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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Regular Calibration vs. Infrequent Calibration
Calibration is the comparison of measurement values of a device against the standard of known accuracy, and then corrects the output of the device under test to match the standard within stated accuracy specifications. That standard could be another measurement device of known accuracy. The purpose of calibrating instruments and devices is to ensure that the measurements being recorded are accurate and within the proper range. If a device is not in calibration, it can return inaccurate measurements, which can have costly implications for a process.
Regular calibration and maintenance programs validate the performance of the sensor and the process. Established programs also yield the most accurate measurements and build confidence in the measurement system. A common practice is to remove the sensor from the moisture measurement system and send it back to the OEM for calibration. The vendor cleans and calibrates the sensor in the same (or similar) system that performed its initial calibration. By calibrating the sensor using an in-house system, the OEM removes any outside variables that could impact measurements, ensuring more accurate results. This practice also allows the OEM to look for changes in calibration that may be caused by outside sources, such as corrosion or contaminants.
Regular calibration is the proactive approach to sensor maintenance. It ensures sensors are clean and ready to respond to changes in water vapor. It also makes certain that the sensor is in agreement with national standards and that it meets quality assurance and quality control requirements. Regular calibration leads to increased confidence in measurements and, compared to delayed or infrequent calibration, provides cost-savings as it likely means companies are addressing concerns before they become problems and they aren't waiting for failures to respond.
Consequences
Infrequent calibration or reactive approaches to sensor maintenance result from inconsistent calibration schedules. This can be a very costly and inefficient approach. Sensors change in calibration over time. The more time that passes in between calibrations, the more uncertainty there is around the accuracy of the readings. This leads to wasted man-power and money spent to resolve the discrepancy of readings. Operators run a higher risk of failure, which leads them to keep more spare sensors on hand to keep the plant running.
Moisture measurement and calibration need to be looked at as tightly integrated processes that cannot be completed without the other. When approached strategically with best practices considered for both, they ensure process integrity, improve efficiency and reduce overhead maintenance expenses.
About the Author
Narge J. Sparages is the Senior Global Product Manager for Trace Moisture Analyzers and Process Analyzer Services at GE Oil & Gas. He has over 25 years' experience in process moisture measurement, systems, and services.
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