It is supposed to be a natural gas pipeline, so why are there liquids in there?!! Where do they come from?
The liquids flowing in many gas pipelines have two sources:
If they were put into the pipeline (psst…That “should” not have happened)
Or, they were created during the operation of the pipeline system. In this article, we will discuss why, and under what conditions liquids are created in gas pipelines.
So, notwithstanding item #1, how do liquids appear in the natural gas pipelines? Yes, they can be created in the pipeline itself. But how?
Let’s look at the natural gas phase diagram again. If you are not familiar with it, we suggest brushing up on the topic, but here is a short overview.
The (Temp. Vs Pressure) phase diagram for a typical natural gas mixture looks like the diagram above. All areas to the right of the diagram represent the pressure and temperature where all of the mixture is in vapor state. If you are a pipeline owner, this is where you want things to be at; all vapor and no liquids. Why is that? Because liquids create all sorts of problems in pipelines, in power plants, in turbines, in measurement, etc.
The area in the middle represents when you have the mixture in a gas+liquid state. There is some liquid and there is some gas. As you move further to the left (i.e. reducing temperature) eventually everything turns into liquid.
The portion of the curve that separates the vapor region from the Liquid+Gas region is referred to as the dew point curve. Each point in that curve is called the hydrocarbon dew point at the given pressure.
The portion that separates the Liquid+Gas region from the all liquid region is called the bubble point curve. This curve is also important but not for our discussion here.
Notice a few of interesting points about this phase diagram. The dew point curve has a positive slope initially; i.e. the hydrocarbon dew point increases with increasing pressure. This is expected and happens for almost all substances. But, the hydrocarbon dew point stops increasing after it reaches the “cricondentherm”. The pressure at which cricondentherm happens is called the “cricondentherm pressure”. If we increase the pressure even further, the hydrocarbon dew point begins receding, or the slope of the dew point curve is now negative. This is called retrograde behavior and is unique to some mixtures, including natural gas.
So, this was a short overview of the natural gas phase diagram, but let’s gets back to the original question: Why and how do liquids get created in natural gas pipeline (or turbines)?
There are several conditions that will lead to creation of liquids in the pipeline.
Decreasing temperature
Decreasing pressure
Increasing pressure (Compression)
1- Decreasing Temperature
Back to the phase diagram again. Let’s assume your gas P&T (pressure and temperature) are represented by the orange circle. Everything is in the vapor state and all looks good for trouble free transport of the gas through the pipeline. But alas, it is colder than usual (rings a bell?) for this time of the year and an unexpected arctic freeze descends in the area and segments of your pipeline are exposed to lowering temperature.
This scenario is represented by the following diagram; as you decrease the temperature, the gas will cross the dew point curve into the two phase Liquid+Gas region and liquids will drop out in the pipeline.
2- Decreasing pressure
Yes, decreasing the pressure may also lead to creation of liquids. Let’s consider the same phase diagram again. But this time, you have your gas at the new point shown by the green circle.
As the gas is traveling through the pipe, it will lose pressure. As it loses pressure, it may cross the dew point curve into the Liquid+Gas region, thus shedding liquids.
3- Increasing Pressure
Let’s look at the phase diagram again in the figure below. Assume you receive gas into your pipeline at a given pressure and temperature represented by the blue dot on the diagram. It is all gas, so everything should be good right? Well not exactly, because now you may have to compress the gas.
If the p&T (pressure and temperature) are below the phase envelope, compression (increasing the pressure) will cross into the Liquid+Gas region. What this really means is that you will create liquids.
-------------------------------------------------------------------------------------------------------------------------------
In all of the above scenarios, liquids will act like liquids do. You can blame Isaac Newton for it, but the liquids will flow to the lowest point of the pipeline. These liquids will cause a lot of issue, most importantly safety issues. Now the useful diameter of the pipeline is smaller. You will need more pressure to move things along. Hydrates may form. Turbines may get damaged. Flow measurements will be affected, etc. etc. etc.
The simple solution
So what is the solution? Well it is not really that hard: Measure your hydrocarbon dew point and make sure it is above the lowest temperature the gas is likely to encounter.
Notice we said “measure” and we specifically did not say “calculate”. The reason is very simple. Years of data, scholarly articles and research has shown that you cannot reliably calculate the dew point of natural gas in the field using composition analysis from a GC.
You don't need to look so hard for evidence. Next time you pig your pipeline (or damage a turbine blade), look at what is coming out and ask yourself why is it there? If you are the inquisitive type and would like further detail, check out the “Technical Resources” page and look up the technical note titled “The Fallacy of Calculating Hydrocarbon Dew Points”.