How to Deliver Liquids and Gases Accurately

In the industrial world, the term custody transfer does not refer to what happened to Dustin Hoffman's "screen" son in the Academy Award winning film "Kramer vs. Kramer." Instead, it describes a method for keeping track of quantities of fluids—liquids, gases, slurries—as they are moved between two locations.

02/01/1999


KEY WORDS

 

  • Process control & instrumentation

  • Custody transfer

  • Flowmeters

In the industrial world, the term custody transfer does not refer to what happened to Dustin Hoffman's "screen" son in the Academy Award winning film "Kramer vs. Kramer." Instead, it describes a method for keeping track of quantities of fluids—liquids, gases, slurries—as they are moved between two locations. The transfer is usually done as part of a commercial transaction (where money changes hands as payment for material delivered) or as transfer of products used internally as part of the overall process.

In either case, the method of determining just how much material changed hands must be accurate or, at least, as accurate as the parties involved in the transfer agree on. Adding to the confusion, however, when consumers or consumables are involved, is the addition of industry and government bodies with the responsibility and authority to set the standards by which custody transfer methods are judged.

According to Sue Lynch, product manager vortex flowmeters at Yokogawa Industrial Automation (Newnan, Ga.), "The definition of custody transfer can be very different from one market to another. This difference is generally driven by the product being bought or sold. Gas, oil, water, and steam are but a few of the materials on the list."

In the real world, methods of keeping track of quantities of moving materials vary from simple to sophisticated. Cross-country pipeline-delivered materials occasionally used a "slug" of material different from the liquid being delivered to mark the beginning and end of the "measured" quantity delivered to a pipeline station. The receiving station simply diverted its premeasured load after detecting the marker material. When the marker reappeared, the diverted was closed. This method worked but accuracy was low and separating out foreign material could be a headache.

Another method of transferring material uses volumetric measurement. Material to be delivered is first premeasured in a tank of known dimension using level sensing technology. Once filled, the reservoir is simply pumped out or gravity emptied delivering a known amount of product. Level sensing accuracy and the degree to which the container can be thoroughly emptied determines this method's accuracy.

When weight is the criteria for measuring pipeline-deliverable material, a similar method can be used. Using load cells, the material is weighted and then delivered in same manner as the volumetric example above. Both methods—popular in batching operations and applicable to gravity-fed bulk-solids material transfer—can be used in nonbatch material transfer applications.

Doing it on the fly

Flow measurement either takes place in closed conduit, usually a pipe of known diameter, or an open channel such as a river or a concrete-lined stormwater runoff channel. In either case, a flow measurement system monitors and controls a process.

Probably the easiest way to "premeasure" material to be sent to a remote location uses flow measurement technology. Flowmeters fall into three categories—those that measure volume, velocity, and mass flow. Volume measurement entails continuous counting of constant partial amounts of flow over long periods of time. Velocity measurement determines the speed of a liquid at a given point of the pipe's diameter. Multiple measurements are used to determine a flow profile. Velocity flowmeters can serve as a quasi-flow measurement or volume counter if the measuring point is positioned at the point in a pipe, where average flow velocity occurs.

In transfer situations, where density changes can occur during transit, mass flow is used. Common continuous mass flow methods measure either the Coriolis force; volume flow and density; and volume flow, temperature, and pressure. Because flowmeters that measure mass flow are generally much more expensive than simple volume counters—three to ten times is not uncommon—they are only used where the materials to be measured are very expensive (chocolate, speciality gases, fine chemicals, aromatic hydrocarbons, etc.).

Legal matters

No matter which type of measurement is used or what type of flowmeter is used, custody transfer requires using any one of these methods still boils down to the accuracy of the measurement and whether or not the recipient is willing to accept that accuracy.

Generally, custody transfer operations are controlled by national laws and regulations. Typical industrial applications include commercial transportation of materials via pipeline and water supply. Although flow measurements are the basis for a commercial transaction, they are also used for levying Customs duties and excise taxes. However, just because regulations are "on the books" regarding custody transfer requirements, process engineers do not necessarily have an accepted approval process to turn to. According to Stephen Ifft, flow measurement engineering manager for McCrometer Inc. (Hemet, Calif.), "Canada has the only government-based approval process of its kind in North America. The United States does not provide this service."

In addition to state and federal laws and regulations, other organizations also certify flow devices. Depending on the intended application, flowmeters may require certification by organizations such as the American Gas Association (AGA, Arlington, Va.) and the American Petroleum Institute (API, Washington, D.C.).

Mr. Ifft continues,"Independent organizations such as the AGA and API are expected to provide such standards and guidelines. The 'approval process' through these organizations are not clearly defined since the standards deal with meter design rather than performance. A patented design could not be covered with such a standard. This may be changing as ultrasonic technology is entering the industry. Ultrasonic meters, while all use the same basic principles, have proprietary designs. Current drafts of standards within AGA and API deal with design and performance issues in different ways," Ifft added.

Some basic requirements

Typical certification requirements include guarantees of accuracy, operational reliability, and protection against unauthorized tampering. Guarantees of accuracy vary with type of flowmeter and the media to be measured. Most organizations and regulating agencies publish recommendations only. As far as operating reliability and unauthorized access issues are concerned, regulations are stringent.

In the case of liquid custody transfer applications, flowmeter faults which cause incorrect measurement and cannot be recognized either by the operator or through the instrument's diagnostics must not occur. Flowmeters also must be constructed so that unauthorized personnel cannot tamper with its operation. Prior to being authorized for custody transfer use, devices are tested for tamperproof design. Calibration to meet a specific application only takes place once this criteria is met.

