Saturated steam flowmeters often use pressure compensation to correct for varying pressure. Measurement of the vapor phase of saturated steam with condensate (wet steam) can usually be done with reasonable error because the volume of liquid represents but a small percentage of the volume measured. Superheated steam applications usually use pressure and temperature compensation to infer mass flow.

Typical vortex shedding and fluidic flowmeter construction is such that the wetted parts include the body and sensor that can usually be made from materials that can withstand corrosion and do not contaminate the fluid. These flowmeters can be applied to measure corrosive fluids, and fluids where contamination is an issue, such as in sanitary applications. However, some vortex shedding and fluidic flowmeter designs do incorporate internal seals that should be considered for compatibility with the process. All-welded designs should be considered for applications where corrosive or hazardous fluids are present.

Vortex shedding and fluidic flowmeters usually require straight run upstream and downstream of the flowmeter. Straight run requirements can reduce the usability of some vortex shedding and fluidic flowmeters in some applications. The supplier's detailed mounting recommendations should be examined to evaluate suitability for a particular installation.

Vortex shedding and fluidic flowmeter technology has Reynolds number constraints, so it can be applied where the liquid viscosity is relatively low. The viscosity of the fluid can vary as long the Reynolds number constraint is not violated. However, as fluid viscosity increases, the pressure drop across the flowmeter will increase and may limit the applicability of the flowmeter.

Excerpted from The Consumer Guide to Vortex Shedding and Fluidic Flowmeters

My findings about the flowmeters were documented in an expert report and a court date was set to testify regarding the performance of these flowmeters - effectively contesting the billing for the amount of sewage generated by the first sewage district. The technical considerations and proposed actions seemed straightforward: correct the flowmeter installations and calibration techniques, as well as adjust previous billing to account for known discrepancies. After these items are complete, additional adjustments to previous billing may be indicated based upon the measurements obtained from these improved measurement systems.

Not so fast. Not every matter is resolved using a fact-based engineering approach. The issues had to be dealt with globally because there was some history involved as the result of a legal proceeding that occurred approximately three years prior to my involvement in the case. At that time, the first district's expert (who was not a flowmeter expert) addressed the flowmeters only once and did not question their performance. By not questioning, the flowmeters were presumed by the court to be functioning properly such that their performance could not be questioned in the future - even if significant issues came to light such as those documented in an expert report.

We will continue this discussion next month.

This article originally appeared in P. I. Process Instrumentation magazine.

1. 5D upstream and 5D downstream
2. 6D upstream and 4D downstream
3. 7D upstream and 3D downstream
4. 8D upstream and 2D downstream

The purpose of straight runs is to develop good velocity profiles entering and leaving the flowmeter. Most flowmeters that require straight run are more sensitive to flowing conditions upstream of the flowmeter and less sensitive to flowing conditions downstream.

There is no definitive answer, but of the installations offered, 8D upstream and 2D downstream (Answer D) represents the longest upstream straight run, which should provide the best balance of velocity profiles - hopefully, without exhibiting the adverse effects of a short downstream straight run. 

Additional Complicating Factors

Manufacturers test their flowmeters in different installations and different operating conditions, which allows them to publish installation guidance to include straight-run requirements. Therefore, the manufacturer can be contacted and perhaps use this test data to better determine how much straight run should be allocated for upstream and downstream straight runs in non-ideal installations.

This article originally appeared in P. I. Process Instrumentation magazine.