Measure moisture accurately

Moisture measurements are critical to the control of many processes. Excessive moisture can result in loss or poor quality of product, significant downtime of a process, or corrosion and damage to critical equipment, resulting in costly repairs or loss of revenue. To minimize process moisture problems, reliable, on-line moisture measurement is needed.

08/01/2002


Moisture measurements are critical to the control of many processes. Excessive moisture can result in loss or poor quality of product, significant downtime of a process, or corrosion and damage to critical equipment, resulting in costly repairs or loss of revenue. To minimize process moisture problems, reliable, on-line moisture measurement is needed. Several basics concepts should be considered to achieve accurate moisture measurements.

Sensor/analyzer maintenance

As part of a normal maintenance cycle, moisture sensors should be inspected and calibrated to ensure that the accuracy and performance of the sensor are maximized. Periodic sensor calibration provides information to identify potential problems with the application, such as sensor corrosion.

Some moisture analyzers are provided with on-board calibration systems, which can be complex, expensive, and require further maintenance. Also, due to the polar nature of the water molecule and its ability to adsorb to wetted surfaces, certified moisture standards in gas cylinders are generally unavailable for field calibration. Consequently, most moisture sensors are typically returned to the manufacturer or suitable third-party laboratory for traceable calibration to national standards.

Although some moisture probes can be installed directly in-line, they will not withstand the long-term rigors of the process. For this reason, most installations use sample-conditioning systems as a means of exposing the moisture sensor to the process fluid. Sample systems provide the ability to isolate the moisture sensor, filter contaminants, meet hazardous area requirements, provide environmental protection, and control process conditions at the moisture sensor, such a temperature, pressure, and flow rate.

Typically, a sample tap is installed in the process pipeline and connected to the sample system by a short, continuous piece of tubing. Stainless steel wetted parts and tubing are recommended as stainless steel has excellent adsorption/desorption properties with respect to water molecules and is not susceptible to permeation of ambient moisture. Sample systems should be periodically leak-tested to prevent leakage of ambient moisture into the system; and filter elements should be cleaned or replaced as part of routine maintenance.

When reviewing a moisture analyzer installation, it is important have a good, practical understanding of moisture measurement units. For gas applications, the two most common units of measure are dew point temperature and parts per million by volume (PPM V ). Depending on the application, one measurement unit may be more applicable than the other for interpreting moisture measurements.

For example, in cryogenic natural gas processes, gas flows through a "cold box" maintained at a very low temperature to recover heavy hydrocarbon products. It would be practical to compare the dew-point temperature to the cold-box temperature to ensure that the dew-point temperature is lower than the cold-box temperature to prevent frost formation within the cold box.

Another example involves comparison of moisture measurements conducted at two pressures. Many moisture analyzers can only measure moisture content at atmospheric pressure, while a few analyzers are capable of measurement at line pressure. Dewpoint temperature is functionally equivalent to water vapor pressure. Since water vapor pressure is one component of the total gas pressure, dew-point temperature will increase or decrease with corresponding changes in total pressure. Therefore, comparing dew-point temperature readings at two different pressures can lead to confusion in understanding the moisture measurement reading. In this situation, it is beneficial to use a PPM V reading, which is not dependent on system pressure, to determine if the analyzers are in agreement. By comparing the PPM V reading, the problems of the dew-point temperature to pressure relationship can be avoided.

Select a reputable vendor

In today's business environment with fewer personnel performing more tasks, most companies have limited resources to invest in the review of available technologies. Therefore, it becomes important to select a reputable vendor with in-depth application experience, technical expertise, and reliable service and support to help ensure successful moisture measurement applications.


Author Information

John T. Kerney is the hygrometry product manager for Panametrics, Inc. in Waltham, Mass. Contact him at kerneyj@panameterics.com For more on moisture measurement, see the August issue online at




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