Chemical & Process Technology

Earlier post "Mal-Distribution of Phases at T-Junction Phenomenon" and "Stagnant Liquid In Inclined Parallel Pipe Downstream of T-Junction" have shown two phase flow mal-distribution and stagnant liquid phenomenon at T-junction or splitting tee.

The flow distribution from a manifold to parallel channels is becoming of interest in predicting the heat transfer performance of heat exchangers. As discussed, due to maldistribution of two phase flow at T-junction, flow rates through the channels to each heat exchanger are not uniform. In the extreme case, there is almost no flow through some of them (i.e. stagnant liquid phenomenon). As of today, there is still no general way to predict the distribution of two-phase mixtures at header–channel junctions (T-junction).

Simulation studies by Bernoux et al. (2001) with two-phase distribution at the inlet manifold of compact heat exchangers, results showed that the vapor distribution to the channels became uniform with the increase of the mass quality (X_v), but the liquid distribution still remained unbalanced. Besides, the liquid distribution through the channels was not much sensitive to the mass flux (M_Flux).

Flow behavior studies (by Osakabe et al, 1999) in a horizontal square header (with each side being 40 mm) connected to four parallel vertical tubes (10 mm in diameter) for the bubbly and slug flows at the inlet, largest amount of the liquid flow into the first tube for a low mass quality (X_v) flow in the header; however, the tendency of mal-distribution is reduced with the increase of the mass quality (X_v) inside the header.

Flow behavior at one junction has no influence on that at the next junction. On the other hand, for the most of the compact heat exchangers, the distance between the channels is comparable to or even smaller than the header size (hydraulic diameter), flow interaction between the junctions has to be taken into account.

Infact, behavior of flow separation in small T-junctions appeared different from that in the large T-junctions. There are strong interaction between each T-junctions. The flow behavior at one T-junction is strongly influenced by other T-junction especially when the distance between the T-junction is equivalent to or smaller than the hydraulic diameter of the header (as reported by Lee & Lee, 2004).


Further investigation on two-phase flow behavior at the upward header co-current flow and horizontal rectangular parallel channels and simulating the corresponding parts of compact heat exchangers shown that intrusion depth of the channels into header affects flow distribution of the liquid-phase. Less amount of liquid was separated out through the channels at the rear part except near the end-partition with the zero intrusion depth, Reversed trend observed for deeper intrusion. This indicated that a uniform distribution could be obtained by adjusting the intrusion depth. Deeper intrusion channel promote mixing effect and hence the uniform distribution to each outlet.

Above shown that mal-distribution of two phase flow at T-junctions affects by many factors i.e. vapor mass quality, flow pattern, distance of T-junction, size of T-junction and header size, penetration of T-junction into header, etc. Infact, above only several factors affecting mal-distribution, there are many more factors affecting it. In a compact heat exchanger i.e. Plate Fin Heat Exchanger (PFHE), Brazed Aluminum Heat Exchanger (BAHX), gas & liquid fraction flow from header to each channel may be different from tube to tube. This could seriously affects the heat transfer performance of the heat exchanger. Not only that performance varies at each channel would lead to change in temperature profile, and hence thermal stress profile of heat exchanger which increase the fatigue cracking tendency and life span of the heat exchanger.


Related Topics

• Stagnant Liquid In Inclined Parallel Pipe Downstream of T-Junction
• Mal-Distribution of Phases at T-Junction Phenomenon
• Problems Caused by Two Phase Gas-Liquid Flow
• Slugging & Slugcatcher
• Liquid Slug Stabiliser - Another type of slugcatcher 
• Assess Potential Piping Failure Due to Valve Quick Opening with Two-Phase Vapor Liquid 
• How Fluid Characteristic affect 2 phase Relief via PSV on Liquid filled Vessel Exposing to External Fire

This has provided some insight and idea to the designer and operator, there is a minimum flow for two phase flow in parallel pipes. Under turndown operation, there is possible flow in single pipe while liquid column stagnant in the other pipe. Whenever increase production, it shall be gradually increase to avoid large liquid volume feed to the downstream equipment and pipe failure due to slugging flow in downstream pipe.

Another potential issue is present of heavy sand in the two phase flow. Heavy sand will tend to stays and accumulates in the stagnant liquid and potentially partially block the pipes. Therefore, this stagnant liquid phenomenon should be checked and taken care during design and operation phase.

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.

Related Topics

• Problems Caused by Two Phase Gas-Liquid Flow
• Assess Potential Piping Failure Due to Valve Quick Opening with Two-Phase Vapor Liquid 
• How Fluid Characteristic affect 2 phase Relief via PSV on Liquid filled Vessel Exposing to External Fire 
• Facts about Erosion & Erosion-Corrosion 
• Erosion & Erosion - Corrosion 
• Several Criteria and Constraints for Flare Network - Process

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Floating Accommodation Unit (FAU)

Floating Gas Turbine Generator Unit (FGTG)