Custody transfer flow measurement: Why can a DP flowmeter be used for gas but not liquid?

This column is moderated by Béla Lipták, automation and safety consultant and editor of the Instrument and Automation Engineers' Handbook (IAEH). If you have an automation-related question for this column, write to [email protected].

Q: I'm interested to know why orifice differential pressure (DP) flowmeters aren't used in liquid metering systems (for custody transfer purposes), whereas they're widely used in gas metering systems? What makes an orifice flowmeter a viable, cost-effective choice for gas metering systems only?

A. Rashimi
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A: The short answer is that orifices are used for both, but because custody transfer (fiscal metering) is such an important topic (see Chapter 2.20 in volume one of my handbook), I will give a more detailed answer.

Flowmeter selection can be based on government regulations, industry or national standards, and contractual agreements, and it can also be subject to the approval of such organizations as API, AGA and ISO.

The acceptable uncertainty in the quantity of transferred liquid or gas determines meter selection. The uncertainty is the sum of the errors of all components of the metering system. In the case of volumetric flowmeters, this includes errors by flow, pressure, temperature, density, composition sensors, their A/D converters, and in calculating the amount of energy (not mass, but energy) transferred. The hydrocarbon industry claims that custody transfer operates at an uncertainty of ±0.25% on liquid and ±1.0% or better on gas service, but I consider these numbers overly optimistic.

Table I lists flowmeters that can be used to measure hydrocarbon liquids and gases. The table also gives the chapter numbers where each is described in my handbook, their accuracies (if they're correctly sized, installed and maintained), and other main features. Table I doesn't list their first costs because, in larger transactions, the cost differences between meters are small in comparison to the cost of measurement errors.

For example, when oil costs $60/barrel, and we're unloading a 500,000-barrel tanker, each 0.1% uncertainty corresponds to $50,000. In cases of smaller quantities, meter cost differences can be considered, and if accuracy is not critical, one can determine the transferred quantity without flowmeters just by measuring the level change of liquids or pressure change of gases.

A typical liquid custody transfer skid includes multiple flowmeters (master and operating meters), flow computers and meter provers. For pipe sizes below 42-in. diameter (1.07 m), onsite provers can be used and API requires prover accuracy to be 0.02%. The meter prover volume is calibrated against Seraphin cans, whose precise volume is traceable to NIST. Recalibrations should be performed frequently, typically before, during and after the batch transfer.

Béla Lipták
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TABLE I: FLOWMETERS USED IN CUSTODY TRANSFER

Table-Flowmeters-used-in-custody-transfer2

A: Orifice meters are still widely used for liquid measurements and have been for many years. The orifice meter accuracy is much affected by the details of installation, and comments about poor accuracy can usually be explained by poor installation.

The advantages and disadvantages of the various meter technologies are well known, and the available technologies have changed. One example is how Coriolis meters have become more popular as the technology has matured and competition has driven prices down. It’s clear that meter selection is heavily affected by pipe size. I find it hard to imagine a Coriolis meter in a one-meter-diameter pipeline. And the cost?

National and international standards can affect decisions. Custody contracts may well have statements limiting options. It can happen that non-technical people write those contracts.

I'm prejudiced, but I have the impression that salespeople tend to suggest the more expensive choices in their catalogs.

In the decision process, it's common to underestimate the costs and details of installation for the various flowmeters. Accuracy is expensive.

I once developed a program to aid in flowmeter selection. The user entered information about the fluid and flows. The program then displayed a list of possible meter types with costs, accuracy and permanent pressure loss. This brought interesting comments challenging estimated costs and accuracy. Support for this program went away as the costs of maintenance would be high as the data changed.

Good question, and we need to discuss these things.

Cullen Langford
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A: Orifice flow measurement is at best ±4% accurate. In spite of this, it was used for custody transfer of liquids for many years until better and more accurate instruments became available. Today, the Coriolis flowmeter is the standard for liquid flow custody transfer due to its high accuracy and often because it directly measures mass flow instead of volumetric flow. It's expensive, so sometimes if liquid is transferred from tank to tank, the before and after tank level measurements are used for custody transfer of liquids. However, that requires very accurate tank level measurement.

Measuring the flow of gases 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 uneconomical to spend extra money on 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 source of the gas, such as a cylinder.

Orifice flow measurement depends on Bernoulli’s law that relates 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 the flow rate, so there's no practical way to measure this pressure drop. We do the best we can, and usually just measure the pressure drop at the flanges that hold the orifice plate in place, and depend on a correlation (approximation) to estimate the pressure drop at the vena contracta, or just assume that the pressure drop at the orifice is the same as that of the vena contracta.

Richard H. Caro, CEO, CMC Associates
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A: One of the biggest reasons why orifice plates are not used for liquid custody metering has to do with the following:

During startup, while the flow stabilizes, the error in measurment is generally unacceptable. The same happens when the system is shut down.
Sizing an orifice plate to guarantee custody transfer precision requirements generally requires the orifice plate to be designed and fabricated to extreme tolerances that not every company can do.
The turndown ratio for liquid orifice plates would require users to have too many orifice plates available for when flow conditions change.

To summarize, though custody transfer can be performed with orifice plates, it’s not recommended due to the fact that signal instability can cause accounting errors; design and fabrication of orifice plates is very expensive; and finally, if the custody transfer is based on varying flowrates, then the need for additional orifice plates is increased.

Alex (Alejandro) Varga
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