Noting that the radar signal travels to and from the material, degradation of the signal strength between the sensors and the material can cause radar level measurement systems to fail to operate. Degradation can occur at the antenna, in transit to/from the material, and at the surface of the material.

Dirt or other coatings on the radar antenna can cause the received signal to be weak. When accumulations over time are normal for the process, routine maintenance may be required to keep the transmitter/receiver operating. In many applications, the sensor may be shielded from the process and/or continuously purged with gas to keep it in operation.

Careful analysis of the sensor design should be performed. Some designs inherently tend to keep the sensor from accumulating material while others tend to inherently accumulate material.

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

The substance of an excerpt from the report reads (in part):

Relatively small measurement errors can have a relatively large economic effect in custody transfer installations for high value products, large pipelines, or both.  For example, assuming that the average annual steam flow rate is 100,000 lb/hr of steam valued at $10.00 per 1000 lb, the flowmeter would pass approximately $8,760,000 of steam per year (100,000 lb/hr * 8760 hrs/yr * $10.00 / 1000 lb). 

The large economic value of the fluid over time means that even small measurement errors can be significant.  In the above example, a 0.01 percent measurement error would result in a billing error of $876 per year.  Therefore, even small errors such as rounding or unit conversion can result in significant billing errors.  Conversely, measurement errors of a few percent can result in billing errors that can approach $1,000,000 per year. 

In many such installations, total steam consumption is more than (the stipulated) 100,000 lb/hr, so the economic implications can be considerably larger.  It takes only a one percent error on another client's billing of $100 million per year for the error to exceed $1,000,000 per year! 

There is much that you can do after you are armed with this information.  Initially, you can evaluate heat and material balances to determine the magnitude of suspected flow measurement errors.  This can be followed by an examination the entire flow measurement system with a "fine-toothed comb".  Be sure to examine ALL aspects of the installation including the upstream and downstream piping, process conditions, flow element, taps, impulse tubing (or other process connections), instrument installation, instrument specifications, calculations, configuration, wiring and communications, indicator, totalizer and other aspects of the measurement as may be required. 

Remember that the idea behind this approach is to find, identify and quantify flow measurement problems.  While cursory investigation may uncover some issues, detailed investigation is advised because errors associated with virtually any of the above aspects of the flow measurement system can result in errors that can exceed $1,000,000 per year in many processes.

This article originally appeared in Flow Control magazine. 

The equations for fluid flow in a pipe are:

Q = A * v

W = rho * Q

where Q is the flowing volume, A is the cross-sectional area of the pipe, v is the velocity of the fluid in the pipe, W is the mass flow rate, and rho is the fluid density.

Some flowmeters (volumetric) measure the flowing volume of the fluid while others measure the mass of the fluid. Other flowmeters measure the velocity of the fluid and infer the flow rate using the cross-sectional area of the pipe (A). Still other flowmeters infer the flow by measuring the velocity head (0.5 x density x velocity²) across the flowmeter.

The flowmeters listed in Answer A are differential pressure flowmeters. They present an obstruction to the flow that creates a differential pressure that is measured and used to infer the flow in the pipe. These flowmeters are not volumetric flowmeters so Answer A is not correct.

The flowmeters listed in Answer B measure the velocity of the flowing fluid. They are not volumetric flowmeters. Answer B is not correct.

With regard to Answer C, positive displacement flowmeters measure the volume of the fluid. However, the remaining flowmeters measure velocity. Therefore, Answer C is not correct. Similarly, Answer D is not correct. This leaves Answer E as the correct answer. 

Additional Complicating Factors

Testing will apparently be administered based on this subject matter. 

This article originally appeared in Flow Control magazine.