Why you HAVE to measure Hydrocarbon Dew Point continuously
Every year, pipeline companies spend millions of dollars pigging (cleaning) their pipelines of liquids that seem to magically appear in their pipelines. Every year, the accountants at the same pipelines wonder why their intake of gas BTUs does not match their output, losing out on millions of dollars.
They just file it under LUAF (Lost and Unaccounted For), an accounting bucket that captures all forms of lost energy.
The reason is somewhat simple. They do not tightly control the hydrocarbon dew point of the gas entering their network.
Yes, there is some sort of a contract with a stated hydrocarbon dew point, but no one at these companies actually measures the gas hydrocarbon dew point before it enters their network.
They trust and they do not verify. (For further info read the blog titled : “Natural Gas Pipelines: Where do liquids come from”)
If you are involved in processing, transport, or use of natural gas you must make sure you know what your hydrocarbon dew point is.
Before we discuss why, let’s take a moment to review what hydrocarbon dew point is.
What is hydrocarbon dew point?
Hydrocarbon dew point is an essential quality parameter for natural gas. It refers to the temperature at which the first hydrocarbons, usually the heaviest ones, begin to condense out of the gas phase into the liquid phase.
That seems simple enough. But, what determines the hydrocarbon dew point of natural gas?
Hydrocarbon dew point depends on two things: 1) The composition of the gas, and 2) its pressure. In general, the higher the percentage of heavier hydrocarbons are, the higher the dew point is.
The effect of the pressure is a little different though. Let's look at a typical natural gas phase diagram:
The blue line is the "phase envelope". The right hand portion of the blue line is the phase boundary between vapor and liquid phases.
In other words, the right hand part is the collection of the hydrocarbon dew points at different pressures.
The "cricondentherm" is the highest possible dew point at any pressure. For most pipeline quality gas, the cricondentherm corresponds to a pressure of ~25-35 bar.
So what is the effect of pressure on dew point?
For a given gas mix, lowering the pressure lowers the dew point, if pressure is under the cricondentherm pressure.
Above the cricondentherm pressure though, increasing the pressure reduces the dew point. This is called the retrograde region.
Now that we know what hydrocarbon dew point is, and how its components and pressure affect it, we discuss why we need to measure it accurately.
Can you calculate the hydrocarbon dew point?
The short answer is NO. Calculations involve very large sources of error (For more information see the blog post titled: “A Fallacy: Calculating hydrocarbon dew points from GC-based measurements”)
Measurement of hydrocarbon dew point has to be done with a chilled-mirror instrument, usually an automated chilled-mirror.
Using a GC to determine the content and using an Equation-of-State to calculate the dew point is not A MEASUREMNT.
READ MORE " What is Hydrocarbon dew point " here.
There is a wealth of data from academic and industrial researchers that clearly and unambiguously show the very large errors in using a GC-based calculation.
So, why do you have to know the hydrocarbon dew point any way?
Imagine you operate a pipeline at 50 barg (~730 psig). You also know that your pipeline may see temperatures as low as -15 °C (+5 °F).
What this means is that if you receive gas with a dew point of -15 °C (+5 °F) or higher, you will have liquids condense in your pipeline, where the pipe’s temperature drops below the hydrocarbon dew point.
Is that a bad thing?!! You bet. Having liquids slush around in your pipeline is one of the worst things that can happen from safety and cost points of view.
If you have liquids, it will affect all of your measurements on gas flow as well as other properties.
Funny thing is that Gas analyzers (including flow-meters) are designed for gases, not a mixture of gas and liquids.
If you have liquids in your gas, at best your measurements will be way off, at worst you will damage the analyzer and will need service on your damaged analyzers.
Now imagine that you operate a natural-gas based turbine power generation plant. If you have liquids entering your turbine, they will cause significant damage to the turbine, typically of the order of millions of dollars.
Perhaps more importantly, you will have to take your power plant off-line for repairs, losing out on even more revenue.
So what is the solution?
Here are your choices:
Trust your gas vendor to always give you the gas with a dew point that they promised? Seriously?! Unless you are the type that likes to buy things without knowing what you are buying, this would not be the best option.
Trust an occasional spot sample run through a GC and then try to calculate the hydrocarbon dew point? Absolutely not! As discussed in the other blog, GC-based guesstimation of hydrocarbon dew points are almost always on the low side, sometimes by as much as 50 °C (90 °F). So you may think you are getting a gas with a dew point of -15 C (+5F) but you are really getting a gas with a dew point of +20 °C (68 °F). Now you will have a lot of liquid in your pipe to get rid of.
Measure the hydrocarbon dew point of the gas that is entering your pipeline network continuously and in real-time. Bingo!! That is the right solution.
Unfortunately, there are many GC vendors who would like to sell you a software package to convince you that it does give you a good estimate of the hydrocarbon dew point.
There are a lot of gas processors who would like you to continue trusting them and not measuring the hydrocarbon dew point.
Who pays the cost for this neglect?
Ultimately, we all do since all costs are eventually passed on to end consumers.
However, the millions of dollars that the pipelines spend on removing liquids that should not have been there in the first place, the millions of dollars of LAUF, the cost of turbine damage, and analyzer errors, damage and repair, could surely be spent on more fruitful endeavors.