"The need to measure, dose, or mix fluids while controlling their flow is omnipresent in industry and society, in general," says Tim Cillessen, technical specialist at Siemens Energy & Automation (Alpharetta, Ga.). "Increasing raw material prices and labor costs have forced industries to produce, process, administer, and manage resources more efficiently and economically. And, despite the multitude of flow measurement technologies that have been developed to assist in implementing needed control strategies, older technologies such as simple volumetric meters have proved indispensable in this role," Mr. Cillessen continues. "Even though a large number of functional (and legal) requirements may restrict use of some flowmeter technologies, there are almost no restrictions preventing the use of mechanical flowmeters."

For example, the inherently reliable mechanical nature of rotary-piston flowmeters allows them to be used for calibrated measurements in the production of precise mixtures and as automatic proportioners. They are suitable for automatic dosing in the production of precise mixtures and are frequently used in mobile applications because they can function without auxiliary power.

Mechanical flowmeters function in viscosity ranges up to 350,000 mPa. Depending on materials of construction, they can handle aggressive substances such as strong acids or bases. They can be used to measure substances without electrical or acoustic conductivity and can be installed directly downstream of flow disturbances. Even though mechanical flowmeters can be used in high shock situations and electrically heated when process conditions require, pressure drops across them can be high. The unit's physical size limits its capacity.

As expected, nonmechanical flowmeters must also exhibit high reliability to be used in custody transfer applications. According to Sue Lynch, Yokogawa Industrial Automation (YIA) supplies vortex flowmeters to companies that sell steam for large-scale commercial and industrial heating. Because heating costs can be very high in these applications and customers demand accurate billing, reliability and accuracy are absolute requirements. "Additionally, vortex meters have no moving parts and offer a wide turndown that accommodates the wide range of usage requirements depending on seasonal requirements," Ms. Lynch adds.

YIA manufactures vortex flowmeters that can be recertified to meet individual state weight and measurement criteria for steam custody transfer. In heating applications, flowmeters used for billing purposes must be recalibrated on a specific schedule in a NIST-traceable flow lab. For example, in the state of Maryland special handling and technical review procedures must be followed to assure the meters are recalibrated in an "as found condition." The lab gave a YIA vortex flowmeter an accuracy statement of

Full steam ahead

In the case of Berghuizer Paperfabrick, a Dutch paper factory in Wapenveld, The Netherlands, measurement of low pressure steam in a 28-in. (700-mm) pipeline was required to perform a complete energy balance. Turbine output steam is used in this paper, envelope, and packing paper manufacturing operation to heat paper machines and glue. The steam condensate was then pumped back to the power plant.

Because the amount of energy supplied to the plant was part of manufacturing expense, it became a fiscal transfer measurement, which can require flowmeter certification. Now a part of the accounting process, the steam transfer measurement required high turndown (1:20), no pressure loss, high repeatability, high accuracy with regard to internal transfer, and high reliability (low maintenance costs and trouble-free operation).

High turndown was needed to cover the wide range of steam flows encountered with swings in production demand. Pressure loss robs energy from the steam and reduces the amount delivered to the user. Because ultrasonic flowmeters offer both high turndown and no protrusion into the pipe to cause pressure drop, they were chosen for this application.

According to Mike Scelzo, vp marketing and product development for Panametrics Inc. (Waltham, Mass.), "The steam flowmeters specified were not certified by an outside agency for custody transfer. But both the buyer and seller in this case agreed on the accuracy of the meters when the technical advantages and low cost of ownership were considered. These factors were enough to make uncertified meters acceptable to both parties."

Because getting flowmeters specified for custody transfer without certification is not necessarily the rule, Mr. Scelzo is quick to point out that Panametrics is proactive in providing accuracy-enhancing multipath configurations in its ultrasonic line should custody transfer be on the horizon.

Running the gauntlet

Not all custody transfer flowmeters get off the hook when it comes to pre-service certification. A case in point was approval process faced by Micrometer Inc. when its V-Cone differential pressure (dp) flowmeter was specified for a Canadian natural gas pipeline custody transfer operation. Even though the V-Cone represented established technology—it is a dp flowmeter based on Bernoullian principles patented in 1986—Measurement Canada, the approving agency, required that it pass the Specifications for Approval of Gas Meters and Auxiliary Devices (LMB-EG-O8). Because the V-Cone is a dp device, McCrometer's intention was to have it certified under the same principles as an orifice plate.

However, after evaluating both product and performance data, Measurement Canada decided that the V-Cone should be evaluated under Section 7 of the specifications regarding turbine meters, due to similarities in accuracy (

Once test criteria were set, a laboratory-based performance test was run. This required a witnessed test run of both the flowmeter and its instrumentation that would support the documented data produced during the evaluation process. It was not until accuracy was verified as

Even with the NOA, only certain models were approved for custody transfer measurements in Canada. ANSI flange types, pressure class, acceptable pipe sizes, and materials of construction were limited under its scope. Strict installation requirements per the manufacturer's recommendation were also mandated. Even the device's nameplate information was specified.

According to McCrometer's Mr. Ifft, "The approval has been significant for the V-Cone device and will have impact well beyond Canada or even North American applications." Dealing with performance rather than design issues has clearly given the approval process credibility in this case, proving flowmeters that are accurate, stabile, reliable, and tamperproof can find their way into a wide variety of custody transfer applications.






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