveying process. Due to the difference in density and viscosity between
water and oil, as well as the influence of unstable operation and height difference, the
actual oil mixing amount will be more than the theoretical calculation amount. At the
same time, the mixture of oil and water is likely to increase due to multiple shutdowns
during the production process [6].
For large drop pipelines, the mixing of oil in the height difference, the stopping of
oil transmission and the oil-water emulsification phenomenon after the shearing of the
pressure reducing valve in the large drop pipelines should be fully considered. According
to the emulsification theory, after the oil/water is vigorously stirred, a part of the work
done by the external force is converted into the interface energy of the system, which
increases the contact interface area, and the emulsion is formed by the dispersion of
one kind of liquid in another kind of liquid with very small particles. According to the
second law of thermodynamics, under constant temperature and pressure, the system of
matter has the tendency to reduce free energy automatically. When oil/water emulsion
is formed, its contact interface and interface energy are both large. From the perspective
of thermodynamics, the emulsion is an unstable system. After the agitation stops, the
internal phase particles collide and merge due to Brownian motion, and soon the oil
and water are layered. Emulsifiers (surfactants) are needed to form a stable oil/water
emulsion. Under normal circumstances, the oil-water displacement process of pipeline,
won’t appear strong oil-water emulsion phenomenon [7]. But the big gap between the
pipe may be after a vigorous stirring, ups and downs along the section of shear diffusion,
pressure reducing valve, and may have some dirt in pipeline (natural emulsifier), make
the oil and water form a stable emulsion, and by gravity settling difficult demulsification,
must be used under the conditions of demulsifier dehydration. During Commissioning,
the pipe with a part of the gas can’t discharge, with oil head moves downstream, the
emulsion will replace the water and the gas contact, friction increases rapidly, so the
alternate with bubble breakage rate, liquid and gas capacity increase, gas rate increases,
along which resulted in increased section of additional pressure loss [8] (Fig. 3).
W. Li et al.
Figure 2 shows the change of theoretical and actual head difference with time during
the actual production process, and the difference is proportional to the gas content, so it
can represent the gas content and gas resistance of the pipeline. It can be seen from the
figure that the head difference of each station changes periodically with time, indicating
that the gas migrates by the way of accumulation, discharge, re-accumulation and redischarge. In the continuous undulating pipeline, due to the influence of topography,
gas is likely to exist in both the up-slope and down-slope sections of the pipeline, and
the presence of gas also greatly affects the flow of liquid. According to the momentum
equation, when the section V present (or ), liquid flow will change, at the same time
because of the existence of gas inside the liquid energy loss cannot be recycled, lead to
the section of the pressure loss is bigger.
3 Impact Analysis of Oil Mixing During Commissioning
The mixing process of oil replacement water in large drop pipeline is similar to that of
sequential conveying process. Due to the difference in density and viscosity between
water and oil, as well as the influence of unstable operation and height difference, the
actual oil mixing amount will be more than the theoretical calculation amount. At the
same time, the mixture of oil and water is likely to increase due to multiple shutdowns
during the production process [6].
For large drop pipelines, the mixing of oil in the height difference, the stopping of
oil transmission and the oil-water emulsification phenomenon after the shearing of the
pressure reducing valve in the large drop pipelines should be fully considered. According
to the emulsification theory, after the oil/water is vigorously stirred, a part of the work
done by the external force is converted into the interface energy of the system, which
increases the contact interface area, and the emulsion is formed by the dispersion of
one kind of liquid in another kind of liquid with very small particles. According to the
second law of thermodynamics, under constant temperature and pressure, the system of
matter has the tendency to reduce free energy automatically. When oil/water emulsion
is formed, its contact interface and interface energy are both large. From the perspective
of thermodynamics, the emulsion is an unstable system. After the agitation stops, the
internal phase particles collide and merge due to Brownian motion, and soon the oil
and water are layered. Emulsifiers (surfactants) are needed to form a stable oil/water
emulsion. Under normal circumstances, the oil-water displacement process of pipeline,
won’t appear strong oil-water emulsion phenomenon [7]. But the big gap between the
pipe may be after a vigorous stirring, ups and downs along the section of shear diffusion,
pressure reducing valve, and may have some dirt in pipeline (natural emulsifier), make
the oil and water form a stable emulsion, and by gravity settling difficult demulsification,
must be used under the conditions of demulsifier dehydration. During Commissioning,
the pipe with a part of the gas can’t discharge, with oil head moves downstream, the
emulsion will replace the water and the gas contact, friction increases rapidly, so the
alternate with bubble breakage rate, liquid and gas capacity increase, gas rate increases,
along which resulted in increased section of additional pressure loss [8] (Fig. 3).