Q: I am an instrument engineer with the Port-Harcourt Refining Company Ltd. (a subsidiary of Nigerian National Petroleum Corp.). We are currently doing a feasibility study for the use of a two-way flowmeter for custody transfer of the following products: diesel, low-pour fuel oil (LPFO), premium motor spirit (PMS) and dual-purpose kerosene (DPK). The selected meter must be repeatable in both directions, accurate to ±5% and durable over its useful life. Could you please suggest to us the flowmeter type that could serve our purpose?
Ideh C. Ebuehi
A: The most popular custody-transfer flowmeter is obviously the Coriolis meter, which, if your flow exceeds the capacity of the single-tube design, is also available in two- and four-tube designs (Figure 1). If your flow is greater, you can put these meters in parallel or, if you can live with lower than custody-transfer accuracies, consider less expensive designs.
In addition to the Coriolis, there are a dozen other bi-directional flowmeter types on the market, and to list all vendors and their pros and cons would take too long. So I will mention only one, which will cost less and will provide pipe-size insert design. One of these is the time-of-passage ultrasonic meter. If you do not want to cut the pipe, this design is also available in a clamp-on version.
Time-of-passage meters come in two varieties, one with an engineered metering tube and the other in a clamp-on form. Some users like the clamp-on one because the piping does not have to be cut, and because it is significantly lower in cost. Naturally, I never use them for financial custody transfer, because to my knowledge, to date, no clamp-on meter has met the accuracy and repeatability criteria for fiscal custody transfer.
Figure 3 shows the installation of an in-line, pre-engineered, transit-time-type, four-transducer unit that comes complete with conditioning metering tubes for better accuracy.
A: From my experience in custody-transfer metering, the Coriolis flowmeter has proven to be superior to other forms of flow measurement, even if it is more expensive. Generally, the accuracy of a Coriolis flowmeter returns its higher initial cost. Having no internal moving parts is also a major benefit. For your reference, the following link is to an excellent application article on Coriolis flow measurement. Look for the paragraph describing bidirectional flow. See bit.ly/1lGUs8G.
A: Not all vendors do business in Nigeria. I would suggest you contact the flow measurement vendors your company already has a good relationship with and ask their sales departments for their recommendation. They are in business to serve your needs and are there to help you.
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.
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.
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.
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.