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Chemical Process Technology

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Sunday, February 28, 2010

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Previous post "Problems Caused by Two Phase Gas-Liquid Flow" has discussed about obvious and non-obvious location where two phase gas liquid can present. Problem caused by two phase gas liquid flow such as Impingement erosion, Splashing erosion, Cavitation erosion, Flashing erosion, Flow induced vibration, Surge / hammer, Noise, Decrease process performance and Phase separation possibly lead to severe consequence. Minimizing / prevention of two phase gas liquid becoming important during design phase.

Prevention Principle
In minimizing / prevention of  two phase gas liquid flow, one may follow below principles :
  • Avoid two phase gas liquid present in process design / simulation
  • Promote phase separation once two phase is present
  • Avoid destructive flow pattern e.g. Slugging flow
  • Proper piping arrangement
  • Improvement of mechanical integrity

    Details Measures
    ELIMINATE - Avoid two phase gas liquid present in process design / simulation
    Process engineer may take extra effort to avoid present of two phase gas liquid during process design. Several methods such as locate level control valve downstream of filter coalescing separator so that saturated fluid is always in liquid form before it is entering filter coalescing separator, provision of flash drum prior to flash off hydrocarbon vapor before feed the solvent to plate frame heat exchanger, provision of condensate flash drum for recovered steam condensate before it is return back to boiler, etc. These are the measures can be considered by process engineer during design phase to avoid possibility of two phase flow in the system.



    SEPARATION - Promote phase separation once two phase is present
    Completely avoid present of two phase flow may not be possible. Saturated liquid flowing pipe, pressure drop (and ambient heating) along pipe may lead to liquid vaporization. Saturated vapor flowing in pipe, pressure drop (and ambient cooling) along pipe may results condensation. Mixing of hot saturated fluid with cool fluid can shift the equilibrium and results two phase vapor liquid flow after the mixing. Thus, avoidance of two phase gas liquid flow entirely may not be possible.Once it is present, one way is to remove of the phase from the other phase. Following measures may be considered :

    Vapor flow with condensation
    • Superheat vapor before it is transferred
    • Provide insulation to minimize heat loss to ambient
    • Provide heat tracing to compensate heat loss to ambient and avoid condensation
    • Provide liquid trap along the pipe to remove liquid from vapor
    • Minimize pipe length to minimize frictional loss
    • Use low surface roughness material (e.g. Stainless steel) for transferring fluid
    • Provision of intermediate vessel to remove liquid
    Liquid flow with vaporization
    • Subcooled liquid by cooling or pressurize prior to transferred
    • Provide insulation to minimize heat input from ambient
    • Provide vapor trap along the pipe to remove vapor from liquid
    • Minimize pipe length to minimize frictional loss
    • Use low surface roughness material (e.g. Stainless steel) for transferring fluid
    • Provision of intermediate vessel to remove vapor
    AVOIDANCE - Avoid Destructive Flow Pattern
    The destructive level of two phase gas liquid flow varies with flow pattern. One shall try to design their system to avoid Slugging flow and/or plugging as slugging/plugging can lead severe vibration and erosion. The destructive level reduce from slugging flow to stratified flow, and minimum at annular, mist and bubble flow. So a process engineer shall design their system to move away from slugging flow. Proper pipe size selection and operation control may be considered to avoid destructive flow pattern.

    DESIGN - Proper Piping Arrangement
    Good piping arrangement may be considered to minimize the impact of two phase gas liquid flow.
    • Avoid / Minimize pocketed line
    • Avoid / Minimize vertical lift
    • Provide low point drains / traps
    • Slope away liquid from source
    • Design piping to avoid liquid accumulation and promote auto-draining
    STRENGTH - Improvement of mechanical integrity
    Once all above measures are implemented and present of two phase gas liquid flow including slugging flow, the mechanical integrity of the piping and support system shall be improved :
    • Use high and reasonable tensile strength material for piping and/or equipment
    • Provision of piping support improved mechanical strength
    • Increase design margin to minimize any uncertainties in design
    • Conduct transient analysis and/or CFD to identify localize high stress area and strengthen weak point


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