The Demo Unit That Was Not Returned - Why the customer loved the portable TDLA For moisture measurement in natural gas
If you work in the natural gas industry or are an industrial user of natural gas, water dewpoint is not only a quality measurement but also a big concern. Water condensate presents serious challenges and issues for your process. It is highly corrosive and will form hydrates, which are ice-like solid molecules that can block the flow of gas in pipelines. In cold weather, it will also freeze reducing the pipe pressure.
There are several sensor technologies that can be used to measure moisture in natural gas.
Let’s start with the CLASSIC - the chilled mirror “Standard Bureau of Mines” (ASTM D1142) analyzer. If you are a carpenter, you will have a hammer in your toolbox; if you are in the natural gas industry, it’s very likely you will have a Bureau of Mines dew point tester. This device is a first principal measurement, where a mirror is chilled using a gas (propane, carbon dioxide, or nitrogen). The temperature and pressure are measured when the first drop of water is formed. These analyzers are portable, using a pelican case with a tripod, but also require a propane gas bottle. They are easy to use but require a trained eye to see when the first water droplet appears on the mirror. Since it requires a user to see the dew form, the technique is subject to operator errors by cooling the mirror too quickly or misinterpreting the dew formed on the mirror. These devices can also detect hydrocarbon dew point or have interference from methanol or glycol which are commonly used as desiccants in natural gas.
In this situation, the user needed to measure water dew point temperature on natural gas pipelines in Alaska. You can image that going out in the field in subzero temperatures can be a challenge during the long winter months. The user had a portable moisture analyzer based on aluminum oxide technology, which has been in the industry for years and basically is a capacitor.
The main issue the user had with the technology was the drying time - it took a very long time to reach equilibrium and have a stable reading. As a result, the user used two analyzers and switched between them to try to improve and reduce the response time. In the cold Alaska climate, the user was spending hours outside waiting for these analyzers to reach the final dew point reading.
The user contacted us seeking a faster, more accurate solution for portable moisture measurements. We recommended a portable analyzer based on tunable diode laser absorption spectroscopy (TDLAS) technology. Without getting too technical, it is basically a spectrophotometer. A TDLAS analyzer performs non-contact measurements, where the sample gas does not touch the sensor. The analyzer, called the 5100P, has a water ampule that is used as reference cell to lock the laser on the proper wavelength where water is detected; unless you have a gross carry over of methanol and glycol, the non-contact measure and the lock-in feature will avoid a false reading.
A demo 5100P was send last November for a two-week test. Almost two months later, the user was still using the analyzer as the performance was a significant improvement over his prior experience with the aluminum oxide-based analyzers. The user wanted to keep the demonstration unit and purchase several additional units for the site. The user was very impressed with how well the 5100P worked and provided data on its usage on site. The figure below shows reading at five seconds interval where the user was testing the analyzer while closing and opening the drain valve. As the valve was opened and closed, the moisture reading would change.