Calibration Is The Insurance Policy for Moisture Measurement

December 23, 2016 By: Narge J. Sparages, GE Oil & Gas

Sensors Insights by Narge J. Sparages

Moisture measurement is essential in many industrial and process applications. It helps ensure the integrity of systems and the processes to which it is applied. Accurate readings instill confidence in the measurement system and improve the overall efficiency and effectiveness of the process. Regular calibration and maintenance programs need to be implemented to ensure the most accurate measurements are realized.

Sensors can change in calibration over time. They are subject to process contamination that may impact speed of response and accuracy. Even the best sample systems are subject to performance upsets such as plugged filters, saturation with water, contamination due to unexpected upsets, and leaks caused by vibration and/or corrosion. All of these factors can lead to operators questioning the validity of the moisture measurement. This punctuates the need for a regular calibration schedule.

Moisture Measurement and Sample System Maintenance

Because moisture measurement is necessary in many industries, the required range of measurement and the conditions in which the measurements are carried out vary widely. For instance, unwanted water contamination can lead to corrosion and oxidation of sample lines and process piping. When combined with solid contaminants, water vapor can aid in the plugging of filters, increasing the plant's operating and maintenance costs.

Plant operators need to know if moisture is present in process streams to mitigate unwanted process by-products. Moisture can combine with chemicals being processed to produce acids that are detrimental to plant and piping systems. These problems are both costly and damaging, if not identified and rectified quickly.

Moisture measurement in instrument air ensures that the plant air delivery system is working efficiently. Wet air can lead to significant damage to pneumatics tools, corrosion in air transport lines, and quality issues with the products/processes that rely on the air quality. If the ambient temperature falls below the dew point temperature there will be condensation in the air pipes, causing corrosions and freeze-ups.

The moisture content of natural gas is an integral component of the gas quality and included in the custody transfer contractual requirements. Natural gas in pipeline applications is typically dried using tri-ethylene glycol (TEG) dehydrators at a gas processing plant and the moisture content must meet the contractual requirements to allow the gas to enter the pipelines network. TEG carry-over from the drying process can potentially cause the moisture reading to be wetter, if not properly removed prior to the sensor. A properly designed sample system with the right filtration can minimize TEG carry-over from adversely impacting the moisture reading. If TEG should break through the filtration, a proper cleaning and calibration will return the moisture sensor to normal operation.

Molecular sieve dryers use moisture sensors on the drying beds and in the downstream combined headers to ensure that the dryers are effectively and efficiently drying the hydrocarbon gases and liquids that will be processed. Should moisture be present at the outlet of these dryers, it will freeze and cause damage to downstream equipment and bring the process to an expensive halt.

In all applications, regular calibration allows the opportunity for regular sample system maintenance (filter inspection, flow settings, pressure regulator effectiveness, as examples), to ensure that the sensor is able to see moisture intrusions, should they occur. In the above example, molecular sieve dust can plug filters and can get on a sensor, keeping the sensor from seeing moisture intrusion.

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About the Author: Narge J. Sparages

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