Selecting flowmeters for custody transfers

A1: An important goal of the hydrocarbon industry is reducing custody transfer costs. Accurate flow measurement is a tool for achieving that goal, but overall optimization requires more. So, I’ll start by discussing the process before focusing on the proper selection of the flow sensors.

Global per capita oil consumption is about five barrels per year (0.5 gallons per day). Known oil reserves can meet that rate of consumption for about 50 years (Figure 1), which is enough time to switch from fossil energy sources to a clean and inexhaustible solar-hydrogen-based energy economy, if the cost of conversion is provided.

During the 20th century, global expenditures for all primary energy was about 4% of the global GDP. Today, oil alone costs about 3% of the global GDP, which at a yearly consumption of nearly 40 billion barrels at $75 per barrel comes to about $3 trillion. That’s about the same as global military expenditures, which are mostly spent on competing for the control of the planet’s remaining fossil and mineral reserves.

Converting to the future (green) energy economy requires a yearly investment of about $20 per ton of greenhouse gas (GHG) emitted for 20 years. Since yearly emissions are 40 billion tons, this total investment is also about $3 trillion or about the same that we spend on oil. If this calculation is correct, what we’re doing today makes no sense, particularly if we also consider the damage that climate inaction causes.

Oil losses in custody transfers are the difference between the sending and receiving amount in a mixing tank. Measurement is needed at every point where ownership changes, whileoil losses usually amount to 5-10%. Such losses can be cut in half by using the best flow and level detectors, and better monitoring practices.

"Best" is a relative term, but to me it means I don’t use devices that have moving parts or DP detectors to measure flow or level. In case of transfer to or from calibrated tanks, I use guided wave radar (GWR) interface sensors. When volumetric pipeline flow must be detected, I use eight-path ultrasonic meters with orthogonal planes (Figure 2). When high-rangeability mass flow measurement is required, I use Coriolis flowmeters  (Figure 3).

Béla Lipták / [email protected]

A2: Orifice meters are widely used for liquid measurements and have been for many years. Their poor accuracy can usually be explained by poor installation.

Available technologies have changed; Coriolis meters matured and competition drove down prices. It’s clear that meter selection is affected by pipe size. I find it hard to imagine a Coriolis meter in a 1-m diameter pipeline. National and international standards also impact meter selection for custody contracts and may have statements limiting options.

I have the impression that sales people often tend to suggest usingmore expensive choices than needed. On the other hand, it’s common to under-estimate the costss and details of installingvarious flowmeters. Accuracy is expensive. 

I once developed a program to aid in flowmeter selection. Users entered information about their fluids and flows. The program displayed a list of possible meter types with cost, accuracy and permanent pressure loss. It generated interesting comments challenging estimated costs and accuracy. Support for developing such reliable data is always justified.

Cullen Langford, control consultant / [email protected]

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A3: Orifice flow measurement is at best ±4% accurate. In spite of this, it was used for liquid custody transfers for many years, until more accurate instruments became available. Today, Coriolis flowmeters are standard for liquid flow custody transfer due to high accuracy, and because they directly measure mass flow instead of volumetric flow. however, they’re expensive, so sometimes if liquid is transferred from tank to tank, the before-and-after tank level measurements are used, requiring very accurate tank level measurements.

Measuring gas flows with an orifice flowmeter with compensation for temperature and pressure is typical for natural gas transmission, but that still doesn’t make it accurate. In most cases, the low value of natural gas makes it costly for more accurate gas-flow measurement. For high-value gases, it’s possible to use Coriolis flowmeters or a high-accuracy positive displacement meter. Sometimes, custody transfer of gases is calculated from a change in pressure of the gas’ source, such as a cylinder.

Orifice flow measurements depend on Bernoulli’s principle relating pressure drop thorough a sharp-edge orifice to volumetric flow rate. The pressure drop is between the upstream pressure (before the orifice) and the pressure at the vena contracta, formed by the increased velocity of the liquid or gas as it passes through the orifice. Unfortunately, the location of the vena contracta varies with flow rate, so there isn’t a practical way to measure pressure drop. We usually measure the pressure drop at the flanges that hold the orifice plate in place, and rely on a correlation to estimate the pressure drop at the vena contracta, or assume that the pressure drop at the orifice is the same as that of the vena contracta.

Richard H. Caro, CEO, CMC Associates, ISA Life Fellow / [email protected]

A4: Orifice plates aren’t used for liquid custody metering because:

• During startup, while the flow stabilizes the error in measurment is generally unacceptable, the same happens during shut down;
• It’s difficult to guarantee custody transfer precision requirements becasue not every company can achieve the needed orifice or fabricate to extreme tolerances; and
• The turn down ratio for liquid orifice plates requires too many orifice plates in parallel to cover the flow range needed.

Though custody transfer can be performed with orifice plates, it’s not recommended due to instability, and because fabricating required orifice quality is expensive. If the flow varies during custody transfer, then multiple prifices are needed.

Alex (Alejandro) Varga, control engineer / [email protected]