Monday, December 12, 2011
Multiphase flow, primarily gas-liquid flow, exists in chemical, power, oil/gas production and refining plants. Multiphase flow is very complex phenomena. Most application has considered that fluid split at T-junction, all phases will be evenly split between the run and branch. In reality, maldistribution of phase occurred at T-junction. Each phase has their preference route. This maldistribution phenomenon has been experienced in many industrial applications.
Mal-Distribution of Phases at T-Junction Phenomenon
In the Oil and gas, refinery, petrochemical and chemical plant, pipes has been widely used to transfer product from equipment to equipment for further processing. Starting from offshore platform, oil & gas produce from reservoir via wellheads, partially stabilized in production separators (sometime produced water knocked-out in the production separator and further re-inject back to reservoir or treated and disposed locally), separated gas and condensate/oil will be transport to onshore via separate long pipelines. Along the pipeline, external cooling by ambient and seawater couple with pressure drop in the pipeline, condensation will occur in some places in the pipeline and two phase flow initiated. Gas with condensate arrived onshore will be dumped into a multi-fingers slugcatcher. Impacting Tee will be used to split the flow between the fingers. Gas-condensate is then separated in the slugcatcher via slight-inclined horizontal pipe with vertical Tee. At the impacting Tee, maldistribution of gas & condensate between the branches are observed in reality. These ended-up some fingers are over capacity and some under capacity.
Production from several wellhead platforms will be send to central processing platform (CEP) for partial separation and stabilization via long subsea pipeline. Due to geographical arrangement of wellhead platforms and well develop at different phases, those pipelines could be mixed at topside or subsea using Tee.
Gaslift used to enhance oil productivity will be supplied from central processing platform via long gaslift pipeline. The gaslift pipeline will be delivered from one platform to another platform. Tee will be used to split the gas flow. In order to avoid condensation, generally the gaslift dew point will be depressed to avoid condensation along the gaslift pipeline. Inefficient performance and mal-operation of topside gaslift dew point control will result saturated gas feed into the gaslift pipeline. Similar to above gas pipeline, condensation occurred in the long pipeline due to external cooling and pressure drop and affect the proper split.
Oil production system producing high viscosity oil, steam will be injected to enhance oil recovery. Multiple steam injection is implemented to ensure proper distribution of steam and increase the oil recovery efficiency. Steam supply from main header will be distributed to all injection points via tees. In some cases, steam is supplied from central utility production unit which is far away from the users, cooling by external (imperfect insulation) and pressure drop along header will lead to two phase flow. Maldistribution of steam-condensate at each split will result oil recovery performance dropped.
The LPG or natural gas will be supplied to users such as factory, household, etc. A lot of pipeline and tees will be used for transfer and splitting the flow. Similar to above gas pipeline, condensation could occur in the pipeline and distribution network, malditribution at the splitting tee and could result some users received large amount of condensate.
In the chemical plant, there are two phase flow gas (with low liquid loading) feeding to condenser. In order to increase operability and turndown, multiple condensers will be installed. When the plant is operate under partial capacity, some condensers will be put in operation. Maldistribution occurred at splitting tee result some condenser over capacity (fed with high liquid loading) and the some condenser under capacity (fed with low liquid loading).
Phase maldistribution has been reported from offshore platforms in the UK North Sea. Two main (phase) vessel separators has been installed in parallel in order to enable production to continue albeit at a reduced level if there was a need for maintenance or modification of a separator. To ensure an even split of the phases, an impacting T-junction was employed. When the system was started up it was found that one separator received most of the gas whilst the other got most of the liquid. Inspection of the pipework upstream of the junction showed that bend located upstream result centrifuging of the phases and presenting each outlet with substantially one phase
There are others phase maldistribution observed on T-junction. For example, phase separation in main coolant piping of light water nuclear reactor (LWR) can play significant role in the effectiveness of emergency core cooling (ECC) system during analysis of Loss-of-coolant-accident (LOCA).
Therefore, in handling vapor near condensation point or two phase flow, phase separation is one of the phenomenon shall be analyzed to minimize mal-distribution.
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