Going Both Ways
Figure 3. Bidirectional ultrasonic flowmeters can be provided complete with pre-engineered conditioning tubes and pressure/temperature detection to improve accuracy.
A: You could use ultrasonic flowmeters. They require very long straight lengths (typically 20 upstream and 10 downstream). Here’s an example by Emerson Process Management: bit.ly/1rWtOuo. Others, such as Krohne, Siemens and FMC Technologies, also manufacture them.
Raj Binney
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A: For custody transfer, the accuracy requirement must be much higher than you indicated (±5%). Is it allocation metering or custody?
The Coriolis meter will be most suitable for this application. It can provide you repeatability in both directions. However, for liquid, you need to ensure the flowmeter is vertically installed with flow upwards for forward measurement. For reverse measurement, please make sure there is a valve at the downstream for throttling. This will ensure full pipe flow. One supplier is Micromotion.
Alternatively, an ultrasonic flowmeter can provide bidirectional flow, but you need flow straighteners and sufficient upstream and downstream run. The overall uncertainty in reading is better with a Coriolis-based custody meter skid.
Debasis Guha
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A: Based on my experience, I would most certainly use a Coriolis meter. They’re approved for custody transfer, but they also must be in-situ, verified/proven by the local weights and measures governing authority. They handle bidirectional flow and measure mass directly, so temperature, pressure and API fluctuations are accounted for, but — there’s always a but, isn’t there? — location and installation are critical.
Whereas the meters are incredibly accurate and repeatable (precision error), a poorly planned and resultant poor installation can have a negative impact on the bias error component of the overall measurement uncertainty (a.k.a. accuracy). In custody-transfer applications, it will be mandatory to offload and complete the transaction from full pipe to full pipe or from a known pipe fill status. Therefore, overall pipe design clarity is important, as are check valves to prevent backflows, location of drain/vent valves and, finally, a well-documented procedure.
Peter Baker
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Also Read: How Can You Measure Viscocity with Coriolis Flowmeters?
A: Béla has asked me to edit the new chapter on bidirectional flow in the fifth edition of the Instrument Engineer’s Handbook. I agree completely with him that the ultrasonic meter is a good choice. From my experience with this meter, I would like to suggest the following additional points to improve performance:
1. Please respect the straight-run distances in both directions. Even so, you will probably have a different meter factor in each direction. Be sure you’re able to prove in each direction. Store these meter factors in your configuration, and use them in your calculations.
2. Each product has a different speed of sound. You can test this parameter by packing the meter in the product at zero flow and collecting the value. Then configure your meter to measure by product.
3. A common mistake is to oversize the meter. Ultrasonic meters work the best and are most accurate in the turbulent range of Reynolds Number > 4000.
Victor Wegelin
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