Dual-element sensors generally consist of two parallel rods separated by a spacer whereby one rod guides the radar signal to the surface of the material while the other rod guides the return signal to the transmitter. The coaxial sensor is a variation of the dual-element sensor where the concentric section takes the place of one of the rods.

Both dual-element and coaxial sensors are available in rigid or flexible designs. The accuracy of flexible sensors is generally inferior to that of rigid sensors. However, flexible sensors are generally often in longer lengths where the shipping and handling of rigid sensors tend to be problematic.

Single-element sensors emit radar energy near the base of the rod into the vapor space of the vessel. Some of the radar energy is absorbed by the material and returned to the transmitter via the rod. Materials with relatively high dielectric constants are generally required in order to obtain reflected signals that are sufficiently strong to achieve reliable measurement.

Excerpted from The Consumer Guide to Capacitance and Radar Contact Level Gauges.

The concept of using variable speed drives to throttle pumps and save energy was received with some apprehension. My peers had no problems with the equipment (there were a handful of variable speed drives in the plant on specialized equipment) and with the concept. However, they wanted assurances that capital expenditures for variable speed drives would result in predictable energy savings. That said, as a relatively new engineer at the plant with limited experience and no published literature/software available to calculate energy savings, I was first asked to demonstrate that I could accurately predict the energy savings.

We all agreed to test my calculations by temporarily connecting an unused variable speed drive to a feed pump for a distillation column. The predicted energy savings using the variable speed drive were calculated to be approximately 45 percent as compared to full speed operation. The energy savings based upon measurements taken during actual full speed and actual reduced speed operation were between 40 and 50 percent.

Now that the ability to accurately calculate energy savings had been validated, variable speed drive projects could be credibly presented for funding. While bothersome, the process of gaining credibility forced me to formalize my calculations and enabled me to write Variable Speed Drives: Principles and Applications for Energy Cost Savings (available at www.isa.org) so that others can have the calculations readily available for use.

This article originally appeared in Flow Control magazine.

All of these flowmeters can be affected by downstream disturbances. To determine the relative effects of downstream disturbances on the performance of each technology, one could analyze the downstream straight run requirements for each technology. Investigation might conclude that (say) magnetic flowmeters require less downstream straight run than (say) an orifice plate, so the orifice plate is more sensitive to downstream disturbances.

However, the noise generated by a control valve downstream of an ultrasonic flowmeter can be so overwhelming that the ultrasonic flowmeter may simply cease to operate. Therefore, it would be prudent to carefully investigate control valves located downstream of ultrasonic flowmeters to determine whether the noise generated by the control valve occurs at the same frequency as the ultrasonic signal. If it does, the control valve should be relocated farther from the ultrasonic flowmeter and/or carefully selected to incorporate special trim that can shift the frequency of the valve noise away from the frequency used by the ultrasonic flowmeter.

Additional Complicating Factors

Paying special attention to these details can avoid costly piping rework. In some installations, the ultrasonic flowmeter can be relatively easily relocated in the field with clamp-on sensors. However, installations with wetted sensors may be required to be relocated at great expenditure of time and money --- especially in custody transfer applications.

This article originally appeared in Flow Control magazine.