If the average weighted chemical cost is USD $1.00 per liter, operating in this manner wastes over USD $100,000 of chemicals annually, aside from making the process less productive and less sustainable. Spending (say) USD $50,000 for a flowmeter to measure the production mass flow and ratio controls to adjust the chemical injection controller setpoints in proportion to the actual measured production mass flow, would result in a simple payback of approximately 6 months. Productivity increases realized by injecting the desired amount of chemicals would further reduce payback time. Projects such as this, with potential payback measured in months, should be aggressively investigated for viability in both new and existing wells.

It should be noted that a given percentage error in the production mass flow causes a corresponding equal and opposite percentage error in the mass concentration of the chemicals in the production flow. In this example, a 10% lower (higher) production mass flow resulted in 10% higher (lower) mass concentrations.

Neither scenario is good, but their stark contrast illustrates the potential economic, protection and sustainability improvements that can be realized using ratio controls that inject chemicals as a percentage of the measured production mass flow, instead injecting a fixed flow. In these scenarios, a ratio control strategy would either save approximately USD $100,000 or provide necessary equipment protection that was lacking, respectively. Between these scenarios, such as when the well is (unknowingly) operated closer to the desired mass concentration of each injection chemical in each location, less chemicals are saved or additional equipment protection is provided, respectively.

Excerpted from Measuring Difficult Flow Streams and More Accurate Flow Control Can Improve Oil and Gas Well Profitability in Processing magazine.

Not so fast. The flowmeter was now reported to be measuring correctly, however it was also dropping out to zero flow. Depending on design, vortex shedding flowmeters require a minimum Reynolds number of (say) 5000 in order to function accurately and reliably. Below their minimum Reynolds number constraint, vortex shedding flowmeters turn off and measure zero flow --- even when the liquid velocity is high.

Calculations revealed that the increased pipe size and relatively high viscosity coupled with a somewhat low flow velocity resulted in an operating Reynolds number that marginally violated the minimum Reynolds number constraint of the flowmeter. In other words, the application of a vortex shedding flowmeter in this application was likely not the best selection.

Note that my client initially did not fully describe the problem at hand. When questioned, he confirmed that the flowmeter was initially measuring incorrectly and dropping out. Not mentioning the drop outs would lead one to believe that the flowmeter measurement was steady --- but incorrect --- so Reynolds number constraints were not initially investigated. Initial mention of the drop outs would have prompted a calculation of Reynolds number and the discovery of a likely violation of hydraulic constraints.

Solving a problem is not a guessing game or a quiz, so you would think that people would transmit all of their observations so as to help diagnose the problem. This may be routine for some, but it is not necessarily practiced universally. The thought here is to keep asking questions until you are satisfied that you have a complete working knowledge of the installation that includes all of the observations.

 This article originally appeared in Flow Control magazine.

All of these flowmeters can be used in some form of custody transfer application. Magnetic flowmeters are often used for the custody transfer of clean water, sewage and wastewater. Orifice plate and positive displacement flowmeters are commonly used for the custody transfer of natural gas. Both can also be used for hydrocarbon liquids and water. Ultrasonic flowmeters have been developed for natural gas and liquid hydrocarbon custody transfer applications. Venturi tubes are commonly used for the custody transfer of water. The correct answers are Answers A, B, C, D and E.

Additional Complicating Factors

There are standards that describe the use of certain flowmeters for the custody transfer of certain fluids. These standards provide guidance and can have the effect of law in some jurisdictions because local statutes often stipulate conformance with a certain standard. This is particularly prevalent when sales are made to the public such as the sale of gasoline, natural gas, and potable water.

Notwithstanding the above, it is my understanding that the custody transfer of fluids (in the USA) can be measured by a flowmeter that is agreed upon by both the Buyer and Seller. In these applications, the actual flowmeter may or may not be well-suited for its service. Needless to say, such flowmeters can cause problems if the Buyer or Seller questions its performance.

This article originally appeared in Flow Control magazine.