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Oil and Gas: Injection Flowmeters (Part 7 of 10) by David W Spitzer
There is significant uncertainty surrounding the actual production mass flow rate, so control strategies such as these that manually set the chemical injection flows at fixed values can have profound effects on well profitability and sustainability.
The typical flow accuracies of these flow control strategies are approximately 20% (anecdotal), 5%, and 1% of rate, respectively. Note that these are the accuracies with which the flow is controlled, and not the accuracy of the process control objective, which is maintaining the desired mass concentration of each injection chemical at each strategic location in the well.
Improving flow measurement accuracy can reduce the unnecessary addition of chemicals and increase profitability. For example, the flow settings for systems with accuracies of 20%, 5% and 1% of rate would be 125, 105.3 and 101 cc/min, respectively, to ensure that the chemical flow does not fall below the desired (say) 100 cc/min. More accurate flow measurement enables the flow to be controlled closer to the set flow rate and potentially reduce chemical consumption, reduce chemical costs and increase sustainability. These chemical savings are largely in addition to the above cited savings associated with controlling the chemical injection flows as ratios to the production mass flow.
Excerpted from Measuring Difficult Flow Streams and More Accurate Flow Control Can Improve Oil and Gas Well Profitability in Processing magazine.
What Would You Do? Designing a Parallel Extractor by David W Spitzer
Sound engineering practice and judgment can yield significant operational improvements. I started my career in the mid-1970s in the central engineering department of a major steel company where nothing was "too big". After about a year of on-the-job training, I was assigned to work on a US$ 400 million expansion. This same expansion would cost well over US$ 1 billion today. Later in my career when working as the only electrical, instrumentation and utility engineer at an operating chemical company, I could suggest and make changes subject to questioning for reasonableness and achievable benefits. The details of implementation were left for me to decide.
The chemical plant had an extractor which is a horizontal tank where relatively clean solvent enters at one end of the extractor and an aqueous solution containing the raw product enters at the opposite end of the extractor. The center section of the extractor contains a rotating mixer that causes the solvent to scavenge the product from the water. Solvent with product and "clean" water exit at opposite ends of the extractor.
When I was upgrading the instrumentation on this unit when I noticed that the existing extractor was controlled using a local pneumatic level controller. Interestingly, the outlet flow of "clean" water at one end of the extractor was controlled using the interface level on the opposite end of the extractor. This did not seem logical because there was a significant distance between the interface level measurement and the control valve that would cause a relatively long response time.
What would you do if confronted with this application? How would you control the extractor? Remember that the existing extractor is installed and operating, but the plant is planning to install an identical refurbished extractor in parallel as part of the upgrade to improve extraction efficiency. The existing extractor will similarly be refurbished afterwards.
... more next month.
This article originally appeared in Flow Control magazine.
Understanding Velocity Profile Distortion by David W Spitzer
Which of the following upstream piping configurations causes the greatest velocity profile distortion?
A. Elbow
B. Reducer
C. Two elbows
D. Two elbows out of plane
E. Two close-coupled elbows out of plane
Most flowmeters require a good velocity profile upstream of the flowmeter in order to measure accurately. A number of techniques can be used to develop a good velocity profile upstream of the flowmeter to include the installation of straight pipe immediately upstream of the flowmeter and/or installing a flow conditioner to remove the distortion present. Another alternative is to select a flowmeter that is designed to tolerate the distorted velocity profile.
Standards published by organizations such as AGA, API, ASME and ISO specify the upstream and downstream straight run requirements for various flowmeters. The diameters of straight run required to mitigate the effects of the above piping configurations tend to progressively increase from piping configuration A thru piping configuration E. Therefore, the correct answer is Answer E.
Additional Complicating Factors
In many installations, the actual straight run is less (and often much less) than the published straight run requirement for a given piping configuration. When piping changes cannot be made to have the straight run in conformance with its requirement, the user may choose to make other modifications such as installing a flow conditioner and/or accepting a less than accurate flowmeter installation.
This article originally appeared in Flow Control magazine.
ABOUT SPITZER AND BOYES, LLC
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 500 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
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