DAVID W SPITZER'S E-ZINE (January 2025)

The material itself can cause the intensity of the reflected radar signal to degrade when the material exhibits poor reflective qualities. Notwithstanding other problems, radar level measurement is typically limited to materials with a dielectric constant greater than approximately 1.5. Liquids with lower dielectric constants do not reflect well and may cause the level measurement to be erratic or fail to operate. FMCW radar level measurement systems can typically operate at lower dielectric constants as compared to pulsed radar level measurement systems. Notwithstanding the above, some non-contact radar level measurement systems can measure the level of materials having dielectric constants as low as 1.05.

Radar level instruments often can measure through foam, so their measurements generally represent the level of the material. However, applications have been reported where the foam either attenuated the radar return signal too much to permit a measurement or was able to cause a large enough "spurious" return to an erroneous level in the vessel.

Excerpted from The Consumer Guide to Non-Contact Level Gauges.

Most flowmeters, including differential pressure flowmeters, do need to be "located a certain length downstream of pipe bends and restrictions (such as control valves)".  Similarly, the "certain length" does make the flow measurement more predictable.  However, technically speaking a "certain length" could mean (say) 3 meters.  In flow measurement, we tend to express these upstream requirements as multiples of pipe diameters, so the "certain length" could be (say) 2 meters for one pipe size and (say) 3 meters for another pipe size.  This is a relatively minor point as is that the "certain length" is inferred (but not stated) to be straight pipe because it is downstream of pipe bends and restrictions. 

However, the statement that the flow is more laminar indicates that the writer does not understand the concepts involved with the laminar, transitional and turbulent flow regimes.  This is not uncommon and is encountered in virtually every flow seminar that I teach.  Approximately 60-80% of my (non-chemical engineer) students proudly raise their hands when asked if they have heard of Reynolds number.  Typically, only 1 or 2 students raise their hands halfway when I ask if they know what Reynolds number is. 

 Reynolds number is a dimensionless number that is the ratio of the inertial forces in the pipe to the viscous forces in the pipe.  In general, the flow is in the laminar flow regime when the viscous forces dominate, the turbulent flow regime when the inertial forces dominate, and the transitional flow regime between the laminar and turbulent flow regimes.  These flow regimes are dependent on the flowing conditions and pipe size, and not dependent on the piping configuration. 

 The laminar, transitional and turbulent flow regimes should not be confused with velocity profile distortion issues that can be caused by upstream pipe fittings and restrictions.  There are a number of techniques that can be used to address velocity profile distortion --- including the installation of upstream straight run.

This article originally appeared in Flow Control magazine. 

Variable speed drives are increasingly applied in many applications to reduce energy consumption.  Reducing the speed of a centrifugal pump reduces the pump discharge pressure by the square of its speed and its energy consumption by the cube of its speed.  Even a small speed reduction such as 5 percent can save almost 15 percent of the energy consumed at full speed. 

Such is not the case of constant torque loads such as positive displacement pumps and positive displacement compressors.  The capacity and energy consumption of constant torque equipment are both approximately proportional to operating speed.  Therefore, at 80 percent speed, the motor will consume approximately 80 percent of its full speed energy consumption.  Answer A is correct. 

At 80 percent speed, the load will operate at approximately 80 percent of its capacity while consuming 80 percent of its full speed energy.  These relatively simple relationships for positive displacement equipment make the operating speed and energy consumption considerably easier to predict as compared to centrifugal equipment. 

Additional Complicating Factors

Many variable speed drive applications and installations require attention to detail to ensure that the motor and its mechanical equipment can function properly in operation.  In addition, failure to attend to certain details of lower speed operation can damage the equipment --- especially when variable speed drives are applied to positive displacement equipment. 

This article originally appeared in Flow Control magazine.