Flow and Pressure Reduction of Gas Pipeline: How to prevent freezing


Flow and Pressure Reduction of Gas Pipeline: How to prevent freezing
1.    Due to the Joule-Thomson effect; whenever gas pressure is reduced, its temperature is also reduced. This may result in freezing of the pipeline or formation of ice in the pipeline. The potential for freezing can be seen by calculating the temperature drop and subtract from initial temperature.
We can use the formula below to calculate the final temperature if we reduce the pressure. From our calculation, if we get a Final temperature below 32˚F, then there is possibility of formation of ice due to moisture within the gas stream, hence we have to stop reducing the pressure of the gas.
However, if from our calculation we get a Final temperature above 32˚F, then we can go ahead with reducing the pressure.
2.    Preheating the natural gas before pressure reduction, we can decrease the Joule Thomson effect. Natural gas is heated in a heat exchanger and then goes through a JT valve where the gas undergoes the pressure and the temperature drop. Line heaters can be used to heat the entire gas stream.  These heaters use boilers to heat a water/glycol mixture, which is circulated through a heat exchanger in the main gas line.  However, there are two important things to consider with such a setup: fuel efficiency and noise generation.
3.    Injection of glycol
To deal with it, hydrates inhibitors (e.g. methanol or monoethylene glycol) can be injected upstream the valve.
4.    minimization of the pressure drop across the valve is the other consideration
5.    Another method, using desiccant, has the ability to produce even drier gas than glycol absorption. This process passes the gas stream through a tower filled with desiccant.  Water vapor clings to the desiccant, exiting with little or no moisture remaining.  This process works well until the desiccant is fully saturated at which point it must be dried by passing hot gas through the tower to dry the medium.  In order to maintain continuous operation, two or more towers must be utilized to maintain a flow of dry gas, one in operation while the other/s regenerate.  This system is better at removing moisture, but it is more expensive to install and operate than glycol systems.
6.    Another method for reducing the occurrence of freezing in a pressure reducing station is to introduce an antifreeze solution.  In this system, antifreeze in a pressurized reservoir is dripped into the process piping by means of a Hex needle valve.
Another similar solution employs the use of another small pressure regulator located between the antifreeze reservoir and the process piping.   The small regulator, with a control line connected downstream of the main valve, is set at a lower pressure than the outlet pressure of the main regulator.  If the pressure downstream of the main regulator is normal, the small regulator will remain closed.  However, if freezing begins to block the piping, the downstream pressure and the control line pressure will begin to fall.  When the pressure falls below the set point of the small regulator it will open and allow antifreeze to enter the piping system.  When normal pressure is restored, the small regulator will close, conserving the antifreeze until it is needed again.

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