 |
|
||||||||||
|
 Inside this issue
 Why Flow Computers? Electronic flow computers play a vital role in the tracking of natural gas and hydrocarbon liquids, as product ownership is transferred through and between pipeline systems, across plant boundaries, in terminals and at the wellhead. In addition to bearing responsibility for the transfer system's fiscal integrity, the flow computer must act as the "brain" of an enhanced system that integrates numerous measurement, control, sequencing, data logging and communication functions into a single stand-alone device. With all that's involved, the industry demands a highly secure, tamper-free, reliable and accurate measurement device that will reduce the risk of systematic errors or increased uncertainties. This has occurred with some unsecured field programmable devices that are controlling pumps, motors and compressors, to lose critical data or measurement signals as a result of accidental resets or reprogramming of equipment.
The flow computer's security regime should include multilevel keypad and serial communication passwords, keylocks, totalizer error-checking, redundant totalizer registers and configuration data, and maintenance of calibration data even when firmware is changed. Data archiving, alarm and configuration events, and stored reports can provide months of backed-up data for audit retrieval. By having on-line configuration access, the field technician or operator can easily adjust his operating parameters without affecting program integrity. This can be of critical importance when a variety of field instruments can individually fail and a quick response is needed. In custody-transfer applications, real-time, high-accuracy measurement and control is being achieved. Because the flow computer is regarded as the "cash register," it is imperative that it retain or acquire the ability to seamlessly interface to devices such as digital transmitters; turbine, ultrasonic, and coriolis mass meters; viscometers; and densitometers. NX-19 and AGA-8: The Difference The American Gas Association Transmission Measurement Committee Report No. 8, has made dramatic improvements over AGA Report NX-19. In 1962, The American Gas Association (AGA) published a report (NX-19) developed for calculations of supercompressibility factor of natural gases. Because of the limited set of input parameters, and the limited accuracy and applicability of previous AGA and ISO Standards, both GERG and AGA provided new technically defensible Standards for the measurement of Natural Gas. These Standards established the most extensive empirical data base (12,000 for AGA-8 vs. 300 for NX-19) ever developed in measurement of gas. Additionally it established the tightest governor, for example, on the equation performance ever developed. This governor dictates that the equation performance by different flow computers must agree within 50 ppm. This tolerance can be met only by performance of the AGA report #3 and 8, 1992 revisions of the equations. AGA-3 uses new techniques (equations) with consideration of Orifice and Pipe dimension expansion from reference conditions based on temperature. Tolerance for concentricity and structural dimensions tightly define manufacture of measurement components. AGA-8 sets specific accuracies associated with gas composition and defines detailed methods of computing compressibility and density for individual constituents. AGA Report No. 8 is dramatically more accurate for high specific gravity and high carbon dioxide content natural gases than NX-19.
Comparison of Computed Compressibility Factors for A.G.A. Reports No. 8 and NX-19 with Experimental Compressibility Factors for Seven Natural Gases. The average absolute percentage deviation (A.A.D.%) the bias percentage deviation (BIAS%) and the maximum percentage deviation (MAXD%) are referred to as their abbreviations. With acknowledgments to Dr. K. Starling, Starling & Associates, Norman, Oklahoma, for published data. Coriolis Meters Some coriolis Mass Meters can provide a density or viscosity measurement in addition to a mass or volume measurement. Most coriolis meters are used for liquid applications in process industries. They have been popularized by not requiring any moving parts. The coriolis force meter provides a direct measurement of the mass flow rate of a fluid. Manufacturers claim that coriolis meter performance is generally unaffected by changes in process temperature and pressure. They can be advantageously used in place of conventional meters in many applications where exposure to hazardous or corrosive chemicals would be harmful to the operator, or when product conditions are not well-known or unstable. Coriolis meters are typically used where product flow is measured or sold on a mass basis. However, some coriolis mass meters also provide an on-line density measurement. Since volume at flowing conditions is the ratio of Mass/Flowing Density, the mass meter can provide a volumetric pulse output similar to a conventional volumetric meter. But process conditions, fluid properties and mechanical system design can affect individual meter performance. So, it may not be just a simple choice between mass and volume. There has been great interest in the use of these flow meters in oil production, transportation and marketing applications. Their use has been approved by various Weights & Measures agencies. API and IP in UK have produced technical papers for the guidance of users. An acceptable API standard for the use of these devices has not yet emerged due to the absence of sufficient user field data from metering systems with varied operating pipeline and process conditions. Omni flow computers already have the necessary firmware to receive both pulse and RS485 communication from Micro Motion transmitters. Omnis can provide selectable mass or volumetric metering and proving with Micro Motion meters, the leader in mass meter installations within the energy industry. Applications at locations as far apart as Thailand, Colombia and Siberia are using Omnis with Micro Motion meters. Omni has also received NMi Approval (Weights & Measures, Holland) for interface to the Rheonik mass meter.
Smart TransmittersDigital transmitters have the ability to provide more than one measurement variable at a time, resulting in reduced costs and improved accuracy. Digital (a.k.a. "smart") transmitter technology has been available in flow-related devices for at least a decade. The ability to integrate field measurement into process control systems on a more cost-effective basis is a growing requirement around the world. Improved measurement accuracy and ease of reliable calibration is a constant goal in a technologically-challenged world that has been used to the 4-20 mA signal standard. Most field transmitters, utilizing capacitance and resistance, communicate with the flow computer via 4-20 mA signal. However, only a limited amount of information (i.e. the measured variable) can be represented by a single 4-20 mA signal. A major benefit of a digital transmitter is the ability to provide more than one measurement variable at a time. Digital communication can significantly reduce the costs of separately connecting multiple measurement devices into a distributed process control system. Smart transmitters improve accuracy by eliminating analog to digital conversions by communicating a direct digital signal. But digital protocols can also take time to send all the process information to the host system. The delay can be more than one second. This can cause a greater measurement uncertainty and may provide little or no gain over the 4-20mA signal. For example, in an Omni, the process input comes in as a continuously monitored frequency and not as a sampling A/D. Because operational features, such as PID, need fast updates, the Omni expects to use that process value every 500 msec. The major obstacles to acceptance of digital transmitters and the proprietary digital protocols being used by individual manufacturers are the absence of a common standard and sufficiently skilled field technicians in various world markets who can adapt to changing calibration requirements and new technologies. A common standard would ensure the development of open architecture systems. A common industry standard such as the one for analog transmitters, would ensure that any product from any manufacturer can work together safely and reliably with any other device. The new digital standard being developed is known as Fieldbus. Omni flow computers can accept RS485 Modbus from Fisher-Rosemount 3095 Multi-Variable Transmitters; and from the Honeywell ST3000 series of transmitters using Honeywell's DE protocol. Omni’s built-in modularity allows Omni to be both Fieldbus-ready and technologically ready for any other advancements without obsoleting the "cashregister"! |
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
| | | | | | | | | | | |
 |
©2003 Omni Flow Computers, Inc. All rights reserved |
