Vapour pressure drop

[Ravind5]

Dear All, Greetings,
I am facing a challenge with a pipeline system that is experiencing pressure drop at a certain point.I am looking for suggestions on how to improve the flow and reduce this pressure drop. As an engineer,I am interested in knowing the best practices and methods that can be used to solve this issue.

We are receiving LPG MIX from A point (2.5KM away) through straight 8" Liquid pipeline and 6" vapor pipeline and meets at mid point controlled by MOV's and goes through a few bends, After the midpoint, the flow goes through a another 1.9KM long pipeline to point B spheres. After completion of liquid, we need to clear the liquid line of vapor pushing and this is where the problem starts as vapor pressure doesn't have a strong flow to push the liquids from the midpoint to the spheres. This is due to compressor used at point A was not designed for vapor pushing.

Please share your thoughts and experiences in this matter, as I would greatly appreciate any insights or recommendations on the best course of action to take.

Thank you

 

[1503-44]

Define your LPG mix. A combination of X% butane and Y% Propane. What range of percentages? Can you receive?

Can you post a PFD of the system please. Show MOVs, one-way check valves, and all control valves.
What exactly is entering each pipeline at the inlets? at what pressure ranges and what flow rates.
What mixture is going into the spheres and at what pressure range?
Temperature along the pipeline is important.
What minimum and maximum temperatures do you have at the pipeline entry points and at the sphere inlets?
Also need to know the pipeline elevations. A profile of each pipeline would be great.
How are you trying to operate the pipelines now? Are you running liquid in one line until pressure gets low and vapor forms,? Then do you open the vapor line with compressor power? How do you operate this system? What exactly are you doing now?

Mixing LPG liquids and vapor is not common practice, it either tries to all turn into vapor, or all liquid at the mixing point. You may have two phase flow going on somewhere. It may not all want to go in the right direction at times. When near the vapor pressure, you may be forming vapor at some places and Liquid at others. The flow and pressure will be different for liquid and vapor cases. Pressure changes can be quick and put strain on the equipment.

Normal practice is to operate above vapor pressure everywhere and keep pressure high all the time to avoid vapor formation, even when the pipeline is stopped. Liquid is never cleared.

It may not be possible to clear liquids quickly. When pressure reduces and you enter the vapor phase, you cannot move much liquid at the lower pressure and increasing the pressure will collapse the vapor back to liquid. You can make no progress. At lower pressure, you must let all the liquid trun to gas, then move the gas at low pressure, low flow and high friction factors. There is little else you can do. That's why these kinds of pipelines are stopped always maintaining all liquid inside the pipeline at all times..

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."